ENVIROI'i11ENTAI. GEOLOGY OF URBAN AND URBANIZD~G AREAS: A CASE STUDY FROM TSE SAN MARCOS A3..EA, ~t'Ek'\S APPROVED BY SUPERVISORY CO~J.MITTEE: Copyright by Thomas Walter Grimshaw 1976 ENVIRONMENTAL GEOLOGY OF URBA?~ AJ.\'D URBANIZI"NC P._1\.EAS: A CASE STUDY FROM THE SA..L~ !1ARCOS AREA, TEXAS by THOMAS WALTER GRD-1SHAW, B • S . , M.A. DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the ~e<;uirt=mt:nts for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT ACSTIN August 1976 PREFACE Beginning about the time of the awakening of environ­mental awareness in the United States during the early 19603, geology has become increasingly recognized for the substantial contribution it can make toward the solution of problems stemming from man's interaction with the earth's physical environment. This increased recognition has led to the establishment and growth of an application of the science that is called environmental geology. This term, and the geologic application it signifies, are becoming well accepted in the geologic profession. This study attempts to formulate and demonstrate a methodology for conducting environ­mental geologic study of urban and urbanizing areas, where en­vironmental problems are most acute. I wish to thank Dr. Keith Young for serving as super­visor for this study and for his invaluable assistance with the field mapping part of the project. ~ wish also to thank the following individuals for serving on the supervising committee. Dr. L. J. Turk deserves credit for upgrading the overall quality of the study by his constructive criticisms and helpful suggestions. Dr. C. G. Groat provided valuable input by critiquing the study from the point of view gained by his environmental geologic work elsewhere in Texas. Dr. E. G. wermund made significant contribu­tions because of his familiarity with environmental geologic problems along the Balcones Escarpment. Peter Coltman, as a planner having considerable knowledge of geology, has helped greatly in the attempt of this study to convert geologic informa­tion into a form which is readily usable by nongeologists. In addition to these committee members one other individual, Mike iv Colchin, deserves commendation for serving as student editor for the dissertation. Mr. Charles Batte, soil scientist for the Soil Conserva­tion Service soil survey party in San ~rcos, familiarized me not only with the soils in the study area, but also with the soil classification system now used by the SCS. In addition, he pro­vided free access to unpublished soils raaps of the area and filled many of the "holes" in the existing data. Dr. Glenn Longley, professor of Biology at Southwest Texas State University, contri­buted freely of his knowledge of environmental problems in the study area and provided much interesting discussion of the cavern system and the unusual fauna of the Edwards aquifer. ~any land­owners have my gratitude for allowing access to their ranches during the mapping phase of the project and for sharing their knowl­edge of various aspects of the area. The Department of Geological Sciences assisted in this study not only by furnishing excellent physical facilities, but also by providing aerial photographs and a Jeep f cr carrying out the field mapping. I also give thanks to the Geology Foundation of the Department for a generous grant for summer field work in 1974 and for a fellowship from the Hogg-Sharp Scholarship Fund in the Spring semester, 1975. Finally and foremost, I thank my wife, Susan, not only for her invaluable editorial efforts on the text, but also for her patien~e, support, and fortitude during the conduct of the study. This dissertation was submitted to the supervisory com­mittee in March, 1976. v FRONTISPIECE The Balcones Escarpment between Austin and San Antonio. The city of San Marcos can be clearly seen just south of the center of the case study area (outlined in white). Note the offset in the scarp in the northern half of the area. This Landsat (ERTS) Band 5 image was made on July 21, 1973. vi vii ENVIRONMENTAL GEOLOGY OF URBAN Ai'ill URB&\IIZING AREA: A CASE STUDY FRmf THE SAJ.\l K.\RCOS AREA, T[XAS by Thomas ~alter Grimshaw A B S T R A C T A major focus of environmental geologic work for the foreseeable future will be on the growing urban areas of the "'-orl Engineering geology pocket Soils .. pocket Resources pocket Processes pocket Landform pocket Current land use pocket Land use control pocket Complete Demand Analysis Hierarchy for a sanitary landfill . . . . . . . . pocket Geologic map of the Kyle section pocket Geologic map of the San Marcos section pocket xx Chapter 1 INTRODUCTION The term environmental geology was coined by Hackett in 1964 (Ivey, 1971, p. 3). Since then, the ter;n has been variously defined by different authors (Hayes and Vineyard, 1969, p. 4; Flawn, 1970, p. xvii; Ivey, 1971, p. 3; Geyer and ~'lcGlade, 1972, p. l; Turner and Coffman, 1973, p. S; Fisher, 1974, p. 1). A concise and adequate definition for this study is as follows: "Environmental geology is the comprehensive use of geologic data, methods, and reasoning for optimizing the relation between man and the geologic aspects of his physical environment." This definition of the term has a strong ecological connotation. Ecology is defined simply as "the study of the relationships bet"Ween organisms and their environments" (Gary, McAffee, and Wolf, 1972). Human ecology is therefore concerned with the relationship of man to his total environment, and environmental geology is that part of human ecology which deals specifically with the interaction of man with his geologic environment. Flawn (1970, p. xvii) early recognized this ecological basis of environmental geology in his definition of the term: "Environmental geology is a branch of ecology in that it deals with relationships bet'Weerr man and his geological habitat; " The ecologic or interactive basis of environmental geology is a key element of this study. The purpose of environmental geology is to avoid or reduce conflict of man's facilities or activities with the geologic setting in which they are located. In theory all subdisciplines of geology, including those involved with the procurement of mineral and fuel resources, could be thought of as comprising environmental geology. In practice much environmental geologic work includes a synthesis of such traditional geologic subdisciplines as engineering geology, hydrology, hydrogeology, and urban geology. This somewhat more re­stricted view of the scope of environmental geology has served to advance the use of geology in such activities as urban and regional land use planning, formulation of land use control measures, and site suitability studies. It has become increasingly apparent that growing urban areas constitute a major focal point of environmental geologic study. The importance of cities was discussed by Flawn (1970, p. xvii): "A little reflection makes clear why the term urban geology is com.~only used interchangeably with environmental geology. In urban areas the use of the earth is most intense and society has the most urgent earth problems." Legget, who believed geology to be so important to cities that he devoted an entire book to the topic, put it this way (1973, p. 3): "The purpose of this book is to show how essential is geology in the planning and development of cities.... the use of geological information, and of geological methods to obtain new in­formation about local subsurface conditions, should ... be an essen­tial part of the physical planning of all cities." The objective of this study is to present a syste~atic method­ology for optimizing the interaction between man and his geologic environment in growing urban areas. Two aspects of this interaction, indicated by the terms urban and urbanizing in the title, will be con­sidered. The urban aspect deals with urban facilities already in existence and involves primarily curative measures for environmental geologic problems of these facilities. The urbanizing aspect has to do with facilities not yet in existence. It deals with the role of geology in physical planning for cities, and therefore involves preventive measures for avoiding future environmental conflicts. As used here, the term urban refers generally to concentra­tions of population not engaged in or supported directly by local agricultural pursuits. The scope of the methodology is confined to the immediate area of influence of the urban center being investi­gated. Distal effects of the city, such as environmental geologic problems of procurement of resources far removed from the city, are not considered. Also, environmental geological aspects of the study area that are not directly related to urbanization are not in general taken into consideration. For example, if a city were located in an area where mining for metallic ore is also taking place, the use of this methodology would not account for the environ­mental geologic problems associated with that mining activity. Be­cause most geologic problems of urban areas are caused by the improper use of land, the focus of this study will be on the envirornnental geologic implications of various urban land uses. Also, the main objective of this study is the mitigation and prevention of environ­ mental conflicts of urban facilities and activities with their physical environment. Consequently, tte avoidance of conflicts among various land uses stemraing from their proximity to each other is not generally accounted for. This study is organized into six chapters. Following this introductory chapter, Chapter 2 presents the methodology for conduct­ing environmental geologic study of urban and urbanizing areas. In Chapter 3 the San Marcos case study area is described in general terms, and the data sources are delineated for use in subsequent sec­tions. Chapter 4 demonstrates the curative part of the methodology and uses the case study area as an exam?le for the investigation of environmental geologic problems of urban facilities already in exis­tence. Chapter 5 demonstrates the preventive part of the methodology by showing how environmental geology can be used in physical land use planning in the case study area. Chapter 6 rounds out the text with a short summary and conclusions, and the geologic report which serves as the backbone of the study appears in the Appendix. The plates referred to in the text are in a pocket at the end of the dissertation. The San Xarcos area was chosen as a case study chiefly be­cause it is situated in the Interstate 35 growth corridor between Austin and San Antonio and because it is not yet fully urbanized. The suitability of the area as a case study is evaluated in Chapter 3. Chapter 2 A METHODOLOGY FOR ENVIRONMENTAL GEOLOGIC INVESTIGATION OF URBAN A.:.~D URBANIZING AREAS A successful methodology for environmental geologic study of growing urban areas should meet three requirements: 1) It should be universally applicable; that is, it should be usable in as many urban areas as possible, regardless of their location or geologic setting; 2) It should be comprehensive; all of the significant environmental conflicts must be accounted for; and 3) It should be systematic, so that it can be readily comprehended and rigorously applied. In addition, the methodology must provide for both the cura­tive and preventive aspects of the environmental geology of growing urban areas, as discussed in Cnapter 1. The objective of this chap­ter is to develop in concept a methodology which meets these criteria. Because of the fundamental ecological basis of environmental geology, the approach will be to emphasize the interaction of urban facilities with their environments. Both the impact of the environ­ment on the facilities and the impact of the facilities on the environ­ment will be considered. The primary emphasis will be on the geologic environment. Factors related to the atmospheric or biologic environ­ment will be given little attention, and the cultural environment is considered only insofar as current land use and existing land use control measures are concerned. Social, political, and economic factors are not accounted for, but their relation to the geologic factors in physical land use planning is described. The procedure of this chapter will be to: 1) analyze the components of urban areas and geologic environments, 2) describe the general na'Cure of the interaction of urban areas with their geo­logic environments, 3) delineate the natural and cultural data sources to be used in subsequent steps, 4) present a method for systematically studying the environmental conflicts of existing urban systems, 5) describe a procedure for evaluating the capability of land to sustain the demands placed on it by various urban facilities, so that future environmental conflicts can be avoided, and 6) present a diagrammatic summary of the methodology . COMPONENTS OF GEOLOGIC ENVIRONMENTS AND URBAN SYSTEMS Before a rigorous methodology for environmental geologic in­vestigation of growing urban areas can be developed, a clear dis­tinction should be made between a city and its geologic environment. Once this distinction is made, effective conceptual tools can be developed by analyzing the components of each. The objective of this section is to develop these tools by introducing organizational schemes for both urban systems and geologic environments. Components of the Geologic Environment A description of the geologic setting of a growing urban area should: 1) present the geological data in a way that reflects both how the geology influences urbanization and how it is influenced by urbanization, 2) concentrate on surface geologic phenomena, since much of the interaction of cities with their geologic settings occurs at the surface or in the shallow subsurface, and 3) be widely appli­cable. A scheme of organization that has these characteristics recog­nizes three broad categories -substrate, processes, and landform. Substrate The subsurface materials of a city fulfill three basic func­tions. First, they bear the weight or loads of buildings and other urban structures. Second, they are the medium that is excavated and used as fill as the land is modified for the city. Finally, the substrate serves as the source of locally derived low-value resources that are essential to the city's growth. The first two functions depend on the physical properties of the substrate, and the third depends on its resource potential. Because of these two vastly dif­ferent aspects of the substrate, it is described below in two cate­gories -physical properties and resource potential. As noted earlier, only the upper few meters of the substrate usually have significant implications for urbanization. Included in this zone are the soil horizons and the upper part of the bedrock. There are exceptions to this generalization, however, as exemplified by prob­lems that are sometimes associated with utilization of ground water resources. Physical Properties The physical properties of the substrate determine how it will respond to the activities and stresses imposed by urbar.ization . Some important physical properties are shear strength and bearing capacity, consolidation characteristics under loading, swelling poten­tial, corrosive potential, and porosity and permeability. These pro­perties in turn depend upon more fundamental characteristics of the substrate, such as lithology and chemical composition, amount and chemical character of subsurface water, large and small scale struc­ture, and amount and interconnection of void space. Physical proper­ties are commonly different at depth in the bedrock than near the surface where bedrock has been altered by soil-forming processes. For this reason the physical properties are described in two parts ­a deep substrate or engineering geology part and a shallow substrate or soils part. Resources Certain natural resources are very important in determining the suitability of an area for urbanization. These resources include the low-unit-value commodities, such as aggregates and water, which are essential to the maintenance and growth of a city. If they are not locally available in sufficient quantity, the cost of urbaniza­tion increases considerably, and if they are present, considerable environmental conflict frequently arises from their utilization. The indication of the resources on a map not only aids in their utiliza­tion, but also delineates areas where urbanization should be delayed until the resources have been utilized. Soil resources and locally derived food resources for the city are not accounted for in this study. Processes The geologic processes at work in an area also have much im­pact on urbanization. The processes of concern are geologic processes which occur at a rapid rate. Examples of important surface processes are fluvial, coastal, and mass movement processes. Subsurface proc­esses of importance include earthquakes, fault movement, volcanism, regional tectonic subsidence, and subsidence associated with the withdrawal of ground water or petroleum. Other processes, such as karst processes and aquifer recharge, take place partly at the sur­face and partly in the subsurface. A map should be prepared to indi­cate where the significant processes are active. Landform As used in this study, the terr:i landform refers to the shape of the earth's surface in the broadest sense rather than to geo­morphic features. The landform of an area does not usually have as great an impact on urbanization as do the substrate and the proc­esses . Also, landform is usually more subject to modification than are the substrate or the processes; excavation and filling operations are conducted in virtually all cities as they are built. Neverthe­less, topography does affect the pattern of land development and contributes to the relative suitability of an area for urbanization. Not only does the landform determine the extent of cutting and filling required as urbanization proceeds, but it also dictates t o a large extent the impact the urbanization will have on the environ­ment. The landforrn can be described in many ways, but one of the most useful for indicating suitability for urbanization is to show the areal distribution of slope. The Urban System Just as the geologic environment can be divided into compon­ents that are useful for investigating the city-environment inter­action, so can cities be classified based on their interaction with the geologic environment. A useful organizational scheme for this purpose views the city as a system having four components -urban situs, urban input, urban output, and transportation. The relation­ships among these components is shown diagrammatically in Figure 2.1. Each of the four components of the urban system is discussed briefly in this section, and the types of environmental problems to be con­sidered in each are outlined. Because many urban environmental prob­lems result from the use or misuse of urban land, the major land uses comprised by each category are also described. Urban Situs The urban situs component includes those aspects of the city­environment interaction which result from the physical location of the city. Tne problems considered in this component arise because the city is located at a particular site; these problems would be different or possibly would not exist at all if the city were located elsewhere. The problems include both the effect of the geologic environment on the city, such as the geologic hazards, and the effect of the city on the environment, such as uncontrolled waste materials released to the local environment. The concept of urban situs will be clarified considerably in later sections where the urban system scheme is actually applied. Because of the strong locational aspects of this component, urban land use plays a strong role in the problems considered. Many urban land use classifications have been formulated, but they all have basically the same categories with different groupings of these categories. Chapin (1965) presents a summary of a widely used systematic classification. Two simpler but useful classifications TRANSPORTATION OUTPUT INPUT Figure 2.1 The Urban System are given here: A. Yeates and Garner, 1971, 234: P· 1. Residential 2. Industrial 3. Commercial 4. Roads and highways 5. Public and semipublic 6. Vacant B. Detroit Metropolitan Area Regional Planning Commis­sion, 1962: 1. Residential 2. Extractive and industrial manufacturing 3. Manufacturing 4. Transportation, communication, and utilities 5. Commercial 6. Personal, business, and professional services 7. Public and quasi-public services 8. Recreation 9. Unused space Some of the categories could be used directly , whereas others would require subdivision to more specific land uses. It would be futile in this study to attempt a comprehensive urban land use classifica­ tion applicable to all cities. Instead, the classification should be tailored to suit the particular urban area under study. Urban Input Urban input consists of the energy and material commodities that are brought into the city for its maintenance and growth. A con­ venient classification of urban i~put consists of two categories ­ finished and unfinished conunodities. The finis~ed commodities are usually manufactured goods and are either transported into the city from some distance or are produced at industrial facilities in the city. Generally, they are not significant to the urban input cate­ gory of this study, although they may be important to other categories such as urban output or transportation. The unfinished commodities comprise the raw materials that are consumed or transformed within the city. These materials can be classed into two categories -goods produced locally and goods imported from some distance away. The imported unfinished goods are generally significant to this study only insofar as their transport into the area is concerned. Tne locally derived, unfinished commodi­ ties therefore have the greatest local envirolli~ental significance. These commodities are the low-unit-value, high-place-value resources (Bates, 1969, p. 7) that constitute the essential local basis for the growth and maintenance of a city. Examples of these resources are aggregates, such as crushed stone and sand and gravel, and water, including both ground water and surface water. On first impression it may seem that the urban input category outlined here is essentially synonymous with the resources category introduced earlier for the geologic environ.~ent. However, there is a great difference in the application of these two. The resources category refers to the presence and location of the resources, whereas the urban input category is used as a vehicle for describing environmental problems associated with actual use of the resources. Many types of resource extraction activities, such as gravel pits and stone quarries, require definite, sole commitment of land use, at least for the lifetime of the operation. Other activities, such as ground water puropage, involve little or no exclusive commit­ ment of land use. Urban Output The output which emanates from cities is of two basic types ­finished goods and wastes. The finished goods comprise much of the economic value produced by the city and often provide the major economic reason for the city's existence, but they usually do not cause severe local environmental geologic problems. However, the wastes generated by a city comprise some of the most severe environ­mental conflicts associated with urban systems. Urban wastes are amenable to many classifications. An impor­ tant distinction to be made is between controlled and uncontrolled waste output. Uncontrolled waste output is released to the environ­ment at or near the point of generation withir. the city . Examples are heat energy released to the atmosphere at industrial plants and solid waste discarded by residents as litter. Environmental degra­ dation caused by this kind of waste is best considered in the urban situs category. Controlled waste output is that which is systemati­ cally collected, perhaps treated, and finally disposed of. ':bis type of waste is the main topic of this section. Controlled waste output can be effectively classified as follows: I. Solid wastes A. Individual and municipal wastes 1. Individual dumps 2. Collective dumps 3. Sanitary landfills B. Industrial wastes II. Liquid wastes A. Individual and municipal wastes 1. Septic tank and drainfield systems 2. Sewage treatment plants B. Industrial wastes III. Gaseous wastes IV. Energy (heat) wastes A. Air disposal B. Water disposal Some of the categories of this classification require a com­mitment of land use at least for the duration of their operation, whereas others require no land use commitment at all. Solid waste disposal sites not only require land use commitment during operation, but also greatly restrict the use of t he land after operations cease. Septic tank and drainfield s ystems, on the other hand, are designed specifically to be used in the subs urface while the surface is used for entirely different purposes. The gaseous and energy wastes often have few environmental geologic implications. Transportation Transportation is classed as a discrete component of the urban system because it ties a city together and links it to the surrounding area. Included in the transportation category are the common means of moving people, materials, and energy. Some of the components of urban input and output, excluded from those categories because they were significant only insofar as their transport is concerned, are indirectly considered in this category. Perhaps the most effective classification of the transportation component is according to the means of transport, as follows: I. Automobiles and trucks A. City streets B. Roads and highways II. Railroads III. Xass transit sys teiilS A. Surface B. Subsurface IV. Airports v. Pipelines VI. Power lines Some of the environmental problems of the transportation facilities that are located within a city (internal circulation) are probably more effectively considered in the urban situs section than in this section. Interaction of the Urban System with Its Geologic Environment In summary , a clear distinction has been made between a city and its geologic environment. The urban system has been subdivided into four components which have different kinds of interaction with the geologic environment, and the geologic environment has likewise been subdivided, as shown in the following outline: I. Substrate A. Physical properties 1. Engineering Geology 2. Soils B. Resources II. Processes III. Landform The degree of interaction between the components of urban systems and their geologic environments can be illustrated by the conceptual interaction matrix in Figure 2.2. Although this interac­tion is shown in simplified form in the figure, it is in fact highly complex because each of the components of the urban system and the geologic environment are themselves highly complex. The crux of the problem of conducting environmental geologic investigation of urban and urbanizing areas thus lies in analyzing the conflicts arising from the interactions indicated in the matrix. An additional complicating factor is the two-fold nature of the problem noted in the introduction--the determination of conflicts between the geologic environment and urban facilities already in existence, and the pre­vention of future problems by incorporating environmental geologic considerations in physical land use planning for future urban sys­tems. After the generation of the data source ~aps is described, the remainder of this chapter will approach each of these parts of the problem separately. GENERATION OF DATA SOL'RCES The first step in the methodology is to define the limits of the study area and generate the data sources that will be used in subsequent steps. The components described in the preceding section will be used to organize the data sources, and the data will be pre­sented as a series of maps. Two broad classes of data -natural and cultural -are important in the environmental geologic investi­gation of growing urban areas, so the maps of this section will be presented in the context of this two-fold classification. A topo­graphic map is probably the best base ~ap for the data source maps . Definition of the Study Area When a growing urban area is selected for environmental geologic study, the limits and extent of the area to be included GEOLOGIC ENVIRONMENT SITUS ~ w I- Cl) >-INPUT Cl) z < CD a: OUTPUT ::::> TRANSPORTATION PHYSICAL PRO PE RT IE S RESOURCES PROCESSES LANDFORM >< x xx 0 00 x ' 0 >< xx x 0 x x x STRONG INTERACTION >< WEAK INTERACTION 0 LITTLE OR NO INTERACT I 0 N Figure 2.2 Interaction between Components of the Urban System and Components of the Geologic Envi~onment in the investigation must be determined. The area to be covered depends on several factors: 1) the areal extent of existing urbani­zation; 2) the amount of area external to the urbanization but still under its strong influence, where land is used for resource procure­ment or waste disposal; 3) the rate or urban growth; 4) the primary projected directions of growth and any extant physical barriers, such as large water bodies; and 5) the length of time for which the planning period extends. Once the boundaries have been set, the scale of the data source maps can be determined. The scale should be as large as possible Nithin the limits imposed by manageable map size, the resolution or quality of the data to be presented, and the economics of map reproduction. Natural Data Sources The natural data sour~es portray the geologic characteristics of the area. In the preparation of these maps the organizational scheme of the geologic environ~ent is used, and a map is prepared for each cf the lowest level categories. All map units of each cate­gory can usually be shown on a single map. However, if these units are numerous and overlapping, which is possible on the Resources and Processes maps, it may be desirable to use more than one map to avoid confusion. This procedure could, of course, be carried to t he extreme so that a separate map could be prepared for each map unit. The Engineering Geology, Soils, and Landform maps will not have overlapping units and are therefore best shown as a single map. Engineering Geology Xap The Engineering Geology map is intended to portray the engineering characteristics of the substrate below the soil zone. In this map geologic units are grouped together into physical property map units, and the location, extent, and distribution of each unit are depicted. The map is backed up by a table of the map units con­taining a qualitative description of each unit and, if data permit, the important engineering properties of the units. The physical properties of the substrate below the soil zone are a function of the bedrock, so the Engineering Geology map is a direct derivative of the conventional geologic map. The type of geologic map which should be prepared for derivation of the Engineering Geology map is discussed in a later section. Soils Map The physical properties of the bedrock are often greatly altered near the surface by soil-forming processes . The function of the Soils map is to express the areal distribution of the physical properties of the substrate above the bedrock. Probably the best type of soil unit to use is the soil series. The current soil classi­fication used by the U. S. Department of Agriculture, Soil Conserva­tion Service (Soil Survey Staff, 1960) is based on objective physical and chemical criteria (Buol, Hole, and McCracken, 1973, p. 198). Soil series recognized on the basis of this classification therefore adequately represent the physical properties of the shallow substrate. Most soil reports now published by the Soil Conservation Service include a tabulation of the physical and engineering properties of each soil series. A similar tabulation should accompany the soils map prepared for this study. Soil series maps are best prepared by professional soil scientists who have a working knowledge of the somewhat difficult current soil classification. A considerable amount of work has been done on the application of soils data in land use planning for both urban and agricultural purposes. In some of this work soils data have been used somewhat unr~alistically, such as for delineation of aquifer recharge areas. The use of soils information in this study is restricted to its more proper role of describing the distri­bution of the physical properties of the upper part of the substrate. Resources Map The Resources map shows the location, extent, and distribution of the low-unit-value, high-place-value resources that are essen­tial to the growth and maintenance of an urban system. This map indicates both the availability of resources and the areas where urbanization should be delayed until the resources have been utilized. The resources shown are potential resources. Some of the resources indicated may not be economic to utilize because of ::Brket factors, and others may be unavailable because they are located on land whose value is too high for change to resource exploitation. Resources are shown irrespective of any use that is made of them; resource utilization and the associated environmental implica­tions are accounted for in a later section of the methodology . The resources are restricted to those which come from the substrate below the soil zone. Soil resources are primarily of agronomic value and are not considered here to be significant enough to local urbaniza­tion to warrant coverage. The resources indicated are further re­stricted to those that are related directly to local urban systems. Other geologic resources which are present but are not essential to urban growth and maintenance are not generally shown, even though they may be important to the economy of the city. Processes Map The Processes map shows the extent and distribution of the active geologic processes which have or are likely to have an impact on urbanization. The most important processes shown on this map are those which pose a geologic hazard to urban systems. Examples of the units depicted are flood-prone areas, unstable slope areas, and active faults. Some processes can be directly interpreted from the geologic map, whereas others, such as flood-prone areas, are determined only with considerable additional field work . Landfor:n :'.'lap The purpose of the landform map is to depict the shape of the land surface in a way that best reflects how that shape will affect urbanization. As noted earlier, one of the most useful maps for this purpose is a slope map. Probably more than any other landform characteristic, slope determines the suitability of a site for various urban land uses and dictates the extent of landform change required to make the site more suitable for these uses. The units depicted on a slope map are areas having a cer­tain range of slope values . The limits of the slope categories used as map units depend on the landform of the area being in­vestigated and on the judgement of the investigator. The Kansas Geological Survey (1968, p. 11) presented a set of slope categories which are very useful for indicating relative suitability for various urban land uses (figure 2.3) . The slope map is derived directly from the topographic map of the study area. Derivation of Natural Data Sources The primary sources from which most of the natural data sources are derived are topographic and geologic maps . Topographic maps are available from the U.S. Geological Survey for most of the major urban areas in the United States. Geologic maps are not available for many urban areas, however, and ~here they are available, often they are not accurate enough or at a sufficiently large scale. Consequently, the preparation of a suitable geologic map is frequently necessary before environmental geologic inves­tigation of an area can begin. Toe type of geologic map to be prepared for environmental geologic analysis depends largely on the kind of geology in the area . A conventional geologic map which uses rock-stratigraphic units as defined in the Stratigraphic Code (Aiderican CoITmission on Stratigcaphic Nomenclature, 1961) may be an excellent map, inasmuch as rock-stratigraphic units are GENERAL RECREATION AREAS GENERAL URBAN USES SEPTIC FIELD SYSTEMS COMMERCIAL CENTERS ENGINEERED STRUCTURES CONVENTIONAL HOUSING AIRPORTS RAILROADS INTERSTATE HIGHWAYS ALL-WEATHER URBAN ROADS SOURCE : KANSAS STATE l I I I I ~ I I I I -I I I I I . I I I l I I I I I I I I -. I I I I I I I I I I ' I I I ~ I I I I I -I I I I I I I---I I I I I I I I I I I I I I I I I I I I I I I I . . I --. . 