Hydrogeology of the Lower Cretaceous 
Edwards and Trinity Group Formations 

Junction (Kimble County) Texas
near 
by 
Stephen Robert Allen, B.S. 
Thesis 
Presented to the Faculty of the Graduate School of 
The University of Texas at Austin 
in Partial Fulfillment 

of the Requirements for the Degree of 
Master of Arts 
THE UNIVERSITY OF TEXAS AT AUSTIN 
May, 1997 
Hydrogeology of the Lower Cretaceous 
Edwards and Trinity Group Formations 

Junction (Kimble County) Texas
near 
Acknowledgments 
I wish to express my appreciation to the many individuals who provided guidance and support to during this degree program. your own
me You have all in special way served as facilitators in the unfoldment of my potential. David Hill, Jun and Bruce
Liao, Darling checked my answers to homework problems during the coursework 
phase. Jim Mayer, Jack Sharp, Clark Wilson, and Norman Van Broekhoven helped to clarify field interpretations of structure and stratigraphy. Philip Bennett allowed me to use lab to
his ion chromatography perform analyses, and Todd Minehardt assisted by setting up the 
1C equipment and explaining the use of standards. Alan Fryar and lan Jones answered questions I had on interpreting chemistry results. Leo Lynch volunteered his help by x-raying sediment samples to determine clay content. Barry Hibbs worked with me to get the numerical model set 
up and calibrated, and Robert Mace discussed how he resolved problems he 
encountered while models. Erica checked the
using ground-water flow Boghici of borehole log cross-sections. was able to include color
accuracy my I graphics in the thesis because I learned to use several software packages under the guidance of 
Reuben Reyes and Koko Kishi. Clark Wilson, Joel Davidow, Richard Weiland, and Tor Steineke edited the thesis. The final thesis review was performed mainly by Barry Hibbs who directed the research on a day to day basis. Larry Land, Peter Rose, Rene Barker, Jack Sharp, and Keith Young evaluated the proposed thesis goals and scope of work before activities were started. My thanks go to the individual landowners in the Junction, Texas area who allowed me to collect data on their properties. I’m especially indebted to 0. C Fisher, former U. S. Congressman who allowed me to stay at his vacation house near London, Texas while 1 worked in the 
field. 
III 
Abstract 
Hydrogeology of the Lower Cretaceous Edwards and Trinity Group Formations near Junction (Kimble County) Texas 
by 
Stephen Robert Allen, M.A. 
The University of Texas at Austin, 1997 
Co-supervisors: Clark R. Wilson, Barry J. Hibbs 
This study describes ground-water flow in the Lower Cretaceous formations Texas. Rock
near Junction (Kimble County), exposures were examined throughout the 150-mile study area to determine the nature and distribution of permeable 
features. Dominant features include nearly vertical fractures and horizontal bedding 
planes in carbonate rocks of the Edwards Group formations (Edwards), and coarse-
grained fluvial channel deposits in the underlying Hensel Sand Formation (Hensei). Static water levels were measured in over one hundred wells and contoured to reveal 
the existence of two separate potentiometric surfaces, one overlying the other. 
but useful estimates of and
Preliminary, transmissivity, hydraulic conductivity, ground-water velocity were derived using specific capacities from eighty-three wells completed in both the upper (Edwards) and lower (Hensel) aquifers. 
At the edge of the Edwards Plateau where the contact between the 
Edwards and the Hensel is exposed on the face of the erosional escarpment, ground 
water discharges from the Edwards aquifer through numerous low volume springs 
IV 
leaks the Edwards aquifer to the underlying Hensel aquifer across a thin low permeability bed at the base of the Edwards which consists of marly, unfractured, nodular 
and seeps. An even greater proportion of ground-water discharge from 
limestone. 
The sum of these two components of discharge is approximately equal to precipitation recharge to the Edwards. 
To gain additional insight into cross-formationai flow an analysis of major and minor ions, redox potential, and dissolved oxygen was conducted for twenty-one water wells which were located along three north-south (inferred flow direction) transects. The Edwards waters were found to be a Ca-Mg-HC03 facies; the Hensel waters, a mixed facies. This difference in hydrochemical facies was initially thought 
to be caused by ion evolution along flowpaths, but it more likely reflects the existence of a regional aquifer below a locally constrained aquifer. High values of dissolved oxygen and redox potential in the Edwards aquifer indicate that recharge is predominant; lower values of these parameters in the Hensel aquifer indicate that this water occurs in an intermediate or discharge zone. 
To test the conceptual model of steady-state ground-water flow, a numerical model was constructed MODFLOW finite-difference
using the computer 
code. Over one hundred trial and simulations were executed to calculate
error 
leakage through the confining bed, discharge from springs, and discharge to the Llano River. In addition, the distribution and magnitude of focused recharge to the Edwards aquifer was calculated, estimates were made for unknown hydrogeologic parameters, and the Edwards aquifer was demonstrated to be fully perched above 
the Hensel aquifer. The increased understanding of the ground-water flow regime resulting from this study will support range management activities and improve the success rate of 
water well drilling. 
V 
Table of Contents 
Acknowledgments iii Abstract iv 
Table of Contents vi 
Introduction and Geology 
1 
Methods and Data 36 
Results of Field Observations and Water Chemistry Analyses 
43 
Conceptual Model of Ground-Water Flow 73 
Mathematical Model of Ground-Water Flow 81 
Conclusions 110 
Appendices 114 Appendix A Water chemistry sampling and analysis procedures 115 119
Appendix B Rainfall record Appendix C Descriptions of soil types 
121 
Table of water levels in wells 123
Appendix D Appendix E Specific capacity and transmissivity calculations 
128 
Appendix F Aquifer recharge calculation 132 Appendix G Table of water chemistry values 136 
Bibliography 145 Vita 155 
VI 
Introduction and Geology 
Research objectives and scope of work 
a
This study describes the ground-water flow regime in 390 square km (150­
square mile) area at the edge of the Edwards Plateau in Kimble County, Texas. 
Along this escarpment the Lower Cretaceous Fort Terrett and Segovia Formations 
of the Edwards Group (Edwards) and the underlying Hensel Sand Formation (Hensel) of the Trinity Group are exposed. The study resulted in the development of a 
conceptual model which describes recharge, discharge, head-dependent flux through a thin low-permeability bed between two aquifers, and flow within the saturated zone. 
controlled
The model explains how hydrogeologic inputs, throughputs, and outputs are by stratigraphy, structure, and topography. Fieldwork consisted, first, in measuring the water levels in 112 wells to 
produce potentiometric surface maps. Next, the water from twenty-one of the wells These two
was analyzed for major ions, minor ions, unstable indicator parameters. 
sources of data guided the mapping of recharge and discharge areas, and supported the hypothesis of two separate aquifers. The finite-difference 
program MODFLOW (McDonald and Harbaugh, 1984) was used to create a quasi three-dimensional, steady-state model for the purposes 
of visualizing the ground-water flow regime, testing the validity of assumptions used to construct the conceptual model, and verifying and adjusting estimated 
hydrogeologic parameter values. 
1 
Location and physiography 
The study area lies between north latitudes 30° 30’ and 30° 42’ 30”, and 
west latitudes 99° 35’ and 99° 50’, at the eastern edge of the Edwards Plateau in 
Kimble County, Texas (Figure 1). It is located 210 km (130 miles) west of Austin 
and 177 km (110 miles) northwest of San Antonio. Surface geology and major 
physiographic features are shown in Figure 2. The edge of the plateau is represented by erosional escarpment having average of approximately
an an relief 60 meters 
(200 feet) (Figures 3 and 4). This escarpment extends across the center of the 
Its existence has
study area and strongly influences the ground-water flow regime. 
the effect of separating the study area into two distinct physiographic regions: a highly dissected, horizontally bedded carbonate upland composed of the Lower Cretaceous Edwards (Figures 5 and 6); and, the Llano River floodplain, which is 
veneered with alluvium atop the siltstone, sandstone, and conglomerate of the Lower Cretaceous Hensel (Figure 7). Relief from 713 meters (2,340 feet) at the
ranges topographic divide to 475 meters (1,560 feet) at the lower (eastern) end of the 
Llano River. 
Significance of the study 
This study investigates the physical and chemical hydrogeology in the Junction, Texas area. It provides a framework from which to understand cross­
formational flow and the origin and distribution of solutes. The increased knowledge of aquifer quality, storage and capacity can also be used by landowners to exploit 
this limited resource more effectively. 
Physical hydrogeology 
Two separate potentiometric surfaces were drawn from water levels 
measured in wells that were located throughout the study area. These potentiometric will establish depths to the
surface maps, when overlayed by the topographic map, 
2 
Figure 1. Map of regional geology in central Texas. The study 
area is located in part, at the dissected edge of the Edwards Plateau; and in part, on the Llano River floodplain (modified from American Association of Petroleum Geologists, 1973). 
3 
Figure 2. Base map of the study area showing surface geology, 
watersheds, the Edwards Plateau escarpment, and the Llano River floodplain. The exact distribution of alluvium in the study area has not been established (modified from Barnes, 1981). 
4 
Figures. Topography in the study area. Elevations range from 475 meters (1,560 feet) at the eastern end of the Llano River to 713 meters (2,340 feet) at the highest topographic divide (modified from Barnes, 1981). 
5 

area study 
mile 

150-square 

the 
of 

Physiography 

4. 
Figure 
Figure 5. Boundary between the flat top of the Edwards Plateau and one of the many deeply incised tributaries that originate at 
the edge of the plateau. 
7 

ephemeral surface. 

Plateau. 
for 
upland 
flat

outlets


Edwards 
its 
as 
via
the 
serve 
of 



edgerocks Formations 
at 
the carbonate Group
head 
its in 

the Edwards
to 
close planes the valley 
bedding infiltrated 

tributary 


horizontal has 
that 
and




steep-walled rainwater 

A 
of
fractures 

6. 
Figure Vertical discharge 
the 

background
is 
along here 
in

floodplain the 
The (seen looms 
Texas. alluvium Plateau 
Junction, with Edwardsveneered the
near 
is of 
River 

which Llano 



escarpment
Formation

underfit 
erosional 
the Sand 
of The



HenselFloodplain river).
the bythe 
7. 
of 
Figure underlain banks 
saturated zone at potential drilling locations. In the past, drillers have 
not understood 
the nature of ground-water occurrence in this region. The relationship between relief and water-table depth in the highly dissected region at the edge of the Edwards has 
not been recognized. This has resulted in the drilling of non-productive wells in places where the saturated zone is thin or non-existent. 
Numerical flow simulation assisted in evaluating the distribution of recharge. Spatially variable recharge was assigned to the model grid, and the simulated water-table contours were compared to hand-drawn contours. The recharge pattern leading 
to the best match was used to calibrate the model and to derive estimates of rate 
and spatial variability of recharge. Observations of springs and seeps, and aerial photos of tree line patterns 
were used to identify areas of ground-water discharge. A comparison of present and historical tree 
coverage (as seen in photographs provided by landowners) shows that an increase in the number of mesquite and juniper trees since earlier in the century 
correlates with a reduction in ground-water discharge from Edwards aquifer springs. The spread of opportunistic vegetation over time (Figure 8), and the related loss of grass cover, are likely to be responsible for a reduction in aquifer recharge, and a concomitant reduction in spring discharge. This observation suggests it be of
may benefit to remove these species of trees to increase the throughput of water. A study by Dugas and Hicks (1994) in the Seco Creek watershed in Texas concluded that spring discharge increased after junipers were removed from a selected area. Chemical hydrogeology The analysis of major ions and indicator parameters (DO, pH, Eh, 
a water
temperature, and alkalinity as bicarbonate) in twenty-one wells establishes baseline. lon concentration data were used to determine the
quality spatial distribution of hydrochemical facies, and to map the locations of aquifer recharge and 
discharge zones. Variations of hydrochemical facies correlated mostly to transitions 
Juniper 

and 

Mesquite 

as 
such 
species 






Opportunistic 

8. 
Figure 
century. 

the 
in 
earlier since 

coverage 

spatial 
in 


increased 

have 
in lithofacies and, to a much lesser extent, to hydrochemical evolution along 
flowpaths. 
Environmental setting Land use and economics The of
economy the study area is based primarily on ranching, outdoor recreation, and interstate travel. Limited withdrawals are made from the Llano 
River to and
irrigate hay fields and pecan orchards near the river. Deer, turkey, numerous exotic species graze on private property where they are harvested by hunters who lease the land. Ground water contributes directly to the as
economy water supply and, indirectly, as a scenic feature, with springs and spring-fed streams contributing to the recreational value of the area. Climate and weather 
The study area is located in a subtropical subhumid climatic region characterized by hot humid summers and dry winters (Larkin and Bomar, 1983). 
Onshore flow of tropical maritime air from the Gulf of Mexico is the dominant control of climate. Intermittent seasonal intrusions of continental exert
regional air, secondary influences on the climate. Droughts and floods occur in Texas on a regular basis. Floods are caused by 
thunderstorms in the spring and tropical storms in late summer. Spring thunderstorms occur along the line of contact between cold fronts and overtopping warm, moist air from the Gulf of Mexico. In late summer, tropical cyclones originate 
in weather systems that have their beginnings in the Caribbean Sea or the Gulf of 
Mexico. Droughts are caused mainly by the extensive subtropical high pressure cell (the Bermuda High) that drifts latitudinally with the passing of the seasons. When the Bermuda High is entrenched over the southern United States the possibility of drought becomes more likely (U. S. Geological Survey, 1989). 
In the Junction, Texas area the average annual precipitation for the period 1939-1993 (Dunk, 1994; Appendix A) 61.34 cm
was 
(24.15 inches). Two graphs of 
precipitation are shown in Figure 9. One shows the average monthly precipitation for this period; and the other, the total rainfall for each year in the period of record. Monthly rainfall maxima occur in late spring and early fall; minima occur in midwinter 
and early summer (Dunk, 1994). The monthly gross lake evaporation rate during the period 1951-1980 ranged from an average of 6.60 cm (2.60 inches) during January to an average of 25.40 cm 
(10.0 inches) during August for an annual average of 177.80 cm (70.0 inches) (Larkin and 80mar,1983). Vegetation 
A variety of vegetation is present on the Edwards Plateau. Prairie grasses, live oak and Spanish oak, and "cedar" (scrub juniper) grow on the limestone upland and marly dissected zones. Lining the banks of the creeks and rivers are cypress trees. Terraces live and
support growths of post oak, juniper, elm, hackberry, cottonwood, sycamore, and willow. Natural grasses include little bluestem, Indian 
grass, sideoats grama, and Texas winter grass. Introduced grasses include coastal Bermuda, plains lovegrass, Klein grass, and King ranch bluestem (Cuyler, 1931). 
Trees, such as the juniper, are wasteful water compared to grasses
users 
because they release moisture to the atmosphere through their stomatas twenty-four hours a day. Some landowners report that where junipers have been removed, 
It
creeks and springs on their property have increased in discharge. is hypothesized 
that excessive growth of juniper and other opportunistic trees have caused a reduction in aquifer recharge, and a concomitant reduction in spring discharge. 
Figure 9. Histograms of rainfall in Junction, Texas from 1939 to 1991. The 
upper histogram shows average monthly rainfall; the lower, total annual rainfall (Dunk, 1994). 
14 
Soils Soils in the study area are divided into four types by the Soil Conservation Service (SCS) (Blum, 1982). This division is based on sets of physical properties that control land use. in soil
Mapped Figure 10, the types in the study area areTarrant, Tarrant-Real-Brackett, Nuvalde-Dev-Frio, and Menard-Hext-Latom. 
Detailed descriptions are included in Appendix C. During the initial phase of this study, the use of soil thicknesses and infiltration rates reported by the SCS was considered 
as 
an approach for estimating the spatial distribution of recharge because these factors are correlated with rate of recharge. This approach was discarded, however, because other factors such 
as 
relief, structure, and stratigraphy were found to be more important. In addition, Chock Woodruff (pers. comm., 1995) performed field experiments which led to the conclusion that soil thicknesses recorded by the SCS for the Edwards Plateau-Hill Country area were underestimated because they had been measured with handtools 
rather than backhoes. These underestimates of soil thicknesses could lead to underestimates of aquifer recharge because thinner soils would likely have a lower infiltration capacity that would result in higher runoff of rainfall. 
Geology 
The surface geology of the study area (Figure 2, Barnes, 1981), is organized by relative age in the stratigraphic column (Figure 11; Maclay and Land, 1984). The the
Edwards Plateau escarpment (Figure 12) exposes stratigraphy represented in Figures 2 and 11. upon a topographically rugged, pre-Cretaceous
These strata rest unconformity named as the Wichita Paleoplain (Hill, 1898). Strata were deposited in conjunction with the eustatic rise and northwestward transgression of the Lower Cretaceous Comanchean Sea. Referring to Figure 12, the lower strata is the Hensei Sand Formation (Hensei) of the Trinity Group, and the upper is the Fort Terrett 
Figure 10. Distribution of soil types in the study area (Blum, 1982). 
Figure 11. Generalized stratigraphic column in the Junction, Texas area (modified from Maclay and Land, 1984). 
12.
Figure Stratigraphy on the face of the erosionai escarpment of the Edwards Plateau. Exposed from bottom to top is the red-orange, friable siltstone at the top of the Hensel Sand Formation, the basal marly limestone member of the Fort Terrett Formation, and 
the fractured limestones and dolomites of the Fort Terrett 
Formation of the Edwards Group (Edwards). The Fort Terrett Formation and the overlying Segovia Formation of the Edwards Group are together referred to in this 
report as the Edwards except in cases where it is important to distinguish between the two. The aquifer occuring in both formations will be referred to as the Edwards 
aquifer. 
Hensel Sand Formation 

