Browsing by Subject "Edwards Aquifer (Tex.)"
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Item Correlation of structural lineaments and fracture traces to water-well yields in the Edwards Aquifer, Central Texas(1990) Alexander, Kenneth Bower, 1961-; Bennett, Philip C. (Philip Charles), 1959-Lineaments are "straight lines visible from afar on the surface of the earth". In the Austin, Texas area, lineaments reflect the structural grain of the Balcones-Ouachita fault zone and may indicate subsurface geologic phenomena such as faults, fractures, and joints. These structural features often represent discrete zones of high permeability, and thus, areas of enhanced flow of groundwater capable of transmitting greater quantities of water than surrounding, non-fractured, rock. For this study more than 900 lineaments and fracture traces, identified in aerial photographs during a previous study, were detected in the Barton Springs section of the Edwards Aquifer. The endpoints of each linear feature were digitized and tagged with a unique identification label. Rose plots, Cartesian histograms, and a series of statistical operations were utilized to illustrate regional trends in the orientation of lineaments. As an indicator of well productivity, specific capacities of 27 wells in the area were obtained. Sixty-one water samples were collected and analyzed to test for possible chemical evidence of lineament-well interactions. The orientations of lineaments and fracture traces in the study area clearly display a bimodal distribution with a primary trend of N 40 E and a secondary peak of N 50 W. A general correlation exists between increased well productivity and decreased distances to the nearest lineament, particularly within 200 feet of lineaments. Also, 10 of the 13 largest specific-capacity values are from wells located southeast of southwest-northeast trending lineaments. Nonparametric statistical methods show that direction from lineaments is a significant factor in predicting water-well yields. Lineaments provide a tool for predicting possible sites of environmental sensitivity with respect to groundwater resources. Examples include the siting of groundwater monitoring wells for point sources of pollution, predicting the likely underground flow paths of a pollution plume or locating dam sites for recharge enhancement. Awareness of the location, orientation, and density of structural lineaments will allow the water-resource manager to identify discrete groundwater flow paths, and, thus, predict contaminant plume migration.Item The Edwards formation in the Balcones Fault zone(1930) Eifler, Gus Kearney; Not availableItem Geologic map of the New Braunfels, Texas, 30 x 60 minute quadrangle(Bureau of Economic Geology University of Texas at Austin, 2000) Collins, Edward W.Portion of a scanned and georeferenced version of Collins, E.W., 2000, Geologic Map of the New Braunfels, Texas, 30x60 Minute Quadrangle: University of Texas at Austin, Bureau of Economic Geology, Miscellaneous Map MM-39, scale 1:100,000.Item Hydrochemical facies in the badwater zone of the Edwards Aquifer, central Texas(1989) Clement, Tonia Judith, 1944-; Sharp, John Malcolm, Jr., 1944-The Cretaceous Edwards and Georgetown Formations and their stratigraphic equivalents form two extensive carbonate aquifers in central Texas. These faulted, cavernous, and highly transmissive formations are the sole source of water for many communities, including the City of San Antonio. At the badwater line, which delineates a hydrogeologic boundary from 1 to 21mi (2-34km) south and east of the outcrop, the total dissolved solids concentration of the groundwater changes abruptly from less than 500 mg/l to over 1000 mg/l. Although the boundary is relatively stable, some water from the badwater zone migrates updip toward major springs and well fields. Unlike the water in the freshwater zone which is predominantly Ca-HCO₃, the chemistry of the badwater zone changes along the strike of the aquifer. From west to east, the first hydrochemical facies is Ca-SO₄ with a low chloride concentration. Faulting is minimal, and chlorides previously trapped in the pores or adsorbed by the carbonates have been flushed. This facies is produced by the dissolution of anhydrite in the badwater zone. As the potentiometric surface declines and the intensity of faulting increases to the east, sodium and chloride concentrations also rise. The hydrochemical facies changes first to Ca-Mg-SO₄ with high sodium and chloride concentrations and then, in the area of maximum faulting, to Na-Cl with calcium, magnesium, and sulfate subordinate. Using PHREEQE computer modeling, the second facies can be simulated by mixing fresh water with a Na-Ca-Cl oilfield brine and saturating it to specific states with calcite, dolomite, and gypsum. The origin of the Na-Cl facies with Na/Cl molar ratios close to one is still problematical. The fourth hydrochemical facies borders the northern segment of the aquifer and is predominately a Na-Cl-SO₄-HCO₃ water, the anions being approximately coequal in milliequivalents per liter. This last facies is almost identical to groundwaters in the underlying Trinity Group which probably have provided some recharge to the aquifer. Leakage from the Trinity Group or updip migration of bad water also may affect the quality of fresh water in this areaItem Permeability structure in fractured aquifers(2000) Halihan, Todd, 1970-; Sharp, John Malcolm, Jr., 1944-Understanding water movement through fractured and karstic aquifers is difficult, but it is important for those managing these resources. Determining which features contribute to flow in these aquifers is important because accurate predictions of flow and transport are most sensitive to variations in the permeability field. A continuum approach to these aquifers has led to two problems that are approached with two new techniques that