0 5 10 15 20 25 30 35 40 45 50 % GRADE GEOLOGICAL SURVEY. 1968 Figure 2.J Optimum Ranges of Slopes for Various Urban Tnstallations and Activities h.l h.l defined on the basis of lithology and mappability. Since many of the properties shown on the natural data source maps, such as physical properties and resource potential, depend directly on lithology, a rock-stratigraphic geologic map would seem an ideal source for the data source maps. However, in some areas, such as where the existing rock-stratigraphic nomenclature does not adequately reflect the lithology of the rocks, other methods of geologic mapping are more effective. An example of improved geo­ logic mapping for environmental purposes in areas underlain by sedimentary rocks is the depositional systems mapping which has been done in recent years by the University of Texas Bureau of Economic Geology (Fisher and others, 1972; Fisher and others, 1973). The primary objective in deter~ining the type of geologic map to prepare for the methodology of this study is to produce a map which best shows the lithology, geometry, spatial arrangement, and areal distribution of distinctive, mappable bedrock units. Cultural Data Sources The cultural data sources express man's current relation to the land, and the cultural factors depicted are the characteris­tics of the land resulting from the previous land uses and other land-related activities of man. Two aspects of man's relation to the land are of foremost importance to environmental geologic investigation -current land use and regulatory measures that have been instituted for future land use. Current Land Use Map The purpose of the Current Land Use map is to show how the land is being used by the owners and occupants of the land. These land uses may be categorized into V.vo broad classes -urban-related and agricultural uses. The latter category generally denotes areas into which urbanization may expand in the future. Land in 24 the urban-related use category can be further categorized accord­ing to the urban systems organizational scheme introduced above. Thus, four major classes of urban land use are recognized -urban situs, urban input, urban output, and transportation. Various land use categories can be delineated within each of these if necessary. Land Use Control Map The Land Use Control map delineates those parts of the areas of investigation in which some form of land use regulation is exer­cised. Such regulation includes measures, such as city zoning, which seek to make the uses of adjacent land areas compatible with each other. Also included are controls which attempt to reduce damages from geologic hazards, such as ordinances prohibiting develop­ment of flood-prone or landslide areas. In addition, some controls may have been instituted to provide a measure of protection for areas of resource value or particular environmental sensitivity, such as aquifer recharge zones and coastal lagoons and embayments. The primary use of this map will be to depict areas which cannot be used for certain land uses under existing regulatory meas­ures. The results of the enviroruaental geologic analysis presented in this study could well be used, however, for recommending changes in these measures. New criteria for land use controls may emerge, and new areas may be found which should be included under existing controls. In addition, some areas may be found which are limited unnecessarily and can safely be removed from control. The Land Use Control map is the most ephemeral of the data source maps; the areas shown are easily changed by actions as simple as a vote of a city council or a directive of a state agency. Derivation of Cultural Data Sources The Land Use Control map is derived by obtaining the data from the appropriate regulatory authorities, such as a city government or a state agency. The Current Land Use map is prepared from several sources. One of the most important of these is the topographic map which is used, if it is available, as the base map for all the data source maps. Xany important current land use features, including roads, railroads, buildings, and water bodies can be found on a topographic map. Another important source of information is a land ownership map, which is often available from the local real estate taxing authority. The land ownership map indicates the degree of land subdivision; large areas of single ownership usually denotes agricultural use, whereas numerous small land parcels indicate a more urban usage . After the topographic and land ownership maps have been utilized, aerial photographs should be examined because they usually indicate land uses directly if they are recent enough. The final step in deter­mining current land use is field verification. The results of the initial steps, which are conducted in the office, are checked, and the areas of uncertainty are clarified. Summary and Discussion In summary, the conceptual tools developed in the first part of the chapter were utilized in this section for organizing the natural and cultural data source maps which will be used in subsequent steps in the methodology. The maps presented are probably best thought of as the "minimum basic" data sources needed. Other ncngeologic factors, such as a biologic assemblages map, could also be generated at this stage for later use. The Soils map, since it is only partly related to the geology, illus­trates the potential for a more interdisciplinary scope of this section of the methodology. Howeve~, this study is restricted primarily to the geologic concerns of urbanization. It will be noted that an environmental geologic map has not been presented as one of the data source maps. It would be possible, of course, to combine the data source maps to produce a composite map illustrating the most significant land characteris­ tics. This composite map would constitute one type of environ­mental geologic map. However, this approach may require deletion of less important but nevertheless still significant factors. The data source maps presented here attempt to depict the physical characteristics separately and in as complete a form as possible. They will be used in the following sections for identifying exist­ing urban environmental geologic problems and for evaluating land suitability for future urban uses. ENVIROID-lENTAL GEOLOGIC PROBLEMS OF EXISTING URBAN SYSTEMS Most urbanizing areas have an existing urban center which serves as the nucleus for urbanization of the surrounding outlying areas. The objective of this section is to utilize the conceptual tools and data sources developed earlier and to construct a syste­matic procedure for investigating the envirolli~ental geologic prob­lems of existing urbanization. This section thus addresses the curative aspect of urban environmental geology described in the introductory chapter. Either a geologic approach or an urban systems approach could be used in this section. If the geologic approach were used, the various components of the geologic environ~ent could be analyzed individually and the interaction of each with the exist­ ing urbanization could be considered. If the urban systems ap­ proach were used, each component of the system could be investi­ oated and the interaction of each with the geologic environment could be described. The urban systems approach is adopted here because it is more likely to be universally applicable. That is, different cities generally have more in common than do different geologic settings. This approach also ensures that no significant urban facilities will be omitted from consideration. Completeness of coverage of the geologic environment has already been provided for in the generation of the natural data sources. Tne procedure of this section is to systematically consider each of the four major components of the urban system and delineate the environmental conflicts associated with the facilities and activities of each. Within each component either of two approaches can be used. If an interactive approach is used, the activities and facilities of the component are considered as a whole, and their interaction with the environment is analyzed. In the subcomponents approach the specific activities and facilities are considered individually and their environmental conflicts are examined. Both approaches will be used in succeeding sections. Regardless of which approach is used, the procedure con­sists of a three-step process. First, the Current Land Use map, which shows the locations of the existing urban facilities, and the natural data source maps are studied to identify potential conflicts . Next, the facilities are visited in the field to confirm or refute the existence of the problems and to identify other problems not anticipated by map study. Finally, recommendations are made either for alleviation of ~he problems or for further study be­fore corrective action is taken. The recommendations for elimi­nating or lessening the conflict may entail such steps as engineer­ing modification of the site or, where unavoidable, actual changes in existing land uses. This procedure in effect identifies environ­mental problems stemming from the inability of the land at the locations of the urban facilities to sustain the demands of those facilities. The procedure in the final section of the methodology attempts to avoid these conflicts by siting future urban facili­ties on land which is most capable of sustaining their demands. Urban Situs As noted in the section where the urban situs concept was introduced, the environmental problems described in this category result from the location of a city in a particular geologic set­ting. Falling within the scope of this category are the urban facilities within the city and the associated urbanization in the surrounding outlying area. As was shown in the land use classifi­cation of cities, most urban areas comprise a large variety of land uses. Consequently, the subcomponents approach, in which each land use is considered individually, is not used. Instead, the interaction approach will be used, and the city will be considered as a whole. First, the impact of the environment on the city will be analyzed and then the impact of the city on the environment will be considered. Impact of the Environment on the City The greatest impact of t r1e environment on a city is in the form of what are commonly viewed as geologic hazards. The hazards posed for a city vary widely across the entire spectrum of geologic conditions in which cities have been built. A few examples, in­cluding floods, landslides, and active faults, have already been mentioned. ~ost geologic hazards stem from geologic processes and will thus most li~ely be depicted on the Processes map. This map can therefore be used to delineate the various parts of the city that are likely to be subject to the different hazards. Field investigation will then document the extent and seriousness of these hazards. Impact of the City on the Environment Cities usually do little to enhance the overall quality of the natural environment. The impacts with which man is chiefly concerned are those which alter the environment in such a way that the environment in turn adversely affects man and his activities. Some typical examples, such as changes in the hydrology of the stream basin in which the city is located, pollution of surface water by uncontrolled wastes, and increased erosion and sedimenta­ tion, illustrate the kinds of impacts that should be considered. Clearly, the impact of a city on the geologic environment is pri­marily on the geologic processes and landfonn and to a lesser ex­ tent on the substrate. Field investigation of the existing urban facilities should identify the major impacts of the city on its geologic environment and document the seriousness of that impact. Urban Invut The environmental conflicts analyzed in this category stem from utilization of the locally derived, low-unit-value resources. Because the types of activity involved in urban input are few, and because their envirorunental implications are usually quite different, the subcomponents approach is probably the most effective. The subcomponents of urban input are the different kinds of re­sources which are used for the growth and maintenance of a city, such as aggregates, water, and sometimes energy. Field investi­gation will usually disclose the environmental conflicts resulting from these activities or from post-operation land use or reclamation. The Resources map is not extensively used in this section. Its primary implication for urban input lies in the control that the indicated resources have exercised on the location of the input activities. Urban Output The environmental geologic problems included in this sec­tion stem from the disposal of wastes generated in the city. As in the previous category, the subcomponents approach is probably more effective than the interaction approach. The classification of urban wastes presented earlier in this chapter should be used, and the environmental problems associated with each category should be analyzed. The location of most of the various urban waste activities and disposal sites are shown on the Current Land Use map as a separate category. Potential problems at these sites can often be discerned by examination of the five natural data source maps. Field investigation is needed to confirm the presence or absence of these conflicts and to discover unanticipated conflicts. In general, these conflicts are primarily in one direction -impact of the site on the environment. The impact of the environment on most output facilities is minimal or of little significance. Transportation The environmental geologic problems associated with tra..~s­portation facilities can probably be delineated equally well using either the interaction or the subcomponents approach. An effective organizational scheme for the categories of the transportation component was introduced earlier. Some of the categories, such as roads and railroads, are similar enough that they could be grouped together and analyzed using the interaction approach. On the other hand, the number of different components is com.~only small enough that they can be efficiently analyzed individually. As was the case for the other components of the urban system, transporta­tion facilities are shown as a separate category on the Current Land Use map. Environmental conflicts should be analyzed as be­fore by comparing the Current Land Use map with the five natural data source maps and confirming the results by field investigation. Summary and Discussion The urban system organizational scheme has provided an effective means of systematically analyzing the environmental geologic conflicts of existing urban facilities and activities. If each of the four components of the urban system is considered in turn, most of the significant environmental geologic problems of a city can be effectively outlined. The procedure was outlined step by step, but in practice some of the steps can be carried out concurrently. For example, site visitation to determine the presence and extent of environmental conflict is often done at the same time the geologic map or the data source maps are prepared. After the analysis is completed, recom­mendations should be made to solve or ameliorate the problems or at least to indicate where further study is needed before a solu­tion can be recommended. As noted earlier, solutions to the indi­cated problems may involve engineering modifications at the sites of conflict or actual land use changes. ENVIRONME::HAL GEOLOGY IN PHYSICAL LAL\!D USE PL-'u'<'NING FOR URB&i GROWTH An important function of environmental geology is to prevent conflicts such as those disclosed in the preceding section from occurring in future urbanization. This function constitutes the preventive aspect of the environmental geology of urbanizing areas, and the objective is to optimize the interaction of the geologic environment and future urban activities and facilities. In this way the expensive and inefficient "patch-up" efforts to correct environmental conflicts can be avoided. The approach of this section will be to evaluate the capa­bility of the land to sustain the demands placed on it by the various facilities and activi ties of the urban system. Once this capability has been deter~ined for the various urban land uses, it should be possible to fit future urbanization to the geologic environment. A successful technique of l and capability evaluation must take several factors into consideration: 1) The various urban land uses must be considered separately, because different demands are placed on the land by different uses; 2) All of the demands of each land use should be considered, and the relative importance of the demands must be taken into account; 3) All of the significant characteristics of the land that determine its ability to meet those demands must be evaluated, and 4) The areal variation of these characteristics must be taken into consideration. The methodology of this section attem?ts to account for these factors by using the following five-step process: 1. Selection of the urban facility or land use for which the land suitability is to be evaluated 2. Construction of the blank suitability score grid 3. Screening procedure 4. Evaluation procedure 5. Verification procedure This process must be repeated for each different facility or land use. The result of this process is a grid map for each projected urban use which shows in terms of a percent the relative suitability of the various parts of the study area for that use. Tnis map of numerical values can then be used as the basic data source for opti­mally siting the various future urban facilities. The following paragraphs outline the five steps in detail and show how the re­sults can be used in formulating a physical land use plan for the study area. Selection of Land Use The first step of the procedure is to select the urbar. use for which the land is to be analyzed. The urban systems organiza­tional scheme provides an effective tool here for outlining the various urban land uses anticipated for a study area. The land use selected determines to a large extent the data source maps that will be used in succeeding steps. A data source map that is used extensively for one land use may be utilized very little or not at all for another land use. Also, the stage at which the data source maps are used will vary for different land uses. For example, a data source used only in the screening procedure for one land use may be used extensively in the evaluation procedure for another use. Construction of the Blank Suitabilitv Score Grid After the urban land use has been selected, the next step is the construction of the blank suitability score grid. This uni­formly sized grid will be used in subsequent steps to overlay the data source maps, and the individual grid squares will be the units used in evaluating land suitability. A grid system has some inherent drawbacks for land evalua­tion. One of the main problems is the use of a rigid network of uniform grid cells to overlay data source maps that portray irregu­larly shaped map units. A corollary to this problem is that of boundaries; many of the grid squares will inevitably fall astride the boundaries between map units. The decision must then be made between which unit will be represented in the square. However, a superior and easily applied alternative to the grid system has not yet been devised (Ferris and Fabos, 1974). Grid squares will therefore be utilized here as the basic land elements in the follow­ing steps. The major problem to be solved in this step is the deter­mination of the co2.rseness of the grid . Two opposing factors have great impact on grid coarseness: 1) The finer the grid is, the more e~act the final results will be. Smaller grid squares divide the area into smaller units, and the areal variation of the land characteristics can be more accurately expressed. Thus, a finer orid enhances the resolution of the final result. 2) The 0 coarser the grid is, the less is the work in applying the procedure. Each grid square is considered many times in the procedure below. If the grid square dimensions are halved, the number of grid squares to be evaluated is quadrupled, thus making more difficult an already time-consuming process. Three major factors must be considered in determining grid coarseness: 1) the size of the study area and the scale of the data source maps; 2) the size of the smallest map units on the data source maps; and 3) the size of land parcel required by the land use for which the land is being analyzed. The maximum grid square size allowable is generally determined by the size of the smallest data source map units and the size of the land parcel required by the land use. The minimum grid size depends on the size of the study area and the extent of use of automation in applying the procedure. The use of computers, as will be suggested below, may eventually provide the solution to most of the problems inherent to the grid overlay approach by permitting the use of very fine grids which can closely approximate actual field conditions. Screening Procedure The objective of the screening procedure is to elininate areas that are totally unsuited for the land use under considera­tion. By removing these areas at this stage the needless effort of subjecting them to the rigorous evaluation of the next step is avoided. Examples of land characteristics which could cause some areas to be screened are zoning restrictions, current land use, susceptibility to floods, unstable slopes, and active faults. It could also be argued that some areas should be screened at this point because they have a potential for a better land use than that being analyzed for. This approach is not used here, however, because the relative suitabilities of all projected urban land uses are to be determined. The trade-offs and compromises necessary in actual land use determination should be made during the 35 formulation of the physical land use plan as described in a later section. The screening procedure is a six-step operation: 1) Decide which factors contribute to screening for the land use under con­sideration; 2) Arrange these factors in order according to the total area covered and the sizes of individual map units; 3) Begin­ning with the first factor, overlay the blank suitability score grid on the appropriate data source ~ap and trace the boundaries of the screened areas; 4) Repeat step three for all the screening factors; 5) Combine the screened areas to depict the total "unaccept­able" area; and 6) Convert the results into the grid format by shading the grid squares in the unacceptable area. These shaded squares are assigned a value of zero for the Evaluation Procedure. Evaluation Procedure In the evaluation procedure the candidate areas left after the screening procedure are evaluated for their relative suitability for the urban land use under consideration. Tne procedure used for this suitability analysis is an assessment algorithm adapted from a decision-making technique developed by :tiller (1970). This tech­nique is a subjective, numerical method of making a decision where complex factors must be taken into account. One of the fundamental features of the procedure is the clear distinction that is made between the demands or requirements of the land use and the ability of the land, based on its physical characteristics, to meet those demands. This distinction makes it possible to outline the de­mands of a land use objectively and without regard to how well the area being investigated can meet the demands. The adaptation of Miller's technique used here has involved some simplification. For example, as originally presented, the method was based on the assump tion that mos t of the data needed to make a decision is available. The adaptation used here determines relative suitability of the land based on available data, which 36 may not be complete. Also, the original version was developed for making a decision only once. As applied here, the same decision­making process is applied many times -once for each square of the grid. Despite these changes, the conceptual soundness of the method is left intact. The procedure for land suitability analysis is outlined step by step in the following paragraphs. The devices used will be introduced and described and will be referred to thereafter with an abbreviation. A glossary of the terms introduced is given ~ith the appropriate abbreviations near the end of this chapter. Step 1. Conceptualize the Overall Objectives In the first step the urban land use for which land is be­ing analyzed is considered generally, and the broad categories of the demands which that use will place on the land are conceived. These categories are listed in random order as they come to mind. Examples of topics which might be included in this list are water quality protection, suitability of engineering characteristics of the substrate, and freedom from geologic hazards of a particular type. This list will be a starting point and #ill undoubtedly undergo many changes in subsequent steps. Step 2. Construct the Demand Analysis Hierarchy In this step the items on the list from Step 1 are subjected to a process of successive subdivision. Each item is divided into its conceptual components, and each component is further sub­divided to a lower level. This process is continued until the lowest level criteria represent demands on the land for which a particular, specific property of the land can be measured to indi­cate how well the land can meet this demand. The result of this subdivision process is a treelike hierarchy (Figure 2.4), designated BROADLY STATED CRITERIA FROM STEP 2 ---t ------------------c AN URBAN LANO USE ---------------------c _j LOWEST LEVEL DEMAND CRITERIA Figure 2.4 Diagram of a Demand Analysis Hierarchy 38 the Demand Analysis Hierarchy (DAH), whose trunk is the land use being analyzed for and whose largest branches are the overall ob­jectives established in Step 1. The outennost branches at the other end of the hierarchy are the Lowest Level Demand Criteria (LLDCs). The generation of the DAH is a dynamic, fertile process in that many changes are made as the subdivision proceeds. Some cri­teria may be included as parts of others, and new criteria may con­tinually emerge. Criteria may also change positions of level within the hierarchy. If the DAH is generated in this manner, nearly all of the demands imposed on the land by the urban use under consider­ation should be determined. Many of these demands, however, are eliminated here from further consideration in subsequent steps. For example, the degree to which some of the demands are met is measured by characteristics which are uniform over the stady area . For rating the relative suitability within an area, there is clearly no point in considering this kind of demand. Also, de­mands which fall beyond the scope of this geologically oriented analysis are not cons i dered . Another type of demand that is eliminated is that which depends on the location attributes of a site. That is, each site (grid square) is analyzed for its in­herent physical characteristics only, and the location of the site with respect to other facilities or activities is not taken into account. Consideration of locational attributes is reserved for the formulation of the physical land use plan as outlined in a later section. The criteria which are deleted from further consideration in subsequent steps in effect constitute "dead branches" of the DAH. In generating the DAH the best procedure is probably to formulate the hierarchy complete with dead branches and then to signify these branches with some type of symbol. A key to the symbols should then be presented to explain the rationale for branch elimination. The DAH is therefore generated in two stages ­formulation of the complete DAH followed by elimination of the dead branches to obtain a condensed DAH. For practical purposes, particularly if subsequent steps are carried out without the benefit of automation, the number of LLDCs which can be considered is limited to about thirty or forty. A larger number requires too much effort in managing the data. If the condensed DAH has greater than this number, then two alterna­tives are available. On the one hand the condensed DAH could be derived as described above and then, after the weighting process of Step 5, the low-weight LLDCs could be eliminated. This solution is not particularly satisfactory, but is relatively safe as long as no more than 10% of the weight is thus deleted. A second alterna­tive would be to abandon formulation of the DAH for this land use and begin anew with two or more subcategories of the land use. For example, one might attempt to construct a DAH for a housing develop­ment only to find that the number of LLDCs generated is far too large. He might then begin again using specific aspects of the development, such as septic tank suitability or foundation suit­ability, as the land use demands. In some hierarchies, there may be a repetition of branches in different parts of the hierarchy. If these branches "work" in the same direction, they will reinforce each other, and if they "oppose" each other, they will cancel each other out in proportion to their weights. In summary the analysis of land use demands by the hier­archy method should be a highly effective approach. The method not only assures that no criteria will be omitted, but also shows how the criteria are related to each other. In addition, the de­mands are subdivided to a level where their degree of satisfaction can be determined by a clear, measurable physical characteristic of the land. 40 Step 3. Select the Physical Performance ~·1easures After the DAR has been completed and reduced to the con­densed DAH, the Physical Performance aeasures (PP:Is) must be select­ed. For each LLDC a characteristic of the land is selected as the best measure of how well the land is able to meet the demand rep­resented by that criterion. It is this characteristic which serves as the conceptual bridge between the demand criteria of the DAR and the actual physical properties of the land in the area under study. In theory, the PP:!s are formulated only after the DAR has been completed. In practice, however, the measures that ~ill ulti­mately be used must be kept in mind during subdivision of the DAR in order to know when the subdivision can stop. In other words the associated PPM helps to define what constitutes an LLDC. The land characteristic chosen as a PP:1 for a partic•Jlar LLDC depends primarily on the LLDC itself, but it is also deter.nined in part by the nature and quality of the information on the data source maps. In an ideal case, where all the data on all land characteristics of an area is available in map fcrrr., a measu~e could be selected which Nould rate the land exactly for the LLDC. In practice, however, such complete data are never available, so an approximate measure must be used. Step 6 provides a ~eans of measuring the relative effectiveness of PPMs for their associated LLDCs. Examples of typical LLDCs are shown in Table 2.1 with some possible associated PPMs. Step 4. Formulate the Suitability Score Functions After the PPMs have been chosen for the LLDCs, a device must be formulated for actually rating the land according to the PPMs. The mechanism designed to fill this need is the Suitability LLDC PPM 1. Excavation potential 1. Depth to bedrock 2. Shape of land surface 2. Slope 3. Ground water proximity 3. Depth to water table 4. Potential for infiltration 4. Substrate permeability 5. Potential for bearing a load 5. Substrate shear strength Table 2.1 Typical Lowest Level Demand Criteria and Associated Physical Performance Xeasures Score Function (SSF). An SSF provides the means for assigning a dimensionless numerical value to a land unit (grid square) which is a direct indication of the ability of the unit to meet the LLDC. As the name implies, an SSF is usually in the form of a mathematical function whose axes are set up in standard Cartesian coordinates. The units of the PP:1 are established along the x-axis, and a scale of score points from 0 to 100 is placed on the y-axis. The relation between the score points and the units of the PP:1 (that is, between the x-axis and y-axis values) is then expressed by sketching a curve of the appropriate shape in the x-y space. The delineation of this relationship is the key element of this step of the evaluation procedure. The shape of the curve is determined on the basis of judgement and experience, and numerous factors must be considered in its determination. Probably the best procedure is to consult the literature and solicit the opinion of several professionals experienced in working with the land use under consideration. As will be shown in Step 7, the SSFs are applied to each grid squar~ and a dimensionless score value is assigned to the square . Several variations of the SSF as described above are pos­sible. Most SSFs will probably use arithmetic scales on both axes, but in some functions semilog or log-log scales may be more desirable. When reliable quantitative descriptions are available for the pp:f, numerical units should be used on the x-axis of the SSF, and the function will then usually be a smooth curve. Some­times, however, only qualitative descriptions are available for the PPM, and the function should then be a step function with score values assigned to different verbal descriptions. Quanti­tative data can also be scored using discrete steps rather than a smooth curve by assigning particular score values to ranges of values on the abscissa. Tnis capability of translating equally well both qualitative and quantitative data into score values is one of the major advantages of the SSF approach to land rating. A~other important advantage is the translation of physical characteristics of many different types into easily compared dimensionless score value units. Examples of some different SSFs are illustrated in Figure 2.5. Ideally, the units of the SSF abscissa should be deter­mined only by the LLDC with which the SSF is associated. In prac­tice, however, these units must to some extent depend also on the data available and the range of data in the area under study. There is little point, for example, in including within the range of the abscissa values that are not represented within the study area. Step 5. Assign Weights to the Demand Analysis Hierarchy After the SSFs have been formulated, it is necessary to re­turn to the DAH to perform a weighting operation. The purpose of this step is to determine the relative importance of the land use demand criteria set forth in the DAH. The condensed DAH is used, and beginning at the highest level of the hierarchy tree, the relative importance is estimated in percent terms for each cri­terion at that level. Tnis process is repeated within subcategories down the hierarchy tree until the level of the LLDCs is reached. The process is always performed within a subcategory and at a par­ticular level on the basis of 100 (percent). Wnen the procedure is completed for all branches of the DAR, the weights are dis­tributed proportionately by multiplication down the various branches. That is, at each level, the weight alloted to that level is dis­tributed among the criteria at that level by multiplying that weight by the percent assigned to each criterion. The final re­sult of this process is the assignment of a weight to each LLDC. The sum of the weights of the LLDCs should, of course, equal 100. An example illustrating this procedure is shown in Figure 2.6. () I 2 3 '+ s /"-¥ /()., Iii~ /li1 I /() /tr /()~ 1tof1r.1 K ("n/,1~c) DEPTH OF SOIL PERMEABILITY r. t. t. ,. f• f• t• L'W H;1 ;, ttr04 I o·.J 1o·;a /." IC! !Oa 10' If (,m/u,) PERMEABILITY PERMEABILITY Figure 2.5 Examples of Suitability Score Functions - LLDC NO. 20 (5.0l 1 25 (25) ------------------180(20.0l 2 25 ( 15) ---------­--­ 15.0 3 20 (9) -------­ 9.0 4 60 (60) -­ 10(2.2) -­ 2.2 5 60 (9.5) 6 50 (22.5) 70 (15.8) 30 (4.7l 7 100 75 (45) -­ 10(1.6) 8 20 (4.5) -- 60 (2.7>l::: 40 (1.8) 9 10 90 ( 1 2.2) 30(13.5> ___ --­--b. 10(1.3) 11 12 10 ( 10) ------------------­ 10.0 13 5 (5) --------------­-­ - - 50 (2.5)' -\so c2.s> 14 15 Figure 2.6 Example of the Weighting of the Demand Anal ysis Hierarchy 46 The first number of each branch is the percent assigned to reflect the importance of that branch relative to its peers. The second number (in parentheses) is the net weight of the branch obtained by multiplying the percent times the weight alloted to all branches at that level. The percentages assigned to the various branches of the DAH are determined from judgement and experience as in the preceding step. Probably the best approach is again to search the literature and to obtain a consensus of opinion from experienced professionals. Step 6. Adjust the Weights It was noted in Step 3 that the PPMs assigned to the vari­ous LLDCs often do not, mostly because of incomplete data, exactly measure the ability of the land to meet the demand criterion. Therefore, an adjustment of the raw weights derived in the preceding step is necessary to allow for the varying degrees of effectiveness of the PPMs. The procedure for this adjustment is to consider each LLDC and its associated PP~ and to make an estimate, in per­cent terms, of the effectiveness of the PPM. This percentage, called an adjusting factor, is then multiplied by the raw weight to yield an adjusted weight. The process is repeated for all LLDCs,and the adjusted weights are then recalculated to a basis of 100, resulting in a net weight for each LLDC. An example demon­strating this process for the hierarchy in Figure 2 .6 is shown in Table 2.2. The net effect of this step is to emphasize those LLDCs for which the data is best. However, it should be noted that, as shown in Table 2.2, although there is sane transfer of weight, no gross changes in the weight distribution has occurred . Thus the original derivation of the raw weights in the preceding step has far greater effect on the final results than the weight adjustments of this step. LLDC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Raw Wei ght 5.0 20.0 15 . 