Report of Investigations No. 141 Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas

UT Librarie

Hydrogeology of Barton Springs, Austin, Texas

The major point of discharge of the Edwards aquifer between the Colorado River and the ground-water flow divide near Kyle, 15 miles south of Austin, is Barton Springs; it comprises five major springs. The Balcones Fault Zone southwest of Austin is the principal zone of recharge to the aquifer. Changes in water levels of wells in the area show good correlation with changes in spring discharge, indicating good interconnection. The potentiometric surface of the aquifer shows a shift from conditions of high flow to low flow. During low flow groundwater flow lines are concentrated in the eastern portion of the Balcones Fault Zone. Water-levels are also significantly lower. The water levels of wells in the Rollingwood area do not follow that pattern. Overall hydrologic parameters of the aquifer were estimated by applying recession curve analyses to hydrographs of the spring discharge and of water-level declines in the potentiometric surface throughout the aquifer. Additionally, a two-dimensional ground-water flow model was constructed for the northeastern part of the aquifer in order to simulate the observed water-level fluctuations in well 58-42-915. The average value of transmissivity inferred from the model agrees well with results based on the recession curve analysis. Storativity, however, differed by about one order of magnitude. The water chemistry of the springs varies also between high-flow and low-flow discharge. The concentrations of Na, Cl, SO4, and Sr increase during low flow, indicating influx from the 'bad-water' zone (water from downdip Edwards with 1000 ppm total dissolved solids or more). This inflow of water from the 'bad-water' zone during low flow is also documented by the water chemistry of well 58-50-216, approximately two miles south of Barton Springs; during dry periods there is a large increase in total dissolved solids in that well. Even though the chemical composition of Barton Springs changes with varying discharge, the general water chemistry in the Edwards limestone aquifer remains constant. The aquifer contains calcium-bicarbonate water that evolves to a sodium-sulphate water and then a sodium-chloride water as it moves dondip. In some locations, however, leakage from the Glen Rose Formation increases the sulphate and strontium concentrations. This leakage occurs along large displacements of faults, where the Edwards Formation is adjacent to the Glen Rose Formation. In addition, carbonate equilibria of selected samples from the aquifer, springs, and creeks were calculated. Creek water is saturated with respect to calcite and dolomite during conditions of approximate steady state flow. During floods after heavy rainfall the water chemistry of most of the creeks, except for Barton Creek, indicate undersaturation with respect to calcite and dolomite. Saturated spring water which occurs only during very high discharge could result because spring flow is sustained to a major part by saturated flood water from Barton Creek. The influx of highly saturated 'bad-water' appears to have little effect on the saturation state of water from Barton Springs.Geological Science

Hydrodynamic Development of the Palo Duro Basin and Other Mechanisms Creating Possible Transient Flow Conditions

Characterization of the regional flow regime in the Palo Duro Basin is assisted by numerical groundwater flow models as described by INTERA (1984), Senger and Fogg (1984), and Wirojanagud and others (1984). In general, the models which incorporate the available hydrogeologic information simulate hydraulic head distribution and fluxes under steady-state conditions. The computed heads are then compared to measured heads in order to evaluate the adequacy of the conceptual model. The conceptualized flow regime in the Palo Duro Basin is generally assumed to represent a steady-state, gravity-driven flow system of the type described by Hubbert (1940). Groundwater flow is governed by the fluid potential along the topographic surface and permeability of the hydrostratigraphic units. In the Palo Duro Basin, extensive modification of the topography as a result of tectonic and geomorphologic processes has occurred within the last 15 million years (McGookey, 1984; Gustavson and others, 1981). Accordingly, it is possible that hydraulic heads observed in the Deep-Basin Brine aquifer represent transient conditions and that they are still equilibrating to modifications of topography in the past. Significant hydrocarbon occurrences in the Texas Panhandle suggest other possible mechanisms creating transient flow conditions as a result of reservoir pressure decline due to hydrocarbon production. The purpose of this study is to investigate transient flow conditions and to identify possible flow patterns resulting from changing hydrologic conditions with time. For this purpose, the model herein is used to simulate possible groundwater flow patterns caused by different tectonic and geomorphologic processes. The hydrodynamic development of the basin is crucial to the origin and hydrochemical evolution of the fluid in the deep basin. Changing hydraulic head distributions with time result in changes of groundwater flow paths. Consequently, transport of chemical constituents could have traveled along ancient pathways much different than is suggested by the present-day hydraulic head distribution observed in the basin.Bureau of Economic Geolog

Identification of Recharge-Discharge Areas of the Palo Duro Basin, Texas Panhandle

This report presents the preliminary results of research to identify the recharge and discharge areas of the major aquifers in the Palo Duro Basin. Recharge has been investigated by studying the isotopic compositions of surface and shallow ground waters in New Mexico and Texas. Discharge studies have focused on identifying the source of the brine discharged in springs and seeps along the outcrop of Permian rocks east of the Caprock Escarpment.Bureau of Economic Geolog