The Hensel, which is Upper Trinity in age, was deposited contemporaneously with the downdip (southeast and east of the study area) Glen Rose Formation. “This is the of three clastic-carbonate
depositional couplet youngest couplets, that
separated by disconformities, reflect a pattern of cyclic sedimentation 
superimposed on an overall transgressive regimen” (Stricklin and others, 1971). Each couplet generally onlaps rocks of the previous cycle and documents a major advance of the early Cretaceous sea terminated by an overall drop in sea level. “Episodic rejuvenation in the source area resulted in an increased supply of elastics 
detrital followed
and a consequent depositional phase, by relatively quiescent sedimentation of carbonate deposits, the latter phase in part contemporaneous with and transitional into the clastic phase” (Boone, 1968). 
During Jurassic time (60 million year period) most of west-central Texas 
was emergent and subject to erosion prior to inundation by the Comanchean Sea. 
Extensive northwest-southeast had been carved out of
trending river valleys that the Paleozoic surface, became filled with terrigenous Trinitian sediments to
age cause a substantial leveling of the ground surface. Eventually, these sediments 
formed a wedge which thickens downdip (toward the southeast). The Hensel 
sediments which fill these valleys were derived from erosion of the Paleozoic 
surface, and from erosion of Precambrian rocks that were exposed as a paleohigh in 
In the the thickness of the Hensel is
the Llano Uplift region. study area average approximately 60 meters (200 feet). 
cross-sectional view
An interpretation of the Hensel paleoenvironment and a 
of the of Lower is
sequence depositional events occuring during the Cretaceous Previous studies
depicted in Figure 13 (Payne, 1982). (Rapp, 1988; Payne, 1982; Barnes, 1981; Hall, and Turk, 1975; Amsbury, and others, 1974; Inden, 1974; and
Stricklin, others, 1971; Boone, 1968; Campbell, 1962; Hughes, 1948; Hill, and Vaughan, 1898) have established the Hensel to be a its
cemented conglomerate at base which fines upward to become a friable calcareous siltstone (Figure 12) at its 
top. The basal conglomerate is attributed to initial high stream gradients, and the 
fine-grained material at the top is attributed to the low stream energy that developed as the relief became leveled (Stricklin and others, 1971). Sedimentary features 
observed in the field such as thin beds of carbonate, cross-bedding, ripple marks, 
crevasse splays, oyster shell orientations, psuedoanticlines, and caliche have lead to arid to semi-arid The Hensel is
the interpretation of an fluvial paleoenvironment. 
described by Hall and Turk (1975) as a dip-oriented multilateral sandstone body 

characterized by two important facies: (1) a meanderbelt sandstone facies 
floodbasin facies
(lenticular coarse-grained framework deposits), and (2) a (finer­grained matrix deposits). architecture within the Hensel results in
The complex facies an equally 
of a that
complex pattern ground-water flow. An example of diagenetic feature contributes to this complexity is presented in Figure 14. This is a photograph of an with extensive nodular caliche that was deposited between individual grains
exposure in the sandstone by circulating ground water. This caliche reduces primary porosity 
and probably increases the tortuosity of ground-water flow. Edwards Group Formations 
During the Lower Cretaceous Period the study area was situated 
approximately 25° north of the equator within the belt of prevailing easterlies. 
at around 30° north resulted in dry adiabatic warming and
Descending air masses 
Figure 13. Fluvial paleoenvironment (upper diagram) and model (lower diagram) of Trinity Group deposition (from Payne, 1982). 
out 
oxidized 
precipitated
The 
has
Formation. that 
Sand carbonate)Hensel 
the 
(calcium

of 
part 
caliche middle with 
the 
in 
Outcrop impregnated 

is 
14. 
groundwater.
Figure silty-sand 
of 
low relative humidity (Rose, 1972). Rainfall, possibly associated with tropical storms was distinctly seasonal. This climate was conducive to the deposition of the 
carbonates and that formed the Edwards These
evaporites Group. sedimentary rocks were deposited in shallow water on the broad Comanche Shelf whose dominant feature the study area) was the Central Texas Platform, a wide
(in elongate positive surface that was leveled by deposition of terrigenous Hensel sediments that filled pronounced valleys on the Paleozoic (Wichita Paleoplain) surface (Figure 15). 
The Edwards is characterized rudist bioherms, carbonate and
by grainstones and down
mudstones, evaporites laid during three transgressive-regressive sequences under predominantly shallow marine conditions of relatively low wave and current energy (Rose, 1972). These relatively thin, nearly flat-lying strata (Figure 16) dip gently southeastward atop massive Paleozoic and Triassic units that dip westward. 
By early Fredricksburg time, an offshore bioherm of rudists, corals and carbonate deposits, named the Stuart City reef trend, had grown to an almost continuous ridge along the seaward edge of the continental shelf in the ancestral Gulf of Mexico basin (Figure 17) (Bebout and Loucks, 1974; Fisher and Rodda, 1969; Rose, 
from
1968). The rapid growth of this reef trend probably resulted a rapid rise in sea level that may have been triggered by an increase in the rate of seafloor spreading (Bay, 1977). This reef trend sheltered intertidal and restricted marine depositional environments on its leeward side (Comanche Shelf) from deep, open marine conditions in the ancestral Gulf of Mexico basin. During periods of especially low sea level and extreme aridity, the crest of the central Texas Platform became a broad, 
sahbka-type mudflat where evaporites, dolostone, and thin-bedded dolomitic limestone Sedimentation was controlled by several
were deposited (Fisher and Rodda, 1969). factors; climate, influx of terrigenous clastic sediment, distribution of tectonic subsidence and uplift, and energy level of wave and current action (Barker, and 
15. Tectonic framework of central Texas. Cretaceous
Figure 
age carbonate rocks of the Edwards Group were deposited 
in a broad shallow sea named the Comanche Shelf (from Rose, 1972). 
undisturbed and thickness,relatively 
lithology,

its 
environment.
Note 
texture,Group. 
the 
depositional
Edwards in 
the
transitions 

the in 
Vertical 

of adjustments
Formation Segovia horizontality. periodic 
the of reflect 
of degreebeds Outcrop high 
individual 
16. and 
of 
Figure nature color 
Figure 17. Lower Cretaceous paleoenvironment in central Texas (from Fisher and Rodda, 1969). 
others, 1994). The resulting tithofacies determined the stratigraphy, and together with the effects of subsequent tectonics and diagenesis, the hydraulic characteristics 
of rocks that today compose the Edwards aquifer area the Edwards two the
In the study Group is composed of formations, Fort Terrett and the overlying Segovia. Rose (1972) elevated the Edwards from 
formation to group status, the Fort Terrett and Segovia from member to formation 
status, and several informal members to member status. These rank changes were 
made because the Fort Terrett and Segovia each contain significant rock units that should be called members because of their thickness and vertical 
variability. 
as
Accordingly, the units comprising them rank formations constituting yet a larger unit, the Edwards Group. 
The base of the Edwards below the Fort Terrett is a
probably disconformity, the rock above being distinctly more marine. For purposes defining
of 
is drawn at
ground-water flowpath boundaries, the base of the Edwards the change upward from recessive argillaceous rock to resistant massive limestone. 
The Fort Terrett, which is approximately 58 meters (190 feet) thick in the Junction area, was deposited in environments ranging from open marine to intertidal, 
all with low wave and current Members composing the Fort Terrett are
energy. from bottom to top, the Basal Nodular Member, Burrowed Member, Dolomitic 
Member, and Kirschberg Evaporite Member. These are described by Rose (1972) as follows: 
Basal Nodular Member-A nodular marly zone up to eight meters 
Exogyra texana, lunatid and turritellid
(twenty-five feet) thick, rich in snails, and protocardid clams. Burrowed Member-Massive, resistant layers of porous, burrowed 
limestone approximately twenty-five meters (eighty feet) thick, thin beds of miliolid and mollusc-fragment biosparite and dolomite, deposited in a 
restricted to open shallow marine environment. This member is the chief 
water-bearing zone of the Edwards, its porosity caused in part by 
preferential leaching and removal of burrow-fillings to produce widespread honeycomb porosity. Dolomitic Member-Massive to thin-bedded, fine to medium crystalline, homogeneous dolomite, with abundant chert and beds of miliolids and rudistid biosparite, deposited in an intertidal to restricted shallow 
marine environment. 
Klrschberg Evaporite-Widespread, twelve to twenty meter (forty to eighty foot) thick disturbed and altered zone that marks the former 
horizon.
presence of a gypsum Massive to thin-bedded, cherty, crystalline 
limestone and travertine containing intervals of limestone, dolomite and chert fragment collapse breccia. Its depositional environment was restricted shallow marine, and in part sahbka-type supratidal and intertidal (Figure 17). 
The Segovia, which is approximately 35 meters (115 feet) thick in the Junction area, was deposited in environments ranging from marine to intertidal.
open It is composed of the Burt Ranch Member, Alien Ranch Breccia Member, Orr Ranch Bed Member, and Black Bed Member. In the Junction area the Orr Ranch Bed and 
does notthe Black Bed are eroded atop the Segovia, and the Allen Ranch Breccia 
extend this far north and west. Unlike the Fort Terrett, which is divided into four intergradational members, the Segovia is subdivided into members on the basis of thin, distinctive widespread key beds. The section in the Junction area is described by Rose (1972) as follows: 
Burt Ranch Member-Persistent and widespread zone of marly limestone, above the Kirschberg Evaporite Member having a thickness of approximately twenty meters (seventy feet). It includes marl, marly micrite, miliolid 
biosparite, and rudist biosparite with a few scattered beds of soft massive dolomite. Clayey or marly fossiliferous zones are particularly common 
near 
the base and toward the top and contain Exogyra texana, and turritellid 
snails. This member reflects distinctly more marine depositional conditions than the underlying Fort Terrett. This claim is supported by its rich and diverse mollusk and ammonite fauna which indicates deposition on a shallow 
shelf.
open Unnamed Member-Atop the Burt Ranch Member in the Junction area is an 
unnamed thin-bedded limestone biomicrite and micrite with isolated beds of dolomite about 180 meters (55 feet) thick. 
Deposition of carbonates continued during Washita time, but over most of the Plateau, as well as in the study area these sediments are now eroded. Toward the 
end of the Washita, in response to an upwarping of the Comanchean Shelf, there was a widespread regression of the shallow sea. The Washita Group strata was stripped away and the Edwards became exposed. Since becoming exposed, the Edwards has alteration such
undergone diagenetic by processes as fracturing, recrystallization, cementation, dissolution, and collapse of These
resistent beds. 
processes have produced variations in porosity and permeability both laterally and vertically, which have influenced aquifer heterogeneity. Alluvium and Colluvium 
area.
Several types of Quaternary surficial deposits are found in the study 
These are classified as either one of several types of alluvium or as colluvium (Barnes, 1981). Deposition of these sediments occured from the Pleistocene to the Recent, but their exact ages and modes of formation are not known because they 
have not been studied (Michael Blum; pers. comm., 1995). 
Close to the Llano River, near the level of the present floodplain, is a ten (33 foot) layer of (Figure 7) some places supports
meter thick alluvium that in 
extent. Three of alluvial
shallow, potable aquifers of limited types deposits were mapped by Barnes (1981) using aerial photos: (1) 
those that formed in Recent times 
terraces
along the sides of tributaries which are perpendicular to the Llano River, (2) 
deposited in Recent times which parallel the present course of the Llano River and, 
(3) remnants of high terraces flanking the edges of the underfit Llano River valley 
that are older than the low terraces. The distribution of alluvial deposits is represented in 
a on
generallized sense the map of surface geology (Figure 2). A photograph of a coarse-grained point bar deposit in a tributary that drains the dissected area next to the Edwards Plateau is 
shown in Figure 18. These rounded gravels which are composed of limestone, dolomite, and chert, were deposited during intense, short duration floods. The source of most of the gravel is probably colluvium that has accumulated at the bottom of terrace
steep slopes of the Edwards higher in the tributary reaches. Exposures of gravels throughout the area reveal that they are constructed of a mixture of well-
rounded chert and limestone A
gravel loosely bound by fine-grained travertine. present day example of fine-grained deposition within the interstices of gravel deposits is shown in Figure 19. This is a photograph of the bed of the Llano River 
showing how calcium carbonate (travertine) precipitates out of solution when carbon dioxide is lost from the river water. The calcium carbonate settles into the 
intergranular spaces of the limestone and chert gravel. The result is a tightly-bound alluvial deposit having a bimodal distribution of grains and perhaps a diminished infiltration capacity. 
Throughout the area, near the transition between the Edwards and the 
occur.
Hensel, widespread fanplains These surface deposits were referred to as 

point
across 
These
ran 
that 
tributary.
waters 
flood 
ephemeral
by 
deposited 

meandering

bar point a 
within 
Coarse-grained floodplain 

River 
18. Llano 
Figure 
the 
downward. 

cuts 
stream 
the as 
terraces 

form 
to 
time over 
up 
build bars 
Figure 19. Bed of the Llano River showing a fine-grained travertine (calcium carbonate) ooze building up between the pebbles and cobbles. 
Travertine is precipitated from the river water when carbon dioxide is lost to the atmosphere. 
32 
“washes” by Hill (1898). Apparently, they are deposited by overland runoff that contains an unsorted mixture of colluvium. 
Previous investigations 
Previous investigations are grouped into two categories: (1) local 
hydrogeologic and surface hydrology studies, and (2) hydrogeologic studies outside of the local area yet within the same physiographic province. Local hydrogeologic and surface hydrology studies 
The ground-water study that relates most substantially to the present one is 
by Alexander and Patman (1969). They “determined the occurrence, availability, dependability,quality, and quantity of the ground-water resources of the County” and provided a qualitative overview of hydrogeology and surface-water hydrology. 
A stream gauging study was completed by Holland and Mendieta (1965). From January 17 to January 24, 1962, the flow rates of the Llano River and its 
principal tributaries were measured at fifty-three points between Junction and Llano, Texas with a portable stream gauge. Holland and Mendieta believed that the flow of 
the Llano River and its tributaries was being sustained contact No
by springs. runoff-producing rains had occured for sixty-six days prior to the investigation. Based on this observation they determined that measured discharge values would 
low-flow conditions. It
quantify was hoped that their results would shed light on interaction to the
surface-water/ground-water support present study but, unfortunately, because the rate of discharge measured from one stream to
gauge the next did not differ by more than five percent, the data could not be used. Five percent is the margin of error for stream gauging with the portable Price current 
meter. 
Hydrogeologic studies outside of the local area vet within the same 
physiographic province 
A number of hydrogeological studies within the Edwards or Hensel in the 
Edwards Plateau/Hill Country region were completed outside the study area. These 
provided insight into the hydrologic regime of the study area, and contained data that were compared with those of this study. These data included transmissivities, hydraulic conductivities, thicknesses, formation thicknesses, spring outlet
saturated 
elevations, well yields, major ion values, and fracture orientations. Previous studies 
include those of: 
(1) Barker and others (1994) who provided an up-to-date overview of the 
hydrogeology of the Edwards-Trinity aquifer by condensing an array of previous studies. 
(2) 
Kuniansky and Holligan (1993) who presented the results of a digital model of the ground-water flow system in the Edwards-Trinity aquifer. They calculated water budgets for pre-development and post-development conditions, and found that ground water development reduced spring flow and leakage to streams. 

(3) 
The Hill Country Underground Water Conservation District (1994) who produced 


the "Gillespie County Regional Water Management Plan." 
(4) Bush and others (1993) who presented a three-sheet map series on the historical 
surface the These
potentiometric of Edwards-Trinity Aquifer System. maps 
portray regional water-level patterns over broad areas using the earliest available 
data at 1,789 well locations. 
(5) Ardis and Barker (1993) who created a two-sheet map series illustrating the 
saturated thickness of the Edwards-Trinity Aquifer System and selected contiguous 
hydraulically connected units in west-central Texas. 
(6) Barker and Ardis (1992) who reported on the configuration of the base of the 
Edwards-Trinity Aquifer System and hydrogeology of the underlying pre-Cretaceous 
rocks, west-central Texas. They reported that the Cretaceous aquifer system is underlain by an extensive complex of rocks ranging from Late Cambrian through Late Triassic in that are typically ten to one thousand times less
age permeable than those comprising the aquifer system. 
(7) Bluntzer (1992) who evaluated the ground-water resources of the Paleozoic and 
Cretaceous aquifers in the Hill Country of Central Texas. He claimed that the projected population increase between 1985 and 2010 would double the demand for 
water from 22,872 acre-feet/year to 47,380 acre-feet/year. 
(8) Abbott and Woodruff (1986) who edited a compendium of papers that addressed 
the geology, hydrology, ecology, and social development in Central Texas near the the is the
Balcones Escarpment. Particularly relevant to present study paper by the Edwards
Rose (1986) concerning the potential impact of pipeline oil spills upon Plateau Aquifer. He concluded that the aquifer is exceptionally vulnerable to pollution from pipeline oil spills because its fracture permeability would allow oil to sink into the bedrock before cleanup crews could respond. 
Ashworth who assessed the of the Lower
(9) (1983) ground-water availability Cretaceous formations in eleven counties in the Hill Country. 
Ashworth described 
the depositional environments, stratigraphy and structure of the Lower Cretaceous 
formations and how these factors control the hydrogeology. 
(10) Walker (1979) who reported on the occurance, availability, and chemical quality of ground water in the Edwards Plateau region of Texas. He defined five aquifers of 
different ages located in the twenty-eight county study area. 
Methods and Data 
Data used in this study came from three sources: existing records, field 
measurements, and laboratory analyses. Data collection and processing, and the methods used to generate new data are described. 
Existing records 
included Llano
Existing records utilized drillers’ reports, specific capacities, River discharge rates, and aerial photos. Water well inventory 
One hundred and twelve water wells were inventoried using data from two State of Texas record archives (Appendices D). Well location maps and drillers’ reports for “plotted” wells were obtained from the Texas Natural Resources Conservation Commission (TNRCC). Well location maps and hydrochemical analysis values for “located” wells were obtained from the Texas Water 
Development Board (TWDB). 
Plotted wells 
well locations
Maps of plotted wells show approximate on U. S. Geological Survey (USGS) 7.5-minute topographic maps, as reported to the TNRCC by individual drillers. These wells assigned approximate locations on Texas Department of Water well contain
Transportation county highway maps. reports may information on specific capacity, static water level, lithology, well diameter, and well 
and screen depths. Located wells 
Located wells are field located by TWDB technicians on USGS 7.5 minute 
In the of these wells
topographic maps. study area, many are easily spotted because use windmills for include information on static
they pumps. Well reports 
water levels, and field-measured values of specific conductance, and total iron 
(Alexander and Patman,l969). Reported iron analyses are ferric values. Ferric iron is the undissolved form of iron contributed by rusty well hardware. 
Specific capacity tests 
Some well reports submitted to the TNRCC included the results of well 
acceptance tests. These data were used to calculate specific capacity, which in 
turn was used to estimate transmissivity and hydraulic conductivity (Appendix E). By pumping for a period of time and noting little or no drawdown of the water table, the driller “accepts” the well for its intended use. The well acceptance test is conducted for a much shorter duration (0.5 to 2.0 hours) than the aquifer pump tests For this
performed by hydrogeologists to calculate hydrogeologic parameter values. study area, specific capacity calculations were the only source of data to make 
estimates of transmissivity and hydraulic conductivity. Fifty-seven specific capacity values from the Hensei aquifer and twenty-six from the Edwards aquifer were used to estimate transmissivity. Hydraulic was then calculated the
conductivity by dividing transmissivity by average saturated thickness of each aquifer. The formula used 
was 
the modified non-leaky artesian formula (Walton, 1962): 
Q/s = T/(26410g(Tt/2693r2S)-65.5) (eq. 1) 
where: 
2
Q/s = specific capacity (L /1) 
T = transmissivity (L2/t) 
S = storage coefficient [dimensionless] 
well radius (L)
r= 
t = time since pumping started (t) 
This equation relates the specific capacity of the well to the transmissivity of the 
aquifer. It assumes that the well is discharging at a constant rate in a homogeneous, isotropic, nonleaky, artesian aquifer, infinite in areal extent. Estimates of S (.01 to .0001) bracketed the range of probable actual values because the degree of confinement within the portion of the Hensei aquifer occuring below the Edwards 
was 
not known. 
There are three reasons why specific capacity calculations must be considered approximations. First, pumping periods in some wells may be too short; thus, the water table is not given an adequate amount of time to become depressed as required by the equation. Second, well reports do not indicate how soon after the cessation of pumping that the static water level was measured. A delay between the cessation of pumping and measurement of the water level could cause an 
erroneously small drawdown which would result in the calculation of an overly large value of specific capacity. Third, the calculated average values of specific capacity probably underestimate T because test results in cases where zero drawdown is 
observed (i.e. higher transmissivities) cannot be processed by the Walton equation to calculate 
transmissivity. Llano River discharge records/precipitation records 
Discharge rates recorded at USGS stream gauge # 08150000 (Junction) were used to estimate baseflow in the Llano River. Data from the months of December and January for the years 1939 to 1993 were used (U. S. Geological 
Survey, 1994, and Appendix F) because they were low-flow months. Surface runoff 
and evaporation during low-flow months are at a minimum; thus, the water in the river is derived almost from baseflow. These data were with
entirely coupled for the same months and for the
precipitation values (Dunk, 1994, and Appendix B) same period of record to produce an estimate of annual ground-water recharge in the The estimated
drainage basin upstream of the river gauge. recharge in cm/year was extrapolated to the study area and used as initial input to the numerical flow model. 
Low altitude aerial photographs Black and white aerial photographs of the study area were obtained from 
the Texas Natural Resources Information Service Distinct
(TNRIS). patterns of tree growth observed on the photos correlate strongly with stratigraphic features that control spring discharge from the Edwards aquifer. 
New data 
acquired in the field and laboratory 
New data acquired included ground-water level measurements, dissolved ion concentrations, unstable constituent values, and results of an x-ray analysis of sediment. 
Ground-water level measurements 
Static water levels were measured in 112 wells (Appendix D). Interpolated 
data points were then used to construct potentiometric surface maps. 
Measurements were made in wells equipped with windmills and submersible pumps, 
and in wells with no installed Before measurements were made, the
pump. pumps were turned off and the water table was allowed to recover. Depths to 
water were 
measured to the nearest .3 meter (one foot) with a chalked steel One foot
tape. was considered sufficiently accurate because topographic relief in the study 
area is 
about 230 meters (750 feet) and ground elevation control is accurate only to about three meters (ten feet) (one-half of a contour interval) on 7.5-minute USGS topographic maps. 
Measurements were taken during an eight month period as access to each 
property could be secured. Repeat measurements of a half-dozen control wells were 
made the and water-level values were to within
during study, replicated approximately one meter. To calculate the elevation of the potentiometric surface 
at each water well, the depth-to-water was subtracted from the ground surface elevation. Ground surface elevations were established from USGS 7.5-minute topographic maps, but where it was not possible to plot a well accurately on the 
an transit was used to from three
topographic map engineering triangulate topographic features to determine location. Sampling and analysis of ground-water chemistry 
Ground-water samples were collected from twenty-one wells. Seven 
samples were taken from the Edwards aquifer, thirteen from the Hensel aquifer, and one from the thin alluvial aquifer next to the Llano River. Collection and analysis followed the set of procedures listed in Appendix A and diagrammed in Figure 20. To summarize, wells pumped until temperature,
were 
dissolved oxygen, and pH achieved temporal stability. These indicator values were measured within a flowcell that isolated the aquifer water and the measuring probes from the atmosphere. Alkalinity was measured in the field by titrating with 1.5 N HCL to the 4.5 pH endpoint. Samples were collected in pre-cleaned polyvinyl chloride 
bottles and preserved for ion analysis in the laboratory. Anion concentrations were measured with the ion chromatograph (IC), and 
cation concentrations were measured using the inductively-coupled plasma spectrophotometer (ICR). Anion concentrations were determined using both the 
conductivity and the absorbance detectors and these calculations were then adjusted by the ratio of the !C measured value of a certified check standard to the 
manufacturer’s measured value. ionic charge balance to within 5.2 percent was achieved for each sample that 
HC0 N0 and N0
was analyzed for Ca, Mg, Sr, K, Li, Na, and Cl, F, S0 4 , 3, Br, 3 , 2 . An ionic charge balance error of less than five is believed to rule out
percent Several constituents not measured in the lab were measured in the
analytical error. 
field with a Milton Roy Mini-20 spectrophotometer. These included sulfide, dissolved 