0 9.0 2.2 9.5 4.7 1. 6 2. 7 1. 8 12.2 1. 3 10.0 2.5 2.5 Adjusting Factor 90 80 95 75 60 80 85 100 65 90 95 50 100 85 75 Adjusted Weight 4.5 16 .0 14. 3 6.8 1. 3 7.6 4. 0 1.6 1.8 1.6 11.6 0.7 10 .0 2. 2 1. 9 85.9 Net Weight 5.2 18 . 7 16 .6 7.9 1. 5 8.8 4.7 1. 9 2.1 1.9 13.5 0.8 11 . 6 2.6 2.2 100.0 Table 2. 2 Example of Adjustment of Weights of Lowest Level Demand Criteria Step 7. Prepare the Suitability Score Grids With the completion of Step 6, the preparatory work for land suitability assessment is finished and the actual assessment process can begin. Three tools that were developed earlier in this chapter are utilized: 1) the data source maps; 2) the blank suitability score grid; and 3) the score functions. The procedure for preparation of the Suitability Score Grids (SSGs) begins with the first LLDC in the DAH. The associated SSF is selected, and the previously screened blank suitability score grid is overlaid on the appropriate data source wap . The data source map selected depends on the ?~1 and SSF of the LLDC under consi~eration. After the blank grid is overlaid in proper register, the scoring of the grid squares begins. Each square is examined and the map unit overlain by the square is noted. The SSF x-axis value associated with that map unit is noted and the corresponding score value (y-axis value) is derived from the SSF. This score value is then assigned to the grid square. The procedure is repeated for all the grid squares of the grid except those that were eliminated and assigned a value of zero in the screening procedure. The entire process is carried out for each LLDC, resulting in a ncmber of SSGs equal to the number of LLDCs. The most important part of this step is the determination of the SSF x-axis values corresponding with the map units of the various data source maps . As noted earlier, the maps used in this step are primarily the five natural data source maps . The scoring process is quite easy if the data corresponding to the abscissa parameters have been tabulated for the data source ~ap units. These parameters can be either qualitative or, preferably, quanti­tative. If such a tabulation is not available, then one of t~ree approaches can be used: 1) An estimate may be made for each grid square directly from the data source map,and the score value can then be assigned; 2) A separate map showing the distribution of the parameter in question can be prepared as a derivative from the data source map; or 3) A score map can be prepared as a t ype of derivative map. The difference among these approaches, however, is more apparent than real. Some of the data source maps, particularly the Engineering Geology and Soils maps, portray map units having a large number of associated characteristics. It may therefcre be expected that these maps will be used for the grid overlay several times during the preparation of the SSGs. Occasionally, an LLDC will have an associated PP:·! and SSF for which none cf the data source maps will provide a value for determining a score. In such cases, there are two alternatives. Either a supplemental data source map which will supply the required values can be prepared or, if this option is not feasible, the con­servative approach can be adopted and a score of zero assigned for that LLDC to all grid squares. Tnis option is equivalent to using an adjusting factor (Step 6) of zero, and has a direct inpact on the validity of the results, as will be shown below. In theory, as described above, one SSG is prepared for each LLDC. However, several LLDCs, some of which are identical and some of which have different names, will use the same pp;1 and there­fore the same SSG. Duplicate SSGs could be prepared for each of these LLDCs, but an equivalent and easier approach is to add the weights of all these LLDCs. In this way, only one SSG need be prepared and the combined weights of the LLDCs used in the next step. When SSGs are prepared manually as they are in this study, the maximum number that can be reasonably considered is approxi­mately twenty, depending on the number of grid squares used in the blank suitability score grid. Step 8. Calculate the Suitability Index (·1ap The final step of the land suitability analysis procedure is the calculation of the Suitability Index Map (SIM) . The data used in this step are the SSGs from Step 7 and the net weights derived in Step 6. The procedure for calculating the SIM is a two-step operation. First, the SSG for each LLDC is converted to a weighted SSG by multiplying the net weight of the LLDC by the score value of each grid ele~ent in the SSG. Second, the SIX is derived by summing the weighted SSGs in stack fashion. That is, the weighted SSGs are stacked one above the other and the values of corresponding grid elements are added together. The resulting sums are the final suitability scores or suitability indexes, and the grid of sums is designated the SIM. A diagram illustrating the two-step procedure is shown in Figure 2.7. The SI~1 should, if all steps have been performed properly, indicate in percent terms the relative suitability of each grid element for the land under consideration. In theory, the SIM should indicate both the absolute suitability of the land and the relative suitabilities of different parts of the land in the area under study. However, some of the simplifications in preceding steps have limited the usefulness of the SIM in an absolute sense. Some of the factors which determine the absolute suitability of the land were deleted when the complete DAH was reduced to the condensed DAH. Also, when the SSGs were formulated, some score values were assumed to be zero where data were lacking for deter­mining t he actual score value. Although the value of the SIM was reduced in its usefulness in the absolute sense by these measures, its value for indicating the relative suita9ilities of the differ­ent grid elements, based on the factors and the data taken into account, has not been affected and remains valid . SUITABILITY SCORE GRIDS ~x ~x ~x WEIGHTED SUITABILITY SCORE GRIDS CNET WEIGHT1s CNET WEIGHT>2 CNET WEIGHT>!< ~ K SI= LW SS·· "•1 ~J SUITABILITY INDEX MAP Figure 2.7 Calculation of the Suitability Index Map Automation of the Evaluation The application of the evaluation procedure is clearly an arduous task requiring manipulation of large quantities of data. Many of the steps of the procedure can be greatly simplified or enhanced by use of a digital computer. The generation and weight­ing of the DAH could be greatly aided if programming were available which would make it possible to do the hierarchy subdivision at a computer console. This capability is especially attractive if many DAHs must be constructed, as is the case when land is evalu­ated for many different urban uses. The programming for hierarchy generation could also be written so that like branches of different hierarchies could be shared, t~us allowing another savings of time. One of the most promising uses of a digital computer lies in the construction of the SSGs. If the evaluation procedure is to become practical and widely applicable, a means must be found for aiding the arduous process of scoring the hundreds or thousands of grid squares in each SSG, and then repeating the process for several different SSGs. Automation of the process is particularly important when fine grids are used to closely approximate actual field conditions. Computers could be used to formulate the SSGs by the following steps: 1) The data source maps could be imaged in computer memory or on magnetic tape by using one of several geo­graphic information systems now available (Ferris and Fabos, 1974); 2) The SSFs could also be stored in computer memory; 3) The blank suitability score grid could be represented in computer memory as a two-dimensional matrix; 4) For each matrix, the computer could be instructed which data source map is to be used and what x-axis value of the SSF is associated with each map unit on the data source map; and 5) The machine could then be made to "look at" each grid square (matrix element), note the data source map unit within the grid square, extract the appropriate score from the SSF, and assign that score to the matrix element. These uses of the computer have great potential for future application, but much programming remains to be done before they are possible. Such extensive programming is not undertaken here. The objective of this section is to present and demonstrate a methodology, not to conduct a large exercise in computer programming. The computer is used here, however, for calculating the SIH as des­cribed in Step 8. High-speed computation capability is essential both for converting the SSGs to the weighted SSGs and for summing the weighted SSGs to obtain the SI~. A flow chart, computer pro­ gram, and deck structure for the calculation of ~he SI:1 are shown in Figures 2.8 to 2.10. This program is written for an SSG having a maximum width of forty elements and a maximum length of fifty elements. If a greater width is needed, another run of the program is required for an adjacent grid. The length of the grid can be increased either by making another run of the program as before or by increasing the parameters of the Dn!ENSION statement of the program. This program is written for a uniform grid size both within each SSG and for all SSGs. It would be possible to develop a program which allows the use of different sizes of grid squares, but the effort of additional prograrm:iing exceeds the effort saved by using the smaller number of grid squares made possible by a variable grid size. Several other uses of the computer in this methodology can be pointed out, but will not be elaborated here. Contouring pro­grams for converting point data to area data in map form are available, for example, and many different display techniques of output of land data have been developed. Interesting examples have been presented by Turner and Coffman (1973) and by Tillman, Upchurch, and Ryder (1975). Verification Procedure After the screening and evaluation procedures have been completed, the results must be verified. 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HltlCATION -•eu•11• t1e•L11enL plu1L111 l. FUllTKU Stu1rr Tr1nouct.!.!..!2.9. -hl1hwaya -n11ro.d» NH~~·~P~~!:e~~~;c~:ololJJ JU 2. Soll• Kap l. &•10t1rce1 H.ap STAil ~: ~:7~;::•H~:p ===========:; Cultura l Data Sourc•• t 1. c...uent Land Uac l'lnp l. La.ad u..• Contro I Hap _........ Pl"oc•dural ConH r uc t lleiola "~ ==:=::) lnpwc "'""i"' C.ld r....1,.IJcra t!on of Urban Si t"& Src o.:eolua Su~p ~Output -=-··o-rcta1 !~~~~"~.!;:1 ===:) 5::~:1!;1 ~~~~le, a:a:i:O fHY~~~A~~o -• lil•id~iitlal fact ..aa ~.!!-~ t -1 r11vd pita a.ivah.IUl"n St•P -at o1·•.: 'fo••1tr rl11• l. ~. l..1.1.IOSult•i>lllty~I AiuJy,.la 2. c.. llrba11 0...1rut l. ,_ -aanltar)' l•11dllll 11 4. a. -11ow~1c ta ..t•cnt pla1ne l!:l_R~~ -l11 11hv.1)'• t -r 0.. 0 0.. 20 191134 + I 17840+/ ;+11111110 155 111120 / 18+-+---__ 111349 14142 / +-+ HAYS C°""'TY + 14915 ~/ 11 850 11447 10 /+2~. 11382 y +---~ 11383 1 • I + / ~---+......_ _ 104----1-1---· / ../\~HAYS COUlfTY '-...+~~~+~8119~ 11343 7859 / / I017 OUTSIDE a.All MA..COS '-, + /,:/, 7811;---+;;:1 I // "'°'3 +...---e:oe -+--­ ---+/ + 41117 5134 ~ + _/ 4071 HISTORIC H47 ./ +/ +~+ ltH +~ 2631 Htt I .­ 1231 0 -+-·--~1741--+----+---+----i----!---t---t---+---+----+-----l----4-----4 1860 1880 1900 1~20 1940 1960 1980 2000 Figure 3.5 San Marcos and Hays County Population Statistics presently accounts for 30-40% of San :1arcos's population, a definite shift of the population from rural Hays County to urban San Marcos is indicated. This trend is also apparent from the upper dashed line which indicates the proportion of the Hays County population living in San Marcos. This percentage shows an increase from about 20% at the turn of the century to about 70% at present. A recent slight reversal of this trend is probably the result of urbanization outside San ~arcos, such as establislli~ent and growth of housing developments, rather than a remigration back to rural agricultural areas. Population projections for the area indicate both continued growth and a continued trend toward increasing population concentra­tion in urban areas. San Marcos' population projections from two sources are shown in Figure 3.5. These sources are in close agree­ment, except that the more recent study indicates a population figure somewhat higher than the earlier projection. Both projec­tions indicate a growth of 100%, to just under 40, 000 , for San Marcos in the period 1970-1990. Total Hays County population is projected to increase by 89% to over 50,000 by 1990. Hays County outside San ~farces will also continue to increase in population, but its percentage of the total will decrease to below 25% by 1990. Governmental Entities The principal goverlli~ental organizations in the study area are Hays County, the city of San ~'!arcos, and the town of Kyle. Hays County is governed by a Commissioners Court, which is not a court in the usual sense, but is an administrative and policy-making body (League of Women Voters, 1973, p. 3). The county seat is in the county courthouse located in the center of downtown San Marcos. The city of San ~farcos has a Council-~fanager form of govern:nent, and the t own of Kyle is governed by elected aldermen. Other governmental authorities of particular significance to the study area are the Capital Area Planning Council (CAPCO), the Edwards Underground Water District, and the Upper San :·1arcos Reclamation and Flood Control District. CAPCO is the regional council of governments responsible for long range planning for a ten county area, including Hays County, in central Texas. One of its prime responsibilities is the regional coordination of federally funded urban facilities such as water and sewage treatment plants. Participation in CAPCO by member governments is voluntary, and the agency has no enforcement authority except the power to deny federal funding of specific projects. The Edwards Underground Water District was created by the Texas Legislature in 1959 to pro­tect the Edwards aquifer from depletion. This organization has a real estate taxing authority and has funded several studies of the aquifer. The primary mission of the Upper San Marcos Reclamation and Flood Control District is to bring the flood problem in San ~arcos under control. Economy Urbanization in the San :'.-farcos area is closely related to the area's economy. In general, the economic health of the area is good, primarily because of the rapid population growth. Also, the economic base has broadened significantly in recent years, thus reducing the vulnerability resulting from reliance on only a few economic sectors (Longley, 1975, p. 71). The economy of an area can be described on the basis of any one or a combination of several aspects, including production, per­sonal income, and employment. The best available report on the economy of San Marcos and Hays County (Savage, ~1organ, and Yeargan, 1971) is based on employment data. In that report the economy of the area is divided into seventeen sectors and each sector is com­pared to the same sector for the national economy. The results of the study are presented as concentration coefficients, which ex­press as a ratio the proportion of the work force employed in each sector relative to the proportion of the national work force em­ployed in that same sector. The analysis shows that seven of t~e seventeen sectors form the economic base of the area. Listed in order of importance, these are: 1. Colleges and universities 2. Federal government 3. Manufacturing 4. Agriculture 5. Amusements and lodging 6. Elementary and secondary schools 7. Construction and mining Colleges and Universities Owing to the presence in San ~farcos of Southwest Texas State University, the colleges and universities sector of the economy is by far the most important in the area. The concentration coeffi­cient is 6.11, which indicates that this sector is over six times more inportant to the local economy than it is to the national economy. Enrollment has almost tripled in the last decade, from 4,461 in 1964 to 12,894 in 1974. Federal Government The second most important economic sector is provided by the Federal government, with a concentration coefficient of 3.98. Nost of this contribution is from the Gary Job Corps Center, which is located a few miles east of San Marcos. This facility, the largest of its kind in the country, is a voluntary national train­ing center for economically disadvantaged young men from around the country who are out of school and out of work. About 3,000 men are usually in residence, but this number has been reduced in re­cent years oecause of budgetary cutbacks. 81 Manufacturing, Agriculture, and Amusements and Lodging The next three economic sectors -manufacturing, agriculture, and amusements and lodging -are about equal in importance, with concentration coefficients very close to 2.70. Sixteen firms in or near the study area employ eight or more people each, and the following five companies have fifty or more employees each: 1. Kerr-Ban Furniture Manufacturing Co. (wooden furniture) 2. Wide-Lite Corporation (lighting fixtures) 3. Thermon :1anufacturing Co. (heating and cooling equipment) 4. Bollman Industries of Texas (scoured wool) 5. Longhorn Hachine Works (steel forgings) The San Marcos Chamber of Commerce continues to try to increase the manufacturing sector by attracting additional small manufacturing firms. Although agriculture used to be the foremost sector of the area's economy, it has been losing ground recently because of urbani­zation. Considerable quantities of land formerly devoted to agri­culture have been urbanized or are being held vacant in anticipa­tion of urbanization (Clark and Holz, 1971, p. 21). The population shift from rural to urban areas demonstrated by population statis­tics is supported by data on the size and number of farms in Hays County. Between 1940 and 1964, the average farm size increased from 323 to 619 acres and the number of farms decreased from 1,233 to 591 (Clark and Holz, 1971). It seems clear that as some of the population leaves the rural areas, the abandoned farms are bought up by th8 remaining residents who thus increase the size of their farms. Two major tourist attractions, Aquarena Springs and Wonder World, are chiefly responsible for the importance of ar.1usements and lodging in San Marcos's economy. Both of these attractions prosper chiefly because of the proximity of Intersta te 35, which is the transportation artery not only between Dallas and San Antonio, but also for much of the interior United States to ~exico. Aquarena Springs is a concession operated on Spring Lake whose main attraction is a glass-bottom boat ride in which the flow of San Marcos Springs can be seen on the lake bottom. About 500,000 tourists visit the concession annually. Wonder World, with one of the larger caves of the Edwards Group as its major attraction, is visited by about 100,000 people annually. Elementary and Secondary Schools The concentration coefficient of the elementary and second­ary schools sector is 1.7, no doubt because two private schools are located in San Marcos. The San Marcos Baptist Academy is a college preparatory school having 1,000 students and forty faculty. The Brown Schools offer care and training for mentally and emo­tionally handicapped children. Two of the five units of the schools are in San Marcos and the remaining three are in Austin. Construction and Mining The construction and mining sector has a concentration co­efficient of 1.45 and is therefore only slightly above the national average. The primary cause of this importance is probably the construction which has been undertaken in recent years at Southwest Texas State University to keep pace with the rising enrollment. Summary The economy of the San ~'farces area is quite healthy, owing chiefly to an expanding population. The original agricultural economy has steadily become more stable through diversification. The economic base of the area is now centered on Southwest Texas State University . The Gary Job Corps Center, manufacturing, agri­ culture, and amusements and lodgings play a lesser but nevertheless important role. Historical Influence of Geoloay on Land Use Geology had a strong impact on the original settlement pat­terns in the San :farces area and since then has exercised strong control on agricultural land use patterns. The original settlement patterns were controlled in large part by the presence of the Balcones Escarpment and by the large springs located at various points along the foot of the escarpment. Wilie (1940) was one of the first to describe the importance of the escarpment and its associaced springs on land use patcerns. Bybee (1952) expanded and published ideas si:nilar to those of Wilie. The original, short­lived. settlement of San :farcos de Neve, as noted earlier, was a short distance east of che escarpment on the San Marcos River at the crossing of the Spanish road El Camino Real. This site was undoubtedly chosen because of the abundant water supply to the river. The Anglo-A.'T'.erican village of San ;-iarcos was established 40 years 1.2.ter nea::::-San ~farcos Springs and the associated river chiefly because of their potential for supplying both water and power. The escarpment itself has also exercised considerable con­trol on the original pattern of settlement. Because the escarpment constituted a considerable barrier to early means of transpo~tation, most early routes were forced into a north-south direction along the foot of the scarp. This pattern was established during the Spanish occupation, continued through the American settlement, and is still strongly expressed today. Hany west·.vard-migrating pioneers who had found travel across East Texas relatively easy were more i~clined to settle on the Blackland Prairie east of the scarp rather than hazard further :ravel across the difficult ter­rain of the hill country. The routes of the major highways -first U.S. 81 and now Interatate 35 -were chosen to follow the long­establishec path at the foot of the escarpment, and the projected area of major growth in central Texas is along this line. The escarpment had a pronounced effect on agricultural land use patterns because of the great discontinuity in soil fertility across the scarp mentioned earlier in this chapter. Westward-bound settlers found that the escarpment marked the western limit of soils suitable for cultivation for cropland, and hence tended to settle to the east on the rich Blackland Prairie. Because of the change in agricultural suitability westward across the escarpment from cropland to ranchland, the scarp eventually came to form the boundary between the Deep South and the Old West. Within the study area the distribution of abundant ground water supplies and rich agricultural soils has given rise to an ironic agricultural situ­ation. West of the escarpment, where the Edwards aquifer provides an abundant source of water, the soils are too thin and poor for crops. East of the scarp, where the soils are rich and highly suitable for irrigable crops, the Edwards and other potential aqui­fers produce only saline, sulfurous water. R. T. Hill first pro­posed using water from the San Marcos Springs and River for irri­gating the Blackland Prairie (Hill and Vaughn, 1898, p. 308), but no serious attempt has ever been made to institute his suggestion. Suitability of the San Marcos Area as a Case Study The San Marcos area is ideally suited as a case study for the methodology being presented in this study. Four major aspects of the area contribute to this suitability: 1) the presence of a variety of existing urban systems; 2) the strong likelihood of continued and accelerating urbanization; 3) the excellent oppor­tunity which still exists for planning for future urbanization; and 4) the strong and dyna~ic role of geology in determining land suitability. Three kinds of urbanization -the city of San ~farces, the smaller town of Kyle, and the outlying housing developments and mobile home parks -are present in the area. Because these three are experiencing a variety of environmental conflicts, they pre­sent an excellent spectrum for demonstrating the methodology. Inasmuch as continued and accelerating growth are alillost a certainty in the study area, the area also provides an excellent opportunity for incorporating environmental geologic considerations into planning for future urbanization. The projected population increases during the next 15 to 20 years were indicated earlier for Hays County (89%) and San ~arcos (100%) (Figure 3.5). Several characteristics of the area contribute to this high growth rate. First, the area possesses most of the amenities of good living, including a mild cliillate, clean air, aesthetic hill country sur­roundings, and a generally more relaxed pace of life than is to be found in metropolitan areas. Second, the basic resources essential for urban growth, particularly water and aggregates, are abundant. Third, there is no effective natural barrier to growth in any direction, although some of the characteristics of the land do limit its capability for certain urban uses. Fourth, the area is advantageously located in the Interstate 35 growth corridor and within reasonable com.~uting distance to Austin and San Antonio. Demographers predict bvo ~egapoli in Texas -one on the Gulf Coast and the other stretching from Dallas to San Antonio (Flawn, 1965, p. 6), and San Marcos lies directly in the path of the latter. Des?ite the existence of a considerable amount of urbaniza­tion, most of the land in the area is still in agricultural use. Consequently , there is still excellent opportunity for intelligent physical planning for the urbanization that is certain to come. An essential part of that planning consists of envirolli~ental geolo­ gic considerations. NATURAL DATA SOURCES Five natural data source maps and two cultural data source maps are presented as Plates 1 to 7. The study area is depicted as two maps on these plates, with the northern or Kyle section on the left and the southern or San Marcos section on the right. All the maps were prepared at a scale of 1:24,000, but have been re­duced for final presentation to a scale of 1:48,000. A natural data source map was prepared for each of the lowest categories in the classification of the geologic environment. Plates 1 to 5 therefore include an Engineering Geology, a Soils, a Resources, a Processes, and a Landform map. Much of the data shown on these maps was derived directly from a conventional geolo­gic map of the area. The geologic map, which was prepared in the initial phases of this study, presents rock-stratigraphic units as defined in the Stratigraphic Code (lunerican Commission on Strati­graphic Nomenclature, 1961). The map appears in two parts (Plates 9 and 10), and a condensed geologic report of the area is given in the Appendix. Engineering Geology Xap ine Engineering Geology map (Plate 1) depicts the location and distribution of the bedrock units having contrasting physical properties. This map is derived directly from the geologic map of the area by grouping together rock-stratigraphic units having similar lithic and engineering characteristics. Fortunately, the rock-stratigraphic units in the area are relatively uniform in composition and physical properties both through t~eir stratigraphic thicknesses and laterally over the extent of the study area, so that no subdivision of geologic map units was necessary. The grouping procedure led to the definition of four units on the Engineering Geology map: hard limestones, mixed hard and soft limestones, clay, and alluvium. 87 Determination of the rock-stratigraphic units to be in­cluded in the four map units was relatively straightforward. The hard limestones unit (LH) includes the limestone, dolomite, and dolomitic limestone strata of the two formations -the Kainer and the Person -of the Edwards Group as well as the limestone and marly limestone of the Buda Formation. The mixed limestones unit (L~) includes several rock-stratigraphic units which are composed of either marls or interbedded hard and soft limestone strata. The rock-stratigraphic units composing the clay unit (C) are mostly calcareous smectitic mudstones, clays, and clay-shales. One of the formations of the clay unit, the Eagle Ford, consists of calcareous siltstone and sandstone flags through about 25% (about 1.2 meters) of its thickness. The alluvium unit (A) includes the floodplain and terrace deposits of the intermittent and perennial streams in the area. The four map units of the Engineering Geology map are tabu­lated in Table 3.1. For each unit, a brief lithologic description is given, the stratigraphic units included are listed, and approxi­mate values of the more i~portant engineering characteristics are presented. The values listed for the engineering characteristics are taken from previous work on rock-stratigraphic units about 50 kilometers to the northeast of the study area in the vicinity of Austin. However, because of the strong lateral uniformity of the rock-stratigraphic units in the central Texas region, the values are believed to be fairly representative for the study area. Much work remains to be done to determine these properties more exactly for the study area, but such additional work is beyond the scope of this study. The distribution of the Engineering Geology map units over the study area is largely controlled by the Balcones fault zone. The Kyle section is underlain mostly by hard limestones in its western half and by mixed limestones in its eastern half. A complex Hap LJ tho logic Stratigraphic Unit Ho is tu re Triaxial Unconflned Plast l city Adsorpt. Adso rµt. Unit Description Uni ts Weigh t (% by Compr~ss. Cou1pr~ss. Index Swell Pressu~e Included (lb/ft3) volume) (T/ft ) (T/ft ) (:4) (lb/ft ) LH Hard carbonates , includ i 1111 Kka Kainer both limestone and dolomite; Kpe Person 140 N/A unk. 50-200 N/A N/A N/A hard may contain chert nodules Kbu Buda (e~ t .) limestones and solution porosity LM Mixed hard and soft lime-Kgr Glen Rose stones,marls, and dolomites; Kgt Georgetown 80-130 N/A unk. 10-300 N/A N/A N/A mixed individual beds or fault Kau Aus tin limestones blocks range from very hard to very soft c Clay , mostly smcctite , usually Kdr Del Rio cal careous , approaching a Kef Eagle Ford 80-130 10-50 0.1-8.0 0.9-25.0 10-70 0.1-9.0 800­clays marl in places ; may ~onta1n Ksp SprJ nkle 6f>OO a 5-foot thicknesq of f laggy Kpg Pecan Gap limestones Kbe . BergstC"om Keo Co rsicana A. Alluvium, gcner3 lly unconsoli-Qht High dated silty limestone gravel terrace 80-110 3-70 0.1-'>.0 0.1-7 .0 4-60 0-5 2­alluvium with lesser quantities of Qal AlJuvluru 6000 sand; locally with caliche-and low cemented congloir.erate cap terrace Compiled from: Rodda, Garner, and Oawe, 1970 Garner, 1974 Garner, 1975 Tahle 3.1 Engjneering Characteristics of the Engineering Geology Map Units CY;) co zone of irregularly shaped patches of all four units separates these two halves. A broad terrace of alluvium covers part of the mixed limestones, and the southeastern corner of the area is under­lain by clay. The San Marcos section is divided into two distinct parts at the Balcones Escarpment. Southeast of this boundary the area is underlain by alluvium in the north and by clay in the south. Northwest of the boundary the substrate consists mostly of hard limestones with irregular patches of clay. A strip of mixed limestones parallels the escarpment in the western half of the San Marcos section. Soils ::1ap The Soils map for the study area (Plate 2) depicts the areal distribution of the physical properties of the shallow sub­strate. This map was prepared from data provided by personnel of the USDA Soil Conservation Service. Part of the area, indicated by the mapping credits on Plate 2, was mapped and published as a planning tool for the immediate vicinity of the city of San Harcos (Lowther, 1972). The data :or the remainder of the area is based on unpublished mapping on aerial photos provided by the Soil Con­ servation Service Soil Survey Party in San Marcos. Soils mapping is in progress for Hays and Comal Counties, and a combined county soils report should be available by 1980. Part of the study area has not yet been mapped, and the mapping in the remainder of the area not included in the published report is derived in part from work for the current project and in part from earlier work which was done on a farm-by-farm basis and by several different soil scientists. Also, the soil series have not been correlated with other series in the region and are therefore subject to change. Despite those limitations the data presented in Plate 2 are the best currently available. Plate 2 should be replaced by the Hays­ Comal County soil report as soon as the report becomes available. The map units on the Soils map are soil series because, as noted in Chapter 2, the series is now defined on the basis of objectively determined physical and chemical properties of the soils. The twenty-four soil series in the area are shown in Table 3. 