quantify the permeability. First, continuum hydraulics does not allow an understanding of which individual features in an aquifer provide flow. This permeability structure problem is manifested in scale dependent permeability (larger permeability values for larger scale tests). Interpretations of these aquifers are limited by ignoring small-scale data in addressing larger scale problems. The Edwards aquifer of central Texas was used to determine if data sets of matrix permeability, fracture aperture, and conduit size from cores and outcrops can be effective utilized to interpret permeabilities measured at the small-, well-, and regional-scales. The results demonstrate that by quantifying permeability on the small-scale, larger scale interpretations of the aquifer are possible and have a stronger quantitative basis by utilizing geologic information from the aquifer. The second problem is that standard hydraulic measurement techniques are optimized for porous media. This approach does not allow individual features and their connections with other features to be easily evaluated in these aquifers. Fractured carbonate aquifers in Wisconsin and Australia were evaluated using asymmetric dipole-flow tests to determine if the structure of permeability could be determined more effectively. Dipole-flow testing, analogous to resistivity dipole testing, is a relatively new technique that was developed from the use of recirculation wells in contaminant remediation. This asymmetric technique may overcome many of the problems inherent in other testing strategies. Asymmetric dipole-flow tests provided rapid testing and demonstrated the ability to quantify heterogeneities. The results demonstrate that boreholes can be connected in complex geometries with drawdown occurring above and below areas of pressure buildup.Item The potential for springflow augmentation at Comal and San Marcos Springs, central Texas(1995) Uliana, Matthew Martin; Sharp, John Malcolm, Jr., 1944-The Edwards aquifer, a regionally-extensive carbonate aquifer in Central Texas, is the sole source of water for nearly two million people, including the City of San Antonio. Pumpage from this aquifer is jeopardizing springflow from Comal Springs (in New Braunfels, Texas) and San Marcos Springs (in San Marcos, Texas), two of the largest springs in the state. These springs provide a habitat for a number of endangered species, and are an integral part of the local economies and the overall distribution of water in Central Texas. For these reasons it is important that flow from these springs be maintained, either through aquifer management plans that limit the withdrawal and usage of water from the aquifer, or through physical augmentation of the springs. In this thesis, the feasibility of maintaining springflow by augmenting discharge from the springs is investigated. Five potential methods of springflow augmentation (enhanced recharge, subsurface flow barriers, direct addition of water to the spring lakes, injection wells, and infiltration galleries) are presented and described. Based on present knowledge of the springs, the effectiveness of each method is evaluated, and the uncertainties associated with each are discussed. Computer models of the aquifer in the vicinity of the springs are constructed and used to model the effects of two of the augmentation methods (injection wells and infiltration galleries) on aquifer levels and spring discharges. The model results are used to estimate the efficiency of each of these methods and to develop general trends related to how the aquifer responds to the introduction of water through wells and infiltration galleries. Based on model results and the hydrogeology of each spring, an assessment of the each of the augmentation methods is presented, and recommendations are made as to the appropriateness of the various technologies at each spring.Item Stygobite phylogenetics as a tool for determining aquifer evolution(2005) Krejca, Jean Kathleen; Hillis, David M.; Hendrickson, Dean.Abstract: The use of aquifer-dwelling organisms (stygobites) for learning about past and present subterranean hydrologic connections was evaluated in the Edwards (Balcones Fault Zone), Trinity, and EdwardsTrinity (Plateau) aquifers of Texas and adjacent areas in north Mexico, an area with complex karst groundwater flow and sociopolitical problems stemming from overuse and contamination. A priori predictions of subterranean hydrogeologic history were made based on a literature review, and these predictions were compared to phylogenies of two aquifer dwelling isopods created based on mitochondrial gene sequences (16S ribosomal RNA and cytochrome c oxidase subunit I). Using likelihood and parsimony-based comparisons, Cirolanides (Isopoda: Cirolanidae) was found to have a phylogenetic history congruent with a priori predictions of subterranean hydrogeologic history in its terminal nodes. Conversely, basal branches of the phylogenetic tree had placement that was not predicted by this history, a phenomenon that may be indicative of a lack of understanding of subterranean hydrogeology of the area. Lirceolus (Isopoda: Asellidae) had a phylogenetic history congruent with an alternative hypothesis of water flow, namely the patterns of surface drainages. This difference of patterns for two species that both live in the aquifer is probably related to their ecology and evolutionary history, with Cirolanides having invaded the cave habitat as a single marine population and Lirceolus invading the cave habitat as a freshwater migrant with possible pre-existing genetic structure determined by surface drainages. This study pioneers testing of a priori biogeographic hypotheses using phylogenies of aquifer organisms and the creation of hydrogeologic histories in a karst setting, and supports the use of these methods to aid in understanding biogeography and aquifer evolution.