Figure 20. Flowchart of procedures for the measurement of unstable constituents in the field, and collection of ground­water samples for major ion analysis in the laboratory. 
oxygen, total Fe, ferrous Fe, phosphate, and ammonia. Results of all water 
analyses are listed in Appendix G. X-ray diffraction analysis 
to classes
Samples of fine-grained Hensel strata were x-rayed identify of clay minerals. Clay mineralogy is an important factor in controlling the major ion of water in sedimentary deposits due to the high surface area available
chemistry for chemical exchange reactions. X-ray diffraction results are presented in the next 
chapter. 
Results of Field Observations and 
Water Chemistry Analyses 
Field observations 
Observations and measurements made in the field provide a basis for 
developing a conceptual hydrogeologic model of the study area. Field observations were made of topography, soils, streams, storm runoff, springs, vegetation, and rock outcrops. Measurements included static water levels, saturated thicknesses, major ion concentrations, and unstable constituent values. These measurements, and 
interpretations of existing data, were used to create maps of potentiometric surfaces, flowlines, recharge distribution, hydraulic conductivity distribution, redox These
potential, hydrochemical facies distribution, and locations of contact springs. results were used up the finite-difference model of ground-water flow.
to set 
Summaries of results follow. 
Topographic divide on top of the Edwards Plateau of the Edwards Plateau
The relatively flat-lying upland surface (Segovia Formation) is a geomorphically old terrain in comparison to the highly dissected 
zone 
near the edge of the Plateau. Pictured in Figure 21 is the topographic (and 
hydrologic) divide at the higher elevation boundary of the study area. Years of 
overgrazing on open rangelands stripped grasses, soil erosion, and soil compaction. This created the type of landscape locally referred to as a “goatscape.” Surface 
water runoff is probably higher now than it was in the past, even though focused 
is to be on the flat surfaces of the
recharge hypothesized prevalent plateau. Widespread shallow depression storage is observed to occur on this surface after 
storms, so it is likely that some of the trapped water percolates downward to 
at are estimated in the
recharge the Edwards aquifer. Recharge rates the plateau mathematical flow model chapter. 
is
Grazing This Group). erosion. (Edwards soil extensive 
to
Formation 
aquifer. Segovia contributed the unconfined
has 
of 
century upper
surface 
for 
area
Flat-lying mid-nineteenth the 
21. 
the 
recharge
Figure since the 
Fractures and bedding planes in the Edwards Group Formations Vertical fractures are observed throughout the Edwards Plateau (Figure 22). The dominant trend directions are approximately N4O°E and N4O°W. These match the dominant fracture trends inferred from lineaments mapped on aerial photos in the 
southern part of the Edwards Plateau (Wermund, and others, 1978). The outcrop in Figure 22 is typical for the Edwards Plateau; it shows the interconnectivity that exists between fractures and bedding planes which gives the Edwards an effective 
porosity of approximately one percent (Robert Mace; pers. comm., 1995). Secondary porosity forms along fractures and bedding planes, and develops in some horizontal beds by dissolution of burrow fillings and by karst breccia (Sharp, 1990; Maclay and 
Small, 1984; Rose, 1972). Primary porosity exists within grainstones in the form of Interflow and diffuse flow, move in an
intergranular pores. ground-water approximate stair-step fashion down fractures, then laterally within permeable beds and infiltration of rainwater is favored where fractures
along bedding planes, intersect the ground surface. Collapse breccia in the Kirschberg Evaporite Member 
A collapse breccia zone occurs at the top of the Fort Terrett Formation 
This breccia formed when resistant beds of limestone collapsed in upon
(Figure 23). 
a dissolution zone previously occupied by gypsum that had accumulated in the Lower 

Cretaceous Kirschberg Lagoon (Figure 17). After the gypsum was dissolved by circulating ground water, the less soluble overlying beds collapsed into the space vacated by the evaporites to form a breccia with a higher permeability (Peter Rose; This breccia allows water to infiltrate the formation at locations 
pers. comm., 1995). where it has become exposed by erosion. Locally, however, porosity and be reduced where voids have been filled with caliche and soil.
permeability may the
Because gypsum is completely dissolved in these zones, the hydrochemistry of Edwards aquifer does not reflect contact with gypsum. 
45 
Figure 22. Hydraulically connected fractures and horizontal bedding planes in the Edwards Group serve as conduits for flow in the unsaturated and saturated zones. A pocket knife at the bottom center of the photograph serves as scale. 
(Edwards Kirschberg 
the 
of
Formation 
Terrett dissolution 
Fort 
to 
the due beds. of here
top 
overlying
the exist 
at of 
occuring rainwater collapse 

of 
breccia 
infiltration consequent
collapse 
the the for and
of 
Zone Conduits horizon 
23. 
Figure Group).Evaporite 
Contact springs in the Edwards Group Formations 
to
In the foreground of Figure 24 is a concrete structure that serves capture intermittent spring flow from the Segovia Formation. The concrete structure fills with spring water, and then the water is piped downhill to a stock tank (Figure 25). The source of this water is a contact spring that discharges water from the foot of 
a colluvial slopewash deposit upslope of the collection structure (Figure 26). This small spring and numerous others occur where bedding planes and/or fractures 
The
intersect the sloping ground surface that truncates the ground-water flowpaths. total discharge from these springs is impossible to measure due to the small size of the their schedules, and physical
springs, übiquity, unpredictable discharge inaccessibly. Instead, an estimate of spring discharge as a percentage of recharge a finite difference flow
will be made later by assigning drains to a line of cells within model. Discharge from these springs has declined over the last several decades possibly due to the spread of juniper and mesquite trees which use large amounts of soil moisture The of trees observed in
(Figure 8). sparse coverage photographs taken earlier in the century, in conjunction with landowner interview reports of higher and more consistent spring discharge in the past, support the contention that spring 
time due to these trees. A similar conclusion was
discharge has declined over reached by Dugas and Flicks (1994) for the Seco Creek Watershed in Texas. They removed trees from an area in the watershed and measured spring discharge before and after the removal. Spring discharge increased after the trees were removed. 
48 
Figure 24. The concrete structure in the foreground was 
designed to capture and pipe spring water to a cattle reservoir. Reduced spring flow to this structure in recent decades, possibly 
due to the growth of phreatophytes, and the associated increased evapotranspiration, necessitated the drilling of a well. 
49 
Figure 25. Conceptual diagram showing how the limited volume of water exiting a contact spring in the Segovia Formation (Edwards Group) is captured to fill a livestock reservoir. 
Figure 26. Spring water exits the Segovia Formation (Edwards Group) onto a dry streambed from beneath a coiluviai slopewash deposit. 
51 
Aerial photographs of tree growth patterns on the sides of mesas Trees growing in distinct patterns are observed in aerial photographs (Figure 
27). This pattern is probably related to springs (ground water discharge) and seeps (interflow discharge) along fractures, bedding planes, and higher permeability beds exposed on the mesa slope faces (Figure 28). An example of a discharge point on the 
face of a tributary canyon is pictured in Figure 6. Note the staining beneath 
fractures and bedding planes that has been caused by precipitation of minerals from The heaviest concentrations of trees occur on the southeastern
discharging water. 
sides of the mesas because ground water flows along gently dipping bedding planes in 

that  direction.  Evaporation  from  the  ground  surface,  together  with  
evapotranspiration  from  these  trees,  constitutes  the  largest  sink  in  the  overall  
hydrologic cycle here.  
Exposure of Lower Cretaceous geological contacts  

The outcrop in Figure 12 shows exposures of the fractured Fort Terrett Formation at top, the unfractured Basal Nodular Member of the Fort Terrett in the middle, and the calcareous siltstone of the Hensel Sand Formation at the bottom. The Basal Nodular Member a more consistent density and lithology than
possesses the overlying Burrowed Member of the Fort Terrett Formation (Rose, 1972). 
It is the lowest occuring bed of several low permeability beds in the Edwards Group that lower value of vertical 
causes bedding anisotropy (low K /K ). An even anisotropy
zx 
would exist if vertical fractures were provide conduits for downward
not present to percolation. Llano River The Llano River is a tributary of the Little Colorado River (Figure 29). It 
receives water from two mam tributaries, the North Llano River and the South Llano The
River, which together drain an area of 4,789-square km. (1,849-square miles). Llano is a perennial, predominantly effluent river, fed by spring discharge from the 
area. study 
the 
of 
portion 
a 
of 
photograph 

aerial 
altitude 

Low 
27. 
Figure 
related 
is 
pattern 
this that 
believed 

is 
It 
growth. 

tree 
of 
pattern 
the 
Note 
rocks. 
flat-lying 

the 
in 
springs 
contact 

via 
water ground 
of 
discharge 

the 
to 
Figure 28. Conceptual diagram showing how phreatophytes grow in distinct rings around mesas of the Edwards Plateau to exploit contact springs that discharge from horizontal bedding planes. 
54 
River 
Colorado 

the 
to 
tributary 

This River. Llano 
The 
29. 
Figure 
system. 

groundwater 

the for 
area 
discharge 

primary 

the 
as 
serves 
Edwards aquifer, and baseflow seepage from the Hensel aquifer. It maintains an average daily flow during the low-flow months of December and January of approximately 233,800 cubic meters (8,256,072 cubic feet) at the Junction, IX 
stream located 
area
gauge, a short distance upstream of the study (Appendix F). 
The amount of baseflow seepage added to the Llano River as it flows past the study 
area is estimated in the mathematical model chapter. Potentiometric surfaces 
Static water levels measured in hundred water wells
were over one 
completed in both the Hensel and the Edwards aquifers (Figures 30 and 31, and Appendix D). Water table elevations were contoured and two separate potentiometric surfaces were defined (Figure 32). Several water wells, completed within both aquifers, were excluded from the dataset because their water levels 
represented a composite of the potential from each aquifer. Study objectives required development of maps and datasets for distinct aquifers. water tables were in the one for the Edwards
Separate mapped area; River
aquifer (under the Plateau), and one for the Hensel aquifer (beneath the Llano the Hensel
floodplain). It was not possible to measure the potential in the part of aquifer that occurs beneath the Edwards because there are no wells that penetrate 
the and from Hensel
the entire thickness of Edwards, draw the aquifer only. 
in of the Hensel had to be with the
Hydraulic heads this part aquifer predicted numerical flow model. Measured water table elevations in the Edwards aquifer ranged from 584 to 628 meters (1,918 to 2,060 feet). Water table elevations in the Hensel aquifer ranged from 463 to 529 meters (1,520 to 1,737 feet). Farther under the Plateau,
back 
toward the point occuring directly below the topographic divide, it is possible that the potential of the Hensel aquifer increases to an elevation equal to or exceeding that of the bed at the base of the Edwards, but not as high as heads in the
confining 
Figure 30. Locations and project numbers of water wells used to measure 
the depths to water for water table elevation calculations. Well specifics are listed in Appendix D. 
57 
Figure 31. Water table elevations calculated using steel tape These elevations
measurements of depth-to-water in wells. were used to draw potentiometric surfaces. 
58 
Potentiometric contours for the unconfined Edwards
Figure 32. 
Group aquifer and the unconfined portion of the Hensel Sand 
Formation aquifer. Contours are clashed where inadequate 
well control exists. 

59 
overlying Edwards aquifer, it is hypothesized, therefore, that there is leakage downward from the higher potential Edwards aquifer to the lower potential Hensel 
aquifer through this confining bed. This hypothesis is tested in the mathematical model chapter. 
The 
average hydraulic gradient in the Edwards aquifer is calculated to be 0.0045, and the average ground-water gradient in the Hensel aquifer is 0.0055. Saturated thickness averages approximately forty meters (130 feet) in the 
Edwards aquifer, and thirty meters (100 feet) in the Hensel aquifer. Using estimates of 0.0045 for hydraulic gradient, 3.5E-04 cm/sec (one foot/day) for hydraulic conductivity, and 0.01 for effective porosity, a gross estimate of ground-water 
=
velocity using the Darcy seepage velocity equation (v ki/n ) is (3.5E-04 cm/sec * 
e 
=
0.0045)/0.01 1.6E-04 cm/sec. (0.45 feet/day) in the Edwards aquifer. For the 
Hensel aquifer, velocity is estimated at (1.75E-03 cm/sec * 0.0055)/0.10 or 9.7E-05 cm/sec. (0.28 feet/day). 
Ground-water flowlines 
A map of approximate ground-water flowlines is presented in Figure 33. Flowlines were drawn orthogonal to potentiometric surface contours (Figure 32). the flow field in
Orthogonal flowlines accurately represent horizontally isotropic media. In this case, the Edwards is an anisotropic aquifer, and the Hensel is an anisotropic and heterogeneous aquifer. Therefore, the flowlines are approximations. 
Flowlines Edwards aquifer converge the steep tributary valleys
in the at 
incised in the Edwards. Flowlines in the Hensel converge at the Llano River. Plotted 
on the map are the approximate locations for several strong springs that discharge from Edwards aquifer.
the Note that these occur where there is a dense 
of flowlines.
convergence 
60 
Figure 33. Generalized ground-water flowlines within the 
Edwards Group aquifer and the Hensel Sand Formation aquifer. 
Flowlines diverge from recharge areas and converge toward discharge areas such as the three mapped springs. 
61 
Water chemistry analyses 
Twenty-two wells were selected (Figure 34) for analyses of major ions (Ca, Mg, Na, K, Cl, and S0 ), unstable parameters (pH, Eh, dissolved 0 temperature,
4 2, 
and alkalinity as HC03), and additional constituents related to REDOX state, flowpath interpretation, and anthropogenic influences (Fe N0 NH, IDS, hardness,
2, 3, 
and total sulfides). Wells tested included seven wells in the Edwards aquifer, thirteen 
welts in the Hensel aquifer, and one well in the shallow alluvial aquifer next to the 
Llano River. Water chemistry analyses were performed to test for evolution of ion concentrations along inferred pathways of ground-water flow, to determine if separate aquifers existed based on the measurement of distinct hydrochemical facies, and to support the mapping of recharge and discharge areas. 
Field and laboratory procedures used to collect and analyze water samples are listed in Appendix A. The table of analytical values is presented in Appendix G. The error in charge balance between anions and cations for each sample ranged from 0.7 to 5.2 %. A charge balance of 5.0 % percent is considered an acceptable 
margin of error so the data was judged sufficiently accurate. One of the important purposes of chemistry analysis in this study
water 
was to detect evolution of major ion and unstable parameter concentrations in wells 
believed to be located along lines parallel to ground-water flowpaths (Back, 1966; Chebotarev, 1955). Chebotarev explained that ground water tends to evolve He observed that this evolution was
chemically toward the composition of seawater. 
normally accompanied by the following regional change in dominant anion species: 

travel along flowpath (increasing age) . 
—.
HCO~—*HCOSO]~—. SOj~+ HCO ~ —.so;"+ Cl~—*Cl~+ S0 2 CT 
4 
Figure 34. Locations of wells used to sample ground water for major ions, selected minor ions, and unstable constituents. 
63 
Wells located along three transects extending from the upland area to the 
center 
of the floodplain were included in the analysis (Figure 34). These transects were: 1) wells 1-2-3-(inaccessible zone)-15-16-18, 2 wells 4-7-10-14-19, wells S-6-8-9­11-12-13. 
it was determined that the hypothesis of ion evolution could be tested here in a limited way only because the flowpaths were too short, and because the abrupt change in hydrochemical facies occuring at 
the transition from the Edwards to the 
Hensel wells hides the more subtle changes that could be attributed to anion evolution. 
A marked contrast in 
water chemistry between the two aquifers is depicted graphically in the Piper diagram which displays the relative of major
concentrations ions (Figure 35, Piper, 1953). Two distinct hydrochemical facies are apparent; the facies of the Edwards aquifer, and the mixed facies of the Hensel
Ca-Mg-HC03 aquifer. Some evolution apparently occurs within the Hensel; this can be seen in the cation the the vector Hensel
triangle of Piper diagram. Along representing the increases in Na concentration from the edge of the floodplain to the
aquifer, water center of the floodplain. This is probably caused by cation exchange of Na ions for 
silt at
Ca ions on smectite clays. Samples of the fine-grained calcareous the top of the Hensel were x-rayed to test this hypothesis (Figure 36). It was determined that 
does exist
smectite, with its high cation exchange capacity, in these samples (Leo evolution occurs as a result of this
Lynch; pers. comm., 1995), thus, probably to is observed that not only is there a high Na
phenomenon. Referring Figure 37 it concentration in the Hensel aquifer Edwards, high Cl
relative to the but also a 
concentration. High Cl is most likely caused by the dissolution of halite. Likewise, the 
halite dissolution. The
high measured Na concentrations are probably due in part to 
Hensel semi-arid to arid, near-shore environment where sahbka 
was deposited in a conditions existed periodically (Stricklin, and others, 1971). It is probable that Na, Mg, Cl, and S0 in the Hensel aquifer (Figures 37 and 38) are derived from the 
4 
64 
Figure 35. Piper diagram illustrating the relative concentrations of major ions measured in ground water from twenty-one wells in the Edwards aquifer and the Hensel aquifer. 
65 
Figure 36. X-ray diffraction patterns for samples of red siltstone (top) and siltstone (bottom) from an outcrop
green at the top of the Hensel Sand Formation. 
66 
Figure 37. Concentration ranges and median values of major ions in the Edwards and Hensel aquifers. Hensel water samples have higher IDS. 
67 
Figure 38. Geology map with Stiff diagrams that represent the spatial distribution of absolute ion values and IDS. The higher the IDS, the larger the size of the diagram. 
68 
dissolution of disseminated veins of evaporitic minerals within low permeability, fine-grained, overbank deposits that have withstood flushing. 
Elevated concentrations of 
the ions Mg, Ca, and HC0 exist in both aquifers as a result of dissolution of 
3 
carbonate rocks by ground water 
that increases in acidity as it passes through the soil zone and dissolves C0 Drillers’ indicate that thin 
.
2 logs and geologic exposures 
layers of carbonate and disseminated caliche exist in the Hensel and yield Mg, Ca, 
and HC0 ions to the ground water dissolution. The
upon comparatively higher
3 
concentrations of HC0 
3 in the Hensel wells could be caused by bacterially-mediated reduction of sulfate which yields hydrogen sulfide gas and HC03 Hydrogen sulfide 
. 
gas occurs commonly in Hensel wells and can be detected by smell at extremely low 
concentrations. 
The distribution of electrochemical and dissolved
(Eh) potentials oxygen concentrations are The both
mapped in Figure 39. relatively high magnitude of parameters in Edwards wells, coupled with the highest hydraulic heads in the study area, demonstrate that this is a zone of recharge. The ground water is probably younger and the aquifer more open to the atmosphere in the Edwards. The Hensel waters reflect reducing conditions; both dissolved oxygen and Eh values are lower. 
a
This water is probably older and the TDS higher as result of longer ground-water residence times (Figure 38). Ground-water samples in Hensel wells have an iron 
result of the reduction of iron hydroxide and
taste and a hydrogen sulfide smell as a 
sulfate by bacteria in the presence of organic matter. 