2 along with ·most of their important engineering properties. These data are taken chiefly from Lowther (1972). The data for series not recognized in Lowther's report are taken from the county soils report for Travis County (Werchan, Lowther, and Ramsay, 1974) , which is located immediately to the north of the study area. Most of the soils in the study area are residual and therefore strongly reflect the lithology of the underlying geologic parent rock. The Balcones Escarpment marks a sharp discontinuity in soil type which is expressed on the state soils map as a change from the Houston Black -Heiden -Austin Association in the east to the Tarrant -Brackett -Speck Association in the west. The distribution of soil series in the area strongly reflects this regional pattern. Where slopes are steep and the soils are under­lain by the hard lL~estones of the Edwards Group in the northern and western parts of the study area, the soils are primarily the Eckrant and Speck Series. Southeast of the Balcones Escarpment and south of the San Marcos River, thick black vertisols com­prising chiefly the Houston Black and Heiden series have developed on the Upper Cretaceous clays. In the east-central part of the area and extending a considerable distance upstream on either side of the Blanco River, several series, including the Branyon, Bosque, and Lewisville series, have developed on the alluvial deposits of the broad floodplain and terrace of the Blanco and San Marcos Rivers. In the central part of the area along the Balcones Escarp­ment, the complex faulting of the Balcones fault zone, with the resulting irregular, patchy occurrences of geologic substrate, have given rise to a complex distribution of soil series. Soil Sedu ... s,..bo1 3 Afi'°roziaate ~th.to "!.id.rock" 4 U• u.al 8-trock or Pu•at e 8 Cor rod..!!!I, Uat.r-ted Co!llcr•t• StHl 7 tl•pth !l.44a• tor eot_. 8 9 Clu•1f1ca t1oo. Uu.ifhd USDA Tutur• 10 At terbera Litiuid Lia.it 11 U.ait• Pl.utic t.t.ai t 12 hr-ability (iocbu/ hr) 13 ''"l-..c.tioa" (pli) Material 8 t o 13 ( iJlChH) .u >10 feet C.lcareou• clay1 Modent. t.qw ._. 6-JS }5-60 0-28 CL or CL or CH ct. di sdry clay SUty clay Silt y c!Ar Chy .. 1-51 41-51 )0-40 H -96 20-n 20-31 l~-23 48-6.S 0.6)-2.0 0.6J-2.0 fl.63-2.0 0.06 1.9. 8.4 7.9-3. 4 7 . 9 -g 4 O.l -7.8 20 t o 40 28-)2 FTactured l t.ae1cooe ...... lo lpsbct >10 h•t C•kar.aua lo.-y lUgh CL CL Lou; shy :i... Clayey lo• JO:ll') Jo-40 11-20 11-23 0.6)-2.0 Q,63-2.0 7.!i-~ 7.9 -8.4 el !Hy1nm Iatu~.dded SC Gvi'l r.lay Loa :JO 0.2....,J .6J 1.9 -A.4 10 co 20 li.autooe iliih SC Clay lo­ 29 0.2~.63 7.9 -8.t. inch•• aad oMrl >lD'hat Clayi!:y Var;-0-60 CH Clay 60-80 JS-50 0.06 1.9 -8.4 1li1N h;a b18b 0-)6 Slty clay; clay 50-io )0-45 6.06-0.20 '. ~ -3.4 Deatoa Oe 20 to 40 36-40 Y&thbl• lpcbu 0-... t CL GYiy •lty c!ar 40:50 10:10 •J.2-o.63 7.C) -d.4 no.. Do 11-~ Hard•oed l1M.9 tone 10 f HC Cl01yey iUgh 0-60 CL or CH Sllty clay 1) . 2--0.63 7. 9 -8.4 al UVi.\J:ll Calcareous Raidec .. >10 !ut ahaly iUgh Lov 0-60 Cl! Clay 55-30 '.ll-50 0.06 ; ,9 -8.4 '-• (;ara:>Ua Bou•t.OI\ Bo >10 fact 1ha.ly !Ugh 0-<0 CH Clay 60-80 36-50 0 .06 7.9 -8 . 4 • k c!ay.s Clay~y tune• Righ Lo• 0-60 CH Silty Clay 51-71 29-45 0 . 2--0 .6) 7.9-8.4 ;tllu.Yi Clayey CL, Cl! Silty clay 40-6~ Z.5-·~0 O.S3-Z.O 7.9 -8.4 turac• High >10 fut CL Silt y c.!.ay 35-45 17-)0 0.63-Z.O 7.9 -3.4 .s.Llu.V1U1l C't. S1!ty dav 35-!o~ li-30 0.63-2.0 7.] -8.4 ~. GP, 0-60 Qt, SP, Vary gvly nonplutic 6.3-20.0 7 .9 -8.4 On t Or )10 fut Gravelly I.ow CH Clay lo-51.-71 30-.\0 O.Z-0.6J 7.9 -8.4 0-7 SC , CL,GC Gvly loaa 30-45 1.2-20 0.6-2.0 7.9 -8 . 4 C'!u,.eny Qu 4 fO l2 l;ravdly Moderat e i-U l:i.d u::atad cal lth• na :ia r.a n.a ~~~~~~~...!.oua.L-~~..'1..,.,..U...~~~~~~~~~_.~t-~60,,_~~--"''-"''-"''---->.,.il£l:l..!n~·~ad~d~~21-~30,___~~Ql~l~~...L~•~'~~-~'&O-·lL-~~~·•L:.-~'~·''­ O-J6 CL Cly lOM; le-12-45 1.4-25 0.6J-2.0 7.9 -il.4 S• >10 fut Calcaraou.s Lo• J6-60 CL, CL-!il. Cly lou: lou 20-35 5-15 0.63-2.0 7.9 -8.4 0-6 CL Clny; c!7 lou '.l0-48 l..!.-25 0.2--0.63 6.l -7.6 Sp•tk Sp l C:> l Hard Klgh 6-12 C!I ::i.y 51-91. 28-6j 0.06-0 • .? 6.l -7.8 f uc li:11.,;11tone 0-17 CL or CH Slcy cly loam 48 0. 2-J.63 7.9 -8.4 St ll to 20 Chalk. Calcueo:.J• 0-13 CL !.JD; slt;t cly hi ~0-40 7-1.8 0.6-2.iJ 1.9 -d.4 s\lnav Su )111 fut io-v cunc• Mcaarate t.ow 18-16 CL la; .ilty cl; l:a 25-40 :-1e 0.6-2.0 7 .9 -9 .~ 7.9 -il.4 ~~~~~~~~~~~~~·lll>~"'!"'~·~~~~~~~~~--''~·-~·~o~~~~QCI~~_J.'~··._...,1~c·~1~<~'x~l~•L-O'~Q~-~,)~--''~-~ l8c__~----2.:..!!::l.O Cli.ysy !"r1o1ty TT )lO f ..t: &lkal tee H111,h Lo• Cl! Clay 55-75 32-49 1L06 7.9-8.4 ~~~~~~~~~~~·~~iU!l!!Q..~~~~~~~~~--,,-.,,,-~~~..,-::-~-::::-~-:::~-,-:-,-~~..,.,-~~~,,-~~--,,-..,..-,,-.,.,~~~~~~ 0-22 ~ or CH Slcy dy ~ 56 26 0.2-0.61 1.9 -8 ... '/ol.anu V? 3 co 4 Alluviu.11 High Low Z2-36 CH Slty clay 53 30 0.2-0.63 7.9 -3.4 ~~~~~~~...:..""-~~~~~~~~~~~~~~~--''~'-~'~'~~~'-'--"'-'......_~~·'".2~~~-~63'--~--'·'·' '~Y~c~la~y'--~-_;Q__.~~~'--~~~0~'~I....:.·~•~::._ ~ac:a are fro. Lowther, 1972 unlul:I ochervise noted 11 Daca fro. \lerchao and ochacJ:, 1~74 •rormcrl y ta:lotd T..1nanc Table 3.2 Tabulation of Soil Series and Associated Engineering Properties 92 Resources Map In the San Marcos area the most important resources for urbanization are the plentiful supplies of water and aggregates. The location, extent, and distribution of the various categories of these resources are shown on the Resources map (Plate 3). In general the resources indicated on this map should be considered as potential resources whose existence and value at a particular location would have to be confirmed by detailed study at the site in question. The map units shown on the Resources map are derived from several sources. The areas of potential aggregates resources are taken directly from the geologic map, and the availability of sur­face water is indicated on U. S. Geological Survey topographic maps. Areas where significant quantities of ground water are available were determined by study of the geologic map and by con­sultation of published and unpublished hydrogeologic studies of the Edwards aquifer. rne water and aggregates resources are generally described irrespective of their actual utilization; the use of resources for urban input and the resulting environmental conflicts are re­served for coverage in Chapter 4. Only the resources of direct significance to urban maintenance and growth are discussed despite the presence of several potential resources which could in the future have great economic imporcance to the cities. These poten­tial resources include petroleum, limestone as a source of lime or cement, and a small deposit of sulfur. In general these re­sources are not in danger of being eliminated from utilization by the growth of urbanization over the resource deposits. Water Resources Both surface water and ground water resources are indi­cated on the Resources map. Surface water resources are readily 93 available for urban use from the almost-perennial Blanco River and the perennial San Marcos River. Other surface water bodies in the area, such as artificial reservoirs, are too small to be con­sidered for urban uses and are not considered. Although little urban use is presently being made of surface water, the channels of the Blanco and San Marcos Rivers are indicated on the Resources map as potential surface water resources for urbanization. An abundant supply of high quality ground water is probably the most significant natural resource for urbanization in the San Marcos area. Nearly all of this water comes from the prolific Edwards limestone aquifer. With its annual water budget of about 500,000 acre-feet, the Edwards is one of the most important aquifers in the southwestern D.S. Because of its importance to the economy and well-being of south-central Texas, the Edwards has been studied and described at considerable length (see, for example, Klemt and others, 1975, Abbott, 1975; Abbott, 1973; Alexander and others, 1964; DeCook, 1963; and DeCook, 1960). The study area lies over the northeastern tip of the well-recognized major portion of the aquifer. San Marcos Springs, which has an average discharge of about 265 cubic meters per hour (155 cubic feet per second) flows from the Edwards. The areas where potable ground water can be obtained by drilling wells into the Edwards are indicated in three zones on the Resources map. The wide dashed lines indicating the boundar­ies between the three zones are highly generalized and are in­tended primarily to show that the aquifer is quite variable in productivity. Much additional work remains to be done on this intensely faulted, highly nonuniform and anisotropic aquifer before its hydrogeologic characteristics, including the areal distribution of its productivity, are known in detail. The zones on the Resources map should not be construed as more t~an broad 94 guidelines to be used until more detailed and accurate maps of the aquifer properties are available. The boundary indicated as Gl on Plate 3 is the so-called "bad-water line" which marks the limit of the aquifer in the San Marcos area. Southeast of this line Edwards water is not potable because it is charged with as and has 2a dissolved solids content of more than 1,000 mg/l. This line is taken from maps in the U.S. Geological Survey Water Resources Division office in San Antonio. The boundary indicated as G2 is the regional transmissivity concour of 1 million gallons per foot per day as delineated by Klemt and others (1975, Figure 9). The zone between line Gl and line G2 is the most prolific part of the aquifer, where wells producing over 230 cubic meters per hour (1,000 gallons per minute) can be drilled in most places. Tne line indicated as GJ is based primarily on general observations of the productivity of existing wells. Wells drilled in the zone from the G3 line to the G2 line typically will produce between 23 and 230 cubic meters per hour (100 and 1,000 gallons per minute). Northwest of the G3 line wells will generally produce less than 23 cubic meters per hour (100 gallons per minute). ~ear the Blanco River trench the Edwards is essentially dry because re­charge is drained out by springs in the trench walls. In that area even small domestic wells for residences in the Highlands development must be drilled into the Glen Rose Formation. Some water is also pumped from minor aquifers in the Glen Rose Formation, the Austin Group, the Taylor Group, and the Quaternary alluvial deposits, but these are greatly overshadowed in importance by the Edwards aquiier. These minor aquifers are not indicated on the Resources map for three reasons: 1) Their distribution is too irregular and too unpredictable to indicate with confidence on a map, 2) They are usually not productive enough to support population densities associated with urbanization, and 95 3) The water is often too highly mineralized for satisfactory domes­tic use. Aggregates Resources The San Xarcos area possesses abundant aggregates resources. Considerable parts of the area are underlain by limestone that is suitable for crushed stone or by alluvium which can provide sand and gravel. These resources are indicated by three map units on the Resources map and are significant from two standpoints. First, they indicate where the potential resources may be found and utilized, and second, they indicate where urbanization should be delayed until after the resource has been mined, thus allowing multiple sequential use (Flawn, 1970, p. 91). The areas underlain by substrate potentially suitable for crushed stone lie generally west of the Balcones Escarpment, and essentially coincide with the hard limestone and mixed hard and soft limestones (LH and L~) of the Engineering Geology map . Because of the widespread occurrence of these units, and because t heir thicknesses usually exceed the depth limit of economic quarrying, this resource is virtually inexhaustible. The sand and gravel resources in the area are largely alluvial floodplain and terrace deposits of the Blanco and San Xarcos Rivers (Figure 3.6). These potential resources are shown on the Resources map as two units. The lower terrace and flood­plain deposits are mapped separately from the upper terrace de­posits because the lower deposits are more widespread and abundant and are of higher quality for use as aggregates. rne lower de­posits are generally uncemented and in places exceed 9 meters in thickness, whereas the upper terrace deposits are usually capped by well-cemented caliche conglomerate and are generally less than 5 meters thick. Except for the difference i~ degree of cernentation, Figure 3.6 Blanco River Alluvium. This deposit, composed of about 6 meters of limestone gravel overlain by about 3 meters of sand, silt, and clay, is the best source of sand and 0aravel in the area. The contact between the gravel and the finer material is indicated by the arrow. This site is in UTM 3313-605. The view is eastward across the Blanco from a wet-operation gravel pit now in operation. Figure 3.7 Recharge Point for the Edwards Aquifer. Note the cave and the smaller scale "burrow" porosity. The bur­row porosity results from the solution of burrow fill formed by infauna during the deposition of the limestone. The 5-foot pole indicates the scale. The location is in the channel of Purgatory Creek in UTM 3305-598. Figure 3.3 Sinkholes in the Edwards Group. a. Aerial view of a sinkhole used as a stock tank. Tne black specks, which are cattle, indicate the scale. This sinkhole is in UTI1 3322-597 and the view is to the north. b. Ground view of another typical sinkhole. This sink is also used as a stock tank when it contains water. :fote the pond remnant in the left center of the photo. The sink is in UTI1 3312-597 and the view is eastward. c. Collapsed sinkhole in the Edwards Group. This sinkhole cross section is exposed in a quarry wall in UT~1 3309-603. The easily eroded Del Rio Clay, visible in the center of the photo, has collapsed into the Edwards limestones and overly­ing Georgetown marl. The arrows indicate the edges of the sinkhole. This ·1iew is to the northwest. Figure 3.9 Unstable Slope in the Pecan Gap Formation of the Taylor Group. This slope has ~een oversteepened natur2.lly along a cut bank of the San Narcos River in UTI1 3303-605. This northwestward view shows tilted juniper trees and hummocky topography typical of slump terrane. A new house is under construction just off the left side of the photo. Figure 3.10 House Built on the Balcones Escarpment. This house was located to take advantage of the excellent east­ward view over the Blackland Prairie. The home is in San ~farcos in u1-:·1 330 7-603. ; 3.8a 3.8c 98 both types of deposits yield a high-quality aggregate; the clasts are composed mostly of limestone with a small admixture of dolo­mite and chert. Energy Resources The local availability of water power has been of great historical importance to urbanization of the San Marcos area. \fater power could still be taken irom both the Blanco and the San ~arcos River, but it is not presently considered a significant resource, so it is not indicated on the Resources map. Energy input to the area is now derived from distant sources. Processes Map The geologic processes of significance to urbanization are fluvial, karst, mass-movement, and shrink-swell processes (Plate 4). Other much slower but geologically important processes, such as erosion of the limestone uplands west of the Balcones Escarpment, are not mapped because they have little impact on urbanization. Tectonic processes that cause seismic activity are not active in the study area; the Balcones fault zone has not been active in historic time, and the central Texas region is in zone Zero of the seismic risk map of the United States (Oliver and others, 1969). Fluvial Processes The most significant f luvial process in the area is flood­ing. The flood-prone areas are derived in part from maps and air photos showing the "100-year" floodplain as delineated by the U.S. Army Corps of Engineers (1971) and the U.S. Geological Survey (1973). Other flood-prone areas were approximated by : 1) inter­polation of flood elevations and flood depths between the mapped areas; 2) use of geologic evidence, particularly the recent alluvial 99 deposits; and 3) comparison of drainage basins in the area with basins in nearby quadrangles in which the 100-year floodplains have been mapped by the U. S. Geological Survey. Comparison basins were chosen that were similar with respect to size, topography, and geology to the basins in the study area. Plate 4 shows that most of the flood-prone area is in the San Harcos section and includes the broad, relatively flat area around the eastern half of the city of San ~farcos. This large area extends eastward to the eastern edge of the study area and upstream on either side of the Blanco River to the vicinity of Kyle. The remainder of the flood-prone area includes a narrow zone further upstream along the Blanco River and numerous narrow strips along the intermittent streams. These strips are generally shown upstrean to the point where the floodplain width is less than about 100 meters. Karst Processes Aquifer recharge and sinkhole collapse are the significant karst processes in the study area. Aquifer recharge is included here as a karst process because the aquifer involved is the Edwards limes tone aquifer. ~fos t of the recharge occurs in s treambeds (Figure 3.7), but some water also enters by direct infiltration through fractures and sinkholes. The areas of this recharge are im­portant to urbanization from t~o standpoints. First, because the Edwards is a limestone aquifer having solutional porosity, it probably has poor potential for renovation of polluted recharge water and is therefore believed to be very susceptible to degradation of ground water quality by urban runoff. Second, the availability of an abun­dant supply of potable water from the Edwards aquifer is essential to future urbanization in the area. Therefore, the residents of the area have a large stake in protecting the water quality of the aqui­fer. Recent studies of the tritium content of water from San ~larcos 100 Springs (Pearson, Rettman, and Wyerman, 1975) indicate that a sig­nificant part of the Edwards water in this area is derived from local recharge. The Edwards recharge zone in the study area is shown in two parts on the Processes map -a primary and a secondary zone. The primary recharge zone coincides with the outcrop of the Edwards Group on the geologic map. The secondary recharge zone includes two types of areas: l) outcrop areas of the Georgetown Formation and of alluvial deposits overlying the Edwards Group, and 2) areas which lie outside the primary zone but which lie in the drainage basins whose streams cross either the primary zone or the secondary zone as defined in 1). In general, the primary recharge zone in­cludes the western half of the Kyle section and the northwes tern corner of the San ~arcos section of the study area. The secondary zone takes in a wide band which lies proximal to and east of the primary zone. Sinkholes are common in the Edwards Group limestones (Figure 3.8), but they are not particularly noted for a tendency to collapse. However, the presence of post-Edwards strata in some sinkholes (Figure 3.Sc) shows that collapse can occur, and at least one historic example of sinkhole collapse in central Texas has been documented (Hunt, 1973, p. 233). For these reasons the sinkholes in the study area are shown on the Processes map as sites of poten­tial collapse. These sinkholes occur within the outcrop area of the Edwards Group limestones in the western half of the Kyle sec­tion and in the northwest corner of the San !farcos sec tion of the study area. They are found mostly in flat upland areas where stream dissect~on has not destroyed them, and they are particularly aboodant in the nortb;est corner of the Kyle section on a karstic plain. 101 Mass Hovement Processes Several areas around San Marcos have unstable slopes where clay substrate and steep slopes occur together, such as along cut banks of streams (Figure 3.9). Tne unstable slope unit on the Processes map includes only those areas which are experiencing active movement; areas which are underlain by clay substrate are not included unless movement is taking place under natural condi­tions. Slopes that are not actively moving but which may become unstable if they are oversteepened for cuts during construction can be delineated by using the Engineering Geology map and the Landform map (described below) together. Clay units indicated on the Engineering Geology map may be regarded as potentially unstable if they coincide with slopes greater than 15%. Most of the areas of active mass movement are east of the Balcones Escarpment. Shrink-Swell Processes The term shrink-swell is used here for the cyclic change in volume of expansive clay substrate under varying moisture condi­tions. Tne seasonal shrinking and swelling of the clay causes a self-churning action of the soils. In undisturbed natural condi­tions the churning action creates micro-relief (gilgai) on the sur­face, but this feature has been mostly destroyed in the study area by cultivation. The great soil group term for these soils is vertisol in the modern soil classification system (Buol, Hole, and McCracken, 1973, chapter 16). On the Processes map the unit used to delineate areas where this process is active is termed high shrink-swell. This unit can cause severe structural damages to foundations unless special construction techniques are used. The areas where the shrink-swell process is active include most of the clay substrate terrane east of the Balcones Escarpment. 102 Landform Hap The landform of the San Harcos area affords both oppor­tunities and limitations for urbanization. One of the major oppor­tunities is the availabilit y of excellent scenic vistas along the Balcones Escarpment (Figure 3.10). Major limitations are also presented to the construction of various types of urban facilities. As noted in Chapter 2, a slope map is probably the best means of portraying the landform insofar as its implications for urbaniza­tion are concerned. Four categories of slope, shown below, are depicted on the Landform map (Plate 5) for this study: Category Slope 1 <2% 2 2-5% 3 5-15% 4 >15% These categories are adopted primarily from the Kansas Geological Survey (1968, p. 11) because they are based on the slope require­ments of several major urban land uses (Figure 2.3). The slope map on Plate 5 was derived from the U. S. Geo­logical Survey 7-1/2 minute topographic maps having a contour interval of 10 feet. This slope map clearly shows the profound influence of the Balcones Escarpment on the landform of the area. The section east of the escarpment has mostly slopes in the range of 0 to 5%, whereas the section west of the scarp is dominated by slopes over 5%. CULTUEAL DATA SOURCES Current Land Vse Map The Current Land Use map (Plate 6) shows the locations and distribution of present urban and agricultural uses. This map was compiled from several sources. The U.S. Geological Survey 7-1/2 minute topographic quadrangles served as the base map and showed the locations of most major features, such as the cities, highways, buildings, and cemeteries. The San Marcos North and San i'.'farcos South quadrangles are dated l964, and the ~1ountain City quadrangle is dated 1968, but all three were photo-revised in 1973. Two sets of aerial photographs were another prime source of land use data, particularly for agricultural uses. One set was flown in 1965 for a scale of 1:20,000 and the other was flown in 1973 for a scale of 1:48,000. Land ownership maps, which were obtained from the Hays County tax assessor's office, were also very valu­able. The land use information from these sources was compiled in the office and then confirmed by field observation. The land use categories shown on the Current Land Use map are shown in Table 3.3. Six major categories are used -one for agricultural uses, one for each of the four major urban system components, and one for miscellaneous uses. A total of thirty­three subcategories are reccgnized. The distribution of land use is strong.i.y controllS:d by the long-established agricultural use and by the locations of urban centers. Table 3.4 shows the major land use categories and the area occupied by each as well as the percentage of the total study area occupied by each category. The major land use is clearly still agricultural, since this category takes in over 80% of the area. Mixed rangeland and pastureland are dominant west of the Balcones Escarpment and mixed cropland and pastureland dominate east of the scarp. ine land use becomes generally more urban toward the city of San Marcos and, to a lesser extent, toward Kyle. Outlying developments now occupy a considerable portion of the former agricultural lane wes t of the escarpment. Agricultural AR__ Rangeland; grassland with mostly uncleared growth of oak and juniper AP Pastureland; rangeland cleared of trees or cropland returned to grassland ARP Mixed rangeland and pastu::eland; usuall_y rangeland with some cleared areas APC Mixed pastureland and cropland; use frequently changes from year to year AI Agribusiness; industrial activity re:aced to agricul tural products AU Agricultural land use, undifferentiated Urban Situs UC Incorporated city or town, no activities differentiated UF Suburban or fringe areas; usually developed areas near city limits UD Developments; mostly in outlying areas, thinly urbanized UI Industrial areas outside city limits UTP Outlying mobile home parks UU Urban land use, undifferentiated Urban Input IP Active sand and gravel pi:s IPA Abandoned sand and gravel pits IQ Active crushed stone quarry IQA Abandoned crushed stone quarry ..-.-4 Dam constructed to utilize 'Jater power Urban Output OS Active solid waste disposal site OSA Abandoned solid waste disposal site OLT Liquid waste disposal site; sewage treatment plant OLI Industrial liquid waste disposal site OA Automobile graveyard Transportation Roads, streets, and highways; public roads only shown TR Railroads TA TH Airfields; public airports and private :anding fields - Pipelines; regiooal petrole~~ product lines . Power lines; major transmission lines; 0 _= subs~a~ion Undifferentiated or combined transportation facilities TU Miscellaneous Uses v Vacant land R Parks or other recreational land use p Public land c w CemeteriesWater bodies; perennial rivers and natural and artificial ponds with areas greater than 1 hectare (2.5 acres)· Table 3.3 Categories of c~rrent Land Use Land Use Total Area (km2) Total Area (mi 2) Agriculture, water bodies, transportation, most of the vacant land 276.9 106.9 Incorporated community, adjoining areas, some public land, undiffer­entiated urban areas, some vacant land in cities 32.1 12.4 Outlying developments mobile home parks and 21. 0 8.1 Other uses: input activi­ties, output activities, cemeteries, recreation, some public land 3.9 1. 5 333.9 128.9 Table 3.4 Approxi mate Areas Devot ed to the Major Land Uses in the San Marcos Ar ea 105 Percent of Study Area 83 10 6 1 100 Land Use Control Map Land use control measures that have been instituted for some parts of the study area will have profound influence on the future uses of the land affected. The Land Use Control map (Plate 7) depicts four kinds of areas where governmental regula­tions control land use. These are: 1) the Edwards aquifer re­charge zone; 2) the city limits of San :iarcos; 3) the extraterri­torial jurisdiction (ETJ) of San Marcos; and 4) the city limits of Kyle. The land use control measure having the greatest impact on the study area is Texas Water Quality Board Order Number 75-0128-20, which was adopted in January, 1975. This order recog­nizes a recharge zone of the Edwards aquifer and sets forth cer­tain land use controls within that zone. The specific controls may be found by referring to a copy of the order, which can be obtained from the Board. The area covered by the order is shown on 7-1/3 minute topographic maps that are on file at the Board, and the parts of the study area affected are shown on Plate 7. When this area was delineated by the Board, it was intended to coincide with the outcrop area of the Edwards Group limestones, so it should be a replica of the primary recharge zone depicted on the Processes map (Plate 4) of this study . A comparison of Plates 4 and 7 shows that the areas match generally, but there are some discrepancies. The area outlined by the Water Quality Board is based on earlier, less detailed geologic data than the maps prepared for this study. The city of San Marcos exercises zoning control over the area falling within its city limits. These limits, which are shown on the Land Use Control map, were adopted effective January 1, 1975 and are current as of this writing (February , 1976). A zone map of the affected area can be obtained from the city goverlli~ent, and the controls exercised over the various zones can be deter­mined from city ordinances. The extraterritorial jurisdiction (ETJ) extends for one mile beyond the city limits and is approximately indicated on the Land Use Control map . The city has formal con­trol only over land subdivision in this area, but some informal control is also exercised in the form of requests and reconunenda­ tions. A similar type of informal control is exercised by the town of Kyle over land which falls within its city limits. The Kyle city limits shown on the Land Use Control map were adopted in 1968 and are current as of this writing. Chaoter .'.; E"NVIRON:IB:HAL GEOLOGY Of ::XISTUG CR3~~ SYSTE:·lS 1::-; THE SA.'l :1.ARCOS A.."R.::.A With the background information and the natural and cul­tural data source maps developed in Chapter 3, it is now possible to apply the curative part of the methodology outlined in Chapter 2 to the San ~Iarcos case study area. The procedure of this chap­ter will be first to describe the urbanization in the area and then to investigate the environmental geologic problems of this urbanization using the urban systems organizational scheme. URBAN SYSTE~·1S IN THE SA!.~ ~1ARCOS AREA Table 3.4 shows that about 16% of the land surface in the San Marcos area is now devoted to urban-related uses. Three cate­gories of urbanization are recognized: 1) the city of San ~areas; 2) the town of Kyle ; and 3) the outlying housing developments and mobile home parks. The City of San ~areas The city of San lfarcos is in the southern half of the study area at a pronounced break in topography along the Balcones Escarpment (Figure 4 .1). The city presently has a population of about 25,000. The corporate limits include about 22.7 square kilometers (8.8 square miles), but the Current Land Use map shows that urbanization in the city and surrounding area covers 30 . 4 square kilometers (11. 7 square miles). With the exception of heavy industry the city possesses the activities and facilities common to most urban systems. Urban situs, urban input, urban output, and transportation are all relatively well represented. The city has a well-defined central business district, many resi­dential areas, and a growing light industrial section. A large section of the city adjoining the central business district on the north is occupied by the campus of Southwest Texas State University (Figure 4.la). The city has had both a public water supply system and a public sewage collection and treatment system for many years. llO Figure 4.1 City of San Marcos a. Northward and downward view of the central part of the city. The central business district surrounds the Hays County Courthouse in the center of the photo, and the campus of Southwest Texas State University acjoins the CBD on the northeast. Several residential areas surround the city center, and Spring Lake and the San Marcos River can be seen in the lower right portion of the photo. b. Southwest aerial oblique view . The Balcones Escarpment is clearly visible on the left side of the photo. The central part of the city is on the escarpment on the right side of the photo, and the thinly urbanized portion of the city east of the scarp can be seen in the foreground. An ar~ of Spring Lake appears on the right side of the photo. c. Light industrial area in the southern part of the city along Interstate 35. Tnis industrial park is gradually being occupied by small nanufacturing firms. Figure 4.2 Town of Kyle a. Northeast aerial oblique view of the town. Interstate 35 can be seen on the eastern edge of the town in the upper part of the photo. b. Westward view of the small central business district. Figure 4.3 Mobile Home Park. This facility, Woodland Hills mobile home park, has been ex­panded to nearly twice t he size shown here. The park is located about 3. 2 kilometers south of Kyle in UTI1 grid 3314-607. This aerial oblique view is to the north. 4. la 4.lb 4.lc 4.2a 112 The distribution of the urban activities of the city is strongly influenced by the Balcones Escarpment. The light indus­trial areas and most of the COIT!Inercial areas are located either at the foot of the escarpment or further to the east (Figure 4 .lc), whereas the more aesthetic surroundings of the hill country in the western part of the town are used mostly for residential areas. During the strong growth period 1963-1968, 69% of the residential construction was in the western part of the city and 31% was in the eastern part (Lockwood, Andrews, and Newnam, 1969c, p. SJ). The Town of Kvle Kyle is in the northeastern corner of the study area at a point on the Balcones Escarpment where the topographic expres­sion of the scarp is relatively weak (Figure 4.2a). The population of Kyle is approximately 1,500, and the corporate limits include about 2.6 square kilometers (1.0 square miles). However, the Current Land Use map shows that only about 1.4 square kilometers (0.5 square miles) have actually been urbanized. Most of the town's area is devoted to r esidential use, but there is a small central business district (Figure 4.2b). Xany of the residents of Kyle are employed elsewhere, particularly in Austin and San Marcos, so the town is at least partly a bedroom community. One manufacturing firm engaged in the production of steel forgings is located north of town. The town has had a municipal water system for several years and in the past decade has also built a public sewage collection and treatment system. Almost all of Kyle is situated on the mixed hard and soft limestones of the Austin Group . This stable substrate, combined with the flat topography, small town size, and primarily residen­tial land use, have resulted in relatively minor enviror..mental problems. 113 Outlying Housing Developments and Mobile Home Parks Of the 16% of the study area devoted to urbanization, 6%, or 21 square kilometers (8.1 square miles), is devoted to housing developments and mobile home parks. No population data are avail­able for this type of urbanization, but at the present rate of growth, the population is undoubtedly increasing almost monthly. A few mobile home parks, one of which is shown in Figure 4.3, have been established in the proximity of San Marcos to meet housing demands in excess of housing availability in the city. Many of the units in these parks are occupied by students of Southwest Texas State University. Six housing developments outside of existing coITu.~unities have also been started in the area. Virtually all land use in these developments is residential. Most of the housing develop­ments are built by subdivision of the aesthetic hill country ranch­land west of the Balcones Escarpment. When such a subdivision is built, commonly a road network is first constructed to provide access (Figure 4.4), and then lots are sold individually in sizes ranging from 0.4 to 4 hectares (1 to 10 acres). Many of these lots are purchased by people living outside the study area, par­ticularly in Houston, Austin, and San Antonio, for vacation homes and for real estate investment. In all but one of the subdivisions each landowner must install his own water supply and sewage dis­posal system, usually with a well and septic tank. These require­ments, coupled with the location of most of the developments in environmentally sensitive areas, have resulted in considerable environmental geologic conflict. ENVIRONMENTAL GEOLOGIC PROBLEMS OF EXISTING URBAN' SYSTE:!S Because of long exposure the residents of the San Marcos Figure 4.4 Outlying Housing Development. a. Entrance to the Valley View development, showing the avail­ability of lots and an advertisement of features. A house built on one of the lots can be seen in the background. The development is located west of the Balcones Escarpment in CTI1 3312-601. b. Southward aerial oblique view of Valley View. (~ate that few houses have yet been built in the development. The entrance shown in the preceding picture is in the upper center of this photo. Figure 4.5 The 1970 San Marcos Flood. a. Floodwaters at and below Spring Lake Dam. The overtopped dam can be seen in the upper left corner of the photo, and the large buildings are the Clear Springs Apartments. The arrow pointing to the building on the right shows the high water mark at about the top of the lower floor. Tnis aerial oblique was taken in a northeasterly direction. (Courtesy of San Marcos Record) b. Floodeci housing development in eastern San Marcos. This development is in U'D1 3307-604, and the aerial oblique view is eastward. (Courtesy of San ::1arcos Record) Figure 4.6 Flood-Damaged Trees Below Spring Lake Dam. Note the height at which branches have been broken off by flood waters. Tne view is eastward, and the scale is 2 meters high. This scale is also used in several photos hereafter. 