Wellhead Protection Strategies for Confined-Aquifer Settings

Improper management of contamination sources has resulted in numerous cases of groundwater contamination of public water supply wells. One approach toward preventing contamination of public water supplies is to protect the areas that recharge precipitation and surface water to the aquifer near the wells. This zone of protection is referred to as a wellhead protection area (WHPA). The potential for contamination is typically less in a confined aquifer than in an unconfined aquifer. Nevertheless, contamination of confined aquifers has occurred. Wellhead protection areas should be developed for all aquifer settings. A confined aquifer is an aquifer overlain by low-permeability strata. The presence of the low permeability material reduces the risk of a surface contaminant reaching a producing well. The potential for contamination of a confined aquifer is controlled by two factors: (1) The presence of permeable pathways (for example, faults, fractures, permeable sands, or unplugged abandoned boreholes) that permit contaminant migration and (2) the existence of appropriate hydrologic conditions (for example, downward flow) that cause contaminants to migrate through the low-permeability strata. Confined aquifers occur pervasively from coast to coast in the United States. The coastal plain aquifers along the Atlantic Ocean and Gulf of Mexico represent some of the largest confined aquifer systems in the United States. There are numerous other smaller aquifers which exhibit confined conditions.Bureau of Economic Geolog

Modeling the Effects of Regional Hydrostratigraphy and Topography on GroundWater Flow, Palo Duro Basin,Texas

A cross-sectional ground-water flow model was constructed of the Palo Duro Basin to analyze available hydrogeologic data and better understand the causes of underpressuring below the Evaporite Aquitard and mechanisms of recharge and discharge to and from the Deep-Basin Brine Aquifer. Various effects of lithostratigraphy and topography on subhydrostatic conditions in the deep section were investigated in different simulations. The model indicates that the subhydrostatic pressures beneath the Evaporite Aquitard are caused by segregation of deep and shallow flow systems by the low-permeable evaporite section and drainage of the deep system by relatively permeable granite wash deposits. The Pecos River, which allows underflow of some groundwater recharging in the New Mexico area to the west, enhances underpressuring beneath the western half of the High Plains by serving as a discharge area for water that would otherwise move downdip into the Deep-Basin Brine Aquifer. In addition to this recharge, about 26% of the groundwater in the Deep-Basin Brine Aquifer originates from leakage through the evaporite section, assuming K2 = 2.8 x 10-4 md, the upper limit of aquitard permeability suggested by the model. The groundwater flow pattern within the Deep-Basin Brine Aquifer is governed by the spatial distribution of more permeable strata, particularly the granite wash deposits. In the cross-sectional model, most of the groundwater in the Deep-Basin Brine Aquifer discharges laterally through the eastern boundary and eventually by upward leakage in the easternmost part of the cross-section. Ground-water travel times through the Deep-Basin Brine Aquifer from the westernmost recharge area in New Mexico to the eastern boundary of the model range between 1.2 and 4 million years, depending on the flow path depicted by the streamtubes and average porosities of the different units.Bureau of Economic Geolog

Geologic and Hydrologic Investigations Reese Air Force Base, Lubbock, Texas

The Bureau of Economic Geology has conducted a characterization of the geology and hydrology of the Ogallala and Blackwater Draw Formations at Reese Air Force Base to establish baseline information for better delineation of contamination on the base. Analysis of five cores revealed two predominant depositional facies: eolian sands and fluvial gravels. The gravels seem to extend continuously across the base and likely constitute the primary water-bearing unit, potentially serving as the main pathway for contaminant migration in the groundwater beneath the base. Continuous water-level recorder data from water-supply wells and monitoring wells on the base indicated a mix of confined and unconfined conditions. The degree of confinement may be influenced by the wells' proximity to the playa lakes on the base or by the relationship of the water levels in the wells to the top of the water-bearing gravel unit. This variability in confinement suggests a hydrological setting that is more intricate than typically recognized for the Ogallala aquifer.Bureau of Economic Geolog

Hydrologic Characterization of Ground Water Resources in South Central Hudspeth County, Texas