An exception to this overall pattern is observed at the northeastern end of the floodplain (Figures 38 and 39). Here, the dissolved oxygen is intermediate and the Eh is contours exist here, but
potential high. Closely-spaced potentiometric 
that
without the higher TDS that might be expected in water is moving through less As it turns Hensel thins in this area as it gets closer to
permeable media. out, the 
Figure 39. Spatial distribution of dissolved oxygen and Eh. High 
values of both parameters indicate an oxidizing environment; low values, a reducing environment. 
70 
the Llano Uplift, and there are beds of carbonate rock (recorded in drillers’ logs) that may serve to slow ground-water flow. At the southwestern end of the floodplain the Hensel is thicker, there is more rock (recorded in drillers’
fine-grained sedimentary logs) with disseminated minerals available for dissolution, more coarse-grained
and 
fluvial channels to support rapid throughput of ground water. Analytical results are evolved
most easily explained by assuming that ground-water chemistry by cross-bed flow that led to mixing (Figure 40). 
Figure 40. A contrast in potentiometric surface contour densities, and anomalous TDS concentrations, occur in the ground waters from one end of the Llano River floodplain to the other due to a 
change in lithologic facies. Pictured is the mechanism believed responsible for the co-occurence of high K and high TDS at the 
southwest end of the floodplain. 
72 
Conceptual Model of Ground-Water Flow 
Physical hydrogeology 
is
Figure 41 a conceptual hydrogeologic cross-section that illustrates the 
architecture, and the inputs (red), throughputs (green), and outputs (blue) of the ground-water flow system. It is not determinable whether the Edwards aquifer is 
partially or fully perched because there are no water level data for the portion of the Hensel aquifer that underlies the plateau. Numerical simulations are run to 
attempt to answer this question. A revised conceptual cross-section of flow based on numerical simulation results is presented in the mathematical model chapter. Rainfall infiltrates portions of the exposed surfaces of the Edwards and Hensel to recharge the aquifers within these formations. The amount of rainfall 
at
recharging the aquifers is calculated approximately 5.06 percent or 3.10 cm/year of mean annual B and The
(1.22 	inches/year) precipitation (Appendices F). overland
remaining rainfall runs off, evaporates, or transpires. Runoff occurs as flow and as channel flow within ephemeral tributaries that meander toward the Llano 
River. 	and from
The largest portion of the rainfall evaporates from the soil surface trees and plants as they transpire. 
Rainwater that infiltrates the surface of the Edwards downward in
moves 
the unsaturated zone via vertical fractures and zones of collapse breccia, 	and then 
horizontally along bedding planes and permeable beds, until being intercepted by other vertical fractures. Movement therefore, occurs generally in a stair step pattern in the downgradient direction. Where structural or stratigraphic pathways intersect the 
water exits 	If
escarpment surface in tributary valleys, there in the form of 	seeps. 
rainwater infiltrates the flat top of the plateau surface close to the edge of a mesa (thus having a short flowpath) it may be short-circuited (as interflow) to one of these seepage points. The infiltrating rainwater that does not seep out of the 
73 
Conceptual cross-section of the study area
Figure 41. 
illustrating the architecture, and inputs (red), throughputs 
(green), and outputs (blue) of the ground-water flow system. 

74 
system at these points continues to move downward until it recharges the unconfined aquifer. Rainwater that infiltrates the surface at the topographic divides between subwatersheds the It is that these
probably reaches aquifer as recharge. likely summits serve as areas of focused recharge to the aquifer because beneath the 
divides the distance infiltrating water has to travel to reach the underlying saturated zone is less than the distance it would have to travel to arrive at the face of the erosional escarpment to be discharged as seeps. This hypothesis of focused recharge is verified using the numerical model of flow. Results are shown in the 
mathematical model chapter. 
Water that does reach the saturated zone of the Edwards probably moves 
as laminar flow from the carbonate upland toward the Llano River which meanders 
along the axis of a wide floodplain. There are no large springs in the area which would existence of conduits which would
suggest the large subsurface support turbulent flow. Moving laterally along horizontal bedding planes, or downward within 
vertical fractures, some ground water discharges at the face of the escarpment in contact or fracture springs that occur atop low permeability beds. Some ground water is discharged by phreatophytes, and the remaining ground water recharges the underlying Hensei aquifer by cross-formational flow. Spring discharge and cross­
formational flow are quantified in the numerical model. 
Downward leakage of ground water to the Hensei aquifer occurs through a fifteen foot thick marly limestone bed (Basal Nodular Member) which is present The Hensei Sand Formation
throughout the region forming the base of the Edwards. forms a sediment wedge that is approximately 20 meters (66 feet) thick below the and increases to more than 300
main west-east topographic divide in the study area, 
meters (1000 feet) in thickness southeast of the Plateau region (Cartwright, 1932). 
As the saturated thickness of the Hensei aquifer decreases toward the 

a result, northwest until the formation pinches out beyond the study area. 
75 
It was not possible to estimate precisely the saturated thickness of the 
Edwards because wells do not the saturated and
aquifer fully penetrate zone, because the thickness of the host formations is variable. A of 20 to 64 meters
range (66 to 210 feet) was calculated by subtracting the average elevation of the flat-lying Edwards-Hensel contact from water-table elevations measured in the Edwards 
aquifer wells. Ground-water flow in the Hensel aquifer at the western end of the floodplain 
occurs by preferential movement along relatively high permeability sand and gravel fluvial channel deposits encased within lower overbank and sahbka
permeability deposits (John Ashworth, pers. comm., 1994). Ground water exits the Hensel aquifer by seeping into the Llano River which operates as the regional base level across 
which no ground water can flow. A small part of its flow is captured by low 
livestock and domestic wells which occur at a density of less than
capacity one/square mile. It is not a volume of the Hensel water moves
likely that great ground downward into the Paleozoic rocks because the Paleozoics are less permeable than 
the Cretaceous rocks; and they host aquifers which are highly compartmentalized (Barker and Ardis, 1992). Gamma-neutron logs from oil tests fifty miles to the 
The cross-section of these
southwest provide independent evidence for this claim. below the Cretaceous below the Wichita
logs demonstrated that (i.e., Paleoplain 
most of the 150 meters of
Unconformity), the log response over upper (500 feet) Paleozoic rocks surpasses the magnitude established as the “shale line.” Therefore, 
the permeability of the Paleozoic formations is probably low. Contrasting this, the within the overlying Cretaceous rocks is
magnitude of the gamma-neutron response 
uniformly lower than the “shale line” value (Erica Boghici, pers. comm., 1995). The saturated thickness of the unconfined portion of the Hensel aquifer could 
not be determined exactly. A range of 15 to 70 meters (50 to 225 feet) was 
76 
calculated by subtracting elevations of the Hensel/Paleozoic contact from 
elevations of the water table. 
Chemical hydrogeology 
Figure 42 is a conceptual cross-section of the study area which shows the dominant hydrochemical species found in each aquifer and the hydrochemical 
mechanisms believed responsible for their occurance. 
in the Edwards, water infiltrating the surface dissolves carbon dioxide in the soil zone to reduce the pH of the water that recharges the aquifer. This ground water then slowly dissolves the limestones and dolomites of the Edwards to yield low concentrations of dissolved Ca, Mg, HC0 and total dissolved solids (TDS, 279 to 
3, 
418 mg/l). High dissolved oxygen (DO) (6.0 to 8.6%) and high electrochemical potential (Eh) (328 to 488 mV), in conjunction with the highest hydraulic heads in the study area, demonstrate that this is a recharge zone (see Figures 32 and 39). In the Hensel aquifer, low concentrations of dissolved Ca, Mg, and HC0 are 
3 
attributed to the dissolution of the carbonate beds and caliche that can be seen in 
and cross-formational transfer of Edwards waters. Elevated
outcrops, by concentrations of Cl are derived from dissolution of halite that accumulated in nearshore Cretaceous sahbkas (Stricklin and others, 1971). Elevated concentrations of Na are caused by dissolution of halite, and also by ion exchange. Smectite clays 
are found throughout the Hensel (see diffractogram, Figure 36). These clays possess a high cation exchange capacity, and are probably exchanging bound Na for Ca and Mg in solution. Dissolution of plagioclase feldspar is a possible source of smaller amounts of Na. High concentrations of SC are caused by dissolution of 
4 
the absence of associated high concentrations of Ca is probably due to loss
gypsum; by adsorption. 
77 
42. Schematic diagram of the gross mechanisms believed to control the occurances and relative 
concentrations of major ions in the Edwards and Hensel aquifers. 
78 
The Hensel aquifer is more isolated from the oxidizing influence of the 
atmosphere than the Edwards aquifer. The iron taste and hydrogen sulfide smell observed in many of the wells, coupled with the low measured concentrations of DO 
to andEh to228 of
(1.1 2.1%) (99 mV), support the interpretation a reducing environment. sulfate and being reduced
Apparently, iron hydroxide are during the bacterially mediated oxidation matter. These reactions
of organic yield hydrogen sulfide, ferric iron, and bicarbonate to the ground water. 
Elevated concentrations of TDS (404 to 1603 mg/l) are probably caused by 
contact with disseminated evaporites, and possibly by a longer ground water residence time in the Hensel compared to the Edwards. Measured concentrations of DO, TDS, and Eh may underestimate actual reducing conditions in the aquifer. This is because the annular spaces around wells over the entire length of the
are open 
wells. As a result ground water in reduced zones will mix with and become diluted by 
water in zones that are more oxidized. 
the
In the Hensel aquifer the distribution of hydrochemical facies is related to architecture and mineralogy of the rocks through which the ground water flows 
The higher average value of TDS measured in ground water from wells
(Figure 38). 
at the west end of due to water contact with
the floodplain is probably ground overbank clays as it flows preferentially through sinuous sand and gravel deposits that are juxtaposed against these clays (Figure 40). This interpretation is supported by lithologic log data which indicates the existence of these deposits. It is supported 
also by potentiometric contours which are widely spaced. This wide spacing sands and
indicates that flow is occuring through the more highly permeable fluvial gravels recorded in drillers’ logs. portion of the aquifer at end of the
The the east 
with a lower TDS, would be expected to have
floodplain, where wells yield water 
lower density potentiometric high hydraulic conductivity.

contours indicative of 
Drillers’
However, unexpectedly, the contours are tightly spaced (Figure 40). logs 
79 
show that there is a lithologic facies change to sand and silt with interbedded carbonates, thus there is a west to east transition to a more marine 
paleoenvironment. During calibration of the numerical model, the process of estimating the spatial distribution of K in the Hensel to produce a potentiometric contour match is very important. 
80 
Mathematical Model of Ground-Water Flow 
Introduction 
For this study a deterministic numerical model used to simulate ground-
was 
water flow in the Edwards and Hensel aquifers. The model simulates flow in a 
regional system utilizing a set of fundamental approximations. First, the porous medium is assumed to be a continuum that real of
replaces the complex system solids and voids. 
Second, ground-water flow is assumed horizontal because the ratio of aquifer thickness to horizontal length is small. Third, Darcy’s Law is assumed. 
This equation is: 
-
+ + w=0
o>/<fy|-Kyydh/<fy j
(eq. 2) 
where: 
and z are cartesian coordinates
x, y, axes of the flow system, KKK
with hydraulic conductivity tensors (L/t)
xx, , zz
yy 
h is hydraulic head (L) 
w is the volumetric flux per unit volume representing 
sources and sinks (L3/1) 

Finite-difference ground-water flow simulation 
U. S. Geological Survey modular flow model (MODFLOW 
The computer code used for the analysis was the U. S. Geological Survey Modular Flow Model (MODFLOW) (McDonald and Harbaugh, 1988). MODFLOW 
simulates ground-water flow through porous media using a finite-difference 
81 
approximation to solve the steady-state or transient ground-water flow equation at iterative
discrete and regularly arranged points (nodes) by an algorithm. The region being modeled is divided into a grid of cells with a node at the center of each cell, and each cell is defined to be either no-flow, variable-head, or constant-head. The model calculates a value for hydraulic head at the nodes of all variable-head cells, whereas head at constant-head cells is specified by the user. Cells are designated as no-flow ceils if they contain impermeable material or are unsaturated. MODFLOW can 
simulate flow in two or three dimensions, under confined or unconfined conditions, in have
formations that are heterogeneous and uniformly anisotropic, and in those that irregular boundaries. Special notation is used to describe the position of nodes in finite-difference 
so can be addressed when the finite-difference is solved
grids they equation The method of solution is to make an initial of the
iteratively (Figure 43). guess value of head for each of the nodes in the mesh. Based on these head values, the 
finite-difference equation is solved for each node by using the values at the 
surrounding four nodes: 
22 + -2h..+h.. -R/T
=
(h. ~-2h..+h. . .)/(Ax) (h.. ,)/(Ay)
v 1-1 ,j I.J i+l ,J i.J-1 i.J i.J+l 

(eq. 3) 
where: 
h 
is hydraulic head (L) 
R is used to simulate both distributed and point 
sources/sinks (L/t) 

T is transmissivity (L2/1) 
This each node in the finite difference mesh using an initial
equation is solved for estimated value at the node. Once the heads at each node have been recomputed, 
82 
Notation for the finite-difference grid. Spacing between
Figure 43. 
nodes is 506 meters. 

the difference between the initial value and the recomputed value is calculated. This process is repeated until the maximum difference in head values from one iteration to the next is less than a criterion. When the solution has
preset convergence The solution
converged, the equation has been solved. is necessarily approximate because there is some finite value to the criterion.
convergence 
Numerical model architecture, grid orientation, and boundary conditions model constructed
A two layer, quasi three-dimensional, steady-state was 
for this study (Figure 44). Layer one represents the Fort Terrett and Segovia Formations (Edwards Group) which hold the Edwards aquifer. Layer two represents the Hensel Formation which holds the Hensel aquifer. It is hypothesized that the thin 
basal marly limestone (Basal Nodular Member) of the Edwards serves as a confining bed that leaks water from the Edwards aquifer to the underlying Hensel aquifer. so thin compared to the two layers it separates, it is
Because this confining bed is simulated as a conductance value rather than as an individual layer. This type of model is referred to as quasi three-dimensional rather than fully three dimensional. 
across the
This layer functions as a head dependent flux boundary because the flux bed is a function, in part, of the difference in head between the two aquifers 
(Anderson and Woessner, 1992). Confining layers are not explicitly represented in quasi three-dimensional models, nor are heads in confining beds calculated. Quasi three-dimensional models are preferred over fully three-dimensional models when the difference in hydraulic conductivity between the confining bed and the aquifer host 
rocks is at least two orders of magnitude (Anderson and Woessner, 1992). 
Finite difference grids for layer one (Figure 45) and layer (Figure 46)
two 
consist of 56 rows and 55 columns to produce a total of 3,080 model blocks (cells), 
1,456 of which are active. Model blocks are square with 506 meter (1,660-foot) sides. 
Because MODFLOW is a block-centered model, the values of hydrogeologic 
84 
Figure 44. Idealized block diagram of the finite difference grid and associated boundary conditions for the numerical model of 
ground-water flow. 
Figure 45. Finite-difference grid for layer one -Segovia and Fort 
Terrett Formations (Edwards Group), showing no-flow boundaries 

and drain cells. 
86 
Figure 46. Finite-difference grid for layer two -Hensel Sand Formation showing river cells. 
parameters and calculated state variables, are assigned to the center points (nodes) of each block. Columns of cells were aligned parallel to the major direction of inferred ground-water flow which is down the topographic slope. Also, columns and rows of cells were aligned subparallel to air photo lineaments which are the surface of fractures in Edwards
expressions orthogonal found the Group formations throughout the Edwards Plateau (Figure 47, Wermund, and others, 1978). The upper boundary of the model is a free surface boundary. Its physical 
expression is the water table in the Edwards aquifer (layer one), and the water table in the Hensel aquifer (layer two). The procedure for assigning starting heads to the variable head nodes is to the finite-difference hand-drawn
lay grid atop of the water and then transfer
potentiometric contours tables, interpolated head values to the model grid nodes. The base of the aquifer system occurs at the contact between the Hensel and the underlying Paleozoic strata (Wichita Paleoplain). It was treated as a no-flow 
boundary (a specified flux boundary with a flux of zero). The configuration of the base was entered into the model by transferring interpolated elevation values from 
a 
structure map of this surface (Barker and Ardis, 1992) to the model grid nodes. 
A value of elevation was used for the Edwards/Hensel contact
single because the contact is relatively horizontal throughout the Edwards Plateau (Rose, 1972). This value was calculated by averaging the elevation of ten contacts 
observed on the face of escarpments circumscribing the study area. The elevations were read from USGS 7.5-minute topographic maps and found to vary from 1,820 to 
was
1,860 feet so average elevation of 1,840 feet assigned to all grid nodes to
an 
represent this contact. Areas of greatest ground surface elevation coincide with both surface-water 
divides and ground-water divides. Ground-water divides on the Edwards Plateau 
Figure 47. Orientation of the finite-difference grid. Columns are aligned parallel to the topographic slope. Columns and rows are aligned sub-parallel to lineaments mapped in the southern portion of the Edwards Plateau (Wermund, and others, 1978). 
function as no-flow boundaries along the northwestern end of the study area. 
The Llano River, which cuts into the top of the Hensel aquifer in the Llano 
River valley, is simulated as a head-dependent flux boundary. At this boundary the rate of of water between river is the
seepage the aquifer and the dependent on 
difference in hydraulic head between the aquifer and the river, the permeability of the river bed, and the surface area of the river bed. 
Hydrogeologic parameter values -starting estimates for model input 
In moving from the microscopic to the macroscopic level of describing ground­water flow, several coefficients are employed. Ideally, these are obtained from field 
and laboratory measurements; but in cases where these methods cannot be used, 
parameter values are estimated using literature values. Another approach is to use 
numerical simulation to back-caiculate values. 
Hydraulic conductivity 
Hydraulic conductivity (K) is the coefficient of proportionality describing the rate at which water can move through a permeable medium along a sloping hydraulic gradient. For this study, initial K estimates were made by analyzing specific 
capacity tests using Equation 1 (Walton, 1962). Twenty-six specific capacity values were used in
from Edwards aquifer wells, and fifty-seven from Hensel aquifer wells, 
the equation to calculate transmissivity (T) (see discussion in Methods chapter, and 
calculations in Appendix F). Estimated K values of 3.53E-05 cm/sec (.1 foot/day) 

for the Hensel were then
for the Edwards and 3.53E-04 cm/sec (one foot/day) 
calculated by dividing the arithmetic of T the estimated saturated
averages by thicknesses. These values compared reasonably well with those calculated in other 
studies in the Edwards Plateau/Hill Country region (Table 1). 
The condition under which the hydraulic conductivity of an aquifer varies with direction is termed anisotropy. Horizontal anisotropy occurs when the value of 
90 
K /K is more or less than one. Simulations were horizontal
run initially using an 
y 
anisotropy of one (no anisotropy) because at the regional scale the system can be visualized as an equivalent isotropic porous medium. Confining bed vertical conductance (leakance) Vertical leakance is equal to the ratio of the estimated vertical hydraulic the thickness For model
conductivity of the confining bed to of the confining bed. 
input, an initial leakance estimate of 7.0-E-10 cm/sec (2.0E-06 feet/day) was assigned to the grid nodes. This was calculated by dividing an estimated hydraulic conductivity (Table 1) of 7.0E-09 cm/sec (2.0E-05 feet/day) by an estimated confining bed thickness of 3.1 meters (10 feet). 
Dram conductance 
Drains were assigned to the nodes of 138 cells that are crossed by the 565 
meter on the face of the erosional