4.4a 4.4b 4.5a 4.5b 4.6 116 area have grown somewhat accustomed to urban ?roblems related to the geologic environment. :'fost of these problems are associated with the city of San Marcos because it is the largest urban center, it has the greatest variety of urban land uses, and it is located where a diversity of land characteristics give rise to a variety of environmental problems. Neither the town of Kyle nor the out­ lying housing developments exhibit the intensity or variety of enviroTh!lental conflicts found in and around San Marcos. The procedure of this section will be to consider each of the four components of the urban system in turn and examine the environmental conflicts of the various facilities and activities of each category . The interaction approach will be used for the urban situs category , and the subcomponents approach will be used in the input, output, and transportation categories. The Current Land Use map (Plate 6) shows the locations of many of the exist­ing urban facilities, and the locations of features described in the text are indicated by use of the Universal Transverse Her-ca tor (lJTH) grid system. This grid is taken from the U. S. Geological Survey topographic maps and is depicted on t he Current Land Use map. The five physical data source maps illustrate the geologic settings of the various facilities, but most of the observations of this section resulted from field visitation of the facilities. Hany of the problems described in this analysis are indi­cated rather than confirrned problems, and much of the information presented is qualitative and descriptive. Host of the problems would require additional work to evaluate their seriousness and determine the steps needed to solve them. This additional effort is beyond the scope of this study because the objective here is to demonstrate an organizational methodology for delineating and describing potential environmental geologic problems. Additional work necessary for their validation and mitigation should be 117 undertaken either by those who are causing the problem or those who are affected by the problem. Urban Situs Urban situs comprises two facets of the interaction of urban systems with the geologic envirolliuent -impact of the environ­ment on the cities and impact of the cities on the environment. Impact of the Geologic Environment on Urbanization The San Marcos area is rather remarkable for the severity of both insidious and catastrophic hazards that are posed for urbanization. The most important of these are the problem of flooding and problems caused by outcrops of clay substrate in some parts of the area. Flood Hazard Owing to its location over the Balcones Escarpment, the area of study is in one of the most flood-prone areas in the U.S. (Baker, 1975), and flooding is without doubt its most serious environmental geologic problem. Tne cs~arpment produces an oro­graphic lifting effect on inlaGd-Qoving maritime air masses (see Chapter 3) and influences cold f ronts that push their way south­eastward across the state. These fronts are sometimes stalled in the vicinity of the scarp or vacillate back and forth across it, thus causing major thunderstorms. Both the local, relatively small intermittent streams and the Blanco River, whose drainage basin includes about 1,100 square kilometers (430 square miles) in the eastern edge of the Edwards Plateau, flcod frequently, The Processes map (Plate 4) indicates the flood-prone areas by depicting the "100-year flood;:>lain." Most flood damages are caused by the Blan~o and San Xarcos rivers and Sink Springs, Purgatory, and Willow Springs creeks. llS Although the topography at Kyle is relatively flat, some rather serious flooding occurred in November, 1974, and the town applied for eligibility for federal flood insurance in January, 1975 (San Marcos Record, 1975). Most of the housing developments in the area have not been greatly affected by flooding in the past, although parts of some of them are in flood-prone areas. At least one example of road damage caused by flooding in a subdivision was observed, but overall flood damage to subdivisions has been relatively minor because most of the subdivided areas have not yet been fully urbanized. Historically, the city of San ~farcos and the inunediate vicinity have suffered the most serious flood damage, and they com­prise the area most seriously threatened by future floods. The original Spanish settlement of San Marcos de Neve was abandoned primarily because of a flood, and the present city has been flooded several times since its establishment. The most serious floods occurred in 1913, 1921, 1929, 1952, and 1970 (U.S. Army Corps of Engineers, 1971, p. 4), and "nuisance" floods commonly occur two or three times each year (Hays County Citizen, June 12, 1975). The highest magnitude flood recorded occurred in September, 1921, but the Hay, 19 70 flood was the most damaging because of increased urbanization of the floodplain (U.S. Army Corps of Engineers, 1971, p. 6). In the 1970 flood two lives were lost, and damage was estimated at over $3 millicn (Longley, 1975, p. 139). A compari­son of the flood-prone areas shown on Plate 4 with the urbaniza­tion in those same areas as indicated on the Cur rent 1and Use map shows that most of San ~farcos east of the Balcones Escarpment is subject to flooding. Figure 4.5a shows the floodwaters of the 1970 flood as they appeared just downstream from Spring Lake after the flood had abated considerably , and Figure 4.Sb shows a housing development in San Marcos that was in process of being occupied when the flood occurred (San Marcos Record, 1970, p. 6). 119 San Marcos is subject to flooding both by local intermit­tent streams and by the more regional Blanco River. Blanco River floods reach San Marcos at least in part by 11blackflow11 up the San ~farcos River from the confluence of the two rivers. Al though the intermittent streams have much smaller watersheds, they flow directly through the city and occasionally experience very high discharges. The 1970 flood was caused primarily by overflow of Sink Springs, Purgatory, and Willow Springs creeks. These streams also show evidence of high discharge both by the presence of trees that are damaged at considerable heights (Figure 4.6) and by large, meter-long limestone boulders that are in the stream channels. These boulders are clearly transported only when discharges are very high. Floodplain management and flood prevention measures are both used to alleviate the flood hazard. In the floodplain manage­ment method a floodwater elevation map prepared for the 1970 flood (U.S. Department of Agriculture, Soil Conservation Service, 1971) is used, and lando~'ners are required to construct structures in the flood area a minimum of one foot above the 1970 flood elevation. Flood prevention measures consist of the construction of several dams. Floods caused by precipitation in the upper part of the Blanco River basin will presumably be brought under control by con­struction of the Cloptin Crossing Dam about 19 river kilometers upstream near the conununity of \Emberley (U.S. Army Corps of Engineers, 1964). This dam will not, however, prevent floods re­sulting from intense precipitation events between the dam and San Harcos (Victor Baker, personal communication). !r;Jo types of flood prevention methods have been proposed for the intermittent streams in the area -the bypass method and the up-basin retention method. Yne bypass method, proposed by the U.S. Army Corps of Engineers (1971), consists of several measures, including channelizing long segments of the streams and excavating a channel from Sink Springs Creek at a point just upstream from Spring Lake eastward to the Blanco River. This ~ethod was re­jected by the city in favor of an up-basin retention method pro­posed by the Soil Conservation Service Watershed Planning Cnit for the Upper San Marcos Reclamation and Flood Control District. The primary feature of this method is the construction of seven darns on Sink Springs, Purgatory, and Willow Springs creeks and their tributaries not only for retaining floodwater, but also for re­charge of the Edwards aquifer. This plan has been accepted by t he city, and the acquisition of land easements for t~e dams and reservoirs is expected to begin in the near future. The project is expected to eli:ninate between 90 and 95~~ of the flood damages to urban property in San Marcos (Longley, 1975, p. 185). In general, the proposed measures for flood prevention in the San Marcos area should be adequate for controlling smaller magnitude floods. However, additional steps should probably be taken to more clearly delineate the areas subject to the l ow-frequency, extremely high-magnitude floods characteristic of the Balcones Escarpment. Not only should future development be cur­tailed in such areas, but steps should probably be undertaken to make some changes in current land use. The "100-year floodplain" as demarcated by the U. S. Army Corps of Engineers and the U.S. Geological Survey probably is not adequate for delineation of areas affected by these very high magnitude floods (Baker, 1975, p. 13). Clav Substrate Problems "Poor soil conditions," as they are called locally, have long been a troublesome problem in the San Xarcos area. One of the four Hays County courthouses that have been built in San Marcos , for example, was abandoned and destroyed in the 1800s after only 10 years of use because of "earth shiftings" at the courthouse site 121 (from Texas Historical Commission historic marker). The primary causes of these problems are the clay units depicted on the Engineering Geology map (Plate 1). These clays cause a variety of problems because of their low shear strength, their tendency to consolidate under a load, and their susceptibility to cyclic volume changes (shrinking and swelling) with seasonal variations in soil moisture. The occurrence and distribution of the clay substrate units, as shown on Plate 1, is different on either side of the Balcones Escarpment. The entire area east of the scarp is underlain by Upper Cretaceous clay units except where these units are mantled with alluvium. The composition of stratigraphically equivalent units further north in Travis County was found to be dominantly calcareous Ca-montmorillonite having poor engineering properties (Funk, 1975; Tipple, 1975). Unpublished work by the author indicates that the results of the Travis County work can be reasonably extrapolated this far south. west of the Balcones Escarpment the clay substrate units occur in an irregular, patchy distribution in a dominantly hard and mixed li~estone terrane. Two stratigraphic units -the Del Rio Clay and the Eagle Ford Formation -comprise the clay sub­strate in this area. The Del Rio at depth, where it is unweathered, is composed of kaolinite, illite, and a small admixture of mixed­layer illite-raontmorillonite and is apparently stable. In the weathered zone, however, the illite and mixed-layer clay are con­verted to montmorillonite, and the engineering properties are cor­respondingly degraded. About 25% of the 7.5-meter thickness of the Eagle Ford Formation is not clay, but consists of thin calcareous siltstone and limestone flags. The remainder of the for~ation is a bentonitic smectite clay having poor engineering properties. Despite the much wider occurrence of clay units east of the escarpment, most of the probl~~s associated with clay substrate occur at or just west of the escarpment. This anomalous situation 122 is explained by the location of most of the present urbanization near the escarpment and by the fact that the presence of clay units and their poor properties east of the scarp are well recog­nized by most of the inhabitants. West of the escarpment, how­ever, most of the substrate is hard or mixed limestone, and the complicated Balcones faulting leaves the residents at a loss to predict where they will encounter the erratic clay units. The city of San :rarcos is the urban area most seriously affected by unstable clay substrate. Kyle is underlain by rela­tively stable mixed hard ~nd soft limestones. :1ost of the outlying housing developments are in the hard limestone terrane west of the escarpment, and the few developments that encompass substantial areas underlain by clay substrate are not yet sufficiently urban­ized to be greatly affected. Although some parts of the eastern half of San Marcos are underlain by clay, the impact is somewhat subdued because land uses that are compatible with the clay sub­strate are usually chosen. Ynus, most of the conflicts are in the western part of San Marcos, where the city is located on the Balcones Escarpment. There the Del Rio Clay forms a nearly com­plete circle around the flanks of the large hill that underlies most of this part of the city, and the Eagle Ford Formation caps this hill. Most of the city at the foot of the escarpment is also underlain by clay . A variety of problems has been caused by these clay substrate units. Shrinking and swelling has caused widespread buckling of sidewalks and road destruction which consti­tutes a continuous and expensive maintenance problem. Minor structural damage of the t ype illustrated in Figure 4. 7 is also quite common. This example is believed to have been caused by shrinking and swelling in combination with consolidation of the clay substrate. Unstable slopes comprise t he most serious and hazardous problems caused by the clay substrate. These problems occur al­most entirely in Del Rio Clay outcrops in steep slopes in the Figure 4.7 Structural Damage Attributed to Clay Substrate. a. Cracks in the side of a brick church in UTI1 3305-601. These cracks are almost certainly caused by heaving clay substrate. b. Foundation damage in same building caused by pressures from swelling clay. Note that the foundation has pushed out slightly beyond the wall. Figure 4.8 Failure of a Retaining Wall. This failure is caused by downslope creep of the Del Rio Clay. Also note the tilted power pole. This view is southwestward along Burleson Street in UTI! 3305-601. Figure 4.9 Sessom Street Slump a.. The slump as it appeared shortly after failure. b. The slump as it appeared one week later. This slump in the Del Rio Clay was caused by oversteepening of a slope for a roadcut. Both views are to the south across Sessom Street in UTI1 3307-602. 4.7 a 4.7b 4.8 4.9a 4.9b 125 western part of San ~1a:-cos. Fai.ii..i"?:e of tnese slooes is either by creep (Figare 4. 3) or, mo!:."e co;:irr:only, ·oy a slump or related bype of shear failure. Figure 4.9 shows a classic slump caused by oversteepening of a slope during construction of the extension of Sessom Street. A cut made for a nearby apartment ccmple:~ is shown in Figure 4.10 before and after a slump-like failure. Also shown is the retaining wall which was later installed by the de­veloper at an unanticipated cost of more than $10 , 000. Like the in situ clay, spoil material from exca-.1ations in the Del Rio can also be unstable when placed in overly steep slopes as show-n in Figure 4.ll. The engineering behavior of the Del Rio Clay is strongly affected by weathering as noted earlier. The preceding e:..;:amples of failure occurred in the Del Rio where it is exposed at the sur­face and is therefore deeply weathered. \v'here the clay is pro­tected from weathering by the overlying 3uda limestones, it may be quite stable. The very steep slope in the Del Rio shown in Figure 4.12, for example, has been standing for over 25 years, and except for minor surficial sloughing, gives no indication of instability. Impact of Urbanization on the Geologic Environment Within the San Marcos area the most important impact of urbanization on the geologic environment is probably the reduc­tion of surface water quality caused by runoff from the urbanized areas. Other less significant impacts are changes in strean basin hydrology and problems of increased erosion and sedimentation. In general, the impact of urbanization on surface water quality is negative and results, for example, in increased bacterial, dissolved solids, and turbidity content and in decreased dissolved Figure 4.10 Slope Failure in the Del Rio Clay at an Apart­ment Complex. a. Freshly cut, over-steepened slope before failure. The cut was made to create a pad, shown in the foregound, for the new apartment building. b. The same slope two days later after a rainstorm and sub­sequent failure. c. Retaining wall which was put in to remedy the problem. All views are southeastward. The site is in UTN 3307-602. Figure 4.11 Slope Failure in Del Rio Clay Spoil Material. The spoils from the excavation in Figure ~.iC wece pushed out on the hillside to create this parking lot. Note the steep slope on the right side of the photo. The view is south­eastward. Figure 4.12. An Oversteepened but Apparently Stable Slope in the Del Rio Clay. a. Southwest frontal view showing the contact with the over­lying Buda Formation. The folding is the result of fault drag associated with the nearby San ~arcos Springs fault. b. Northwest side view showing the steepness of the slope. This outcrop is on Ed J. L. Green Drive near Spring Lake (UTM 3307-602) . 4.lOa 4.lOb 4. lOc 4.12b 4.12a 128 oxygen content of the water. ine water quality characteristics of urban runoff in the San Marcos area have not been studied, but a few generalizations can be stated. Because the land use in the housing developments is primarily residential and because the de­velopments are not yet fully urbanized, the quality of their runoff is probably not greatly different from its original predevelopment character. San Marcos and Kyle are more densely populated than the developments and have a better representation of urban land uses, so they should be expected to produce a relatively typical urban runoff. The impact of urbanization on surface water quality in the San Marcos area is significant not so much because of a par­ticularly large volume or special toxicity of the urban runoff, but rather because of the extraordinary sensitivity of some parts of the area to poor surface water quality. Two features of the area -the Edwards aquifer and the biologically unique Spring Lake and San ~farcos River -account for this sensitivity. The high flow volume, constant temperatures, and high water quality of the San Marcos Springs have resulted in the development of both lentic and lotic ecosystems that are unique at least in the state, if not in the country. Many of the unusual and even unique species thrive in the reservoirs and watercourse of the San }1arcos River because of the good quality of water and may therefore be highly susceptible to pollution. Consequently , the watershed of the San Marcos River, at least to the confluence of the Blanco River, should probably be considered highly sensitive t o urban runoff. The sensitivity of the Edwards aquifer recharge zone to polluted water was described in Chapter 3. The town of Kyle poses little or no threat to either of the two sensitive areas. The drainage of the t own is r.ot into the San Harcos River watershed and does not cross the Edwards recharge zone. Several of the outlying housing developments are located in either or both the San Marcos River Natershed and the Edwards recharge zone, but as noted above their impact on runoff is probably not yet significant. These developme:its may pose a threat to the Edwards aquifer, however, because of the widespread use of septic tanks as described below in the urban output section. Urban runoff from the city of San Narcos may present a serious threat to the San Xarcos River and, to a lesser extent , the Edwards recharge zone. Virtually all of the city lies in the river's watershed at and around the river's source at Spring Lake. The resulting implications for the unusual fauna and flora of the lake and river are being studied by personnel at Southwest Texas State University (Glenn Longley, personal communication, 1975). J.>1uch of the western part of San ~farcos lies in the primary or sec­ondary recharge zone of the Edwards aquifer, as can be seen by comparing the Current Land Use map with the Processes map. The potential damage to the aquifer caused by recharge of polluted urban runoff in the vicinity of the city is all the more serious considering the intense use of water from the aquifer by the city for its municipal water supply. More complete study of the poten­tial pollution of the Edwards aquifer by urban runoff is clearly indicated. Besides affecting the quality of runoff of the stream basins in the study area, urbanization may also affect the hydrology of the stream basins. Leopold (1968) summarized the effects of urbanization on stream hydrology . This impact of urbanization on streams in the San Marcos area has not been fully analyzed, but a few generalizations can again be made. The outlying housing de­ velopments are generally too small and too vacant to have had appreciable impact on the hydrology of the basins in which they are located, and Kyle is also probably too small to have effected great changes in the hydrology of local streams. Probably only the city of San Harcos is large enough to have caused changes in the local strea~ basins. However, the natural hydrology of streams west of the Balcones Escarpment (Baker, 1975, p. 3) is such that 130 urbanization may not have great impact. Thus, that part of San Marcos located east of the escarpment is the ~ost likely to have experienced changes in stream basin characteristics. Two meander loops in this part of the city have in fact been cut off arti­ficially, and efforts to "channelize" the local streams continue. Increased erosion and sedifilentation caused by urbanization are evident in the San Marcos area but do not appear to be a major problem at present. In the city of San ~farcos excessive erosion may occur on the slopes of Del Rio Clay if they are cleared of vege­tation and not reseeded in a short time (Figure 4.13). Also, local examples of stream rejuvenation and downcutting apparently caused by urbanization (Figure 4.14) can be seen. At least one example of increased sedimentation caused by urbanization is present. A small tributary that rises within the city limits of San Marcos has built a gravel deposit out into the San Xarcos River just be­low Spring Lake dam (Figure 4.15) and has caused some concern for the river's biota (Glenn Longley, personal co:nmunication, 1975). It is possible that this increased sedimentation occurred during construction of an apartment complex upstream on the tributary. In summary, it may be concluded that the urbanization in the study area does not in itself present any special hazards to the geologic environment, but the geology in the area make it unusually sensitive to the effects of urbanization. Further study is clearly needed to document the problems discussed and to deter­mine the steps needed to solve or mitigate them. Urban Input The San Marcos area is rich in the bas ic resources that must be derived locally and are essential for urban maintenance and growth. Three t ypes of urban input -aggregates, water, and energy -have the greatest environmental geologic significance in the San Marcos area. Figure 4.13 Rill Erosion of an Exposed Clay Slope. a. Fresh slope exposed in a roadcut for Sessom Street in UTil 3307-602. b. The same slope about 18 months later, showing the deep rills that have formed. The first view is to the southwest and the second view is southward. Figure 4.14 Channel Scour Attributed to Urbanization . The downcutting of this stream is probably the result of in­creased runoff caused by a housing development constructed in the drainage basin. The development, part of which can be seen in the background, is in U'L1 3307-600. The exposed bedrock in the channel is the Buda Formation. The view is to the southeast. Figure 4.15 Sedimentation in the San Marcos River. This gravel deposit, which is located just below Spring Lake Dam, was built out into the river by a small stream that en­ters the river through the large culvert . The entire length of the stream is within the city limits of San Harcos. This view is westward. Figure 4.16 Unreclaimed Crushed Limestone Quarries. a. Abandoned quarry in the upper Buda and lower Eagle Ford Formations. This quarry is located about 2.8 kilometers southwest of Kyle i~ U'Lf 3317-605. The aerial oblique view is southward. b. Ground view of the quarry shown in a. The line is drawn at the contact of the Buda and Eagle Ford Formations . Note the calichification of the lower clay unit on the hillside. The view is northward. c. Intermittently operated quarry in the upper Person and lower Georgetown Formations. The quarry is about 4 kilo­meters northeast of downtown San Harcos in UTM 3309-603. The aerial oblique view is southwestward. d. Ground view of the quarry shown in c. The cattle near the highwall indicate the scale. The view is southeastward. 13 2 Aggregates Input Two kinds of aggregates -crushed limestone and sand and gravel -are mined in the San Marcos area for urban related uses. Most of the environmental geologic conflict associated with the utilization of aggregates is expressed in one direction only -the impact of the operations on the environment. Aside from determining their locations, the geologic environment has little impact on the operations chief l y because of the relatively small permanent capital investment that is subject to geologic hazards. Crushed stone The Resources map (Plate 3) shows that potential crushed limestone resources are to be found over most of the area west of the Balcones Escarpment, and the locations of the quarries as shown on the Current Land Use map (Plate 6) strongly reflect this availability. Crushed limestone is taken from quarries in the Edwards Group, the Georgetown, Buda and Eagle Ford Formations, and the Austin Group. Most of these quarries were opened during the construction of Interstate 35 and were abandoned when the highway was completed. The primary environmental problem of the quarries lies in their reclamation, or rather, their lack of reclamation. Some of the quarries are still operated intermittently, and their owners are reluctant to close them down completely for reclamation. Also, the quarries are relatively difficult to reclaim. Because of the hardness of the limestone, the high walls that are left when operations cease can be reduced only with difficulty and at considerable expense. In addition, the original soils on the limestones at the quarry sites are mostly very thin, thus making revegetation of the sites very difficult. The chief reason for the lack of reclamation, however, is economic. The land values 134 in the area are generally not great enough to economically justify the costs of reclamation, and no state legislation or tax incen­tives have yet been instituted to require or encourage reclamation efforts. For these reasons few of the quarries have been re­claimed, and they remain for the most part as large scars on the countryside (Figure 4.16). Fortunately, with two or three excep­ tions, the abandoned quarries are not misused as waste disposal sites. Waste disposal in quarries in the Edwards Group would likely be particularly damaging because of the potential pollution of the Edwards aquifer. Sand and Gravel Sand and gravel are presently the most important form of aggregates input into urban systems in the San ~farcos area. As shown on the Resources map (Plate 3), the available sand and gravel deposits are floodplain and terrace gravels of the Blanco and San l-1arcos Rivers. Host of the gravel pits of consequence are shown on the Current Land Use map. Two types of gravel pit operations ­wet operations and dry operations -are present in ~he area. The wet operations are the larger of the two types. Two of these are in production on the Blanco River (Figure 4.17), and a third, abandoned pit is also present. Unlike the wet-operation pits, the dry-operation pits are not restricted in their locations to the irTL~ediate proximity of a wa ter source and are therefore found throughout the area where gravel is available. The wet-operation pits are restricted to the floodplain gravels along the Blanco, but the dry-operation pits are in both the floodplain and the higher terrace gravels . Most of the dry-operation pits are either abandoned or are oper­ated only intermittently (Figure 4.18). Environmental geologic problems of three different types are associated with sand and gravel operations in the study area: l) the problem of "urbanizing over" the gravel deposits before Figure 4.17 Wet-operation Sand and Gravel Pits. a. Active oit located on the Blanco River about 5.3 kilometers southwe~t of Kyle in UD1 3313-605. Five-~ile Dam is only about 1 kilometer downstream. The single arrow indicates the pond which is used as a silt trap. The double arrow indicates the direction of river flow. Tnis aerial ob­lique view is to the north. b. Active pit (inside meander loop) and abandoned pit (fore­ground). These pits are also on the Blanco River about 4 kilome ters southeast of downtown San ~arcos in UTI1 3303-605 and 3304-605. Tne single arrow indicates the location of the pipeline. The double arrow indicates the direction of river flow. ~ote the abandoned equipment in the pit in the foreground. The aerial oblique view is northward. Figure 4.18 Dry-operation Gravel Pit. Aggregates taken from this pit are not washed and screened be­fore use. The pit, which is operated only intermittently, is in allu·Jium of the Blanco River in UD1 3307-606. The view is northward. Figure 4.19 rse of an Abandoned Gravel Pit as a Silt Trap. a. Pipeline across the Blanco River for transporting silt­laden water from the current operation on the far side of the river to the abandoned pit. r:1e location of this pipeline was shown in Figure 4.17b. This •1iew is to the north. b. Discharge point of the silt-laden water in the abandoned pit. The view is to the south. Figure 4.20 Abandoned Gravel Pit Now Used as a Xobile Home Park. The pit is slowly being filled with relatively inert waste material, such as fill dirt and construction and demolition wastes. The pit is in UTI1 3307-605, and the view is north­ eastward. Figure 4.21 Abandoned Gravel Pit Being Reclaimed by Filling With Solid Waste . The view is southeastward, and the dump truck (arrow) indi­cates the scale. The site is in UTI1 3307-605. 136 137 they are utilized; 2) the problem of strear;i and reservoir siltation by the wet-operation pits; and 3) the problem of post-operation use and reclamation of the pits. Within the limits of the study area, considerable areas of sand and gravel deposits have been eliminated from utilization because they have been covered by urbanization. A comparison of the Resources map wi th the Current Land Use map shows that most of eastern San Marcos is located over potential sand and gravel resources. The result of this urbanization is to upgrade the land values in that area so that land use for sand and gravel operations is now economically un­feasible. In addition, the noise and ugliness of these operations makes it politically unlikely that they could be located this close to the city. Further loss of these valuable resources that are located closest to the city where transport costs are the lowest should be prevented in the future by instituting multiple sequential land use. The wet-operation pits in the area pose a potential silta­tion hazard to the Blanco River. lne fines that are left after the sand and gravel are washed and screened ~ust be disposed of in some manner and, until recent years, the practice has been simply to allow these tailings to flow into the river. The wet-operation pits are located just upstream from dams on the Blanco, so the chief problem is one of lccal siltation of reservoirs rather than pollution of surface water by increased turbidity . Considerable siltation of the reservoir behind Alvord Dam no doubt occurred when the now-abandoned wet-operation pit at lJ'D-1 3303. 5-605 . 3 was in production, but the extent of filling of the reservoir has not been documented. Siltation of this reservoir would still be taking place because of the wet-operation pit across the river from the abandoned pit were it not for the discharge pipe (Figure 4 .19a) that has been installed across the river. This pipe trans­ports the tailings from the present operation across the river 138 for deposition in the abandoned pit, which is thus being used as a silt trap (Figure 4.19b). Probably the most serious siltation problem in the area is caused by the wet-operation pit at UT'i-1 3313.3-605.8 (Figure 4.17a). This facility is just upstream from Five-mile Dam and the associated Dudley Johnson County Park, one of the more popular recreation spots in the area. Considerable siltation of the reservoir from t he dam has been reported by park users and a small island has formed in the reservoir just upstream from the dam. An embankment has been constructed in the channel of the Blanco to trap silt from the operation, but this measure is only a short-term expedient because the embankment will undoubtedly be destroyed during the next large flood of the Blanco and the silt will be released to the river. The flushing action on the silted reservoirs by Blanco River floods has not been evaluated. If flood waters flush much of the silt the filling of the reservoirs is only a short-term problem, but water quality implications for the river because of increased turbidity may then arise further downstream. Clearly, more work remains to be done before the impact of the silt pro­duced by the wet-operation pits can be evaluated for the river generally and for the reservoirs specifically. The post-operation use and reclamation of the gravel pits is one of the primary environmental geologic concerns of sand and gravel utilization. As was the case for the crushed stone quarries, little or no reclamation is done solely for the sake of reclamation. However, a variety of post-operation uses are made of the pits. Unfortunately , with the exception of the use of an abandoned wet-operation pit for a silt trap as described earlier, the best post-operation uses at present consist of no use at all. The primary reason for the lack of pr oper reclamation measures is again economic. The lack of sufficiently high land values, 139 tax incentives, or mandatory requirements make reclamation econo­mically unfeasible. Post-operation use of the dry-operation gravel pits consists primarily of merely filling them with what­ever materials are most easily obtained, including urban solid waste . At one site (Figure 4. 20) near San :rarcos , a mobile home park has been built around an abandoned pit which is now full of water. Although this pond could potentially be developed into a duck pond or other aesthetic feature of the park, it is instead merely being filled with relatively inert construction and building demolition wastes. At a nearby site which has no associated trailer park, another pit is being filled (Figure 4.21). Although only inert materials are supposedly allowed at the site, in fact almost all types of urban wastes are de?osited, and the pit has degenerated to a virtual dump . Water Input Water use in the San Marcos area is restricted to ground water because of the abundance, widespread availability, and high purity of water from the Edwards aquifer. Both San Marcos and Kyle have municipal water supply systems which take water from the Edwards aquifer (Figure 4. 22) and subject it only to chlorina­tion (Figure 4. 