Regional and local hydrologic investigations have been conducted in Trans-Pecos Texas at the principal study area for a low-level radioactive waste repository. The area is approximately 40 mi (65 km) southeast of El Paso in the Hueco Bolson, a fault-bounded desert basin that developed in the late Tertiary. Groundwater in the principal study area is found in Hueco Bolson silts and sands at depths of 361 ft (110 m) and 478 ft (146 m), and at depths of 592 ft (180 m) in Cretaceous limestones. The unsaturated zone consists of approximately 50 ft (15 m) of alluvial silt, sand, and gravel underlain by 300 to 500 ft (91 to 152 m) of lacustrine and fluvial clay, silt, and fine sand. The scope of this investigation included (1) evaluating groundwater resources in the area, (2) determining groundwater flow paths and velocities, and (3) testing hydrologic hypotheses using groundwater flow models. Development of groundwater resources in the vicinity of the principal study area is limited by two key factors: (1) costs of drilling and completing wells and of producing water at depths typically greater than 400 ft (122 m), and (2) the very low productivity of aquifers. Transmissivities of aquifers in bolson and Cretaceous strata, as revealed by 11 aquifer tests, range from approximately 0.19 to 290.0 ft²/d (0.018 to 26.9 m²/d); corresponding permeabilities range from 0.0015 to 2.82 ft/d (0.0005 to 0.861 m/d). A composite potentiometric surface based on water levels measured in all available wells and on the hydrologic interconnection of the Diablo Plateau aquifer, Hueco Bolson silt and sand aquifer, and Rio Grande alluvium aquifer indicates that groundwater is recharged on the Diablo Plateau and flows to the south and southwest toward the Rio Grande beneath the bolson pediment. The inferred distribution of permeability zones focuses flow from the eastern Diablo Plateau toward Cretaceous outcrops along the Campo Grande fault, creating an observed potentiometric high. The relatively low hydraulic heads near the principal study area are caused by preferential drainage along relatively permeable bolson deposits to the west and southwest toward the Rio Grande. Water chemistry data, particularly on tritium, carbon-14, and total dissolved solids, generally support the interpreted flow pattern; some discrepancies can be related to paleohydrologic effects associated with the incision of the Rio Grande during Quaternary time.Bureau of Economic Geolog

Hydrologic Investigations of the Saturated Zone in South Central Hudspeth County, Texas

Hydrologic investigations are in progress in Trans-Pecos Texas at the proposed site of a low-level radioactive waste repository. The site is approximately 40 mi (65 km) southeast of El Paso in the Hueco Bolson, a fault-bounded desert basin that developed in the late Tertiary. Groundwater in the area of the proposed site is found at depths of 478 ft (146 m) and 592 ft (180 m) in bolson silt and sand and Cretaceous limestone, respectively. The unsaturated zone consists of approximately 50 ft (15 m) of alluvial silt, sand, and gravel underlain by 450 ft (137 m) of lacustrine and fluvial clay, silt, and fine sand. High-priority tasks for characterizing the groundwater regime include (1) evaluating groundwater resources in the area, (2) determining groundwater flow paths and velocities, (3) testing hydrologic hypotheses using groundwater flow models, and (4) determining groundwater hydrochemistry. The objective of this report is to evaluate the groundwater resources and to discuss our current understanding of groundwater flow paths and velocities. Groundwater resources in the vicinity of the site are limited by two key factors: (1) costs of drilling and completing wells and of producing water at depths greater than 400 ft (122 m) and (2) very low productivity of aquifers. Typical transmissivities of aquifers producing from bolson and Cretaceous strata, based on four tests, range from approximately 4.3 x 10^-2 ft²/day (4.0 x 10^-3 m²/day) to 2.9 x 10^2 ft²/day (2.7 x 10^1 m²/day). A composite potentiometric surface has been mapped on the basis of static water levels measured in all available wells. Regional groundwater flow is interpreted from the potentiometric surface to be generally south-southwest toward the Rio Grande.Bureau of Economic Geolog

Geology and Hydrology of the Northern Segment of the Edwards Aquifer with An Emphasis on the Recharge Zone in the Geogetown, Texas, Area

In March 1986, the Bureau of Economic Geology was contracted by the Texas Water Development Board to conduct geologic and hydrologic investigations of the northern segment of the Edwards aquifer along the Balcones Fault System in the Georgetown-Round Rock area, particularly to elucidate the processes and areas of recharge for the Edwards in the Georgetown area. Geological mapping, combined with analysis of fractures in Comanche Peak, Edwards, and Georgetown limestones (Edwards aquifer strata) in the vicinity of Georgetown and Round Rock, Texas, was conducted to provide data useful in identifying potential recharge areas, assessing local groundwater flow, and enhancing our understanding of the geology of the Balcones Fault Zone. Cretaceous Comanche Peak, Edwards, Georgetown, Del Rio, Buda, Eagle Ford, and Austin strata dip gently (1°) eastward and are overlain in some places by terrace deposits and alluvium. Several major normal faults, downthrown to the east, strike northward across the area. Gentle flexures, possibly related to faulting, parallel the faults. Minor normal faults and joints are most abundant in areas adjacent to major faults and flexures. These fractured-strata zones likely parallel the length of the faults or flexure axes and may be as wide as 1.6 km. Most minor faults strike between 340°-040°, have displacements less than 2 m, and dip from 40°-80° both eastward and westward. Most joints strike between 340°-020° and 260°-300°, and fracture densities range from 4 joints per meter to 1 joint per 5 meters in 1 to 2-meter-thick beds.Bureau of Economic Geolog