(1855-foot) topographic contour escarpment (Figure 45). These drain cells were used to simulate the discharge of Edwards ground water from numerous small contact springs whose exact elevations could not be established due to property access limitations, or because their discharges were too meager or too sporadic. Drain cells simulate the removal of ground water at a and
rate proportional to the difference in elevation between the water table elevation a fixed drain elevation. The 1855-foot elevation was used for mode! input because it 
is the approximate elevation of the top of the ten-foot thick low permeability marly limestone layer that forces overlying ground water toward spring outlets. The starting drain conductance (hydraulic conductivity of the drain material multiplied by 
2 
the length of the drain) used in the simulation was .0139 m 2/ day (.150 feet/day). 
River conductance 
cells across which the Llano River flows were assigned an
Seventy-two initial hydraulic conductance value of 93 m2/day (1000 feet2/day) in the river package input file. River cells simulate baseflow seepage across the aquifer-stream 
91 
Table 1. 
Ranges of hydraulic conductivity used to produce estimates for initial input to the numerical model of ground-water flow. 
92 
interface. The rate of seepage is proportional to the difference in elevation between the Hensel aquifer water table and the stage of the river. Elevations in both the aquifer and the river decrease from the west end of the study area to the east. 
Sources and sinks Estimates for initial model input were made for and sinks including
sources 
precipitation recharge, discharge  to  the  Llano  River,  well  pumpage,  and  spring  
discharge.  
Precipitation recharge and discharge to the Llano River  
Initially, the same  recharge value  was  applied to  the highest active  cell  in  

each vertical column of the model grid. This value was estimated by calculating the 
percentage of rainfall that recharges the aquifer in the adjacent drainage basin which drains the North and the South Llano Rivers in a large area upstream of the point 
to form the Llano River. In making this calculation it 
was
where they converge assumed that the mechanics of recharge are similar in both this drainage basin and 
This is 
a
the study area because of similar topography, stratigraphy, and structure. 
initial estimate.
valid assumption because the calculated value was only used as an 
A flow on the Llano River positioned just below the confluence of 
gauge these two rivers records all ground and surface water exiting the large drainage system. During the winter months when precipitation (Figure 2) and 
evapotranspiration are low, the water in these rivers is derived almost entirely from baseflow conditions this is
seepage. Under steady-state ground-water discharge The rainfall in the area for the low-flow months
equal to aquifer recharge. average of December and January for the period 1939 to 1988 was 138,685,128 cubic 
meters/month (112,419 acre-feet/month). This value was divided into the average 
to
monthly river gauge discharge of 7,014,505 cubic meters (5,686 acre-feet) yield a 
discharge of 5.06 % of rainfall. The value of recharge assigned to model cells in the 
93 
x = 3.10
study area was therefore, .0506 61.34 cm/year (24.15 inches/year) rainfall 
cm/year (1.22 inches/year). 
Well pumpage Total pumpage from approximately 150 domestic and livestock wells in the 
389-square km (150-square mile) area was ignored in the model because with a 
was
density of one well/square mile, well pumpage calculated to be insignificant 
relative to the annual throughput of water. The Texas Natural Resource 
Conservation Commission suggests a value of 5.67E-01 cubic meters/day (150 
gallons/day) as an estimate of domestic water use by a family of four. Total 
domestic use in the study area would be 5.67E-01 X 150 X 365 = 31,043 cubic 
The volume of the area is estimated to be 3.10E-02
meters/year. recharge to 
X 389 km X 1000 meters X 1000 meters =
meters/year square 12,059,000 cubic As a of well would be
meters/year. percentage discharge, pumpage 31,043/12,059,000 or .26%. 
There are no irrigation wells that draw from the Edwards or Hensel aquifers; that exist is either
the source of water used to irrigate the few hay and pecan crops 
the Llano River or the saturated alluvium immediately adjacent to the Llano River. Discharge by evapotranspiration (ET) ET refers to the combined effect of plant transpiration and direct 
evaporation from the ground surface. A quantitative estimate of ET was not and
required for this study because recharge was not estimated by subtracting ET the and distribution of recharge
runoff from precipitation. Instead, magnitude 
assigned to the calibrated model was determined by trial and error simulations. 

Steady-state simulation and model calibration is assumed to operate under steady-state flow
This ground-water system are constant with
conditions because magnitudes and directions of the flow velocity 
94 
time. Under steady-state conditions there is no change in storage in the aquifer. This is a 
the
reasonable assumption because of the lack of aquifer development in 
area. 
Calibration of the model was achieved by comparing simulation output with 
available data or state conditions that the model simulates. After each of 
numerous 
trial and error simulations, adjustments were made to the magnitude and spatial 
distribution of model input values until calibration goals were met. Calibration goals 
required that (1) simulated potentiometric surface contours closely matched hand-drawn interpretive contours, and (2) model-calibrated parameter values matched 
field-measured parameter values reasonably well. 
Potentiometric contours were first matched as closely as possible, then a 
key parameter value (vertical conductance through the confining bed) was adjusted above and below the value used for the simulation run which exhibited the good 
contour match. By using this method an estimate could be made of the of
range volumes entering and leaving layer one of the model. This approach is diagrammed in The simulation with
a series of simulated contour comparisons (Figures 48A-48E). 
that
the best fit (48C) calculated the percentage of Edwards recharge discharged 
from springs to be 13.7%. The percentage of Edwards recharge that discharged to of cross-formational leakage was calculated to
the underlying Hensei aquifer by way be 86.3% (Figure 49). To estimate throughput in the Hensei aquifer, the simulated leakage per year added to the estimated recharge per year applied to the
to the Hensei aquifer was Hensei aquifer in the floodplain. This sum equaled the model-calculated seepage into 
the Llano River minus the model-calculated seepage out of the Llano River (Figure 
49). The of matching simulated contours to hand-drawn interpretive
process contours to calibrate the model required approximately one hundred simulations. The 
95 
Figure 48A (Simulation A) The potentiometric surface of the Edwards aquifer is simulated in a series of five model runs as the vertical conductance (leakance) of the confining bed is decreased. The assigned 
leakance is too high in Simulation A; too much of the precipitation recharge leaks to the underlying Hensel aquifer. 
96 
Figure 488 (Simulation B) The potentiometric surface of the Edwards aquifer is simulated in a series of five model runs as the vertical 
conductance (leakance) of the confining bed is decreased. The assigned leakance is still too high. This causes a leakage of too much precipitation recharge to the Hensel aquifer but not as much as in Simulation A. 
97 
Figure 48C (Simulation C) The potentiometric surface of the Edwards aquifer is simulated in a series of five model runs as the value of vertical conductance (leakance) of the confining bed is decreased. Simulation C produces the best match between simulated and hand-drawn contours. 
It is believed to be the most accurate simulation of the groundwater system 
98 
Figure 48D (Simulation D) The potentiometric surface of the Edwards 
aquifer is simulated in a series of five model runs as the vertical conductance (leakance) of the confining bed is decreased. The assigned leakance is too low in Simulation D. As a result, the confining bed does 
not leak enough water to the underlying Hensel aquifer. This causes the simulated water table of the Edwards aquifer to be too shallow. 
99 
Figure 48E (Simulation E) The potentiometric surface of the Edwards aquifer is simulated in a series of five model runs as the vertical conductance (leakance) of the confining bed is decreased. Simulation E 
a value of leakance lower than the one used in Simulation D.
uses 
Simulation results are unacceptable because not enough water is leaked to the underlying Hensel aquifer. 
100 
Figure 49, Water budget for the ground-water system (feet3/year(m3/year)) 
one
parameter deemed most important for calibrating layer (Edwards Formation) was the distribution and magnitude of focused recharge. The model tested the 
hypothesis that focused recharge occurred in the flat upland areas, and that little 
or 
no recharge occurred on the steep slopes where runoff and spring discharge would be expected to predominate. The final distribution of focused recharge is mapped in 
Figure 50. The amount of recharge applied to the focused recharge cells ranged from 
to
1.00 cm/year (0.40 inches/year) 8.13 cm/year (3.20 inches/year). Recharge 
applied to cells on the sloping surfaces of the limestone upland was 1.25 cm/year 
The excellent match of in
(0.50 inches/year). potentiometric contours Figure 48C lends support to the focused recharge hypothesis. 
A K value of 3.5E-04 cm/sec (one foot/day) was applied to all cells of layer one. Hydraulic conductivity values vary more in the vertical direction than in the 
horizontal direction in the Edwards Group due to cyclical deposition upon a broad flat 
horizontal from
marine shelf. Changes in the magnitude of hydraulic conductivity place to place were thought to be less of a controlling factor than recharge or in contour pattern observed in the
topography producing the wavy potentiometric Edwards aquifer. deemed a more important calibration
was
Hydraulic conductivity parameter 
indicate that the
than others for layer two. Lithologic descriptions in drillers’ reports transition
Hensel Sand Formation is heterogeneous. In addition, there is a in lithologic facies from a more coarse-grained fluvial nearshore paleoenvironment at 
the west 
shallow marine at the east end.
end of the area to a paleoenvironment was the domain in layer two as simulations were
Consequently, K adjusted across performed. Hydraulic conductivity values ranged from 1.06E-04 cm/sec (.3 
feet/day) at the east end to 1.73E-02 cm/sec (49 feet/day) in the west (Figure 51). 
Simulated contours for the Hensel aquifer beneath the floodplain match hand-drawn contours very closely (Figure 52). Simulated contours for the Hensel aquifer 
Figure 50. Distribution and magnitude of focused recharge to active cells of layer one on the flat upland areas which divide drainage basins 
103 
Figure 51. Hydraulic conductivity values used in the calibrated run 
-
of the finite-difference grid representing layer two Hensel Sand aquifer 
104 
Figure 52. Comparison of the simulated potentiometric surface of the Hensel aquifer to the potentiometric surface drawn using available water levels in wells. This good match constitutes attainment of one of the calibration criteria set for accurate simulation of the Hensel simulation. 
105 
in the region beneath the plateau establish that the Hensel aquifer is unconfined near the edge of the plateau, and that the overlying Edwards aquifer is fully perched (Figure 53). This is an important conclusion that could not be made by measuring water levels. There are no wells in this area that fully penetrate the Edwards 
Group, to draw from the Hensel aquifer alone. of
A final uniform recharge rate 1.25cm/year (.5 feet/year) was assigned to model cells in layer two. A single value was used because unlike the varied pattern of recharge in the upland area which is caused by contrasts in topography and uniform. The
structure, recharge in the floodplain is probably more exposed surface of the Hensel in the Llano River floodplain is gently sloping, and composed of friable siltstone and sandstone. 
Calibration sensitivity analysis 
The integrity of the solution can be judged by the accuracy of the input parameters that the solution is most sensitive to. Using the calibrated model, input varied within a realistic and simulations are run to
are
parameter values range, 
observe the effect of these changes upon the potentiometric surface. If a large 

a dominant one. If its
effect is seen it means that the parameter being adjusted is value is inaccurate, uncertainty in model results is increased. in this study, the model parameters judged to have the greatest potential for 
contributing to uncertainty are horizontal anisotropy and vertical leakance (Table 2). When the values of these parameters were adjusted during trial and error simulation, a pronounced effect was observed in the simulated potentiometric surface, and the model-calculated water balance. There are no straightforward ways of determining the anisotropy ratio, so it was maintained at 1:1. might be
The estimate of leakance 
on undisturbed oriented
improved by running laboratory permeameter tests 
samples of the confining bed material, but the methodology is complicated, and the 
106 
the and 
Edwards 

the 
in 
tables water 
simulated 

of 
Cross-section 

53. 
Figure 
Terrett 
Fort the 
of 
base 
the 
at 
bed 
confining 
regional 

The 
aquifers. Hensel 
aquifer. 
perched 

fully 
a 
as 
operate 

to 
aquifer Edwards 
the 
causes 

Formation 

107 
Relative effect on Relative effect on potentiometric potentiometric 
contours of the contours of the Edwards aquifer Hensel aquifer 
Parameter adjusted 
Anisotropy very high very high 
Vertical leakance of confining bed at 
high medium to high
base of FortTerrett Formation 
Edwards Group hydraulic conductivity high low to medium 
Hensel aquifer recharge low to medium high 
Hensel Formation hydraulic conductivity low medium 
Edwards aquifer recharge low to medium low 
Edwardsaquifer recharge 
mediumto high low 
(focused recharge cells) 
Edwards Group drain conductance low low 
River conductance (Llano River) low low 
Table 2. Qualitative sensitivity analysis of model input parameters. 
108 
results uncertain, because these are not in-situ measurements. 
Simulation results were sensitive to two other whose
very parameters assigned values are believed to be more accurate than the two discussed above. These are the Edwards Group hydraulic conductivity and the Hensel aquifer exists to estimate the
recharge. Actual data (specific capacity) Edwards Group hydraulic conductivity so the model’s sensitivity to it is not of major concern. The estimated value of recharge applied to the Hensel aquifer is believed to be accurate because the distributed value of recharge calculated for the adjacent drainage basin, with values calculated at another
and then applied to this one, compares favorably site in the Texas Hill Country (Ashworth, 1983). to the Hensel Formation Edwards
Adjustments hydraulic conductivity, aquifer recharge, and Edwards aquifer focused recharge, had a medium effect upon the simulation results. These are parameters that had been estimated using are thought to be fairly accurate.
independent sources of data, thus they The model results to in the values of drain
sensitivity of adjustments conductance and river conductance was low. This means that uncertainty in their discussed
values is more tolerable than uncertainty in the values of the parameters earlier. 
109 
Conclusions 
1. 	Two separate unconfined aquifers are defined based potentiometric surfaces
on 
constructed from static water levels in wells. These are the Edwards aquifer 
which occurs beneath the Edwards Plateau, and the Hensel Sand aquifer which occurs below the Edwards Plateau and the Llano River valley. 
2. 	Based on numerical simulation the water table of the Hensel
results, Sand aquifer extends below the Edwards Plateau where it is overlain by an 
unsaturated zone that thins toward the interior of the Plateau the Hensel
as 
Sand Formation thins. The Edwards aquifer overlies this unsaturated zone 
where it 	limestone Nodular
is fully perched atop a thin, marly (Basal Member) 
which is the lowermost member of the Fort Terrett Formation. 
3. 	Ground-water movement in the Edwards aquifer occurs principally within fractures and solution-enhanced bedding planes of the Fort Terrett and Segovia Formations. The Edwards aquifer is a diffuse-flow aquifer. Evidence for diffuse 
flow includes the relative steepness of the hydraulic gradient, the absence of uniform
observable karst features, low average specific capacity, and spatially 
hydrochemistry. 

4. 	coarse­
Ground-water movement in the Hensel aquifer occurs principally within 
grained fluvial paleochannel deposits encased within finer-grained overbank 
matrix deposits. 
the 	base of the Fort Terrett
5. 	The low permeability, marly limestone member at Formation slowly leaks water from the perched aquifer
Edwards to the 
underlying Hensel aquifer. 
6. 
Total precipitation recharge applied to the active cells of the numerical model grid was5,080,147meters3/year(179,380,000feet3/year). Ofthistotal,2,759,783 meters 3 3was the Edwards aquifer, of
/year (97,447,971 feet/year) applied to 
which 378,589 meters 3/year (13,368,000 feet3/year) (13.7%) was calculated 
by the mode! to be discharged as springflow, and 2,381,194 meters3/year 
(84,079,971 feet3/year) (86.3 %) was calculated to be discharged as cross­
formational flow. Precipitation recharge applied to the Llano River valley 
portion of the Hensel Sand aquifer was 2,320,363 meters 3/year (81,932,029 feet3/year). The discharge from the Hensel Sand aquifer via seepage to the Llano River was calculated to be 4,955,819 meters3/year (174,990,000 
3
feetThis rate of to the Llano is the sum of three
/year). seepage equal to components of recharge: (1) cross-formational flow to the Hensel aquifer, (2) 
precipitation recharge to the Llano River valley portion of the Hensel aquifer, and 
(3) seepage of Llano River water into the Hensel aquifer. 
7. Based on the results of numerical simulation, focused recharge to the Edwards 
aquifer occurs where rainwater infiltrates the flat ground surface near the 
topographic divides above and between two watersheds. 
8. River
Recharge to the portion of the Hensei aquifer that lies beneath the Llano valley occurs by infiltration of rainfall through the silty sand surface of the Hensei Sand Formation. Focused recharge may occur by infiltration of flood waters through the streambeds of ephemeral tributaries, but this hypothesis is not tested. Additional recharge occurs by cross-formational flow to that part of 
the Hensei aquifer which underlies the Edwards Group Formations. 
9. Mesas at the margin of the Edwards Plateau are substantially dewatered 
because ground-water flowlines converge to form springs upgradient of mesa locations at the heads of ephemeral streams that have cut steep-walled canyons 
into the edge of the plateau. 
to have declined over time.
10. 
Spring discharge from the Edwards aquifer appears This may be caused by the proliferation of mesquite and juniper trees that 
extract potential recharge water from the soil zone. Additional field research is 
needed to test this hypothesis. 
11. The Edwards aquifer is a Ca-Mg-HC0facies water, and the Hensel is a mixed 
3 
facies water. Even though there is cross-formational flow between these 
these
aquifers, the existence of two distinct hydrochemical facies suggests that aquifers operate semi-independently. 
12. Based on field measurements of high dissolved 
oxygen (6.0 to 8.6 mg/l), high Eh (404 to 488 mv), and the highest water table elevations in the study 
area, 
in the Edwards aquifer is determined to occur in a recharge zone
ground water 
which is open to the atmosphere. 