23) before distributing it in the city water mains. One of the housing developments has a collective water supply sys­tem operated by a private company (Figure 4.24), but the residences in the other developments have individual wells and water supply systems (Figure 4.25). ~ost industries in the area use municipal water supplies, but a wool-scouring plant pumps its own water from the Edwards at Sink Springs (Figure 4. 26) for use in some of its plant operations. Little environmental conflict is associated with ground water usage as long as wells are properly constructed. Well com­pletion practices now used by water well drillers, combined with Figure 4. 22 Municipal Water \~ells . a. San Marcos municipal well near Comanche Street in UTI1 3306-601. This well is one of two at this site. Three additional wells are located near Spring Lake of Ed J. L. Green Drive. b. Kyle municipal well near the intersection of Moore and Meyer Streets in UTI1 3317-607. This well is one of two used by the town. Figure 4. 23 Water Treatment System for the City of San ~farcos. The bottle contains chlorine gas. The chlorine metering de­vice can be seen in the upper right cor~er of the photo. Tnis system is at the pump station on Ed J. L. Green Drive. Figure 4.24 Pumphouse and Water Tank for a \v'ater Well Operated by a Private Company. This facility, which supplies water to one of the outlying housing developments in the area, is one of several operated by the company. The view is to the south, and the site is located in CTI1 3302-598. Figure 4.25 Typical Water Well Installed in the Outlying Developments. These wells are usually used by one to three residences. Well diameters are about 15 centimeters, and down-hole submersible pumps are used to pump water into the pressure tank. lnis installation is in the northwestern part of the area in Ur.1 3318-593. Figure 4.26 Pumphouse at Sink Springs. The arrow indicates the intake pipe. The water withdrawn is used in operations of a nearby wool-scouring plant. The water surface in the sink is at the static level of the Edwards aquifer. This view is generally westward, and the site is at UTI1 3307.3-604.0. 4.22a 4.2 3 4. 24 4.26 4.2 5 county regulations on well construction, ensure that at least the newer wells are environmentally secure. The Edwards aquifer is in some danger of being polluted by urbanization, but this danger is not related to water use for urban systems. Depletion of the aquifer is also a possibility, but this problem can be attacked only by regional aquifer management, and not by local action alone. Energy Input Energy input into urban syst~~s in the San ~areas area is of several types, including electricity and liquid and gas petrol­eum products. These are all imported from sources outside the area, and their primary local environmental significance lies in the facilities that are used to bring them in, such as highways, pipelines, and power lines. One of the primary attractions of the site at San Marcos Springs for building a city was the ready availability of water power, which was one of the most widely used forms of energy at the time of Anglo-A.~erican settlement. Five dams were built on the San Marcos River to make use of this energy source, and most of these dams are still in place. The dams and their associated reservoirs are now used primarily for recreation. They were originally constructed for utilization of water power for urban­related activities, however, so their environnental geologic problems will be discussed in this section. Two of the dams were used for generation of electricity, and the remaining three transformed the water power into mechanical energy for running mills of different types. Spring Lake Dam, the uppermost of the dams, impounds Spring Lake (Figure 4.27) which inundates most of the San Marcos Springs's discharge points. This dam was originally built in 1895 to power a water wheel for a mill, but was later converted to electric power generation. Power from this generator was used Figure 4.27 Spring Lake and Spring Lake Dam. a. Spring Lake. The discharge points of San "'.-larcos Springs are inundated by the west arill of the lake (shown on the right in this photo). The Aquarena Springs concession is on the peninsula beQNeen the two arms. Spring Lake Dam is at the upper edge of the photo. 1ne view is to the southwest. b. Western segment of Spring Lake Dam. Although the upper surface is covered with concrete, the dam proper is con­structed of rock and earth. Tnis view is northeastward. Figure 4.28 Structure Remaining from a Power Generating Plant on Spring Lake Dam. a. Upper part of the facility, showing the concrete structure and part of the mechanical gear. I:1e view is generally westward. b. Lower part of the facility, showing the discharge of San Marcos Springs. The sunbather indicates the scale. The view is northeastward. Figure 4.29 Rogers Dam and Associated ~arks. a. Aerial view of dam and reservoir. Rio Vista city park is on the right bank of the river (left side of this photo). The arrow indicates the canal which directs part of the river flow into a mill race. The dar:i is in U'2·1 3305-603, and this view is northwestward. b. Xill race and abandoned mill at Rogers Dam. Some of the old mill works are still in place and can be seen in the lower part of the mill. The view is to the southeast. 4.28b 4.29a to supply electricity to San Marcos as late as 1932 (Dobie, 1932, p. 3), and much of the structure of the plant is still in place (Figure 4.28). The second dam downstream is Rogers Dam (Figure 4.29), which was built to provide mechanical power for a nearby mill that is now abandoned. The reservoir behind the dam is presently used as part of a city park. The next two dams are Upper and Lower Cape dams. Both are in a state of near total disrepair and do not impound significant reservoirs. The upper dam does, how­ever, divert a significant part of the flow of the San Marcos River into a canal (rigure 4.30). Tnis canal in turn feeds a mill race for a large cotton gin that was abandoned within the last 20 years. The furthest downstream dam on the San Marcos River is Alvord Dam, which was built early in this century for electric power generation (Figure 4.31) and was operated as recently as about 1960 by the Lower Colorado River Authority. Although the dam is in need of some repair, it appears to be structurally in­tact. No apparent use is presently being made of the dam or the reservoir. Since these dams are no longer fulfilling the function for which they were originally built, it would at first appear that only their impact on the environment should be of concern. However, the reservoirs of some of the dams have assumed a new importance for recreation, so the impact of the environment on them must also be considered. The primary impact of the geologic environment is the flood hazard posed to the dams. Probably because it is the furthest upstream dam, Spring Lake Dam appears to receive the mos t damage from the periodic floods of Sink Springs Creek. The dam was almost completely destroyed by flood­ing at least once since it was built, and considerable repair is required after almost every sizeable flow of the creek. Alvord Figure 4.30 Upper Cape Dam and Associated Canal. The straight arrow points to the rock dam and the curved arrow indicates the diversion of flow into the canal. Note that the undiverted flow overtops the dam embankment. This aerial oblique view is generally southward. The dam is in UTI1 3304-603. Figure 4.31 Alvord Dam and Associated Works. a. Distal view showing the dam and powerhouse. The dam is in UL1 3303-604, and this view is southwestward. b. Downward view of the dam from the powerhouse. Note that the dam is constructed of concrete. All the flow of the San !.'farcos River is over the top of the dam. c. Remains of the power generator left in the powerhouse when the plant was abandoned. Both the armature and the field coils have been removed, presumably for salvage of the copper. Figure 4.32 Individual Dump. The site is on the property of the persons who are using the dump. The view is generally southeastward. The site is in UDf 3315-599. Figure 4.33 Use of a Cave for Solid Waste Disposal. Tne cave, which is behind and beneath the visible trash, has apparently been filled to the level of the land surface with waste materials. This site is in UTI1 3323-598 and the view is to the east. 4.30 4.31a 4.3lb 4.3lc 4.32 4.33 148 Dam, which is below the confluence of the Blanco River, has in the past also been subjected to considerable flood damage. The present dam is in fact a replacement for a dam which was destroyed by a flood in the 1920s, probably the 1921 flood. All of the dams in the area are quite small and, with the exception of Alvord Dam, are built of rock, earth, or a com­bination of rock and earth, so there has apparently been little problem with foundation failures. Alvord Dam, despite the fact that it is constructed of concrete and is built at least in part in clay substrate, appears to be structurally intact. The impacts of the dams on the geologic environment are somewhat difficult to assess because they have been in place for so long that little data on pre-darn conditions is available. The same generalization may also be made for their impact on the un­usual biologic assemblages of the river. The reservoirs of each dam downstream from Spring Lake extend u~stream almost to the tailwaters of the next dam (U.S. Army Corps of Engineers, 1971, p. 23), so the original flow pattern of the river has been greatly altered. The mixed fast-water and slow-water flow has given rise to a combination of both lotic and lentic ecosystems, as noted earlier. The primary environmental geologic effect of the dams is the impact of Spring Lake on San Marcos Springs. Some of the springs' discharge points have been inundated by up to 13 meters of water, and the resulting head on the springs may be decreasing the spring flow. An early attempt to increase the water power at Spring Lake by increasing the height of the dam reportedly was abandoned because the springs ceased flowing altogether. Unfor­tunately, no pre-dam spring flow data are available to assess the effect of the lake on the springs. Urban Output Urban waste products are the form of urban output having the oreatest environmental geologic significance. In this section 0 149 the urban waste classification presented in Chapter 2 will be used and the environmental implications of the various waste cate­gories in the San Marcos area will be discussed. The environmental effects of the wastes are almost entirely in one direction -im­pact of the wastes on the environment. Two major categories of wastes -solid and liquid wastes -are recognized because they have generally quite different environmental implications. Little environmental geologic effect results from energy and gaseous wastes produced in the area. Solid Wastes Virtually all of the solid waste output of urban systems in the San Marcos area is either waste produced by individual residences or municipal waste that is typical of small cities. The industrial wastes produced are not distinctive, and no hazard­ous wastes are recognized or disposed of by special methods. Three methods of solid waste disposal are used in the study area ­individual dumps, collective dumps, and sanitary landfills. Individual Dumvs The dumping of solid wastes at or near the place where they are generated no doubt began during the early days of settle­ment in the area, when transportation was difficult and local dis­posal was necessary and expedient. Unfortunately, this practice continues to the present at some locations (Figure 4.32). The environmental degradation caused by these dumps could be quite serious. ~ot only are they aesthetically unpleasing, but they may also be lowering the quality of the water of the stream basins in which they are located. If these drainage basins are in the Edwards aquifer recharge zone, as most of them west of the Balcones Escarpment are, then the surface water quality threat rnav also become a problem of ground water quality. 150 Ranchers owning land in the cavernous limestone terrane of the Edwards Group west of the escarpment have frequently used caves as solid waste disposal sites (Figure 4,33). This practice could result in degradation of water quality in the Edwards, but definitive data are lacking. Fortunately, the use of caves as disposal sites has been largely discontinued, but those caves that have been filled have not been cleaned out. East of the Balcones Escarpment solid waste is often dumped into ravines in an attempt to control erosion (Figure 4.34). Poor agricultural practices in the past, such as planting row crops (mostly cotton) with the rows oriented in the down-slope direction, have resulted in severe soil erosion problems in the clayey soils and substrate. Although these practices have been mostly eliminated by soil conservation measures, the excessive erosion which was set in motion in earlier years is very difficult to bring under control. Solid waste dumping is a common but highly questionable means used to stop the gullying, both from the stand­point of effectiveness and envirorunental protection. The primary environmental effect of these dump sites in the eastern and south­eastern part of the study area is their possible degradation of surface water quality. The practice of disposing of solid wastes in i~dividual, widely dispersed sites should be discontinued, and in areas of particular environmental sensitivity such as the Edwards aquifer recharge zone, steps may also be needed to clean up existing sites and remove the wastes for more environmentally secure disposal. However, further study is needed before these expensive measures are undertaken. In addition to the common dur.ips, which are composed mostly of household trash and wastes of agricultural activities, two other types of dump sites are present in the area. A now­abandoned industrial dump is located north of San ~farces on a Figure 4.34 Use of an Individual Dump to Retard Erosion. The arrovo has been filled with trash for a length of about 100 mete~s. This site is in UTI'l 3301-604. The view is northeastward. Figure 4.35 Abandoned Industrial Solid Waste Disposal Site. The dump is near the top of the hill (arrow). The site is in UTI1 3312-605 and the view is to the west. Figure 4.36 Junked Automobile Graveyard. This site is at the edge of San Marcos in UTI1 3308-605. The view is northeastward. Figure 4.37 Abandoned San :1arcos City Dump. a. Aerial oblique view showing the dump mass (outlined by dashed lines). Note the mesquite growth on top of the dump. The land disturbance caused by removal of earth for cover material for the top of the fill can be see~ in the foreground below the face of the dump. rne dump is in UN 3310-602. rnis view is ge.nerall:.r westward. b. Ground view showing the exposed face of the dump. The view is to the west. c. Leachate flowing from the toe of the dump. The dark tone (arrow) indicates grass which is green and thriving because of the moisture provided by the leachate. Else­where (double arrow) the grass has apparently been killed by the pollutants, leaving bare earth spots. I 4.34 4.35 4.36 4.37a 153 hill above the Blanco River (Figure 4.35). This site was used by a wool-scouring plant for dumping what appears to be relative­ly inert materials. For this reason, and because the dump has apparently been abandoned for some time, the environmental ef­fects of this site are probably not serious . The second type of site is the junked automobile graveyard (Figure 4.37). Two or three of these are present in the area, and no significant attempt to reclaim the scrap iron is apparently being made. These sites are not aesthetically pleasing, but their environmental im­pact probably is not particularly serious . No data are available to document possible contribution of heavy metal cations, especial­ly iron, to surface water. Collective Dumos Until recent years the urban systems in the San ~arcos area have used collective dumps for solid waste disposal. At least three of these dumps are present in the area, but two are no longer used and t he third is about to be abandoned. The oldest collective dump is in U'r.·1 grid square 3310-604. It was apparently active around the turn of the century, but it has not been used for many years. The dump is apparently almost completely decomposed and stabilized, although it illay still be releasing iron and possibly other heavy metal cations . The chief environmental significance of the site lies in the demonstration of the ability of the hill country west of the escarpment to recover naturally from environmental degradation. The site is almost completely groT..m over with the natural vegetation of mostly junipers with a few live oaks . All that remains to visibly docu­ment the existence of the dump are isolated concentrations of glass and rusted cans. rne presence of the dump would not be suspected without careful examination of the site. Perhaps the remainina alass and tin can should be recovered and disposed of 0 0 154 properly, but no other action at the site is apparently called for. The city of San Marcos operated an open-burning dump north of the city near Lime Kiln Road (Figure 4.37) for many years before abandoning it in about 1969. It appears that during opera­tion, the solid waste was dumped near the top of a hill, burned in the open air, and then pushed over the side of the hill with a bulldozer. The resulting configuration of the dump, in which the mass of the dump rests on the side of a hill, gives rise to a hydrogeologic situation that is apparently ideal for producing leachate. Leachate can in fact be observed emanating from the toe of the dump (Figure 4.37c). This leachate may pose a water quality threat to the local drainage and the Edwards aquifer, since the site lies in the secondary recharge zone of the aquifer. Drainage from the site flows into Sink Springs Creek in a reach that almost certainly contributes significant aquifer recharge when the creek is flowing. Several aspects of this dump, however, prevent it from being a much greater environmental liability than it is. The substrate at the site are the Del Rio Clay and over­lying Buda Formation, so that clay of the Del Rio effectively prevents direct ingress of the (l . lJ Po tenti11l (or Pr(!Vl·nt ion of G,1 ::;: ~:sc:ape into Preve nt ion of Envlronrnc ntal Surrounding Substr;itt~ ( 1. J) Conflict Pn::vcntion of l\ir Pollution 15 (8.4) Pn:v1·111 ion of Alr Pollution h;t I .-~ndf ill G;rnc~; ~() ( 11 1) Avail~1h ility of Cove r '.-: ·'.l t ·~ r i ;II (4. 0 Potential for Pn•v('nti (2 .2) Prcvc·nt ion of Watt,r Pollution '-------~---­ 90 (Li0.4) :' ;cvt~nt icn o f Water i-'ollulinn D:ir('Ct]y by Potential of Post- Potent' idl for 1-'rcvC'nt ion of Sl 1•\H' uf Lanclfi I: Sit t> i.;.mdfi ll ;·lat t• ri :;l Cpe rnc ion ~l;i~;s l':ovM7l(•nt ( 2 2) fa.~ ltiri> of l.;m rJf i 11 M.'ltt·r i.:t J (SI ;1bi 1i.ty) l'\c:ces::;i li1ity o f Landfill llis tHnCl.' to S i~·.nit i ".'.l nt liO (0.9) Ga::;es to S11rfacc-Water !::)ur fare W:1tC'r Body Pre vr;nt ion of Ha t\.!r l'rl?sencr· or Ahseni·t' of Po llution by L.andr i 11 Cn:;es ····-···· ·-···········-·····---········-······--········ ·····--··-··-····-············· Pott:!nt ial for Leachate 40 ( 16 . 2) RennvatJon Prevent ion of Wate-r ·wc6. l ) Pollution b y l.l·Mhatf~ ··-­ Pou~ntfol fo r l.t·.:icha t t· 'iO ( 20 /.) r-'-fo_v_cm_•_n_t_______ 70( [Ii, 1) J'rt·Vl·ntlon of Ll.'Hchat f• JO (/• . O) (;c ncrnt ion of r;:1s ~:sc:ape i ntn 15 (l .2) Surrounding Suhstr.1tc l'r1·v<'nt ion o f f!'om G<1~; Odors r orPntia l for Prc:vent ion of (;as F.sco1pc O'i ( 6 . 8) hO Potent i.a.l fo r l'rl•vent ion (l. . l) f\vai l.1 h! l i ty of r.,,..,,.r· Mil l f'r i .1 1 o f Cas l~st';:ap1· into ~-------­,'\tmosphcrt·'------------­ 1,0 ( 2. 7) ~'-"-'­"'_'•_<_Y _'_' r_c_·o_v_'"_'_ __·c,;iJ ··· FIGURE 5.2 CONDENSED DEMAND ANALYSIS HIERARCHY FOR A SANITARY LANDFILL for C:rJV(•r --·····-· ·· ·-···-·-····.. ·-··-----···-----·······---·········-·····-·· ..••••••••••.•.•••••••~:.-~-'·o_cm_ n_S_e_a_i ______ Acccss ib il il"I nf 1.anacha t t• Fi ltrllt ion lt·xlurc of Suln; t:i t e 'HI (8.1) C.:lpacity •if Suhst r:1te f'r..:nn Ahund·uH·c ol Ads111'!i iu~ ri() (8 . 1) C:npnr.i ty nf SubstrntP Cl:iys in Suhstr<.ll<' l'otent ial for l're vcnl ion of J)q~n·<· <'•f Top11g n1ph i r: Pre vc:nt ion of Surfare Water 20 ( 1. 2) Lcach<1 t" l.C':1biw Points Disst·t·t i•m Pollution by J.\:m.:hate Esc:J.pe to Surf ar e flydrorl;,"!:1mir t:t•n.J it itn)s f11r Slop1• of !.:_mdfill Si t• 80 (4 .9) Lcach.itc Fl 1 1~i1i11p, 10 Surf<-H'l' (11.vcfrodynami o ;) Prevc~nt ion of Ground Water Potcnti<.d fo r Prv v('nl· ion nf Pollution by Leachate l.t.'iH:h:1t <' SC<'fl<.IP,(' inl n E~c.:ipe to Suhs1.1rface ( J 11 . l) S11bs tr.1t C' Suhstr,1u~ Pt•rme:thility f,(J Av:1il.1bility of Cover i\v;dlahiJit}' of CovC'r L>'!plh ut 1:nr1•:1 ail idatt"d Pn tC'n t i.il f1,r l'rl•V!:!nt ion (2.li ) :1:1tl·rial (2 4) Material Sub~ar.1ll· (c,1vC'r :-1:tt1·r i ;JI) of Sui f c1cc-Wat<·I' Ent ry in t.i i.. 101dfi I I Sui t a hi Iity •>f Cove r · .... ,\J":!11,1cy of Cuvf'r .as n Mat;.•r i .:il fnr Cumpncc ion to 'Jt :~ I 011·1· " I i-l:it L' f' i .1l ll~;C'd !,O ( 1 .f1) S1:al ( l .fi) Fnrm ;1 'il•;il l or C11 v1•r E-i:.; e df r.x1• :1v:1t h•n anns11 I idn! iegret~ of Topo~r;1ph it· (7.0) Tn'i.t ia l Landform 1rr('gul:ir i ! y .................. ---------­ l'ot<•ntL:ll for Prcvlm l ion of C::is Escape into (1.2) Surrounding Subslrnte 5ulrntr.1t<' l'('nnt··1h!lity f\va i l abi lil y o f C:ovli' r IJc!pth of 11111·1111.'·ml id;1r "d (4. l) Materi;1J Suh!-lt r,111• (Cuv1·r tla! f'1·i.1I) Suitability of CoV('r Material for Co1np;w t ion to Ti.:x tun' ol Motc ria l IJM: _r Pot(•nl' Lil for Prl'Vt·nt ion 192 very limited use of a computer. One of the six factors of the master list, transportation, was eliminated entirely during the condensing of the DAH because it involved primarily locational at­ tributes of the various sites. Step 3. Selection of t he Physical Performance Neasures For the twenty-five LLDCs of the condensed DAR, sixteen different Physical Performance Measures (PPMs) have been selected to rate the land. The same PP:ls are used for repeated LLDCs, and two of the LLDCs that are different in name are also able to use the same PPM. The PPMs selected are shown with their associated LLDCs in Figure 5.2. These PPMs are chosen primarily on the basis of the LLDCs, but also in part based on the type and quality of the data available on the data source maps. The relative effectiveness of each PPM for its associated LLDC is considered below in Step 6. Step 4. Formulation of the Suitability Score Functions For each PPM a Suitability Score Function (SSF) muse be constructed for use in assigning score values to the land. The SSF serves to convert the units used to describe the land character­istics of the PPM, such as length, substrate texture, or permea­bility, into a dimensionless number, the score. This number indi­cates in percent form the relative ability of the land to meet the associated LLDC. The sixteen different SSFs used for a sanitary landfill are shown in Figure 5.3. Most of these are close approxi­mations of mathematical functions, but some have been taiLned for special needs which cannot be filled by normal functions. All of them have been set up to be used easily with the data source maps. Figure 5 . 3 Suitability Score Functions for a Sanitary Landfill (The names are the Physical Performance Measures from Figure 5.2) 1. Depth of Unconsolidated Substrate (Capacity) 2. Abundance of Expanding Clays in Substrate 3. Depth of Unconsolidated Substrate (Cover Material) 4. Texture of Material Used for Cover 5. Substrate Per~eability 6. Depth of Agriculturally Productive Soil 7 . Slope of Landfill Site (Stability) 8 . Distance to Significant Wa ter Body 9 . Presence or Absence of Aquifer Accessible to Landfill Gases 10. Texture of Substrate Proximal to Landfill Base 11. Abundance of Adsorbing Clays in Substrate 12. Degree of Topographic Dissection 13. Slope of Landfill Site (Hydrodynamics) 14. Degree of Consolida~ion to Depth of Landfill Excavation 15. Substrate Lithology (Trafficability) 16. Degree of Topographic Irregularity 100 100 100 w ao/+ 100 lUlU a: 80 er EG -+-f-S 0 0 0 !SOIL< 2'1 u en enen EG+EG.S 50 50 50 50 u ,,) u a: 3 30 1 4 20 10/ SAND SILT + 10 0 0 0 CLAY 3.0 4.6 6. ' 7.6 a.' 0.3 0.6 0.9 1.2 1.5 CLAY MIXED HARO SOIL TEXTURE DEPTH LIMESTONE DEPTH SLOPE NOT SCREENED SUBSTRATE LITHOLOGY 100 ...__________ 100 10o­ •00 lUlU erer ao 00 u u enen 50­ 50 10 9 10 0 I 0 MIXED HARDCLAY !Cl ALLUVIUM CAJ LIMESTONE LIMESTONE LIMESTONE LIMESTONE (LMJ ILHl (I.Ml flHI SUBSTRATE LITHOLOGY SUBSTRATE LITHOLOGY 100 100 50 50 50 15 16 10 0 0 --1---1----+--+---+-_;;_-~_;;_~~ 0 SWAMPY HIGHLY MODER. SLIGHT FLAT OR ROLLING LIMESTONE LIMESTONE LIMESTONE LIMESTONE DISSECT. DISSECT. DISSECT. SMOOTH (LM) (LH) CLHl (lMJ SLOPE SUBSTRATE LITHOLOGY SUBSTRATE LITHOLOGY TOPOGRAPHIC CHARACTERISTICS 195 Step 5. Assign.'1lent of \.;'eights to the Demand Analysis Hierarchy In this step the relative importance of the various LLDCs is determined by performing a weighting process on the condensed DAH. The procedure begins at the highest level (the left side) of the DAH in Figure 5.2 and proceeds to successively lower levels. In the first stage the five categories at the highest level are rated in percent terms according to their relative importance. The same exercise is performed at each level in the hierarchy, always on the basis of 100; that is, all the categories at a given level within a branch of the DAH are rated relative to each other in percent terms. The results are shown without parentheses throughout the DAH in Figure 5.2. In the second stage the original 100 points at the highest level are distributed down the ~ierarchy by a series of successive multiplications of the percentages from the first stage. The final result of this process is the assignment of a raw weight to each of the LLDCs. The weights of the branches of the sanitary landfill DAH are shown in Figure 5.2 as numbers with parentheses. The sum of these numbers at the LLDC level is, of course, equal to 100. Step 6. Adjustment of the Weights The PPMs selected for the various LLDCs are highly vari­able in their effectiveness. That is, the PPNs, which are selected in part on the basis of the type and quality of the information on the data source maps, may do a perfect job of rating the land according to their associated LLDCs, or they may be highly im­perfect. An adjustment of the raw weights of the LLDCs is there­fore needed to account for these differences in the effectiveness of the PP'.'.-ls. The procedure for this step is to consider each LLDC -PPM pair and make an estimate in percent terms of how well 196 the PPM fills the function. This percent, termed the adjusting factor, is multiplied by the raw weight of the LLDC. The procedure is repeated for all the LLDCs, resulting in a tabulation of adjusted weights. These adjusted weights are then recalibrated to a sum of 100, which results in a list of net weights for the LLDCs. A tabulation of the raw weights (from Figure 5.2), adjusting factors, adjusted weights, and net weights for the LLDCs of the sanitary landfill DAH is given in Table 5.2. 1ne net effect of this step is to shift the weights somewhat in favor of LLDCs for which the best data is available. However, it may be noted in Table 5.2 that although some shifting of the weights has occurred, the pattern of weight distribution established in Step 5 is not greatly changed. Step 7. Preparation of the Suitability Score Grids With the completion of Step 6 preparation for the land suitability assessment for a sanitary landfill is finished and the actual assessment begins in this step. The procedure begins by considering the first LLDC and its associated PP~[. The appropri­ate data source map is selected, and its map units are examined in conjunction with the SSF associated with the PPM. A score value is then assigned to each map unit by first noting the SSF abscissa value represented by that map unit and then reading the score value for that abscissa value . This score value is assigned to the map unit and the process is repeated until the data source map is in effect converted into a score map . The screened Suit­ability Score Grid (SSG) from the screening step is overlaid on this score map, and a score value is assigned to each grid square based on the score of the map unit overlain by the grid square. The screened grid squares all retain the score value of zero that was assigned to them earlier in the screening step. The scoring procedure is repeated for each PPM because a separate SSG is LLDC Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Raw Adjusting Adjusted Weight Factor Weight 5.0 100 5.0 3.5 95 3.3 6.3 100 6.3 4.2 85 3.6 1. 3 90 1. 2 4.3 100 4.1 2.8 85 2.4 1. 7 so 0.9 1.1 80 0.9 2.2 80 1.8 0.9 70 0.6 1. 3 70 0.9 8.1 100 8.1 8.1 95 7.7 1. 2 65 0.8 4.9 80 3.9 14.1 100 14.1 2.4 100 2.4 1. 6 85 1.4 7.5 95 7.1 2.5 100 2.5 7.0 85 6.0 1. 2 90 1.1 4.1 100 4.1 2.7 85 2.3 100.0 92.5 Table 5.2 Adjustment of the Weigh ts of the Lowest Level Demand Criteria Net Weight 5. 4 3.6 6.8 3.9 1. 3 4.4 2.6 1. 0 1. 0 1. 9 0.6 1.0 8.9 8.3 0.9 4.2 15.3 2.6 1. 5 7. 7 2.7 6.4 1. 2 4.4 2.4 100.0 198 required for each of the sixteen different PP~s. A list of the PPMs used in the land suitability analysis for a sanitary landfill is given in Table 5.3 with the data source map or maps used for each. All of the sixteen SSGs produced in this step cannot be shown here because of reproduction costs, but Figure 5.4 illus­ trates a typical example. The maxir.ium score that any grid element can receive is 100, but the highest score assigned is 99 because only two-digit numbers can be used in the computer program in the next step. However, this introduced error has negligible effect on the final results. Step 8. Calculation of the Suitability Index ::rap The final step in land suitability assessment is the calcu­lation of the Suitability Index Map (SIM) from the SSGs produced in the preceding step. The first stage of this calculation is to multiply all the grid elements of each SSG by the weight of the associated LLDC. T~is process is repeat2d for all the SSGs, and the resulting weighted SSGs are then summed in stack fashion in the second stage to give the SEL Because of the large number of data elements involved in this process (up to 47,000 for each 2-1/2 minute sector), the computer program introduced in Chapter 2 (Figures 2.9 to 2.11) can be used effectively to perform the numer­ous elementary calculations. In the application of this program, one simplifying step can be taken. Instead of repeating some of the sixteen SSGs where necessary if all twenty-five LLDCs are considered separately, the weights of LLDCs having the same SSGs can be added. In this way only sixteen SSGs rather than twenty-five LLDCs need be weighted and summed, which reduces the number of data elements to about 32 ,000 per 2-1/ 2 r;iinute sector. The final sn!s for the three sectors analyzed are shown in Figures 5.5 to 5.7. The shapes of the SI:Ls have been distorted by the computer output, but 199 Physical Performance Measure 1. Depth of Unconsolidated Substrate (Capacity) 2. Abundance of Expanding Clays in Substrate 3. Depth of Unconsolidated Substrate (Cover Material) 4. Texture of Material Used for Cover 5. Substrate Permeability 6. Depth of Agriculturally Productive Soil 7. Slope of Landfill Site (Stability) 8. Distance to Significant Water Body 9. Presence or Absence of Aquifer Accessible to Landfill Gases 10. Texture of Substrate Proximal to Landfill Base 11. Abundance of Adsorbing Clays in Substrate 12. Degree of Topographic Dissection 13. Slope of Landfill Site (Hydrodynamics) 14. Degree of Consolidation to Depth of Landfill Excavation 15. Substrate Lithology (Trafficability) 16. Degree of Topographic Irregularity Data Source Maps Engineering Geology Soils Engineering Geology Soils Engineering Geology Soils Engineering Geology Soils Engineering Geol ogy Soils Landform Current Land Use Processes Resources Engineering Geology Engineering Geology Landform Topographic Landform Engineering Geology Engineering Geology Landform Topographic Table 5.3 Data Source Maps Used in the Preparation of the Suitability Score Grids Figure 5.4 Typical Suitability Score Grid for a Sanitary Landfill This score grid is from Sector 6, and t he Physical PerforEance Measure is the Depth of Agriculturally Productive Soil. The blank grid cells are in the screened portion of the sector. The score values in the candidate grid cells are from Suit­ability Score Function no. 6 (Figure 5.3). Note that 99s are used in grid cells receiving a score of 100. The data source map used in the preparation of this score grid is the Soils map (Plate 2) . 2 3 4 5 b 1 8 Q lll 11 12 13 14 15 lb 17 18 19 20 21 22 23 24 25 2b 27 28 29 lll 31 12 31 34 35 lb 17 ]8 ]~ 4~ I II 0 II I! '1 0 p. ~. ,. il 0 I! 0 0 0 0 0 4'1 49 49 49 4'1 49 0 " 0 0 ~ e 0 e e e I/ " 0 II " " e 2 ll II ll II 0 II 0 II Ill ii 0 ii 111 0 " 50 50 50 s0 50 0 0 0 0 0 0 0 0 e 0 0 II 111 r.i e " l 0 0 Ill II ll II I' II ll " ll II 0 0 ii Iii " 0 0 0 A I! " " 50 50 S0 50 0 0 " 0 0 0 " ll 0 e e 4 0 ii 11 II "0 0 0 1:1 II 0 0 0 50 50 50 0 0 0 0 0 0 e 0 e e " " 0 ll "' vi ~ " " " ~ 0 ll " " " ll s ~ ;., ~ ,.. v1 ~ [' II 0 r "l'l il 0 "0 ~ 0 0 0 0 50 50 0 111 ~ ~ 0 0 p Vo ~ 0 0 "' " 0 7 ,, II 0 ll 0 ~ "' "'0 "0 "' 0"' 0 I!"' "0 0 5111 s0 s0 0 0 0 0 0 0 0 e ll b 0 II 0 I' "' ~ ., ~ ~ " v ~ v " ~ "~ "'e ~ " 11 11 0 ll 0 5"' 50 50 " " 0 0 0 e e ~ e 0 0 " 0 0 0 0 "Id 0 0 lcl II "0 " "0 "0 "Iii "~ "0 II 0 "' "0 0 0 Ill "0 " 0 I! 16 50 50 0 0 0 0 II 0 0 2 e 0 " 0 0 I' "ii 0 ll ll '1 II 0 0 0 0 0 " ill 0 0 0 " 0 0 0 50 50 0 0 0 0 0 e 0 e e 0 " " 10" II ll II 71 71 ll 71 0 "ii " 0 "0 0 " 0 " II 0 0 0 50 50 0 0 0 0 0 0 111 0 ll ll 0 " II 0 0 0 "' "0 II 0 II 71 71 71 71 0 " " 0 "' "0 .. "II "0 "i1 Iii 0 0 0 50 50 0 " ~ 0 " 0 0 0 ... " 111 " ll 0 l'I 0 "II 0 0 II 71 71 71 71 71 e 0 0 11 0 "II ii 0 0 0"' "' 0 0 0 " 0 0 "' 0 0 0 0 0 0 Iii ll ll ll il II 0 ll 3<1 0 lll 71 e 01 0 11 0 0 0 0 "0 0 "' " 0 "0 0 0 0 0 0 0 0 A 0 0 e e 0 0 14 0 ll ll " "0 0 b I Sb 0 i! II b4 e S4 0 71 71 "e 0 ll ~ "ll ii 0 0 0 0 0 0 0 0 0 A 0 e 0 0 15 II II 0 "0 0 b4 bl II ~ 35 II 54 71 71 71 0 il " 0 Ill "0 "0 0 0 0 0 0 A 0 0 0 111 e 0 ~ e II> ll "0 0 b4 blJ 52 II l5 "II Iii 5U 71 71 0 II 0 " ii 0 "' 0 0" 0 0 0 A 0 0 0 A 0 0 ~ 0 ~ e ll I! " 17 ll 0 ll ll 0 II b4 b4 bl " l3 71 71 71 0 0 "' 0 "ll "ii 0 0 " 0 0 0 0 0 A 0 0 e 0 P e il " 1~ 0 A bu oll b4 5b Iii 0 " 4l 71 " 0 " 0 " 0 "0 Ill 0 0 "0 0 0 0 e 0 0 0 0 0 0 0 0 e 0 ll l'I " !'I 0 V. II 0"' 0 ll bU b4 b~ bl e e 11 0 11 ll 0 "ii II "0 0 II 0 0 0 ~ 0 0 0 0 0 0 0 0 0 e 0 e r 21' " e 0 0 0 ll b~ II P. 0 " " 0 b7 71 0 0 Iii 0 0 " " 0 " 0 0 0 0 0 0 0 0 0 " 0 0 0 ~ ll " il ll II ll II "'0 " 0 0 II 0 0 0 0 0 0 0 0 11 0 0 0 0 0 0 0 ~ e " ll b7 71 71 " " 0 0 " " 22 il II 0 "ll 0 0 0 II 0 11 " b1 11 71 71 b1 il 0 "0 0· 0 0 0 0 0 0 0 0 0 0 0 0 e 0 e e 2l (l 0 "11! "'0 II 71 0 0 0 71 0 0 "' " 0 I! 0 0 0 0 11 0 0 0 0 11 "' 0 111 e 0 e b1 " ii 0 " ll 14 " 24 0 ~ 11 II 0 0 0 p 0 0 0 0 0 " 0 0"' 0"' 0 0 "' 0 0 0 0 0 0 0 0 0 0 0 0 se 08 e b1 0 " Ill 25 ll v. ll " 0 II II ii II 0 0 0 ~q 71 0 0 0 0 ll 0 0 0 0 0 0 111 0 0 0 0 0 47 4o 0 50 b9 bb a e Zb "II 0 II 0 II ll 0 " 0 0 0 0 0 0 0 11 0 0 0 49 0 0 0 46 so 11 "' 10 oa e oil " I! ll 0 0 0 " 0 l1 ll 0 ii 0 e 0 0 0 0 0 5~ 49 49 49 qq 49 119 0 51 b0 b2 ~5 0 0 b8 0 b8 a " 0 0 0 " 0 " "' ll " 0 0 " 28 A 0 II 0 ll 0 I! 0 ll 0 0 ll 0 0 0 0 "0 0 11 0 0 0 4b 0 47 4b 0 5o b2 Sb 71 0 0 r. 0 " " ll 29 " 0 0 II ll II 0 II II " 0"' 111 0 0 ll 0 0 0 0 0 0 ~ 0 0 0 0 59 ol s1 11 0 b2 00 11 11 0 e e 30 il 0 11 il II 0 II II "il "0 I! 0 0 0 14 0 0 "0 0 II 0 bl bl 0 bl bl bl bl b4 55 7l 57 0 bl 55 71 0 70 69 31 "ll e 0 0 II 0 (! It 0 0 0 I! 0 0 0 il " bl b2 bl bl 0 b2 bl b2 b2 55 7l 57 b~ 0 58 71 0 0 0 0 " " l2 0 0 il ll e " lil 0 0 ii II II 0 0 0 0 oil bZ bl bl bJ 0 b2 b4 b4 •1 75 5b 0 ~2 0 Sb e e 0 0 " ll " B 0 II Ill I! il 0 II II 0 " 0 0 0 0 0 "0 0 0 0 b2 b2 bl 0 0 A b4 bb bb 75 58 b4 0 b4 0 0 ~ ii ]4 ll II ii II 0 " ll ~ e "ll· Iii 0 0 "I! "0 ii 0 0 b2 b2 bl A 0 0 0 bb b4 7~ b3 bll b4 0 bl 52 0 "0 " " ll 35 0 0 ll "0 ll 1:1 v. 0 0 ll 0 ll 0 0 0 111 0 0 0 0 0 ~ 0 o4 bl 57 bl b~ b4 b4 e bll e 0 " 0 14 " 3b " e e "0 "e ii " 0 11 0 0 0 ll 0 ll 0 0 0 0 0 0 0 0 SJ 55 57 b] bij bu b4 b4 0 0 0 17 ll ll 0 0 il "il 0 0 "0 0 0 " 0 ii " " Iii" 0 0 0 0 0 0 0 Sb 57 b2 b4 b4 b4 bl 57 0 e I! "' " " ll " 38 ll e 111 0 II 11 ii 0 0 II " 0"' 0 0 "0 0 II 0 0 0 0 0 0 0 511 o2 o4 bZ bl 5S 71 71 0 0 " 0 0 " H 0 11 II ii il "0 "Ill 0 0 "~ 0 0 0 .0 0 0 0· 0 0 0 0 40 0 0 59 b4 b2 SS 71 b6 52 e ll " " 4" " 0 ii 0. 