13. Based on field measurements of low dissolved oxygen (1.1 to 2.1 mg/l), 
iron taste, sulfide odor, and the lowest
comparatively low Eh (99 to 255 mv), 
water table elevations 

in the study area, ground water in the Hensel aquifer is determined to be more isolated from the oxidizing influence of the atmosphere, 
and is located either in an intermediate zone of regional flow or a discharge zone. 
Higher values of dissolved mg/l) and Eh (393 to 461 mv) at 
oxygen (5.0 to 7.1 the eastern end of the floodplain are believed related to a facies change to more 
calcareous sediments, and to a thinning of the Hensel Sand Formation in the 
in the
direction of the Llano Uplift area. This thinning results a shallowing of 
aquifer as well. 
This is
14. The Llano River is a gaining river on average. demonstrated by a 
of “V’s” that in the downstream
baseflow potentiometric surface pattern open direction. The flow of this perennial river is sustained by surface waters that 
from
begin as Edwards aquifer springs, and by baseflow seepage the adjacent 
Hensel aquifer. 
15. 	Calculated using Darcy’s Law, the average linear ground-water velocities are approximately 1.6E-04 cm/sec (0.45 feet/day) in the Edwards aquifer, and 9.7­-05 cm/sec (0.28 feet/day) in the Hensel Sand aquifer. 
Appendices 
Appendix A 
Water chemistry sampling 
and analysis procedures 

Water chemistry sample collection and 
analysis procedures (Field, laboratory, and computer processing) 
Field 
1. 
Immerse Eh Zobell solution bottle, pH buffer bottles, and HCL alkalinity titrant 
bottle in bucket of sample water to bring them to temperature equilibrium. 
2. 	Turn on windmill or submersible pump 
and let water run strongly for 20 minutes. Note 
any smells and tastes in water especially hydrogen sulfide smell and/or iron taste which might indicate reducing conditions. 
3. 	
Check batteries in pH-Eh-temp meter, dissolved oxygen meter and 
spectrophotometer. 


4. 	
Check filling solutions in Eh probe and pH probe. 

5. 	
Calibrate pH meter with 4pH and 10 pH buffers. 

6. 	
Attach fiowcell to water supply after water has run for 20 minutes. 


Reduce flow to minimum volume to keep fiowcell 100% full
necessary 
(completely removing contact with the atmosphere). 

7. 	
Insert pH, temp, and dissolved oxygen probes into fiowcell. 

8. 	
Monitor temp, pH, and dissolved oxygen until stability occurs. Record 


readings. 
9. 	Using a .45 urn filter fill one 60 ml plastic bottle with well water for cation 
analysis after adding 2 drops 6N reagent-grade nitric acid (reduces pH to 3 to prevent precipitation of metal ions). 
10. Using a .45 urn filter fill two 60 mi bottles with well water. One is for 
pvc 
anion 	analysis, the other is a duplicate. 
11. 
Put sample bottles in cooler keeping them at 4° C using “blue ice”. 
12. 
Turn on spectrophotometer and warm for 10 minutes. 
13. 
Using a pipet transfer 100 ml of filtered (.45 urn) sample to a volumetric flask for alkalinity (as HC0 ) titration with 1.5 N HCL. Using a burette 
3 
titrate to the 4.5 pH endpoint using magnetic stirrer, stirring bar, and pH meter. Record ml of titrant required and calculate alkalinity as 3 
HC0 
using the formula: Alkalinity as mg/l HC0 = (normality of titration acid)
3 
* (volume of titration acid in ml near pH=4.5) * (61000/volume of sample 
in ml). 
14. 	Measure dissolved0 with CHEMetrics chemet kit # K-7512 (0-12 ppm)
2 
and/or # K-7501 (0-1 ppm) by snapping a vacuum ampule inside a flow cell opening (to eliminate atmospheric interaction). Record reading. 
15. 	Measure manganese with CHEMetrics chemet kit # K-6502 (0-2 ppm). 
Record 	reading. 
16. 	instrument to correct
After spectrophotometer has warmed, set wavelength to measure transmittance for each of the following constituents. Prepare each sample using the specialized CHEMetrics vacu-vile kits. Use calibration charts provided by CHEMetrics (specific 
to the Mini Spec-20 spectrophotometer) to convert transmittance to concentration in mg/l. 
16a. 	Measure phosphate (ortho) with # K-8503 (0-40 ppm). Record reading. 
16b. 	Measure sulfide with # K-9503 (0-1.6 ppm). Record reading. 
16c. 	Measuresilica(SiC2)with#K-9010(0-10ppm) afterdiluting5:1with ultra-pure distilled water. Record reading. 
16d. 	Measure total iron with # K-6003 (0-5 ppm). Record reading. 
16e. 	Measure ferrous iron with # K-6003 (0-5 ppm). Record reading. 
16f. 	Measure ammonia with # K-1503 (0-7 ppm). Record reading. 
Laboratory 
17. 	Measure anions S0 Cl, F, N0 N0 Br, and P0 using the ion 
4, 2,3, 	4 
chromatograph(lC). Prepare fresh supply of recommended eluent, use 
new(cleaned) sample vials for placement of sample within the automated 
wisp system sample tray, and set instrument for proper eluent pump flow rate. 
Pick an appropriate analysis method from existing methods that have worked successfully in the past. Integrate peaks using data acquisition software. Adjust computer-generated values by comparing 
them to 	a certified check standard. Record readings. 
18. 
Measure cations Mg, Ca, Li, Na, K, Ba, and Sr using the inductively-coupled plasma spectrophotometer (ICR). Record readings. 
Computer processing 
19. 
Calculate ion charge balance. Milliequivalents of cations must equal If the
milliequivalents of anions within 5% to rule out analytical errors. 
balance 	exceeds 5% investigate the cause and reanalyze if necessary. 
20. 	Enter data into Microsoft Excel spreadsheet to make additional calculations like IDS, hardness, and ion ratios. 
Appendix B Rainfall record (1939-1993) 
119 
Rainfall records (in inches) collected by the Dunk family, 
Junction, Texas for the National Weather Service{l939-1993) 

Year  Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec  Year  
1939  2.51  1.31  0.70  1.12  1.36  4.25  1.11  1.31  0.00  3.09  1.25  2.29  20.30  
1940  0 58  2.14  2.07  2.83  2.83  11.71  0.00  3.55  0.61  3.56  2.07  2.27  34.22  
1941  0.45  2.20  3.60  2.23  1.22  1.44  2.35  3.28  4.89  8.95  0.00  0.78  31.39  
1942  0.00  0.38  0.10  4.84  0.00  1.08  1.90  6.52  2.57  3.72  1.92  0.85  23.88  
1943  0.32  0.00  1.88  1.23  2.15  4.33  1.81  0.00  7.58  1.95  1.98  1.90  25.13  
1944  4.10  2.15  1.71  0.35  6.60  1.21  0.29  4.28  2.42  0.28  2.56  1.55  27.50  
1945  1.78  1.55  2.72  2.24  0.77  3.01  2.61  1.48  1.90  1.77  0.21  0.36  20.40  
1946  1.91  1.02  0.56  3.25  3.07  1.13  1.34  0.64  4.15  2.90  2.23  0.41  22.61  
1947  4.18  0.38  1.74  1.16  3.34  6.84  0.90  1.17  0.52  1.09  2.60  1.60  25.52  
1948  0.98  0.75  1.03  3.60  1.93  6.79  2.93  0.10  2.70  0.27  0.11  0.07  21.26  
1949  3.46  4.51  0.60  4 60  3.12  5.19  1.47  4.60  4.45  5.87  0.00  2.23  40.10  
1950  0.62  1.09  0.00  1.66  3.11  2.90  3.37  2.61  3.16  0.00  0.00  0.00  18.52  
1951  0.00  0.57  1.75  0.99  2.49  2.26  0.00  3.31  0.35  0.55  0.00  0.33  12.60  
1952  0.09  0.80  1.46  2.67  2.71  0.56  0.35  0.77  2.14  0.00  1.65  3.02  16.22  
1953  0.00  1.04  2.03  0.72  4.13  0.00  1.97  0.82  1.09  1.19  0.30  0.30  13.59  
1954  0.46  0.39  0.00  2.19  3.82  2.61  0.00  0.31  0.37  2.46  0.23  0.00  12.84  
1955  1 43  0.45  0.76  0.07  5.22  2.09  2.97  2.96  6.12  0.08  0.87  0.41  23.43  
1956  0.31  1.17  0.00  1.03  1.28  1.26  1.38  0.00  0.20  0.70  0.30  0.80  8.43  
1957  065  2.21  2.55  9.47  7.78  1.27  0.45  2.07  3.13  9.38  4.29  0.47  43.72  
1958  2.96  2.81  3.18  2.00  3.89  4.78  0.21  3.06  6.46  3.90  0.55  0.69  34.49  
1959  1.43  0.00  3.75  2.40  0.00  6.76  1.11  0.97  0.00  3.49  1.44  2.34  23.69  
1960  1.29  2.03  1.15  0.25  0.90  0.24  3.99  5.55  0.77  3.77  0.74  4.03  24.71  
1961  2.44  1.77  0.62  0.00  2.06  8.37  1 93  2.01  1.46  0.82  1.51  0.55  23.54  
1962  0.18  0.36  0.46  2.90  1.26  1.70  1.06  1.44  2.72  2.99  0.79  0.63  16.49  
1963  0.00  1.11  0.17  0.85  4.79  2.05  0.00  6.71  0.98  1.20  2.81  0.43  21.10  
1964  1.30  1.32  1.83  3.04  1.65  1.03  0.73  5.99  6.19  1.72  1.27  0.55  26.62  
1965  2.29  3.73  0.00  1.65  7.25  1.96  0.89  0.28  2.58  0.99  0.48  1.28  23.38  
1966  0.87  1.18  1.03  4.34  3.87  0.56  0.36  5.22  6.03  0.31  0.00  0.00  23.77  
1967  0.25  0.93  0.40  1.01  2.31  0.67  3.40  0.80  2.08  2.41  3.25  1.68  19.19  
1968  6.05  2.03  2.53  4.12  1.61  2.97  0.78  0.60  3.54  0.44  3.91  0.00  28.58  
1969  0.25  1.00  1.40  3.58  1.59  3.38  2.03  1.76  1.62  6.93  1.39  1.81  26.74  
1970  0.39  3.07  1.96  0.60  4.59  2.03  0.80  2.40  3.17  1.25  0.00  0.00  20.26  
1971  0.00  1.02  0.00  1.70  1.82  1.69  2.12  8.24  0.98  4.55  0.63  1.12  23.87  
1972  0.77  0.24  0.39  1.17  4.51  1.69  0.72  5.45  1.92  1.41  0.65  0.00  18.92  
1973  2.27  2.64  2.08  1.22  1.08  3.21  5.69  0.43  1.79  8.94  0.31  0.00  29.66  
1974  0.60  0.20  0.54  0.93  2.87  0.98  2.10  8.56  2.42  0.00  2.80  1.55  23.55  
1975  000  2.40  0.53  2.06  6.30  3.13  2.13  0.55  0.71  1.68  0.67  0.41  20.57  
1976  0.46  0.00  1.64  5.13  1.96  4 55  6.35  1.17  3.67  4.75  0.67  1.59  31.94  
1977  1.17  0.55  1.97  4.89  3.75  0.23  0.00  0.00  1.78  2.47  2.05  0.00  18.86  
1978  0.47  1.25  0.60  0.75  4.61  1.99  1.71  1.91  3.49  1.59  3.37  1.06  22.80  
1979  0.71  1.70  1.58  5.27  1.93  2.49  2.86  5.07  0.00  0.00  0.50  1.76  23.87  
1980  1.64  0.65  1.14  0.43  4.12  1.43  0.00  0.85  13.20  2.45  2.56  1.23  29.70  
1981  093  0.69  3.51  5.59  3.18  9.37  0.00  1.97  0.58  5.97  0.42  0.00  32.21  
1982  0.00  1.69  0.00  1.34  5.14  4.34  1.66  4.40  0.65  0.99  3.81  1.47  25.49  
1983  0.82  2.62  2.94  0.00  3.08  2.25  5.11  2.13  1.36  0.00  1.07  2.02  23.40  
1984  1.18  0.00  0.00  0.30  1.80  1.37  1.91  0.15  1.51  3.44  2.02  7.48  21.16  
1985  1.02  0.78  1.52  1.22  2.32  1.75  1.92  0.00  5.19  3.10  0.59  0.00  19.41  
1986  0.60  2.24  0.13  0.31  5.03  2.71  1.32  3.70  5.10  7.38  1.94  3.24  33.70  
1987  0.00  2.65  0.94  1.26  4.37  6.43  0.65  1.59  2.59  0.15  1.12  0.89  22.64  
1988  0.14  0.00  0.77  0.60  3.26  2.75  7.06  0.43  4.18  1.63  0.09  0.54  21.45  
1989  2.97  3.02  1.43  3.16  3.12  3.17  0.35  3.00  0.92  1.57  1.10  0.00  23.81  
1990  0.51  5.22  2.50  2.52  3.75  0.00  5.85  2.27  2.42  2.25  2.30  0.33  29.92  
1991  1.60  0.94  0.40  1.21  1.36  3.50  1.87  2.62  4.24  2.21  0.65  5.50  26.10  
1992  2.66  4.79  4.71  3.06  4.32  4.88  0.80  0.99  0.63  0.00  2.82  1.96  31.62  
1993  1.28  1.29  1.56  2.17  3.21  2.95  0.00  0.70  3.47  1.45  0.17  1.15  19.40  
Average  1.19  1.49  1.36  2.17  3.08  2.97  1.76  2.41  2.70  2.47  1.33  1.22  24.15  

120 
Appendix C Descriptions of soil types 
121 
Descriptions of soil types 
Tarrant soils 
Found 
on limestone hills and ridgetops. These are very shallow to shallow clayey soils with limestone cobbles on gently sloping to steep slopes. 
These soils form in residuum weathered 
from the underlying fractured limestone bedrock. They are well drained, surface runoff is rapid, infiltration is moderately slow, and available capacity is very They
water low. are 
well suited for vacation homes, wildlife habitat and rangeland but not to recreation and urban uses. They belong to hydrologic unit group D which has a very slow infiltration rate and high runoff potential. Clays with a high shrink-swell potential are common. 
Tarrant-Real-Brackett soils 
Very shallow to shallow soils on upland ridges and foothills. They found along the base
are of limestone hills that slope toward the Llano River and found the larger creeks.
are 
along form in material weathered from interbedded limestone and calcareous loam.
They clay Slopes range from 1-50%. They are well drained, runoff is rapid, and available water capacity is very low. These soils are not suited to cultivation; they are used as rangeland and wildlife habitat. 
Belonging to hydrologic group C-D, they have a slow to very slow infiltration rate and high runoff rate. 
Menard-Hext-Latom soils 
Moderately deep to deep, form on upland plains from either loamy calcareous sediment of ancient alluvium or weakly consolidated sandstone. found on and knolls
They are low ridges with from 1-12%. These soil
slopes of They are well drained and moderately permeable. 
but are well suited

zones are used mainly as rangeland to grains, fruit, and vegetables because of high natural fertility. They are well suited to recreational uses, rangeland, and 
wildlife habitat, and moderately suited to urban uses. They belong to hydrologic group B-D which has a moderate to very slow infiltration rate. 
Nuvalde-Dev-Frio soils 
Deep, gravelly and loamy, nearly level to gently sloping, and well drained. They are found 
on 
bottomlands formed from
upland outwash plains formed from ancient alluvium, or on recent calcareous, loamy, and gravelly alluvial sediment of recent origin. Surface runoff is slow, These well suited
permeability is moderate, and available water capacity is low to high. are to corn, and alfalfa, and are used for wildlife habitat as well as
grains, pecans, wheat, 
a
urban and recreation purposes. Assigned to hydrologic group A-B they have high infiltration rate and low runoff potential when thoroughly wet. 
122 
Appendix D Table of water levels in wells 
123 
Table of water levels in wells 
Project well #  Date measured (1994)  Windmill (w) Submersible pump (s)  Ground elevation (feet asl)  Depth to water (state data) (feet)  Depth to water (measured) (feet)  Water table elevation (feet asl)  Total depth (state data) (feet)  Total depth (measured) (feet)  Bottom of hole elevation (calculated) (feet asl)  
1  13-Feb  s  1620  - 21  1599  104  104  1516  
2  13-Feb  s  1620  - 24  1596  40  1580  
- 
3  13-Feb  s  1665  41  1624  
- - - _  
4  13-Feb  s  1690  48  1642  80  1610  
- - 
5  13-Feb  w  1750  - 24  1726  100  1650  
- 
6  13-Feb  s  1700  58  1643  90  1610  
- - 
7  13-Feb  w  1640  50  1590  90  1550  
- - 
8  13-Feb  w  1620  21  1599  100  1520  
- - 
9  13-Feb  s  1690  58  48  1642  76  1566  
- 
10  27-Feb  s  1645  64  1581  
- - - - 
11  27-Feb  w  1860  240  1620  
- - - - 
12  5-Mar  s  1750  129  1621  183  1567  
- - 
13  5-Mar  s  1705  83  1622  115  1590  
- - 
14  5-Mar  w&s  1780  123  1654  155  182  1598  
- 
15  5-Mar  w  1730  68  1662  90  1620  
- - 
16  5-Mar  s  1725  73  1652  102  1627  
- - 
17  6-Mar  s  1625  31  1594  50  1575  
- 
- 
18  6-Mar  s  1590  35  1555  55  1535  
- - 
19  6-Mar  s  1580  49  1531  120  1460  
- - 
20  6-Mar  w  1749  87  1662  115  158  1591  
- 
21  6-Mar  w  1862  187  1675  220  220  1642  
- 
22  6-Mar  w  1783  121  1662  150  135  1648  
- 
23  6-Mar  w  1833  124  1707  180  245  1588  
- 
24  15-Mar  w  1760  95  1661  159  180  1580  
- 
25  16-Mar  w  1647  39  1609  >285  <1347  
- - 
26  16-Mar  w  1665  45  1621  75  1590  
- - 
27  16-Mar  w  1675  45  43  1632  100  1620  
- 
28  16-Mar  w  1735  94  86  1649  100  1635  

124 
Table of water levels in wells 
Windmill  Bottom  
Project well #  Date measured (1994)  (w) Submersible pump (S)  Ground elevation (feet asl)  Depth to water (state data) (feet)  Depth to water (measured) (feet)  Water table elevation (feet asl)  Total depth (state data) (feet)  Total depth (measured) (feet)  of hole elevation (calculated) (feet asl)  
29  16-Mar  w  1740  124  1616  140  1600  
- - 
30  16-Mar  w  1700  90  1610  170  1600  
- - 
31  16-Mar  w  1688  72  68  1620  80  1598  
- 
32  16-Mar  w  1716  105  104  1612  115  1601  
- 
33  16-Mar  s  1770  146  148  1622  200  155  1615  
34  17-Mar  s  1655  42  1613  120  1535  
- - 
35  17-Mar  s  1840  103  1737  150  113  1727  
- 
36  17-Mar  w  1913  222  1691  450  280  1633  
- 
37  17-Mar  w  1840  110  104  1736  127  130  1710  
38  17-Mar  w  2056  100  1957  180  1876  
- - 
39  17-Mar  s  1645  31  1614  80  1565  
- - 
40  17-Mar  s  1666  39  1627  90  1576  
- - 
41  18-Mar  s  1650  60  1590  110  1540  
- - 
42  19-Mar  w  2018  68  1950  120  1898  
- - 
43  19-Mar  w  1910  15  1895  60  1850  
- - 
44  19-Mar  s  1845  208  1637  250  1595  
- - 
45  19-Mar  w  2020  370  1650  >400  1620  
- - 
46  20-Mar  w  2052  120  1932  168  165  1887  
- 
47  5-Apr  s  1704  - 100  1604  - - - 
48  5-Apr  s  1640  - 41  1599  - - - 
49  5-Apr  s  1660  - 54  1606  - - - 
50  6-Apr  s  1680  68  58  1622  101  - 1579  
51  6-Apr  s  1695  - 73  1622  - - - 
52  6-Apr  s  1680  - 61  1619  - - - 
53  7-Apr  s  1680  - 62  1618  - - - 
54  7-Apr  s  1700  - 83  1617  - - - 
55  7-Apr  s  2010  - 92  1918  - - - 
56  8-Apr  s  1670  27  1643  100  1570  

125 
Table of water levels in wells 
Project well #  Date measured (1994)  Windmill (w) Submersible pump (s)  Ground elevation (feet asl)  Depth to water (state data) (feet)  Depth to water (measured) (feet)  Water table elevation (feet asl)  Total depth (state data) (feet)  Total depth (measured) (feet)  Bottom of hole elevation (calculated) (feet asl)  
57  8-Apr  w  1640  57  1583  90  1550  
- - 
58  8-Apr  s  1642  55  54  1588  105  72  1570  
59  8-Apr  s  2050  198  1852  450  400  1650  
- 
60  25-Apr  s  2170  188  1982  239  1931  
- - 
61  25-Apr  s  1996  - 174  1966  - 200  1796  
62  25-Apr  w  1670  73  51  1619  110  - 1560  
63  25-Apr  w  110  190  
- - - - - 
64  25-Apr  w  1990  10  8  1982  20  20  1970  
65  26-Apr  s  1760  - 43  1717  54  - 1706  
66  26-Apr  s  1760  32  1728  200  1560  
- - 
67  26-Apr  w  - 55  60  - 100  - - 
68  26-Apr  s  2083  100  132  1951  210  200  1883  
69  1-May  w  2320  - 260  2060  340  - 1980  
70  1-May  s  2225  140  217  2008  220  270  1955  
71  1-May  w  2150  149  146  2004  200  235  1915  
72  1-May  s  - - 34  - - 80  - 
73  1-May  w  2230  - 228  2002  380  285  1850  
74  1-May  w  2151  - 126  2025  - 150  2001  
75  14-May  w  2160  191  1969  350  - 1810  
76  14-May  w  2064  - 121  1943  200  220  1844  
77  14-May  w  2185  - 185  2000  - 225  1960  
78  15-May  s  1720  - 96  1624  - 130  1590  
79  15-May  w  1720  - 93  1627  - 110  1610  
80  15-May  s  1782  - 161  1621  - 190  1592  
81  17-May  w  2188  - 156  2032  170  240  1948  
82  17-May  w  2185  - 187  1998  - 260  1925  
83  17-May  s  2310  - 266  2044  - 275  2035  
84  17-May  s  2071  101  93  1978  200  204  1867  

126 
Table of water levels in wells 
Windmill  Bottom  
Project well #  .ate measured (1994)  (w) Submersible pump (s)  Ground elevation (feet asl)  Depth to water (state data) (feet)  Depth to water (measured) (feet)  Water table elevation (feet asl)  Total depth (state data) (feet)  Total depth (measured) (feet)  of hole elevation (calculated) (feet asl)  
85  17-May  s  2088  225  122  1966  125  206  1882  
86  17-May  s  2010?  88  1922?  250  1760  
- - 
87  17-May  w  2080  102  97  1983  225  166  1914  
88  17-May  w  2194  240  - 1954  225  275  1919  
89  17-May  w  2190  - 184  2006  - 205  1985  
90  state data  s  2140  155  1985  240  1900  
- - 
91  state data  w  1797  183  1614  240  1554  
- - 
92  state data  s  1666  66  1600  118  1548  
- - 
93  state data  s  1720  96  1624  150  1570  
- - 
94  state data  s  1600  46  1554  120  1480  
- - 
95  18-Jul  w  2270  280  1990  320  1950  
- - 
96  18-Jut  w  2190  178  2012  195  1995  
- - 
97  18-Jul  w  2209  205  146  2063  220  225  1984  
98  18-Jul  w  2165  195  1970  254  1911  
- - 
99  18-Jul  w  2135  142  144  1991  200  1935  
- 
100  18-Jul  w  2064  50  2014  200  1864  
- - 
101  18-Jul  w  2070  149  1921  220  1850  
- - 
102  state data  w  2253  238  2015  250  2003  
- - 
103  state data  w  2250  256  1996  >280  <1970  
- - 
104  state data  s  2190  186  2004  264  1926  
- - 
105  state data  s  1680  38  1642  104  1576  
- - 
106  state data  s  1665  40  - 1625  65  - 1600  
107  statedata  s  1640  42  - 1598  60  - 1580  
108  state data  s  1620  40  - 1580  60  - 1560  
109  state data  s  1617  40  - 1577  50  - 1567  
110  state data  s  1646  88  - 1558  220  - 1426  
111  state data  s  1580  60  - 1520  324  - 1256  
112  state data  w  2092  126  1966  240  1852  

127 
Appendix E 
Specific capacity and 
transmissivity calculations 

128 
Specific capacity/ transmissivity calculations 
(based on driller sc test values used within the Walton (1962) equation) Edwards Group Formations 
Transmissivity (feetA 2/day) assuming the  
following values of storativity:  
Specific  
capacity=  
rate of  
Well #  Pumptime (days)  Well radius (leet)  discharge(Q) in gpm/  S = .02  S = .002  S = .0002  S = .00002  
drawdown(s)  
in feet  
56-27-1 a  0.0139  0.29  0.0340  1.9  3.3  4.7  6.0  
56-27-2  0.0208  0.21  0.1000  9.6  13.6  17.5  21,4  
56-19-4a  0.0417  0.25  0.2500  35.5  39.4  49.1  58.6  
56-19-4a  0.0833  0.25  0,0667  7.2  9.9  12.6  15.1  
56-19-7b  0.0208  0.25  0.1692  16.7  23.5  30.0  36.6  
56-19-7b  0.0208  0.25  0.0465  3.5  5.5  7.3  9.2  
56-19-7a  0.0208  0.25  0.0543  4.1  6.4  8.5  9.6  
56-19-5a  0.0208  0.25  0.1300  12.6  17.9  23.2  28.4  
56-19-5b  0.0417  0.25  0.0170  1.2  1.9  2.5  3.2  
56-19-71  0.0313  0.25  0.0400  3.6  5.5  7.2  8.9  
56-19-71  0.0208  0.25  0.1200  10.9  15.9  20.7  25.2  
56-19-7e  0.0208  0.25  0.0100  0.4  0.9  1.3  1.7  
56-18-6c  0.0313  0.25  0.1000  9.6  13.6  17.6  21.5  
56-18-6a  0.0833  0,25  0.1091  12.6  17.0  21.1  25.2  
56-18-6a  0.0417  0.25  0.3000  35.0  46.7  58.1  70.1  
56-18-9e  0.0208  0.30  0.0667  6.3  8.9  11.7  14.2  
56-18-9c  0.0208  0.20  0.0042  0.3  0.4  0.7  0.9  
56-18-9c  0.0104  0.25  0.0057  0.1  0.4  0.7  0.9  
56-18-9c  0.0104  0.25  0.0042  0.1  0.4  0.5  0.7  
56-18-9c  0.0104  0.25  0.0083  0.1  0.1  0.1  0.1  
56-18-9b  0.0417  0.25  0.1406  15.1  20.8  26.2  31.5  
56-26-3a  0.0417  0.25  0.0571  5.2  7,5  9,7  12.0  
56-19-8a  0.0208  0.25  0.1100  9.9  14,4  18.8  23.0  
56-19-8g  0.0208  0.25  0.0300  2.0  3.2  4.4  5.6  
56-19-8d  0.0208  0.25  0.2300  23.9  33.2  42.2  50.9  
56-19-8c  0.0208  0.25  0.0500  3.7  5.9  7.9  9.7  
sums:  231.1  316,3  404.5  490.4  
averages:  8.9  12.2  15.6  18.9  

129 
Specific capacity/ transmissivity calculations 
Hensel Sand Formation 
Transmissivity (feetA 2/day) assuming the  
following values of storativity:  
Specific  
capacity=  
Well#  Pump time (days)  Well radius (feet)  rate of discharge(Q) in gpm/  S = .02  S = .002  S = .0002  S = .00002  
drawdown(s)  
in feet  
56-27-6)  .0417  .2080  .4167  56.0  72,4  88.6  104.4  
56-27-6e  .0208  .2090  .2375  26,6  36.3  45.5  54.8  
56-27-6e  .0208  .2090  .2500  28.3  38.4  48.2  57.6  
56-27-6e  .0208  .2090  .2368  26.5  36.1  45.4  54.4  
56-27-6C  .0208  .2090  .1111  10.9  15.5  19.9  24.1  
56-27-6C  .0208  .2090  .1333  13.5  19.0  24.1  29.2  
56-27-6b  .0208  .2920  .0615  4.6  7,3  9.7  12.1  
56-27-6f  .0104  .2090  .0167  0.8  1.6  2.3  3.0  
56-27-6g  .0104  .2090  .0822  6.6  10.0  13.2  16.6  
56-27-81  .0208  .2090  2.0000  298.8  376.9  453.6  530.1  
56-28-1 d  .0208  .2290  3.3333  516.7  646.8  773.8  900.3  
56-28-1 d  .0208  .2500  .0909  10.6  11.8  15.3  18.8  
56-28-1 f  .0208  .2290  2.8571  435.4  546.8  656.1  764.7  
56-28-1 g  .0417  .2920  5.8333  980.5  1206.6  1428.7  1648.6  
56-28-1 h  .0833  .2090  5.0000  942.8  1134.6  1324.2  1512.1  
56-28-1 h  .0417  .2090  10.0000  1886.3  2269.9  2648.6  3024.9  
56-28-1 k  .0208  .2090  .4000  48.7  64.5  80,1  95.4  
56-28-11  .0208  .2500  1.3330  181.6  233.9  285.2  376.4  
56-28-1 m  .0104  .2500  .0833  6.1  9.6  12.9  16.2  
56-28-Ip  .0208  .2500  .1000  9.0  13.2  17.0  23.1  
56-28-2b  .0139  .2770  .0781  5.7  9.0  12.1  15.2  
56-28-2b  .0139  .2770  .1786  16.0  23.4  30.4  37.4  
56-28-4  .0417  .2090  .0247  2.0  3,1  4.0  5.1  
56-28-4f  .0208  .2090  .0147  0.9  1.6  2.2  2.7  
56-28-4f  .0104  .2090  .0222  1.2  2.2  3.1  3.9  
56-28-4h  .0208  .2090  .0400  3.2  4.8  6.5  7.9  
56-28-4k  .0208  .2090  .0455  3.6  5.5  7.3  9.0  
56-28-4p  .0417  .2090  .0054  0.3  0.5  0,7  0.9  
56-27-9C  .0208  ,2500  .0079  0.3  0.7  0.9  1.3  
56-27-9a  .0208  .2500  1.2000  161.0  208.3  254.8  300.5  
56-28-4b  .0417  .2500  .0960  9.7  13.7  17.4  21.1  
56-28-4C  .0417  .2500  .1600  24.1  30.4  36.5  42.5  
56-28-4  .0208  .2500  .0532  4.2  6.3  8.5  10.6  

130 
56-28-7a  .0417  .2500  .8000  111.3  142.8  173.6  204.0  
56-28-7C  .0208  .2500  3.0000  450.1  567.3  682.5  796.2  
56-28-4q  .0208  .2090  .2778  31.9  43.1  53.8  64,6  
56-28-4r  .0208  .2090  .1429  14.7  20.6  26.1  31.6  
56-28-4S  .0208  .2090  .0079  0.4  0.8  1.1  1.3  
56-28-4  .0208  .2090  .0532  4.4  6.7  8.6  10.8  
56-20-6b  .0417  .2500  .5000  65,5  85.3  104.7  123.7  
56-20-7a  .0208  .2500  .0800  6.5  9.7  12.8  15.5  
56-20-8  .0104  .2500  .0900  7.0  10,9  14.7  18.3  
56-20-8  .0069  .2500  3.1300  412.0  535.7  656.7  775.8  
56-20-8  .0104  .2500  1,0000  119.0  158.8  197.8  236.1  
56-20-4b  .0417  .2500  .8500  119.1  152.6  185.3  217.6  
56-20-51  .0625  .2500  .8600  126.8  160.4  193.5  226.1  
56-20-5q  .0208  .2500  ,1700  16,8  23.8  30.4  37.0  
56-20-5p  .0104  .2500  .0200  0.8  1.7  2.6  3.4  
56-20-5m  .0208  .2500  .0500  3.8  5.9  7.9  9.8  
56-20-5h  .0208  .2500  .0900  7.8  11.6  15.2  18.7  
56-20-5g  .0417  .2500  .7500  91.8  121.7  150.9  179.5  
56-20-5g  .0208  .2500  .1900  19.2  27.1  34.5  41.7  
56-20-5e  .0208  .2500  .3400  38.0  51.7  65.0  78.1  
56-20-5e  .0417  .2500  .7000  95.8  123.4  150.3  177.1  
56-20-5d  .0417  .2500  1.3330  196.8  248.9  300.0  350.3  
56-20-5C  .0417  .2500  1.0000  143.1  182.4  220.9  258.8  
56-20-5a  .0417  .2500  1.3300  196.8  248.9  300.0  350.5  
sums:  8001.6  10002.4  11965.4  13952.0  
averages:  140.4  175.5  209.9  244.8  

131 
Appendix F calculations
Aquifer recharge 
132 
Calculation of average monthly rainfall and average monthly Llano River discharge during January and December (low-flow months) for the period 1939-1988 for the combined North and South Llano River watersheds west of the study area. 
Sources of data: 
Rainfall Dunk family for the National Weather Service
-
USGS river # 08150000
-
River discharge gauge 
High discharge values  
caused by storm runoff  
are removed and  
replaced by the ave. of  
Llano River  the remaining monthly  
Month  Year  Rainfall (inches)  Discharge (acre-feet' month)  discharges. This adjusts the database in a way that  
allows the ave. river  
discharge to be more  
representative of gw  
discharge only.  

JAN 1939 2.51 7210 DEC 1939 2.29 6600 
JAN 1940 0.58 6280 
DEC 1940 2.27 5500 
JAN 
1941 0.45 4820 
DEC 
1941 0.78 5610 
JAN 1942 0.00 5270 
DEC 1942 0.85 7360 
JAN 1943 0.32 6650 
DEC 1943 5330

1.90 
JAN 7860
1944 4.10 
DEC 1944 1.55 4830 
JAN 1945 1.78 4920 
DEC 1945 0.36 2890 
1.91 3640
JAN 1946 0.41 3590DEC 1946 
4.18 6060JAN 1947 1.60 3560DEC 1947 
0.98 3490JAN 1948 4800DEC 1948 0.07 
4840JAN 1949 3.46 
6380DEC 1949 2.23 
JAN 1950 0.62 6310 
DEC 1950 3850
0.00 
3730
0.00
1951JAN 0.33 2380DEC 1951 2370
0.09
1952
JAN 
1952 3.02 2280 JAN 1953 0.00 2200 DEC 1953 
DEC 
0.30 2040 
0.46 2050
JAN 1954 
0.00 1920DEC 1954 1990
1955 1.43
JAN 
29801955 0.41
DEC 0.31 2850
1956
JAN 0.80 1560
1956DEC 
1610
0.65
1957JAN 
5800 11140
0.47
1957
DEC 
8550
2.96
1958
JAN 
133 
DEC  1958  0.69  5800  10780  
JAN  1959  1.43  9460  
DEC  1959  2.34  6570  
JAN  1960  1.29  7390  
DEC  1960  4.03  5800  14490  
JAN  1961  2.44  5800  13360  
DEC  1961  0.55  8730  
JAN  1962  0.18  7740  
DEC  1962  0.63  4810  
JAN  1963  0.00  5120  
DEC  1963  0.43  4430  
JAN  1964  1.30  4220  
DEC  1964  0.55  7320  
JAN  1965  2.29  6670  
DEC  1965  1.28  4770  
JAN  1966  0.87  4670  
DEC  1966  0.00  5710  
JAN  1967  0.25  5770  
DEC  1967  1.68  4470  
JAN  1968  6.05  5800  39390  
DEC  1968  0.00  5540  
JAN  1969  0.25  5540  
DEC  1969  1.81  5800  12900  
JAN  1970  0.39  5800  11810  
DEC  1970  0.00  6650  
JAN  1971  0.00  6770  
DEC  1971  1.12  9650  
JAN  1972  0.77  8780  
DEC  1972  0.00  7500  
JAN  1973  2.27  7410  
DEC  1973  0.00  9200  
JAN  1974  0.60  8540  
DEC  1974  1.55  5800  16460  
JAN  1975  0.00  5800  13490  
DEC  1975  0.41  5800  11310  
JAN  1976  0.46  5800  10810  
DEC  1976  1.59  5800  12530  
JAN  1977  1.17  5800  11300  
DEC  1977  0.00  9370  
JAN  1978  0.47  9210  
DEC  1978  1.06  7640  
JAN  1979  0.71  7330  
DEC  1979  1.76  6290  
JAN  1980  1.64  6330  
DEC  1980  1.23  7350  
JAN  1981  0.93  6920  
DEC  1981  0.00  5800  10650  
JAN  1982  0.00  5800  10070  
DEC  1982  1.47  6880  
JAN  1983  0.82  6840  
DEC  1983  2.02  5600  
JAN  1984  1.18  5440  
DEC  1984  7.48  5800  75560  
JAN  1985  1.02  5800  20140  
DEC  1985  0.00  6770  
JAN  1986  0.60  6360  
DEC  1986  3.24  5800  14190  
JAN  1987  0.00  5800  16580  
DEC  1987  0.89  5800  12260  
JAN  1988  0.14  5800  11470  
DEC  1988  0.54  8880  
averages =  2.9 cm (1.14 in.)/  5686 acre-ft/  
month  month  

134 
Estimation of 	annual
average 
ground-water recharge to be used as the initial value for numerical model input 
Calculaterecharge inthedrainagebasin above USGS Llano River # 08150000
gauge (drainage basin next to study area which includes 
drainage from both the North and South Llano Rivers) 
area of drainage basin above 1,849 square miles or Llano River stream 
gauge 	1,183,360 acres 
average monthly rainfall during the low-flow 	1.14 inches or .0950 feet 
months of Jan. and Dec. (1939-1988) 
average monthly rainfall in acre-feet 	.0950 feet * 1,183,360 
acres = 112,419 acre-feet 
average monthly discharge at river gauge 5,686 acre-feet (Jan and Dec, 1939-1988). Anomalously high discharge values which are caused 
by surface water runoff are replaced by 
the average of the remaining stream gauge values 
% rainfall measured at Llano River gauge 	5,686 acre-feet / 112,419 acre-feet = 5.06 % 
5.06 % IS THE PERCENTAGE OF RAINFALL DISCHARGING FROM THE AQUIFER INTO THE LLANO RIVER. BECAUSE THIS AQUIFER IS BEING MODELED AS A STEADY-STATE SYSTEM, DISCHARGE = RECHARGE. 
Now transfer this discharge value to the area under study (which has no river 
gaugedowngradient)tobeusedastheinitialestimateofrecharge in each cell of the numerical model. 
annual rainfall in study area 	24.15 inches 
average 
1939-1988 (Dunk data for the NWS) 