0 II "ll ll "0 e "0 "' 0 0 ll 0 0 0 0 A 0 0 42 51 0 58 0 bl bl b2 bl b6 57 e ll " 0 41 111 e e "iJ il 0 A A 0 "ll 0 " 0 0 Iii "' 0 "ll 0 0 0 0 0 0 0 0 A 42 48 0 56 59 0 bl bl 56 52 54 II Ill 0 0 ll 0 ll 0 0 0 "ll 0 0 0 ".. l! 0 0 0 "0 0 0 0 0 0 0 ~9 0 0 58 SQ 59 0 49 Sl 5l 53 II "' e " 43 0 0 0 0 0 0 0 0"' 0 0 0 0 0 I! Uil 46 51 0 0 0 58 SQ bQ 0 b8 54 S3 ll 0 0 " " " 0 " 411 " 45 b5 "il Iii 11 0 ii II ll" " ll 0 0 Iii " "' ll I.I ii ii 0 ii 0 0 " 40 51 46 0 0 0 59 b] 55 0 53 0 il 45 "n n ii II II " " 0 0 0 Iii "0 Iii" "ii 0 0 0 0 0 0 0 0 0 48 S0 " 0 0 5Q 60 0 5b 0 0 0 4b il " 44 " ll " ii ., " 0 Ill ll ll il 0 0 "'0 0 5Z 111 ii 0 40 4b 0 0 0 b2 0 b8 So bl 0 ~ " " 0 0 47 0 44 b4 i! " 0 "0 0 Iii "Iii 0 "'0 il 0 il ll 0" " 0 0 42 0 0 0 0 4b 0 0 11 0 0 08 Sh 0 0 0 0 " " " 48 ii II il "0 II Id II II 0 16 0 0 0 0 0 0 0 ll"' ill 0 l9 "' 0 0 0 4b 0 0 0 ·0 0 bll 55 ll 0 " " " " " Figure 5.5 N Suitability In 0 0 ll !ti 0 II ll II II Iii 0 0 0 0 0 0 0 "0 0 0 ~9 0 0 0 U8 Sb 71 Iii 10 b6 e oe ll Iii " 0 ll " 0 " Iii 0 0 0 0 " 68 il b8 ii "' ., 0 " 21 0 0 ii Iii l'I 0 0 0 50 aq 119 119 11'1 U9 u'l 0 51 1>0 b2 S5 ll 0 " 0 " 28 ll 16 0 II 0 [I 0 a Iii "0 0 0 0 Iii 0 ab 0 47 Ub 0 So b2 St. 71 Iii 0 Ii! 0 " 0 0 " ii 111 " " ll " ll 29 11 ll ll ll 0 11 II 0 0 Iii 0 ll " Ii'. II' " 0 0 59 bl S7 7l b2 bl! 71 71 ll II ii 0 " Iii Iii Iii " 10 ll " ll "0 0 " Ill 0 0 "' 0 " " 0 ll"' ll Iii " bl 0 bl bl bl bl bu 5S 7l 57 " t.2 55 71 " 7e b~ " ll 0 "' "' " fll 11 11 11 0 ll ll II ii II ii 0 Iii 0 Iii 0 fll bl bl bl 0 t.2 t.2 b2 b2 55 7l 57 bl! 0 S8 " " 0· lil " 71 ll " I! " 0 l2 11 e ll 0 e ll II II Iii 11 0 "0 (! 0 II II i'I 0 "0 0 b0 b2 bl bl b3 0 b2 64 ba b 1 75 So ll b2 II Sb 0 e 0 ll ll 0 Ill ll ll 0 II 0 e 0 ii 0 0 ii 0 ii 0 0 0 b2 bZ bl ii 0 A bU bb bb 75 58 b4 II bu 33 " " e II ~ JU ll " 0 0 e I/I ii " 0 "0 0 0 "0 ll " bl b2 bl Iii ii " 0 bb bU 73 t>l bU ba 0 b2 52 " 111 " "' 0 15 0 0 Iii 0 0 16 Iii 11 Iii 0 0 " ~ " "II lil 0 0 "ii A 0 Iii Iii ll A 0 bU bl S7 b] b~ bll bU Iii bll Ill Ii! 0 " H II 0 0 ii 0 " " "0 0 16 ll 0 ll ll 0 ll 0 0 0 0 0 51 S5 S7 bl ba bU ba u e e 11 0 e "' ll " " 0 37 il 0 0 " "Iii " ll 0 Iii 0 0 0 Ill fil ll 0 0 Iii Sb 57 b2 bU ba bll bl 57 Ill e e 0 " 0 " " ll " " " 311 " 0 0 II 11 0 II I! 0 "0 "ll 0 0 0 0 "0 0 0 ii 0 0 0 " 0 " S0 b2 tiu b2 b I 55 71 71 II l9 " 0 IIll " 0 " ll " 0 Iii 0 p. " " ii Iii Iii Iii ii" 0 0 0 al! " 0 sq bu bl ~s 11 b& s2 e " ii 0 " QP 0 ll " "ll "ll 0 fl 0 " " fil "ii 0 " " Iii 0 0 "' u2 53 0 58 Ill bl bl t.2 bl b6 H ll " " ll " 41 II ~ I! 0 ll Iii " "0 " " Iii " "0 0 " 0" 0 0 0 Iii u2 as ll 58 5q ii bl b2 58 52 ~u " 0 "' " 0 U2 II I! II 0 0 ii 0 11 II "II "Iii "0 " "II 0" "Iii " 0 0" "ll ll 0 " 0 0 4'1 0 0 58 59 sq ii u9 S2 52 '.il II Ul II e l'I II e 0 0 II 0 0 Iii 0"' "0 " II 0 " 0 "0 ll ll 0 0 0 lllil 118 51 0 Ill 0 58 SQ bU 0 b8 Su 53 e " " " " " 0 s1 as au 0 us bS II II l'I II II 0 0 II ll ll 0 0 0 59 bl 55 0 Sl 0 ii 110 ll " " as ll 7l 7l 0 ll II 0 " ii 11 II "' 0 " " II il " 0 Iii ll" " 0 " 0 0 " ua 511 Iii Iii " 0Iii S'l bll il 5b 0 11 " Ub Iii 11 au 0 111 0 ll ii 11 "' ll "0 "ll 0 ll "ll 0 " ll 0 II " "' l'I ll 0 40 Ub ll Iii ll b2 ll b& 5b b2 0 Ill ll " "' 52 47 0 ua ba II II " ll ll 0 II kl 0 Iii II " "' uz Iii 0 0 ll ab 0 " 0 bl! Sb ii 0 Iii " Iii " " ae ll " ll II" "0 ii" 0 "0 " ii 0 ll Iii ll " 11 l9 " 0 a e 11b "Ill Iii" Ill " 0 011 5S Ill e e " ll " " " " "' " " " Figure 5.6 N Sui t abil i ty In72 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 51 51 50 1-50 Subtotal Screened Percent (SI = O) Total Sector 6 Sector 12 0.4 0 0.4 2.6 2.7 2. 7 2.8 2.8 2.9 2.8 3.5 2.8 3.8 3.2 4.0 5.1 4.1 5.1 6.0 15.8 6.9 16.2 8.2 17.4 8.9 17.5 9.3 17.8 9.7 18.3 10.1 27.7 10.5 27.9 11.l 28.6 11. 6 28.7 11. 9 28.9 12.3 28.9 12.7 28.9 12.8 29.4 14.5 29.4 17.2 29.4 17.2 29.4 82.8 70.6 100 100 Table 5.4 Comparison of the Suitability Index Values of Sectors 6, 12, and 17 Sector 17 0.6 37.9 39.3 78.1 78.4 79.6 80.3 80.3 80.3 80.3 81. 4 81.4 81. 6 81. 6 81.8 81. 8 81. 9 I 81. 9 81. 9 18.1 100 207 which is reflected in the good spread of SI values (from 30 to 75), allows the evaluation method to make a good distinction between the different parts of the sector. In general, the sector is not well suited for a sanitary landfill, as indicated at the outset by the fact that 82% of the area was eliminated in the screening pro­cedure, mostly because of thin soils. The only unscreened part which takes in more than 12 hectares (30 acres) is in the southeast quarter, and even there the Sis are not high. The best 6% of the sector has an SI range of 64 to 75. The best 10% has SI values of 58 to 75, and 7% of the sector has SI values of 30 to 58. The best suited part of the sector for a sanitary landfill is between two northwest-oriented country roads about 2 kilometers south of Kyle. The SI values there are mostly in the 60s. Sector 12, which is in the east-central part of the study area, is relatively flat and is underlain mostly by alluvium with thick soils. The northeast corner is underlain by clay. This sector is also not particularly well suited for a sanitary land­fill, as indicated by the fact that over 70% of its area was eliminated in the screening procedure. However, most of the re­maining 30% is distributed in blocks large enough to be used as landfill sites. Most of this unscreened area is in the northeast quarter of the sector. The maximum SI value in the sector is 72, and a significant part of the sector (2.6%) in the far northeast corner has this value. Overall, the best 5% has an SI range of 66 to 72. Sis of 64 or over take in about 16% of the sector, and 14% of the area has Sis of 51 to 64. Clearly, the best part of the sector for a landfill is in the northeast corner where the substrate is clay. There a large block of over 50 hectares (125 acres) has an SI value of 72. Sector 17 is in the south-central part of the study area. It is characterized by gentle slopes and is underlain almost en­tirely by clays that have thick soils. Because of its topographic and substrate uniformity, only a small range of Sis are represented 208 in this sector. 1be sector is well suited for a sanitary landfill, as indicated initially by the fact that only 18% of its area was eliminated by screening. 1be SI values of the candidate areas are also relatively high in that the minimum value is 60. The high­est SI values are from 75 to 79, but they account for less than 1% of the sector and are generally surrounded by areas having much lower Sis. Large blocks of the sector do, however, have SI values of 72. These occur in the southern half of the sector and are surrounded by large areas having an SI of 70 or 71. Over­ all, more than 78% of the sector has SI values of 70 or higher. The 4% that has Sis of 57 to 70 are widely scattered and generally adjoin surface water bodies. The prime part of the sector for a landfill is just southeast of the sector center, where a large block of 125 hectares (325 acres) has an SI of 72. s~aller, bGt nevertheless significant blocks of areas with Sis of 72 are also in the western half of the sector. A comparison of the three sectors evaluated shows that the geologic and topographic characteristics of Sector 17 are clearly the most suitable for a sanitary landfill. More than 78% of Sector 17 has an SI of 70 or higher, whereas only 2.8% of Sectors 6 and 12 have Sis of 70 or more. These results may have been an­ticipated qualitatively considering the flat topography and mostly clay substrate of Sector 17. Sector 12 has a small area in the northeast corner which have SI values as high as those in Sector 17, but the characteristics of that area are about identical to those of Sector 17. Verification Procedure In order to ensure the accuracy of the results of the Screening and Evaluation Procedure, the ffiOSt promising candidate areas must be verified. Only a cursory verification has been con­ducted. More detailed on-site investigations will still be needed before sites are actually used for a landfill. The first step of 209 the verification consists of rechecking the wost promising areas against the seven data source maps. This check reveals that one or two blocks with relatively high scores should have been screened; the largest is a block which is now occupied by the Quail Creek Country Club. The second step is to check the candidate areas on aerial photographs. This check has turned up no obvious errors. The final step consists of field visitation and cursory observa­tion of the candidate areas. In general, all of the most promising areas appeared to be quite satisfactory for ewplacement of a sanitary landfill. One exception is the large block (most of which has SI values of only 50) located just northwest of Kyle. This block is probably too close to the town, and the small stream draining the site flows in close proximity to the southern edge o: the town, thus posing a potential health hazard to residencs liv­ing there. Also, two of the blocks in Sector 12 are probably lo­cated too close to the Blanco River where leachate may have access to the river through the permeable channel gravels of the alluvium. In Sector 17 some of the grid squares adjacent to areas screened because of water bodies apparently should have received lower SI values because of their proximity to the water. Aside from these relatively minor problems, no large or obvious errors in the Suitability Index Maps were observed in the cursory verification. TOWARD A PHYSICAL LA...~D USE PL-\}l FOR THE SAN Vi..·\RCOS ARE..'1. The results of this chapter represent a beginning of the work required to determine land suitability for all projected urban land uses in the study area. Still remaining are not only the evaluation of the rest of the sectors for a sanitary landfill specifically, but also the multiple evaluations for many projected urban uses. The urban system concept provides a good organizational 210 scheme for conducting suitability analyses for these projected uses. The resulting Suitability Index Maps can then be used as input which should be considered with other factors, such as locational aspects, compatibility of adjacent urban facilities and activi­ties, and social, economic, and political conditions. By consider­ing all of these factors planners should be able to for~ulate a physical land use plan. It should be emphasized, however, that the physical suitability of the land as indicated on the Suitability Index ~aps should be given the highest priority as land use de­cisions are made. Chapter 6 SlJMHARY AL'JD CON CLUS rot;s 212 The procedure of this environmental geologic study has been to focus on growing urban areas and to delineate the components of urban systems and their geologic envirorunents. These components were used as conceptual tools for development of a systematic three-part methodology for conducting environmental geologic in­vestigation of growing urban areas. This methodology was then applied to a case study area around San Marcos in the Interstate 35 growth corridor of central Texas. Recognition of the two conceptually distinct parts of the urban-environment interaction -the urban system and the geologic environment -makes the problem of environmental geologic analysis more manageable. The classification of the geologic environment into three broad categories (substrate, processes, and landform) appears to be conceptually sound and sufficiently comprehensive to be ap­plicable to most of the inhabitable parts of the world. Also, the urban system organizational scheme, which uses the four categories (situs, input, output, and transportation) recognized here, ap­ parently accounts for the urban facilities and activities having the most environmental geologic significance. The organization of the procedure of the methodology into three parts -delineation of the data sources followeci by a curative and then a preventive procedure -appears to be a rational and rigorous approach to the analysis of the interaction between cities and their geologic environments. In the derivation of the natural data source maps for the San Marcos case study area, the substrate-processes-landform scheme proved to be highly effective in organizing the geologic and geology-related phenomena having significance for urbanization. The curative part of the methodology has also apparently accounted for all the important environmental geologic conflicts in the area. The preventive part of the procedure was quite effective in determining land capability for the one projected urban land 213 use, a sanitary landfill, for which the analysis was run. The application of this part of the methodology to the San ~arcos area clearly demonstrated the advantages of this rigorous analy­sis. In the first place the procedure goes a step beyond the preparation of the data source maps and converts the information into an indication of land suitability for specific uses. Second, the procedure is based on an established decision-making tech­ nique, so the results (the Suitability Index ~aps) are highly defensible. Finally, the results are easily understood by nontech­ nical people who are likely to be making land use decisions. Appendix GEOLOGY OF THE SAN Y.ARCOS ..'.\REA, TEXAS 215 The primary aim of this appendix is to present a geologic report to accompany the geologic maps of the San :rarcos area (Plates 9 and 10). Toe procedure will be first to describe the regional geologic context and then to cover in turn the three major aspects of the geology of the area: stratigraphy, structural geology, and geomorphology. This report has been prepared in a very condensed for.n, and only the major points are covered in each topic. Toe geologic mapping for Plates 9 and 10 is original to this study, but most of the information in this text is derived from previous studies. These sources are too numerous to cite individually, but local studies by DeCook (1956, 1960, 1963), Davis (1962), and Noyes (1957) have been relied upon heavily. The geologic mapping was done on Agricultural Stabilization and Conservation Service (U.S. Department of Agriculture) aerial photographs that have a scale of 1:20,000. All mapping was accom­plished in the field because frequent spot checking of outcrops was necessary to work out the complex fault patterns. The :nap information was transferred to topographic base maps by use of a mirror-type desk projector (Reed Research :1odel 635B) . .!J..s noted in the text, U.S . Geological Survey 7-1/2 minute topographic quadrangles (scale 1:24,000) were used as base maps. The study area includes the southern one-third of the Mountain City quad­rangle, all of the San Harcos North quadrangle, and the northern two-thirds of the San Harcos South quadrangle. Available subsur­face data on file at the Texas Water Development Board were also examined. :·lost of these data are in the form of water well logs prepared by water well drillers having little training in geology and proved to be of 0nly li~ited value . DeC0ok (1960, 1963) made the best possible interpretations of these data, so his studies are used extensively in this report. Logs of wells drilled since these studies were oublished were also utilized to some extent. 216 THE GEOLOGIC CONTEXT: GEOLOGIC ELEHENTS OF CENTRAL TEXAS The basement rocks of central Texas comprise two primary elements -the Texas Craton and the Oauchita foldbelt. The Texas Craton, a northwest elongated mass of Precambrian, mostly granitic rock, is the most fundamental basement element in Texas. It is bounded on the southeast by the strongly deformed and metamorphosed Paleozoic rocks of the Ouachita fold~elt. The study area lies over the Ouachita foldbelt just southeast of the boundary with the Texas Craton. Unconformably overlying this basement is a southeast dip­ping wedge of Cretaceous sedimentary rocks. This wedge, which thins to the northwest, is approximately 750 meters thick in the San Marcos area. The strata of this wedge are mostly carbonate rocks and fine-grained terrigenous rocks. The Cretaceous wedge is intensely faulted along the Balcones fault zone, which extends in an arcuate band from Del Rio to north of Waco. The location of the fault zone closely follows the line of crustal weakness along the boundary between the Ouachita fold­belt and the Texas Craton. Displacement across the zone is dow-n­ward to the southeast, and the zone is in a graben-like relation to the Luling-Mexia fault system, which is located further toward the Gulf Coast. The regional dip across this hinge-line fault zone increases fron about 1.9 to 5. 7 meters per kilometer northwest of the zone to about 9.5 to 18.9 meters per kilometer southeast of the zone. The displacement across the zone is maximum between Aus tin and San Antonio. .:10s t of the faulting is believed to have occurred during the :·1iocene, and no well documented instances of fault move~ent have been recorded in historic times. The topographic expression of the fa•Jl t zone is well de­veloped in the San Marcos area. The faulting of the soft, clayey 217 Upper Cretaceous strata downward against the more resistant carbon­ate units of the Lower Cretaceous has resulted in the development of the east-facing Balcones Escarpment. The elevation across this fault-line scarp about doubles from about 150 to about 300 meters. Uplifting of the west side of the Balcones fault zone has caused faster erosion there, and the result has been the exposure of generally older rocks there than east of the zone. However, the rate of erosion west of the scarp had been slowed considerably by the resistant Edwards Group carbonate rocks, thus giving rise to a relatively flat, elevated surface known as the Edwards Plateau. STRATIGRAPHY Because of the emphasis of this study on the surface and near-surface bedrock, only rock units that crop out will be des­cribed. As shown on the geologic maps (Plates 9 and 10), the bed­rock in the San Marcos area comprises mostly Cretaceous sedimentary rocks, and most of the Texas Cretaceous stratigraphic section is exposed in the area. Several gravel deposits of Quaternary age are also present in the area, but their discussion is deferred to the geomorphology section of this appendix. Most of the Texas Cretaceous strata were deposited in shal­low marine environments on a broad shelf that extended inland from about the middle of the present Gulf Coastal Plain. The dominant lithologies of the strata are limestones, dolomites, and marls. ~ost of the upper part of the section is composed of terrigenous clays. Highly uniform lithotopes over large areas of the shelf give rise to rock-stratigraphic units which may change materially in thickness and composition regionally but are relatively uniform in areas as large as a 15-minute quadrangle. Some of the rock­stratifigraphic units thin southward across the San ~areas area, owing to the influence of a subtle Cretaceous positive tectonic element, the San Narcos Plat:o".'.Lil, between New Braunfels and San 218 Antonio. None of the units, however, show appreciable lithologic changes across the area. Each of the stratigraphic units cropping out in the area are described very generally in ascending order in the following paragraphs. For detailed descriptions and measured sections of these units, the reader is referred to Decook, 1956, Noyes, 1957, Decook, 1960, Davis, 1962, or DeCook, 1963. Glen Rose Formation The oldest rock-stratigraphic unit exposed in the San Marcos area is the Glen Rose Formation. It crops out in the north­west corner of area in the canyon of the Blanco River, where the upper 12 to 15 meters are exposed. The strata consist of finely crystalline dolomite interbedded with dolomitic linestone and dolomitic marl. Individual beds range from 0 . 6 meters to several meters in thickness (Davis, 1962). Because of its small area of occurrence, this formation does not have great significance for this study. Walnut Formation The next f ormation in ascending order is the Walnut, which was distinguished by Davis (1962) but was included by Rose (1972) in the overlying Kainer Formation. The Walnut cr ops out on the bluffs of the Blanco canyon in the northwest part of the area. Davis (1962) recognizes the Bull Creek and Bee Cave Members in ascenaing'' o~de~,_ ,_, The Bull Creek is 12.0 meters thick and is mostly a massive lirr.es tone and dolo:Lite. The Bee Cave is 2.5 meters thick Like the Glen Rose. the Walnut Formation is and is a nodular marl. not particularly significant to this study, and it is included wi th the overlying Kainer For~ation on the geologic map. 219 Edwards Grouo The Edwards Group is the next rock-stratigraphic unit. Although this unit has been elevated in rank to a group comprising the Kainer (below) and the Person (above) Formations (Rose, 1972), it will be described here as a group primarily because both forma­tions serve as part of the important Edwards aquifer. The Edwards is a nearly pure carbonate unit with beds of hard limestone, dolo­mite, and all gradations bebveen these two lithologies. Isopachous maps by Rose (1972) indicate a thickness of about 100 meters for the Kainer and 43 to 49 meters for the Person in the San Marcos area. The two formations are lithologically similar and are difficult to distinguish unless the distinctive marker bed (the Regional Dense Member) at the base of the Person crops out. For this reason, and because of the intense faulting, no section has yet been ~easured in the area. 1ne Kainer and Person have been mapped separately on the geologic map where it was possible to distinguish between them. Elsewhere, the Edwards is mapped as undifferentiated. The Edwards is ~ne of the most important rock units in the area. It underlies most of the area west of the Balcones Escarp­ment and therefore determines the physical properties of the sub­strate in that area. More importantly, this unit is the aquifer which supplies the significant quantities of ground water in the area and provides the flow from San Marcos Springs. The outcrop area west of the escarpment, because it provides much of the re­charge to this aquifer, should be subjected to rather tight land use restrictions. The Edwards is different both in lithology and in the chemical quality of its contained water on either side of the "bad-water line" sho<:vn on the Resources map (Plate 3). This line approximately demarks the downdip limit of freshwater circu­lation. ~est of the line the water is potable (altho~gh somewhat mineralized), and the rock has been greatly altered oy the rapidly 220 circulating water. East of the line the water is highly mineralized and is charged with hydrogen sulfide . The rock there has not been subjected to the solution, collapse, and recrystallization effects that are characteristic of the aquifer part of the formation, and it is more typical of a deeply buried petroliferous limestone. Two oilfields a few miles southeast of San Marcos have produced a com­bined total of about 270 million barrels of oil from the Edwards. Georgetown Formation The Edwards Group is overlain by the Georgetown Formation, which is from 9 to 12 meters thick in the San Marcos area. The contact between the Edwards and Georgetown is sharp and distinctive and is probably disconfor:nable. DeCook (1956), in a description of a measured section around Sink Creek just north of San Marcos, indicates that this formation includes beds of shale, marl, argil­laceous limestone, and limestone. The Georgetown crops out in several fault blocks in the intensely faulted zone along the Balcones Escarpment, but does not include large outcrop areas. Del Rio Clav The Del Rio is about 15 meters thick and is composed of a clay-shale almost uniformly through this thickness. In the un­weathered state these clays are composed dominantly of kaolinite and illite with a small admixture of mixed layer illite­montmorillonite. In the weathered zone, however, the illite and mixed layer clay are converted into highly plastic montmorillonite. The contact of the Del Rio with the underlying Georgetown is usually obscured by mass movement of these clays. The formation is characterized by gypsum veinlets (at the outcrop) and by abun­dant specimens of the distinctive small oyster, Ilmatogyra arietina. A thin (about 0.3 meter) bed of Ilmatogyra lumachelle occurs at about the middle of the formation. Like the Georgetown, 221 the Del Rio crops out in irregular polygonal fault blocks in the vicinity of the Balcones Escarpment. Despite the small total area of outcrop, this formation has considerable implications for this study, as noted in the text. Buda Formation The next for;nation in ascending order is the Buda, which is a relatively hard, nodular limestone in the lower part and a hard, resistant, thick-bedded limestone in the upper part. It is about 15 meters thick iQ. the San i-larcos area. The Buda, like the underlying Del Rio and Georgetown, occurs chiefly in fault blocks in the intensely faulted zone along the escarpment. The hard lime­stones of this for;nation often form a resistant cap on hills t~at are flanked by the less resistant Del Rio Clay. The contact be­tween the Del Rio and Buca is usually obscured by slump failure of the Del Rio out from under the Buda. Eagle Ford Form3tion The Eagle Ford Formation, which is about 7.5 meters thick, overlies the Buda Formation. The Eagle Ford has three distinct parts -a lower bentonitic shale about 2.1 meters thick, a middle calcareous, flaggy sandstone or siltstone unit that is about 1.2 meters thick, and an upper shale having a thickness of about 4.2 meters. The lower contact between the lower shale and the upper Buda limestones is sharp and distinct. The contact with the over­lying Austin Group can seldom be seen because of slumpage of the upper shale out from under the more competent beds of the Austin. The Eagle Ford, like the subjacent formations above the Edwards, occurs in fault blocks along the Balcones Escarpment, and it also occurs more orominently in the bluffs of the Blanco River in the eastern half of the San Marcos area. Despite the competent flaggy beds in the middle of the Eagle Ford, this formation is considered a clay unit for the purposes of this study. Austin Grouo The Austin Group has been elevated in recent years from formation to group status and has been subdivided into several formations. However, it has been mapped in this study as a siugle unit because the lithologic differences between the various forma­tions are not in general great enough to be highly significant for environmental geologic purposes. DeCook (1963) reports a thickness of about 49 to 55 meters in the vicinity of San Marcos, but the upper contact is not exposed in the area. The lithology is chiefly an argillaceous or chalky limestone. The Austin ~nderlies a large area in the eastern half of the Kyle section of the study area on both sides of the Blanco River. It also crops out in a step fault block between the San Marcos Springs and Comal Springs faults southwest of San Marcos. Interestingly, the Austin also occurs in a fault block in the western part of the Kyle section not far from the highest eleva­tion in the area. 3ecause of its large area of outcrop in the northern half of the area, the Austin is very significant to this study. Tavlor Group The Taylor Group, which is the next rock-stratigraphic unit in ascending order, has a thickness of about 90 meters in the San Marcos area. Like the Austin Group, the Taylor has in recent years been elevated to group status and has been divided into three fonnations (Young, 1965). Ynese formations are the Sprinkle (lo~er), Pecan Gap (middle), and Bergstro~ (upper). All three formations are composed of smectitic mudstone and are distinguished primarily on the basis of calcium carbonate content; the Pecan Gap is more 223 calcareous and more resistant to erosion than the underlvino Sorinkle , 0 • or the overlying Bergstrom Formation. The formations of the Taylor Group crop out extensively in the area east of the Balcones Escarpment. The Sprinkle was recog­nized in only one location southwest of San ~!arcos, but the Pecan Gap underlies almost all of the rest of the area east of the scarp. The Bergstrom is in contact with the Pecan Gap along an inferred northeast-oriented Balcones fault and occurs in the southeast cor­ner of the area. Because of its wide areas of occurrence and poor engineering properties, the Taylor Group as a whole is very sig­nificant to this study. Corsicana Formation The youngest Cretaceous formation in the San ~1arcos area is the Corsicana Formation of the Navarro Group. Outcrops of this formation were observed in the far southeast corner of the area where t~e formation apparently overlies the Bergstrom in normal stratigraphic contact. A regional dip of about 19 meters per kilo­meter was assumed when this contact was drawn. STRGCTURAL GEOLOGY The San Marcos area lies over the Balcones Escarpment in an intensely faulted part of the Balcones fault zone. This zone, as noted earlier, is a system of mostly normal faults having a net displacement dm~1ward to the east and southeast. At first glance, the part of the geologic map northwest of the escarpment (Plates 9 and 10) has the appearance of a shattered mirror or pane of glass, but with closer study a definite pattern emerges. In general, the faults of major displacement strike about N30°E. Near the northern margin of the area the Mustang Branch and Moun­tain City faults are step faults across which the Austin Group is 224 displaced downward to the southeast to about the same level as the Edwards Group. A third step fault, the Kyle fault, is located about 7 kilometers to the southeast near Kyle. It has displaced the Pecan Gap Formation down to about the level of t he Austin Group. A similar relationshiµ exists near the western margin of the San Harcos section of the area. Three step faults (an unnamed zone of faults, the San Marcos Springs fault, the Comal Springs fault) have displaced the Pecan Gap Formation downward to the southeast to an elevation lower than the Edwards Graue. The t wo step fault zones in the northern and southern µarts of the area are not aligned with each other, but are offset by about 4.6 kilometers. The complex faulting and outcrop pattern between these zones is the result of the adjustment ::if tne intervening area to their en echelon relationship. The offset of the zones can be seen quite clearly in the Balcones Escarµment as shown on the satellite photo in the Frontispiece. In broad outline, the Edwards Group outcrops on the upthrown side of the southern set of step faults give way north­eastward to progressively younger strata that are on the down­thrown side of the Mustang Branch and !fountain City step faults (R. 0. Kehle, personal communication). Figure A.l shows in sim­plified diagrammatic form the ramp between the en echelon step faults. This ramp, instead of bending smoothly, as shown ir: the diagram, from the upthro\m side of the southern fault zone to the downthrown side of the northern zone, has been intensely faulted and fractured, resulting in the !llosaic of gravity fault blocks west of the Blanco River in the Kyle section. unfortunately, a key part of the transition has been covered by alluvium of the Blanco River. This relatively straightforward picture is compli­cated somewhat by the intensely faulted graben-like downfaulted wedge in the west-central part of the Kyle section. Similar but somewhat smaller ra~p-like structures can also be seen along the Balcones Escarpment near the eastern margin of the Kyle section . Figure A.l Diagram Showing tr.e Ramp Between Two En Echelon Fault Zones If this block were eroded to a level surface, the strata exposed in the ramp would become progressively younger north­eastward. 226 Three en echelon faults (the Kyle fault, the San ~!arcos Springs fault, and an unnamed fault between t hese two) separate two ramps which dip northeastward. Outcrops on these ramps change north­eastward from the Austin Group to the Pecan Gap For~ation, but the exact nature of the transition is obscured by alluvial cover. Another interesting structural feature along the escarp­ment is the relatively undisturbed 8onolithic block upon which the northwestern half of San :'.