24.15 inches ’ .0506 
average annual recharge in study area 
=1.22 inches or 3.10 cm 
135 
Appendix G 
Table of water 

chemistry values 
136 
Table of water chemistry 
values  
Eh  
Eh of  
Project well #  Formation  PH  H + (mg/l)  Eh measured (mv)  calculated approxi­mation (mv)  Zobell standard (mv)  Temp (°C)  02(probe) (mg/l)  02 (mg/l)  
1  Edwards  7.34  4.57E-08  315  488  255  21.4  8.6  7.0  
Limestone  
2  Edwards  7.29  5.13E-08  248  414  262  20.3  7.0  7.0  
Limestone  
Edwards  
3  7.45  3.55E-08  282  431  279  21.7  8.0  7.0  
Limestone  
Edwards  
4  Limestone  7.29  5.13E-08  195  328  295  22.3  6.3  7.0  
Edwards  
5  Limestone  7.46  3.47E-08  260  416  272  20.5  6.8  6.0  
Edwards  
6  7.35  4.47E-08  251  419  260  22.3  2.9  2.0  
Limestone  
Edwards  
7  Limestone  7.52  3.02E-08  235  404  259  20.6  6.0  6.5  
Hensel  
8  7.10  7.94E-08  101  255  275  20.1  1.3  0.4  
Sand  
Hensel  
9  Sand  7.17  6.76E-08  235  415  248  21.3  5,6  5.5  
Hensel  
10  7.30  5.01 E-08  45  213  260  22.1  1.1  0.0  
Sand  
Hensel  
11  7.16  6.92E-08  325  413  340  21.4  5.0  9.0  
Sand  
Hensel  
12  6.95  1.12E-07  310  461  277  22.5  7.1  
Sand  - 
Hensel  
13  Sand  7.45  3.55E-08  310  413  345  21.5  1.9  0.8  
14  Hensel  7.10  7.94E-08  -75  99  254  21.6  1.7  0.0  
Sand  
15  Hensel  7.24  5.75E-08  -30  108  290  22.0  1.2  0.5  
Sand  
Hensel  
16  7.07  8.51 E-08  -5  151  272  21.6  1.8  0.5  
Sand  
Hensel  
17  Sand  7.20  6.31E-08  59  193  294  21.4  2.1  1.0  
18  Alluvium  7.23  5.89E-08  195  388  235  20.8  1.9  1.5  
19  Hensel Sand  7.05  8.91 E-08  255  360  323  21.1  6.5  5.5  
21  Hensel Sand  7.40  3.98E-08  250  228  400  21.6  1.3  0.3  
22  Hensel Sand  7.45  3.55E-08  178  353  367  22.3  1.6  0.3  

137 
Table of water chemistry 
values  
Project well *  Formation  02 % sat.  Fe 2+ (mg/l)  Fe-tot (mgyi)  NH3 (mg/I)  Sulfide (mg/l)  SI02 (mg/l)  Mn (mg/l)  HC03 (mg/l)  
1  Edwards Limestone  100  .00  .00  0.21  .03  15  .00  269  
Edwards  
2  79  .05  .08  0.36  .03  12  .00  319  
Limestone  
Edwards  
3  .02  .08  0.14  .02  21  .00  302  
Limestone  - 
Edwards  
4  76  .14  .02  0.25  .03  12  .00  299  
Limestone  
Edwards  
5  Limestone  78  .02  .08  0.03  .03  15  .00  236  
Edwards  
6  Limestone  - .11  .11  0.40  .03  14  .00  328  
7  Edwards  70  .08  .05  0.32  .03  10  .00  275  
Limestone  
Hensel  
8  15  .14  .14  0.60  03  14  .00  352  
Sand  
Hensel  
9  60  .02  .05  0.36  .04  15  .00  350  
Sand  
Hensel  
10  14  .66  .86  1.25  .04  9  .00  338  
Sand  
Hensel  
11  Sand  - .05  .11  0.48  .04  12  .00  352  
Hensel  
12  82  .08  .11  0.40  .03  18  .00  446  
Sand  
13  Hensel  22  .05  .14  0.73  .04  6  .00  345  
Sand  
14  Hensel  20  .51  1.85  1.74  .06  0  .00  386  
Sand  
15  Hensel  14  .05  .74  1.35  .03  9  .00  331  
Sand  
16  Hensel Sand  21  .11  .24  1.46  .04  9  .00  345  
17  Hensel  25  .05  .41  1.46  03  9  .00  327  
Sand  
18  Alluvium  23  .05  .14  0.25  .06  12  .00  266  
19  Hensel  77  .14  .27  0.17  .03  20  .00  333  
Sand  
21  Hensel  .05  .11  1.70  .03  12  .00  303  
Sand  - 
22  Hensel Sand  _  .02  .02  1.30  .03  10  .00  368  

138 
Table of water chemistry Table of water chemistry Table of water chemistry 
values  
Project well #  Formation  HC03 (meq/1) (mm/l)  F (mg/I)  F (meq/l) (mm/l)  Cl (mg/I)  Cl (meq/1) (mm/l)  N02 (mg/I)  Br (mg/I)  N03 (mg/1)  
i  Edwards  4.41  0.49  .03  19.74  0.56  0.21  0.20  1.59  
Limestone  
Edwards  
2  5.23  0.72  .04  25.42  0.72  0.20  0.23  11.47  
Limestone  
Edwards  
3  Limestone  4.95  0.80  .04  31.93  0.90  0.20  0.22  4.65  
Edwards  
4  Limestone  4.90  0.40  .02  16.65  0.47  0.00  0.00  2.14  
Edwards  
5  3.87  0.59  .03  31.83  0.90  0.24  0.03  2.13  
Limestone  
Edwards  
6  5.38  1.39  .07  34.74  0.98  0.42  0.01  1.59  
Limestone  
Edwards  
7  4.51  
Limestone  - - - - - - - 
Hensel  
8  Sand  5.77  1.55  .08  61.54  1.73  0,39  0.71  0.62  
Hensel  
9  Sand  5.74  0.50  .03  46.84  1.32  0.44  0.03  9.35  
10  Hensel Sand  5.54  1.78  .09  316.73  8.92  0.85  2.17  0.31  
Hensel  
11  5.77  0.47  .02  38.84  1.09  0,35  0.01  8.13  
Sand  
Hensel  
12  Sand  7.31  0.00  .00  36.30  1.02  0.43  0.00  21.80  
13  Hensel  5.66  1.84  .10  151.01  4.25  0.00  1.05  2.35  
Sand  
14  Hensel  6.33  1.32  .07  156.50  4.41  0.43  0.57  0.26  
Sand  
15  Hensel Sand  5.43  1.72  .09  196.75  5.54  0.53  0.96  0.25  
16  Hensel Sand  5.66  1.37  .07  182.05  5.13  0.52  1.45  0.33  
17  Hensel Sand  5.36  3.39  .18  242.17  6.82  0.94  1.34  0.74  
18  Alluvium  4.36  0.39  .02  32.65  0.92  0.16  0.29  0.53  
Hensel  
19  Sand  5.46  - - - - - - - 
21  Hensel Sand  4.97  2.93  .15  104.99  2.96  0.52  0.54  0.30  
22  Hensel  6.03  3.16  .17  104.74  2.95  0.55  0.52  0.24  
Sand  
139  

values  
Project well #  Formation  N03 (meq/t) (mnritl)  S04 (mg/1)  504 (meq/l)  504 (mm/l)  Li (mg/l)  Na (mg/l)  Na (meq/l) (mm/t)  K (mg/1)  
1  Edwards Limestone  .026  11.2  0.23  0.12  .017  17.876  0.777  1.520  
Edwards  
2  .185  12.4  0.26  0.13  .018  20.640  0.897  0.850  
Limestone  
Edwards  
3  .075  13.7  0.29  0.14  .027  22.190  0.965  0.840  
Limestone  
4  Edwards  .035  7.5  0.16  0.08  .000  13.730  0.597  1.520  
Limestone  
Edwards  
5  Limestone  034  21.5  0.45  0.22  .019  27.910  1.213  1.370  
Edwards  
6  .026  56.1  1.17  0.58  .063  41.650  1.811  4.210  
Limestone  
Edwards  
7  .000  21.640  0.941  1.210  
Limestone  - - - - 
Hensel  
8  .010  110.4  2.30  1.15  .000  68.980  2.999  7.820  
Sand  
Hensel  
9  .151  40.0  0.83  0.42  .035  30.580  1.330  0.700  
Sand  
Hensel  
10  .005  109.6  2.28  1.14  .000  170.360  7.407  12.410  
Sand  
Hensel  
11  Sand  .131  26.9  0,56  0.28  .027  28.750  1.250  1.310  
Hensel  
12  .352  24.9  0.52  0.26  .023  47.780  2.077  7.590  
Sand  
Hensel  
13  .038  63.2  1.32  0.66  .111  160.100  6.961  10.780  
Sand  
Hensel  
14  .004  188.0  3.91  1.96  .082  94.260  4.098  9.890  
Sand  
Hensel  
15  Sand  .004  181.3  3.77  1.89  .000  131.580  5.721  14.710  
16  Hensel Sand  .005  174.9  3.64  1.82  .082  83.100  3.613  9.880  
Hensel  
17  .012  625.3  13.02  6.51  .203  361.650  15.724  14.310  
Sand  
18  Alluvium  .009  22.0  0.46  0.23  .028  22.570  0.981  1.660  
19  Hensel  - - - .015  22.450  0.976  3.680  
Sand  - 
21  Hensel Sand  .005  390.1  8.12  4.06  .143  190.840  8.297  16.380  
22  Hensel  .004  389.3  8.10  4.05  .130  196.200  8.530  18.060  
Sand  
140  

values  
Project well #  Formation  K (meq/l) (mm/l)  Ca (mg/l)  Ca (meq/l)  Ca (mm/l)  Mg (mg/l)  Mg (meq/l)  Mg (mm/l)  Sr (mg/l)  
1  Edwards Limestone  .04  48.58  2.429  1.212  30.84  2.538  1.269  0.2  
2  Edwards  .02  83.40  4.170  2.081  24.28  1.998  0.999  0.3  
Limestone  
3  Edwards Limestone  .02  61.53  3.077  1,535  32.41  2.667  1.333  0.7  
4  Edwards  .04  59.41  2.971  1.482  31.44  2.588  1.293  0.3  
Limestone  
Edwards  
5  Limestone  .04  43.53  2.177  1.086  27.23  2.241  1.120  0.4  
Edwards  
6  Limestone  .11  57.09  2.855  1.424  42.23  3.476  1.737  4.0  
Edwards  
7  Limestone  .03  50.69  2.535  1.265  33.86  2.787  1.393  0.9  
Hensel  
8  Sand  .20  62.39  3.120  1.557  47.63  3.920  1.959  5.5  
9  Hensel Sand  .02  76.80  3.840  1.916  42.20  3.473  1.736  0.5  
Hensel  
10  .32  91.35  4.568  2.279  72.32  5.952  2.975  10.7  
Sand  
Hensel  
11  .03  73.88  3.694  1.843  40.07  3.298  1.648  0.6  
Sand  
12  Hensel Sand  .19  83.90  4.195  2.093  42.08  3.463  1.731  0.8  
Hensel  
13  Sand  .28  44.56  2.228  1.112  24.37  2.006  1.002  2.5  
Hensel  
14  .25  94.08  4.704  2.347  75.02  6.174  3.086  6.3  
Sand  
15  Hensel  .38  78.30  3.915  1.954  66.22  5.450  2.724  13.9  
Sand  
16  Hensel Sand  .25  93.17  4.659  2.325  77.10  6.346  3.172  10.3  
17  Hensel Sand  .37  85.41  4.271  2.131  77.56  6.384  3.190  22.8  
18  Alluvium  .04  65.78  3.289  1.641  23.19  1.909  0.954  1.8  
19  Hensel  .09  87.86  4.393  2.192  26.91  2.215  1.107  0,9  
Sand  
21  Hensel  .42  72.01  3.601  1.797  57.64  4.744  2.371  17.5  
Sand  
22  Hensel Sand  .46  71.42  3.571  1.782  58.01  4.774  2.386  17.6  

141 
Table of water chemistry 
values  
HC03­ 
Project well #  Formation  2(Ca+Mg­S04+.5(Na­Cl))  Ca+Mg  (Ca+Mg­S04+ .5"(Na-CI))  (Ca+Mg­S04+.5(Na­CI))/HC03  Ca+Mg/HC 03  Na/CI  Ca/S04  Ca/HC03  
1  Edwards Limestone  -0.540  2.481  2.475  0.561  0.563  1.398  10.413  0.275  
2  Edwards  -0.853  3.080  3.041  0.582  0.589  1.253  16.139  0.398  
Limestone  
Edwards  
3  Limestone  -0.566  2.868  2.758  0.557  0.579  1.073  10.769  0.310  
4  Edwards  -0.622  2.776  2.762  0.563  0.566  1.273  19.047  0.302  
Limestone  
5  Edwards  -0.413  2.206  2.141  0.553  0.570  1.354  4.853  0.281  
Limestone  
Edwards  
6  -0.610  3.162  2.994  0.557  0.588  1.851  2.438  0.265  
Limestone  
Edwards  
7  Limestone  - 2.658  - - 0.589  - - 0.281  
8  Hensel Sand  -0.230  3.516  3.000  0.520  0.609  1.730  1,355  0.270  
9  Hensel  -0.745  3.652  3.241  0.565  0.637  1.008  4.606  0.334  
Sand  
10  Hensel  -1.170  5.254  3.356  0.606  0.948  0.830  1.998  0.411  
Sand  
11  Hensel  -0.808  3.492  3.289  0.570  0.605  1.142  6.579  0,319  
Sand  
12  Hensel  -0.874  3.824  4.093  0.560  0.523  2.032  8.080  0.286  
Sand  
Hensel  
13  Sand  0.037  2.114  2.809  0.497  0.374  1.636  1.689  0.197  
14  Hensel  -0.314  5.433  3.321  0.525  0.859  0.930  1.199  0.371  
Sand  
Hensel  
15  -0.334  4.678  2.880  0.531  0.862  1.032  1.035  0.360  
Sand  
16  Hensel Sand  -0.181  5.496  2.918  0.516  0.972  0.705  1.277  0.411  
17  Hensel Sand  -1.167  5.321  3.264  0.609  0.993  2.305  0.327  0.398  
18  Alluvium  -0.433  2.595  2.397  0.550  0.595  1.067  7.168  0.376  
19  Hensel Sand  - 3.299  - - 0.604  - - 0.402  
21  Hensel  -0.587  4.168  2.777  0.559  0.839  2.806  0.442  0.362  
Sand  
22  Hensel Sand  0.221  4.168  2.906  0.482  0.691  2.891  0.440  0.295  

142 
Table of water chemistry 
values  
Project well #  Formation  Sr (meq/l)  Sr (mm/l)  Hardness (mg/l)  %Na  SAR  RSC  IDS (mg/l)  Ionic Strength (mm)  
1  Edwards  .005  .002  249  13.4  0.5  -0.6  279  0.008116  
Limestone  
2  Edwards  .007  .004  309  12.7  0.5  -0.9  348  0.009968  
Limestone  
3  Edwards  .015  .007  288  14.3  0.6  -0.8  338  0.009513  
Limestone  
4  Edwards Limestone  .006  .003  278  9.6  0.4  -0.7  292  0.008744  
Edwards  
5  Limestone  .008  004  222  21.4  0.8  -0.5  287  0,007908  
Edwards  
6  .090  .045  321  22.0  1.0  -1.0  418  0.011769  
Limestone  
Edwards  
7  .019  .010  267  15.0  0.6  -0.8  
Limestone  - - 
Hensel  
8  .124  .062  359  29.3  1.6  -1.3  553  0.014851  
Sand  
Hensel  
9  .011  .005  367  15.4  0.7  -1.6  435  0.012438  
Sand  
Hensel  
10  .245  .122  539  40.6  3.2  -5.0  961  0.024178  
Sand  
Hensel  
11  Sand  .013  .006  351  15.1  0.7  -1.2  404  0.011708  
Hensel  
12  .018  .009  384  20.9  1.1  -0.3  502  0.013663  
Sand  
Hensel  
13  .057  .029  215  60.7  4.8  1.4  636  0.014243  
Sand  
Hensel  
14  Sand  ,145  .072  552  26.9  1.8  -4.6  815  0.022507  
15  Hensel  .316  .158  485  37.0  2.6  -3.9  856  0.022026  
Sand  
Hensel  
16  .234  .117  562  24.3  1.5  -5.3  811  0.022231  
Sand  
17  Hensel Sand  .521  .261  559  58.8  6.8  -5.3  1603  0.038414  
18  Alluvium  .041  .020  262  15.8  0.6  -0.8  313  0.008856  
19  Hensel Sand  .020  .010  332  12.7  0.5  -1.1  - - 
21  Hensel Sand  .399  .200  438  48.6  4.1  -3.4  1014  0.025256  
22  Hensel Sand  .403  .201  438  49.2  4.2  -2.3  1050  0.025917  

143 
Table of water chemistry 
values  
Sum of  Sum of  Charge  
Project well#  Formation  Cations  Anions  Balance Error  Na/CI  Mg/Ca  Ca+Mg-S04  Na-CI  .5(Na-CI)  
(meq)  (meq)  (%)  
Edwards  
1  Limestone  5.79  5.25  4.9  1.398  1.047  2.364  0.221  0.111  
Edwards  
2  Limestone  7.09  6.43  4.9  1.253  0,480  2.951  0.181  0.091  
Edwards  
3  Limestone  6.75  6.25  3.8  1.073  0.868  2.726  0.065  0.033  
Edwards  
4  6.20  5.58  5.2  1.273  0.873  2.698  0.128  0.064  
Limestone  
Edwards  
5  Limestone  5.67  5.28  3.6  1.354  1.031  1.982  0.317  0.158  
Edwards  
6  Limestone  8.34  7.62  4.5  1.851  1.220  2.577  0.832  0.416  
Edwards  
7  Limestone  6.31  - - - 1.101  - - - 
Hensei  
8  Sand  10.36  9.89  2.3  1.730  1.259  2.367  1.266  0.633  
Hensei  
9  8.67  8.07  3.6  1.008  0.906  3.236  0.010  0.005  
Sand  
Hensei  
10  Sand  18.49  16.84  4.7  0.830  1.305  4.113  -1.515  -0.757  
Hensei  
11  8.29  7.58  4.5  1.142  0.894  3.211  0.156  0.078  
Sand  
Hensei  
12  Sand  9.95  9,20  3.9  2.032  0.827  3.565  1.055  0.527  
Hensei  
13  Sand  11.53  11.36  0.7  1.636  0.902  1.456  2.707  1.353  
Hensei  
14  15.38  14.72  2.2  0.930  1.315  3.476  -0.310  -0.155  
Sand  
Hensei  
15  Sand  15.78  14.84  3.1  1.032  1.394  2.791  0.179  0.089  
16  Hensei Sand  15.10  14.50  2.0  0.705  1.364  3.676  -1.515  -0.758  
17  Hensei  27.27  25.39  3.6  2.305  1.497  -1.187  8.902  4.451  
Sand  
18  Alluvium  6.26  5.77  4.1  1.067  0.581  2.366  0.062  0.031  
19  Hensei Sand  7.70  - - - 0.505  - - - 
21  Hensei  17.46  16.20  3.7  2.806  1.320  0.107  5.340  2.670  
Sand  
22  Hensei Sand  17.74  17.26  1.4  2.891  1.339  0.116  5.580  2.790  

144 
Bibliography 
145 
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The vita has been removed from the digitized version of this document.