>!arcos is built. This block is en­circled by intense faulting but has not itself been appreciably disturbed except for a slight eastward tilt which may be a result of regional dip. The block stands quite high topographically on the divide between Sink Springs and Purgatory Creeks. Southeast of the Balcones Escarpment the Balcones faulting does not appear to be as intense as northeast of the scarp. How­ever, this apparent difference is due to the difficulty in recogniz­ing and mapping faults in the clay terrane rather than to the ab­sence of faults. ~ot only is it almost i~possible to observe faults in the soft, unifor~ Upper Cretaceous clay units, but the structure is further obscured by the thick black soils that have developed on the clays. ~ore than likely, intense faulting like that northwest of the scarp has also occurred southeastward to the ;<.fexia fault zone (Keith Young, personal communication). The major faults shown on the geologic mao southeast of the escarpment are taken from previous work (chiefly DeCook, 1960, 1963 and Sarnes, 197~), and their existence is inferred primarily from subsurface data rather than surface mapping. The total vertical slip (throw) across the study area from the upper contact of the Glen Rose in the Blanco River canyon to the inferred contact between the Taylor and Navarro Groups in the southeast corner of the area is about 415 ~eters. An unknown part of this displacement is cue to regional dip . The displacenent on ~he major faults in the area is difficult to estimate because 227 the blocks on either side of the faults are themselves. usuallv, broken up by cross faults. The individual blocks are often dis­placed by differiGg amounts. Also, the blocks are usually tilted and bent, which further increases the difficultv of estimatino , ~ displacements. Despite these problems some very general estimates of displacements on the major faults or fault zones can be made. The minimum throw on the combined Mustang Branch and Xountain City faults equals the thickness of the stratigraphic interval bebveen the Edwards and Austin Groups, and amounts to about 50 meters. The Hidden Valley fault, which is reported to have a throw of about 60 meters in Comal County (George, 1952), has a highly variable throw in the San Marcos area. The upper contact of the Glen Rose is apparently displaced only about 9 meters at one point in the Blanco River canyon, but it is displaced by more than 15 meters at another point in the canyon. DeCook (1963) states that this fault dies out to the northeast. The :lorton Ranch fault has maximum displacement along a segment where the Austin Group is faulted down against the Person :ormation, and the minimum displacement there is about 50 meters. Davis (1962) estimates the maximum displacement of this fault to be between 80 and 110 meters. DeCook (1956) estimates the throw of San Marcos Springs fault to be a minimum of about 90 meters. Comal Springs fault decreases in displacement northeastward across the area. The throw is estimated to be about 120 meters near the west edge of the San Marcos section and about 100 meters near San ~areas. Because of the en echelon relationship of some of these faults, their dis­placements sum to a value greater than the total vertical throw across the area. An interesting struct~ral feature southeast of the Balcones Escarpment is the inferred breached dome-like structure between Hunter Road and Interstate 35 southwest of San ~farcos. A lone outcrop of the Sprinkle formation observed near the center of 228 this feature was found to be completely surrounded by outcrops of the Pecan Gap Formation. The interpreted structure, which assumes a reversal of dip near the Comal Springs fault of the type des­cribed by Cloos (1968), is advanced as the simplest explanation for these observations. An earth resistivity traverse was made parallel to ~1cCarty Lane by an Engineering Geology class of the University of Texas at Austin to clarify the structural picture at this location, but the results were relatively inconclusive . The interpretation depicted proved to be compatible with the resistivity observations, but an alternative interpretation which proposes additional faults and a horst-like structure is equally likely. The reversal-of-dip explanation is used here to avoid depicting a minimum of two additional inferred faults, neither of which can be satisfactorily connected to known faults. GEOMOfil>HOLOGY The primary physiographic element in the San Marcos area is the Balcones Escarpment. As noted in the text, the elevation about doubles westward across the scarp within the limits of the area, and the landforms are different on either side. East of the scarp the topography on the soft Up?er Cretaceous clays of the Blackland Prairie is gentle and rolling with low rounded hills. West of the scarp the rugged limestone terrane of the dissected eastern margin of the Edwards Plateau is known as the Texas hill country . fluvial Processes The dominant geomorphic vrocesses in the area at present are the fluvial processes. One of the more interescing aspects of the fluvial geomorphology of the area is the geologic hiscory of R. ·1Joodruff (Baker and others, 1974) describedt he Blanco .iver. . evidence indicating that the Blanco formerly flowed eastward out 229 of the Kyle section into what is now the drainage basin of Onion Creek. He proposed that the scarp (90°) bend in the course of the river in the north-central part of the Kyle section may be an elbow of capture. This elbow was created when the Blanco was diverted by a small stream that was eroding headward more or less normal to the Balcones Escarpment. A faintly visible trace of a short segment of the precapture course of the Blanco can be seen on aerial photos about 1.6 kilometers northeast of the capture elbow on the present divide between Onion Creek and the Blanco River. The increased gradient resulting from the capture may have been the primary cause of the incision of the Blanco to form the canyon where the river flows through the resistant Edwards Group limestones. Evidence for a second capture of the Blanco can also be found south of Kyle, where terrace gravels cap the Austin Group uplands. This terrace stands about 12 to 15 meters above the present floodplain of the Blanco. Koenig (1940), who has studied this terrace, reports a maximum thickness of 12.8 meters for the gravels of this terrace. A thickness of no more than about 4.6 meters was observed during this study. The terrace is deeply dis­sected and retains none of its original landform, but it clearly indicaces that the Blanco formerly flowed eastward south of Kyle into the present drainage basin of Plum Creek (Koenig, 1940). This former course of the Blanco is also clearly indicated east of the Kyle section on the Seguin sheet of the Geologic Atlas of Texas (Barnes, 1974). The alluvial gravels in this former valley are more resistant to erosion than the surrounding upper Cretaceous clays and have resulted in an inversion of topography (Victor Baker, personal co!Tu~unication) . The gravels now cap hills that are higher than the surrounding clay terrane. The present Blanco River and associated features also dis­play several interesting characteristics. About 4.2 kilometers 230 west of Kyle the narrow canyon of the river abruptly opens to a floodplain having a width of about 1.6 kilometers. The river has a meandering course within the limits of this floodplain, which remains relatively constant in ·.vidth downstream to about Interstate 35. East of the Interstate the floodplain again increases in width, to about 5.8 kilometers. These abrupt changes in floodplain width are attributed to changes in bedrock resistance to erosion. The wideJing of the gorge to a floodplain west of Kyle occurs at al­most t he exact point that the river crosses a fault which marks the do\..-nstream limit of the hard Edwards Group limestones. Apparent­ly the sidecutting action of the river is more effective in the less resistant post-Edwards strata downstream from this point. Interestingly, there is also a sharp nickpoint in the river at the same site. An old gravel terrace on the upland west of the river indicates that this location has marked a sharp change in the river's behavior from a primarily downcutting to a sidecutting stream for some time in the geologic past. The second sudden broadening of the floodplain at the Interstate occurs at another change in bedrock resistance, where the nonresistant Pecan Gap clays are downfaulted against the more resistant Austin Group chalks and limestones. The Blanco has incised slightly and has cut into or through the floodplain alluvium. As a result the flat surface along the river is no longer a floodplain in the normal sense of the term, but is rather an "infrequently flooded surface" (Victor Baker, personal communication). The rivE:r has cut into bedrock for most of its course downstream to Interstate 35, exposing in the alluvium a thickness of about 6 meters of gravel overlain by about 3 ~eters of sand, silt, and clay (see Figure 3.6 in text). Because of the exclusively carbonate rock source area, the gravel is composed chiefly of limestone clasts with a lesser amount of chert and dolomite. The succession of gravel overlain by fine­grained sediment is interprE:ted t o be a normal channel gravel ­231 overbank mud sequence. The deposition of this sequence at a par­ticular location probably occurs during the passage of a meander loop which is eroding into the bedrock slightly. As a result, the preexisting alluvium at the location is entirely removed as the meander passes, and only one channel -floodplain sequence is pre­served at the location. This process, which explains the presence of only one sequence in most places, is highly idealized and is not realized everywhere on the floodplain, resulting in several variations on the most com.~only observed succession. Downstream from Interstate 35, the river probably has not cut through the alluvium along most of its length. Several aban­doned channels can be identified in the floodplain almost from its beginning west of Kyle by tonal changes in large scale air photos. The present course of the river near the confluence into the San Marcos River has apparently been assumed relatively recently, as indicated by the steep slopes in the weak Pecan Gap clays along the cut bank there. A minor but interesting fluvial geomorphic feature of the area is just south of San Marcos, where an exaraple of stream piracy in action can be seen. In UTX 3304-601, the 580-foot con­tour line clearly indicates a connecting channel between Willow Springs and Purgatory Creeks. Longitudinal profiles of these creeks downstream from this channel show a gradient of about 3.8 meters per kilometer for Willow Springs Creek and a gradient of about 1.9 meters per kilometer for Purgatory Creek. The U.S. Army Corps of Engineers (19 71, p. 20) reports that water from Purgatory Creek flows through t~e connecting channel into Willow Springs Creek when the Purgatory Creek discharge exceeds 120 cubic meters per second (4,200 cubic feet per second). It seems clear that the higher gradient Willow Springs Creek is in process of capturing the flow of Purgatory Creek at this natural channel. If these streams were left undisturbed by urbanization, erosion during 232 times of flood would almost certainly deepen the channel, and within a few hundred years all of the flow of Purgatory Creek would be diverted into Willow Springs Creek. Karst Processes Karst processes have been important on the Edwards Group carbonate rocks in the western part of the San Marcos area. The present climate in the area is not conducive t o karst processes, however, and the karst features as a whole appear to be undergoing destruction by stream dissection. ~any dolines nevertheless re­main on the uplands some distance away from the major rivers where the dissection is most rapid. The relatively flat upland in the northwest corner of the Kyle section appears to be a fairly good remnant of the Edwards Plateau surface and has dolines in suf­ficient abundance to be termed a karst plain. As noted in the text the most important karst process from an anthropocentric point of view is t~e recharge to the Edwards aquifer. Some of this recharge takes place by direct infiltration and through sinkholes in the uplands, but most recharge is believed to occur in the beds of intermittent streams (L. J. Turk, personal communication) . An example of this stream recharge is depicted in Figure 3.7 in the rext. Abundant caves are a prominent karst feature associated with the Edwards Group. One cave, ~onder Cave, is operated com­mercially, and several others have been named, explored, and mapped. One of the ~ore interesting caves is Dugger Cave (Tarbutton Cave), which is located near the Blanco River channel in UT~! 3316-604. This cave has a vertical entrance, and t he base flow of the Blanco is prevented from being diverted into the cave only by a lip of bedrock and cemented gravel alluvium about 0.9 meters high between the river level and the cave entrance. During flood stages, when 233 the water overtops the lip, part of the river's flow is diverted into the cave, thus becoming recharge to the Edwards aquifer. Interestingly, although small passages of the cave extend under the Blanco channel (Bill Russell, Texas Speleological Survey, per­sonal corrL~unication), an appreciable part of the river's base flow is apparently not lost to the cave. Dugger Cave is probably the furthest downstream point of recharge from the Blanco River to the Edwards aquifer. Sink Springs is a vertical, water-filled cave in UTI1 3308-603 near the channel of Sink Springs Creek (see Figure 4.26 in text). The water level in the cave is near the ground surface and is at the level of the potentiometric surface of the Edwards aquifer. The cave received its name because it takes water from Sink Springs Creek during flood flow (or did before a small dam was constructed between the cave and the creek) and apparently has been known to flow in historic time (Robert Knispel, local resident, personal communication). It is possible t hac the discharge of San Marcos Springs issued from Sink Springs before t he Edwards was breached at the discharge point at Spring Lake, but little or no evidence can be advanced to support this idea. Two other note­worthy observations can be oade concerning the location of San Marcos Springs. First, it may not be fortuitous that the springs are located at the raargin of the undisturbed monolithic block under northwestern San Marcos described earlier. Second, the springs are situated at a large-displacement fault where the Pecan Gap Formation has been downfaulted into contact with the Person Formation. At other points along the scarp, this displacement is distributed over two or more step faults, giving rise to a fault block havino the Austin Group at the surface be8veen the Ed~ards 0 and the Pecan Gap. That is, o~tcrops of Austin occur between the Edwards and t he Pecan Gap on either side of the springs along the Balcones Escarpment, but t he Austin has been faulted out at the springs location. 234 In addition to these karst features in the Edwards Group, two sinks are present in the Austin Group chalks and limestones near the south-central part of the Kvle section. They are of in­terest primarily because sinks have not heretofore been reported in the Austin Group in central Texas (Keith Young, personal com­munication). These sinks, as well as karstic features in other post-Edwards strata (such as Academy Cave and the uevil's Smokehouse, both of which are developed in the u?perrnost Buda Formation) have almost certainly resulted from roof collapse of large caverns in the Edwards. The dolines are thus created by a kind of natural stoping process from caverns in the Edwards upward through the stratigraphic section rather than by solution of post-Edwards linestones. Figure 3.Sc in the text shows a cross-sectional vi~J of a sinkhole in which this type of collapse has occurred. Mass Mover.1ent Mass movement of so~ewhat different types is active on either side of the 3a~cones Escarpsent. Sast of the scarp, the weak Upper Cretaceous clay units are undergoing slumping and creep that give rise to the low, gently rolling hills. West of the scarp, a major type of mass novement is the calving off of lar3e limestone blocks on cut banks of streams. Hills that are flanked by the Del Rio Clay and capped by the overlying Buda Formation often have large blocks of limestone from the Buda sliding do>v~ the hillside on the weak Del Rio. The best examples of such large sliding blocks are northwest of San .'.'1arcos in UT.:1 3308-599 and 600, where a hill is developed on a fault block of Del Rio Clay. This hill was apparently originally protected frcim erosion by a Buda cap, but this cap has become frag~ented and broken U? into large blocks. These blocks are sliding dO\m the nill radially away from the hilltop and are now found at varying el~vations on the hillside. Another notable exa:i1ple or the failure o: the Del 235 Rio out from under the Buda are two large slu.11p blocks that are developed on the cut bank of the Blanco River at Five-mile Dam. The blocks are composed of Buda and Eagle Ford strata and have collapsed because of undercutting of the Del Rio Clay at the river level. SU:lHARY The bedrock units in the Sc.n Marcos area are Cretaceous sedimentary strata composed pri~arily of carbonates and fine­grained terrigenous rocks. The lithology and physical properties of the rocks are chiefly a function of the depositional conditions and source areas that were extant during deposition of the Cre­taceous section, and the present occurrence and distribution of the rock units are controlled by the Balcones faulting in con­junction with the shape of the present erosional surface. Fluvial, ~arstic, and mass-movement processes are the dominant agents that have given rise to the existing geomorphic features, and they con­tinue to shape the land surface today. REFERENCES CITED 237 Abbott, P. L., 1973, The Edwards Lir;iestone in the Balcones fault zone, south-central Texas: Gniv. Texas (Austin), Ph.D. dissert. (unpublished), 122 p. 1975, On the hydrology of the Edwards Limestone, south­central Texas: Jour. Hydrology, v. 24 , p. 251-269. Alexander, W. H., Jr., Byers, B. N., and Dale, 0. C., 1964, Recon­naissance investigation of the ground-water resources of the Guadalupe, San Antonio, a~d Nueces River basins, Texas: Texas Wa ter Comm. Bull., no. 6490, p . 56-70. American Commission on Stratigraphic Nomenclature, 1961, Code of stratigraphic nomenclature: Am. Assoc. Petroleum Geolo­gists Bull ., v. 45, p. 56-70. American Public Works Association, Institute for Solid Wastes, 1970, Municipal refuse disposal: Chicago, Illinois, Public Adm . Service, 538 p. American Society of Civil Engineers, 1959, Sanitar:r landfill: Am. Soc. Civil Engineers, Manual of Eng. Practice, no. 39, 61 p. Baker, V. R., 1975, Flood hazards along the Balcones Escarpment in central Texas -alternative approaches to their recogni­tion, mapping, and management: Univ. Texas (Austin) Bur . Econ. Geology, Geol . Cir., no. 75-5, 22 p. Baker, V. R., Perdue, James, Sansom, James, and Woodruff, C. M., Jr., 1974, Geomorphic and hydrologic features of the cen­tral Texas hill country: Austin Geol. Soc., Field-trip guidebook, 21 p. Barkley, M. S., 1970, A history of central Texas: Austin, Texas, Austin Printing Co., 235 p. Barnes, V. E., project director, 1974, Geologic atlas of Texas, Seguin sheet: Univ . Texas (Austin) Bur . Econ. Geology, Geologic Atlas of Texas . Bates, R. L., 1969, Geology of the indus trial r ocks and minerals: New York, Dover Publications, 459 p. 238 Blair, F. W., 1950, The biotic provinces of Texas: Texas Jour. Sci., v. 2, p. 93-115. Buol, S. W. , Hole, F. D., and :-!cCracken, R. J., 1973, Soil genesis and classification: A.~es, Iowa, Iowa State Univ. Press, 360 p. Bybee, H. P., 1952, The Balcones fault zone -an influence on human economy: Texas Jour. Sci., v. 4, p. 387-392. California Department of Wa ter Resources, 1969, Sanitary landfill studies, Appendix A: summary of selected previous inves­tigations: California Dept. Water Resources Bull., no. 147-3, 115 p. Cartwright, K., and Sherman, F. B., 1968, Evaluating sanitary land­fill sites in Illinois: Illinois Geol. Survey Environmental Geology Notes, no. 27, 15 p. Chapin, F. S., 1965, Urban land use planning: Chicago, Illinois, Univ. of Illinois Press, 498 p. Clark, C. T., and Holz, R. K., 1971, Economic and population growth in the Guadalupe -Blanco River area: Univ . Texas (Austin) Bur. Business Research, Area Econ. Survey, no. 32, 65 p. Clark, T. P., 1972, Hydrogeology, geochemistry, and public health aspects of environ.~ental impair~ent at an abandoned land­fill near Austin, Texas: Univ. Texas (Austin), M.A. thesis (unpublished), 190 p. Cloos, E., 1968, Experimental analysis of Gulf coast fracture patterns: A.~. Assoc. Petroleum Geologists Bull., v. 53, p. 55-72. Cuyler, R. H., 1931, Vegetation as an indication of geologic forma­tions: Am. Assoc. Petroleum Geologists Bull., v. 15, p. 67-78. Dallas :::1orning News, 1974, Texas almanac and industrial guide: Dallas, Texas, A. H. oelo Corp., 704 p. Davis, w. E., 1962, Geology of Lime Kiln quadrange, Hays County, Texas: Univ . Texas (Austin), :M.A. thesis (unpublished), 90 p. of San Marcos Springs quadrangle, DeCook, K. J., 1956, Geology Univ. Texas (Austin), M.A. thesis Hays County, Texas: (unpublished), 86 p. 239 DeCook, K. J., 1960, Geology and ground-water resources of Hays County, Texas: Texas Board of Water Engineers Bull., no. 6004, 170 p. _____1963, Geology and ground-water resources at Hays County, Texas: U.S. Geol. Survey Wa ter-Supply Paper, no. 1612, 72 p. Detroit "1etropolitan Area Regional Planning Commission, 1962, Land use classification manual: Chicago, Illinois, Lincoln Printing Co., 53 p. Dobie, D. R., 1932, The history of Hays County, Texas: Univ. Texas (Austin), M.A. thesis (unpublished), 134 p. _____1948, A brief history of Hays County and San :1arcos, Texas: San Xarcos, Texas, privately pub., 71 p. Environmental Science Services Administration, 1968, Climatologi­cal summary for San :-farcos, Texas: U.S. Dept. of Commerce, Environmental Sci. Service Adm., 1 p. Ferris, K. H., and Fabos, J. G., 1974, The utility of computers in landscape planning: the selection and application of a computer mapping and assessment system for the Metropolitan Landscape Planning Model (XETLA.~D): Massachusetts Agr. Expt. Station Bull., no. 617, 116 p. Fisher, W. L., Brown, L. F., ~cGowan, J. H., and Groat, C. G., 1973, Environmental geologic atlas of the Texas coastal zone --Beaumont-Port Arthur Area: Univ. Texas (Austin) Bur. Econ. Geology, 93 p. e Fisher, W. L., McGowan, J. H., Brown, L. F., and Groat, C. G., 1972, Environmental geologic atlas of the Texas coastal zone --Galveston-Houston Area: Univ. Texas (Aus tin) Bur. Econ. Geology, 91 p. Flawn, P. T., 1965, Geology and urban developruent, in Urban geology of greater Waco, Part I, Geology: Baylor Geol. Studies Bull., no. 8, ?· 5-7. 1970, Environmental geology -conservation, land-use planning, and resource management : New York, Harper & Row, 313 p. T., Turk, L. J., and Leach, C.H., 1970, Geological con­ Fla,.m, P. siderations in disposal of solid municipal wastes in Texas: Univ. Texas (Austin) Bur. Econ. Geology, Geol. Circ. 70-2 , 2 2 p . 240 Funk, A. C., 1975, The relationships of engineering properties to geochemistry in the Taylor Group, Travis County, Texas: Univ. Texas (Austin), M.A. thesis (unpublished), 111 p. Gary, M., ~1cAfee, R., Jr., and Wolf, C. L., eds., 1972, Glossary of geology: Washington, D.C., Am. Geol. Inst., 805 p. George, W. 0., 1952, Geology and ground-water resources of Comal County, Texas: U.S. Geel. Survey Water-Supply Paper, no. 1138, 126 p. Geyer, A. R., and McGlade, W. G., 1972, Environmental geology for land-use planning: Pennsylvania Topographic and Geol. Survey, Environmental Geology Rept., no. 2, 44 p. Godfrey, C. L., McKee, G. S., and Oakes, H., 1973, General soil map of Texas: Texas A & M Univ., Texas Agr. Expt. Sta. and U. S. Dept. of Agr., Soil Conserv. Service, map. Gould, F. W., 1962, Texas plants -a checklist and ecological sum­mary: Texas A & ~Univ., Agric. Expt. Sta., HP-585, 112 p. Hayes, W. L., and Vineyard, J. D., 1969, Environmental geology in towne and country: ~-Iissouri Geol. Survey and Water Resources, Educ. Series, no. 2, 42 p. Hays County Citizen, 1975, Flood waters again rose over South LBJ at Purgatory Creek as heavy rains hit San Marcos Tuesday ~orning (photo caption): San Marcos, Texas, Hays County Citizen, June 12, 1975. Hill, R. T., and Vaughan, T. W., 1898, Geology of the Edwards Plateau and Rio Grande Plain adjacent to Austin and San Antonio, Texas, with reference to the occurrence of under­ground waters: U. S. Geol. Survey, 18t~ Annual Rept., pt. 2, p. 193-321. Hughes, G. M., Landon, R. A., and Farvolden, R. N., 1971, Hydro­geology of solid waste disposal sites in northeastern Illinois: U. S. Environmental Protection Agency, Sw -12d, 154 p. Hunt, R. E., 1973, Round Rock, Texas ~Jew Town: geologic probleras and engineering solutions: Assoc. Eng. Geol. Bull., v. 10, no. 2, p. 231-242. Institute for Solid Wastes, 1970, Municipal refuse disposal: Chicaao Public Administration Service, 538 p. 0 ' 241 Ivey, J. B., 1971, Definition of environ~ental geology and pur­pose of the conference, in The governor's conference on environmental geology: Colorado Geol. Survey Spec. Pub., no. 1, p. 3-7 . Kansas State Geological Survey, 1968, A pilot study of land-use planning and environmental geology : Lawrence, Kansas, Kansas State Geol. Survey, 63 p. Klemt, W. B., and others, 1975, Ground-water resources and model applications for the Edwards (Balcones fault zone) aquifer: Texas Water Devel. Board, unpublished rept., 93 p. Koenig, J. B., 1940, A consideration oi the Blanco River terraces north of San ~1arcos : Univ. Texas (Austin), ~1.A. thesis (unpublished), 42 p. League of Women Voters of San ~!arcos, Texas, 1973, Survey of Hays County and the city of San :-1arcos: San ~farcos, Texas, League of Women Voters, 27 p. Legget, R. F., 1973, Cities and geology : New York, McGraw Hill, 629 p. Leopold, L. B., 1968, Hydrology for urban land planning: U. S. Geol . Survey, Circ. 554, 18 p. Lockwood, Andrews, and Nei;vnar.1, Inc., 1969a, Study of solid wastes disposal for San ~'!arcos, Texas: Haus ton, Texas, Lockwood, Andrews and Newnam, Inc., 17 p. 1969b, Comprehensive plan, San Marcos, Texas, base stud­ies: Haus ton, Texas, Lockwood , Andrews, and ~!ewnam, Inc. , interim rept. no. 1, 110 p. 1969c, Comprehensive plan, San Earcos, Texas, comrm.:nity facilities and capital improvements: Houston, Texas, Lockwood, Andrews, and Xewnam, Inc., interim rept. no. 2, 86 p. Longley, G., 1975, Environmental assessment, upper San ~!arcos River watershed: Environmental Sciences of San ~!areas, unpublished report, 367 p. Lowther, A. c., 1972, Soil handbook for soil survey cf the city of San Marcos, Texas: U. S. Dept. of Agr ., Soil Conserv . Service, Hays County Dist., 66 p. 242 Miller, J. R., 1970, Professional decision-making, a procedure for evaluating complex alternatives: New York, Praeger Publishers, 305 p. National Center for Resource Recovery, Inc., 197 4, Sanitary landfill -a state-of-the-art study: Lexington, Massa­chusetts, Lexington Books, 119 p. Noyes, A. P., 1957, Geology of the Purgatory Creek area, Hays and Comal Counties, Texas: Univ. Texas (Austin), ~.A. thesis (unpublished), 94 p. Office of Science and Technology, 1969, Solid waste management ­a comprehensive assessment of solid waste problems, prac­tices, and needs: Washington, D.C., Office of Sci. & Technology, Executive Office of President, 111 p. Oliver, J., and others, 1969, Seismology, responsibility and re­quirements of a growing science: ~atl. Acad. Sci., pt. 1, 38 p. Patterson, P. E., 1974, Upper San ~arcos River watershed, Hays Co., Texas: An archeological survey of areas proposed for modificati ons : Univ. Texas (Austin), Texas Archeolo­gical Survey, research rept., no. 42, 35 p. Pearson, P. J ., Rett~an, P. L., and Wyerman , T. A., 1975, Environ­mental tritium in the Edwards aquifer, central Texas, 1963-1971 : U. S. Geel. Survey, open-file rept. 74-362, 32 p. Public Health Service, 1961, Recommended standards for sanita~; landfill operations: U.S. Dept. of Health, Educa t ion, and Welfare, Public Health Service. Roberson, W. R., 1972, A study of extinct rural corrununities in the U.S.: a test oi feasibility: Univ. Texas (Austin) , ~1. A. thesis (unpublished), 24 7 p. Rose, P. R., 1972, Sdwards Group, surface and subsurface, central Texas: Univ. Texas (Austin) Bur. Econ. Geology, Rept. Inv., no. 74, 198 p. San Marcos Record, 1936, History of Hays County: San Marcos, Texas, San Marcos Record, Sept. 25, 1936. 1970, The San ::1arcos flood: San ~[arcos, Texas, San :rarc::is Record, May 21, 1970. 243 San Marcos Record, 1975, City of Kyle eligible for insurance: San Marcos, Texas, San Marcos Record, January 23, 1975. Savage, V. H., ~organ, C. A., and Yeargan, H. R., 1971, Economic base, San Marcos, Hays County, Texas (1959-1971): San Marcos, Texas, Southwest Texas State Univ., unpublished rept., 81 p. Soil Survey Staff, 1960, Soil classification -a comprehensive system -7th approximation: U. S. Dept. Agr., Soil Conserv. Service, 265 p. Suhm, D. A., Kreiger, A. D., and Jelks, E. B., 1954, An intro­ductory handbook of Texas archeology: Texas Archeological Soc. Bull., v. 25, 582 p. Texas State Department of Health, 1974, Municipal solid waste regulations: Austin, Texas, Texas State Dept. of Health, 26 p. Texas Water Quality Board, 1975, An order of the Texas Water Quality Board promulgating regulations for the protec­tion of the water in the Edwards Underground Reservoir: Texas Water Quality Board Order, no. 75-0128-20, 25 p. Tillman, S. E., Upchurch, S. B., and Ryder, G., 1975, Land use site reconnaissance by computer-assisted derivative mapping: Geol. Soc. America Bull., v. 86, p. 23-34. Tipple, G. L., 1975, Clay mineralogy and Atterberg limits of the Taylor Group in the vicinity of Austin, Texas: Univ. of Texas (Austin), ~.A. thesis (unpublished), 172 p. Turner, A. K., and Coffman, D. ~., 1973, Geology for planning: a review of environmental geology: Colorado School ~ines Quart., v. 68, no. 3, 127 p. U. S. Army Corps of Engineers, 1964, Survey report on Edwards underground reservoir, Guadalupe, San Antonio, and Nueces rivers and tributaries, Texas: Fort Worth, Texas, U. S. Army Corps of Engineers, Fort Worth Dist., 3 vols., 198 p. 1971, Flood hazard information, San Xarcos and Blanco ~~~~~-Rivers, San ~1arcos, Texas: Fort Worth, Texas, U. S. Ar8y Corps of Engineers, Fort Worth Dist., 35 p. U. S. Department of Agriculture, Soil Conservation Service, 1971, Flood insurance study, San ~areas, Texas: Temple, Texas, Soil Conserv. Service, 14 p. 244 U. S. Geological Survey, 1973, ~ap of flood-prone areas , ~ountain City, quadrangle: Austin, Texas, U. S. Geol. Survey, Water Resources Division, map. Walz, D. H., 1974, Sewage renovation and surface-water quality, Lakeway Resort Community, Travis County, Texas: Univ . Texas (Austin), M.A. thesis (unpublished), 91 p. Webb, W. P., ed., 1952, The handbook of Texas: Aus tin, Texas, Texas State Hist. Assoc., v. 1, 977 p., v. 2, 953 p. Weir, F. A., 1967, The Greenhaw site: burned-rock rr:idden cluster of the Edwards Plateau Aspect: Univ. Texas (Austin), M.A. thesis (unpublished), 164 p. Werchan, L. E., Lowther, A. C., and Ramsey, R. N., 1974, Soil survey of Travis County, Texas: U. S. Dept. Agr ., Soil Conserv. Service, 123 p. Wilie, E. E., 1940, The cultural influence of the Balcones Fault: Univ. Texas (Austin) , M.A. thesis (unpublished), 68 p. Yeates,~. H., and Garner, B. J., 1971, The North American City: New York, Harper and Row, 536 p. Young, K. P., 1965, A revision of Taylor nomenclature, Upper Cretaceous, central Texas: Univ. Texas (Austin) Bur. Econ. Geology, Geol. Circ. 65-3, 11 p. VITA Thomas Walter Grimshaw, the son of Claude and Phyllis Grimshaw, was born in Chamberlain, South Dakota on Xarch 23, 1945. He attended several public schools in South Dakota and graduated ·from Washington Senior High School in Sioux Falls in June, 1963. He entered the South Dakota School of Xines and Technology in September, 1963 and received a Bachelor of Science degree in Geological Engineering from there in June, 1967. In September, 1967 he entered the University of Texas at Austin and received a Master of Arts degree in Geology from that institution in January , 1970. He married the former Susan Stein in September, 1969. After working for a few months as a geologist for Amoco Production Company in Denver, he entered on active duty in the U. S. Army in February, 1970. He received an honorable discharge with the rank of Captain in April, 1972 and commenced study for the Ph.D. at the University of Texas at Austin in June, 1972. Permanent address: 3815 Brookview Road Austin, Texas 78722 This dissertation was typed by Genene Oestrick. KYLE SECTION SAN MARCOS SECTION c c c c PLATE 1 ENGINEERING GEOLOGY EXPLANATION PLATE 3 RESOURCES " T EXAS AT AU$TIN MAPPING CREDITS 1 KM 1 Ml SCALE ,,,. ,/ ,' JUL 2 7 1976 GEOLOGY LIElRARY G 1 G2 G3 BAD WATER LINE FROM U. S. GEOLOGICAL SURVEY OFFICE IN SAN ANTONIO. TRANSMISSIBILITY OF 1 MILLION GAL PER FOOT PER DAY FROM KLEMT AND OTHERS. 1975 THIS STUDY PLATE 3 OF 10 ENVIRONMENT AL GEOLOGY OF URBAN AND URBANIZING AREAS THOMAS W. GRIMSHAW DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN 1976 KYLE SECTION SAN MARCOp SECTION EXPLANATION CSEE TEXT FOR DETAILS> PLATE 4 TEXAS AT AU:iTIN PROCESSES ,,.f../ FLOOD-PRONE AREA JUL 2 7 1976 ~AQUIFER RECHARGE -PRIMARY GEOLOGY LI ORARY PLATE 4 OF 10 MAPPING CREDITS f<.V N ,,;..---AQUIFER RECHARGE -SECONDARY G POTENTIAL SINKHOLE COLLAPSE 1 KM UNSTABLE SLOPES 1 Ml SCALE ?-""" HIGH SHRINK -SWELL (PROCESSES ARE ACTIVE ON THE SIDE MAGNETIC DECLINATION OF THE LINE WITH THE SYMBOL> 1 FLOOD-PRONE AREAS ENVIRONMENT AL GEOLOGY OF 1. U. S. GEOLOGICAL URBAN AND URBANIZING AREAS SURVEY, 1973 2. U. S. ARMY CORPS THOMAS W. GRIMSHAW OF ENGINEERS, 1971 DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN 1976 KYLE SECTION SAN MARCOS SECTION PLATE 5 EXPLANATION 15% 1 KM THOMAS W. GRIMSHAW 1 Ml SCALE DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN MAGNETIC DECLINATION 1976 KYLE SECTION SAN MARCOS SECTION 3308 3306 3304 3302 3300 3298 3296 586 588 590 592 594 596 596586 594 PLATE 6 EXPLANATION CSEE TABLE 3.3 IN TEXT FOR SUBCATEGORIES> A AGRICULTURAL USES CURRENT LAND USE u URBAN SITUS ACTIVITIES I URBAN INPUT ACTIVITIES PLATE 6 OF 10 0 URBAN OUTPUT ACTIVITIES N T TRANSPORTATION FACILITIES v VACANT LAND ENVIRONMENT AL GEOLOGY OF URBAN AND URBANIZING AREAS R RECREATIONAL ACTIVITIES p PUBLIC LAND 1 KM THOMAS W. GRIMSHAWc CEMETERIES 1 Ml w WATER BODIES SCALE DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN MAGNETIC DECLINATION 1976 KYLE SECTION SAN MARCOS SECTION PLATE 7 EXPLANATION SM SAN MARCOS CITY LIMITS LAND USE CONTROL TEXAS AT AU::illM ETJ SAN MARCOS EXTRATERRITORIAL JURISDICTION JUL 2 7 1976 MAPPING CREDITS PLATE 7 OF 10 GEOLOGY LI CRARY K KYLE CITY LIMITS 1. KYLE CITY LIMITS ENVIRONMENTAL GEOLOGY OF FROM THE CITY OF KYLE ER EDWARDS AQUIFER RECHARGE ZONE URBAN AND URBANIZING AREAS 2. SAN MARCOS CITY LIMITS AS RECOGNIZED BY THE TEXAS FROM THE CITY OF SAN MARCOS 1 KM WATER QUALITY BOARD THOMAS W. GRIMSHAW 3. EDWARDS RECHARGE ZONE 1 Ml SCALE FROM UNPUBLISHED MAPS ON FILE AT TEXAS WATER QUALITY BOARD. 1976 DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN MAGNETIC DECLINATION 1976 . DE"D BRANCH CRITERIA Available Area Volumetric Capaclty ---1 Potential Oepth Acceasibil ity from Established Roads l olstance from Solid Waste Sources Prevention of Endangering Prevent ion of l mpa<:t of Environ•ent Landfill Site ----Prevention of Degradation of Substrate Properties by Leachate ---- Preve ntion of Degradation Enviroruaental Public Health Considerations ---­ Prevention of Ga• Explosion Haurd Fire Prevention and Other Safety Considerations rotential for Disease and Vector Control Prevent ion of Air Pollution ~LCB Prevention of Air Pollution by Gas, ..-.GB ---­Prevention of Alr Pollution by Smoke Transport.at Ion Factors Pre11ention of Environmental SANITARY LANDFILL SlJlTA81LITY Conflict Prevention of Substrate Pollution by Gaa Preveation of Pollution of Substrate Prevention of Substrate Pollution Proxi•al to the Landfill by Leachate Pollution Prevention Potential Prevention of Substrate Pollution by Fill Material Directly Prevention of Water Pollution by 1'111 Material Olrectly Prevention of Water Pollution Prevention or Water Pollution by Landf11 l Gasea Prevention of Water Pollution by Leachate ' Ease of Excavatlon and Manipulation Relative Seriousness of Soil Pollution Prevention of Wind-blown Debris Prevention of Failure of Prevention of Poat-operation Maas Landfill Material Movement Prevention of Failure of Earth Material Prevention of Landfill Inundation by Ploods Prevention of P'ailure of Landfill Prevention of Post-Operation Materie.l Mass Movement Prevent ion of Pai.lure of Earth Material Prevention of Surface W1ulh1ng and Erosion Pre11entlon of Cas Prod1.1ction Composition of Gases Produced Prevention of Water Exposure to Gases Co111po1itio11 of Leachate Produced Potential for Leachate Renovation Prevent ton of Leachate Movement Prevention of Leachate Generation Accessibility of Surface Water to Landfill Gases Accessibility of Ground Water to Landfill Gases Leachate Filtration Capaclty of Substrate Contaminant Ad1:1orption Capacity of Substrl[e Prevention of Surface Water Pollution by Leachate Escape to Surface Ptevention of Ground Water Pollution by Leachate Seepage To Subsurface Prevention of Hobili:l:ation of Cont-inants Water Availability for Leachate Generation Substrate Engineering Properties Land Value Factors Acslhetks TEXAS AT AU:>TIM JUL 27 1976 LIGRARY GEOLOGY Ease of Vehicle and Equipment Hoveinent Initial Laltd Co11ts Post-operation Reclamation Value Presence of Offendees Offensive Factors Dry Weather Trafficability ----1Wet Weather Trafficability Proximity of Populat ion Center Wind-blown Debris Prevention Ugly Sile Prevention Prevention of Operational Noise Odor Pt;evention Potential L Temperature lncrea1e by Exothermic Temperature lncrease by Exothennlc Reactions in Landfill Reactions in Landfill " Prevention of Leachate Production by Water Applied for Compaction 0 Prevention of Submergence of Landfill by Ground Water Surface Water Abundance Surface Water Abundance "' (Regional Precipitation) (Regional Precipitation) Surface Water Accessibility Drainage Control Measures Drainage Control Measures to Landfill "' Rate of Surface Water Runoff Rate of Surface Watel' Runoff L_ A Potential (or lmpermcable. Cover 11a a Seal _..LCB t;ase o[ t:xcavatiun and Manipulation Dry Weather Tra[ficabillty 0 Wet Weather Trafficabillty lnltial Land Costs rX Poteutlal Land Value increase from Urbanization Pressure +, Engineering Charactcristks of C011ple ted landfill 0 .. lnitial Landform Final Landfo.-.. Proxillllty of Population Center J< Wind-blown Debris Prevention OG> Ugly Site PJ:'eventlon 0 Prevention of Opr-rationaJ Noise " Pre11ent1on of Odors Fl ll HateriaJ from "'"' Gas Produi;tlon Prevention Gas Production Prevention 0• Composition of Gas Produced Prevention of Gas Escape Gas Escape by Solution in Leachate ---­ Composition of Gas Produced Pre11enlion uf Gas Escape by Sol utlon in Leachate (.)~ s L_ rre11ention of Gas Escape intoGas Escape into Surroundlng Su\>strate Surrounding !:iuOstrate Gas Escape into Atmosphere --LCB Adequacy of Cover Vegetation Adequacy Of Cover Vegetation OQ Availability of Cover Material Availability of Cover Material Adequacy of Cover as a Seal L Co11.. r Thlclmess frequency of Covering Adequacy of Cover Compaction ---­---­ Cover Thickness r·requency o f Covering Adequacy of r.ovcr Cn,.p:iclion Q Q 0 Suitability of Cover Material for Sui tabii lty of Cover H:1terL:ll '°' Compact loo to Form a Seal ----Compaction to rorn e Seal THOMAS W. GRIMSHAW DEPARTMENT OF GEOLOGICAL SCIENCES THE UNIVERSITY OF TEXAS AT AUSTIN