Bureau Research into groundwater resources looks at such practical questions as how much water can be pumped from an aquifer, how much recharge is there, and how much water will be left in 50 years. Researchers answer such questions via numerical modeling, original field research, and data collection and analysis. Numerical or computer models become the tool-of-choice for making long-term water resource predictions. Field studies help to document recharge rates and flow paths of groundwater in the recharge zone. Collection and analysis of data are required to better understand all aspects of an aquifer including the geologic distribution of hydraulic properties (hydraulic conductivity and storativity), recharge and discharge, exchange or interaction of groundwater and surface water, and the movement of groundwater across the boundaries of an aquifer.
For more information about these reports, please contact the respective authors.
The objective of this study was to quantify the distribution of groundwater arsenic in the major aquifers in Texas and assess linkages to populations using this water. Groundwater arsenic data were compiled from 10,489 wells sampled between 1992 and 2017. The spatial distribution of elevated arsenic concentrations was mapped by aquifer using indicator kriging based on two threshold concentrations: 5 μg/L representing nominal background concentration and 10 μg/L representing the EPA Maximum Contaminant Level (MCL). The current number of non-compliant PWS systems and associated populations were obtained from EPA listings and the estimated populations with non-compliant non-PWS system water (domestic/self-supplied systems) were obtained from the U.S. Geological Survey water use data. Results show that a total of 733 samples exceeded the arsenic MCL of 10 mg/L, representing 19% of all analyses with detectable arsenic and 7% of all samples used in this study 6,667 samples (64% of all samples) were below the detection limit. The remaining samples with detectable arsenic levels (3,822 samples, 36% of all samples) have a median concentration of 4.3 μg/L arsenic. The range of arsenic concentrations is fairly narrow.
Evaluation of Groundwater Nitrate Contamination in Public Water Systems and Major and Minor Aquifers in Texas
The Bureau of Economic Geology conducted a study to evaluate groundwater nitrate contamination in public water systems and in major and minor aquifers in the state to better understand the spatial distribution of groundwater nitrate contamination. Results of this study can be found in this report:
- Reedy, R. C., and Scanlon, B. R., 2017, Evaluation of Groundwater Nitrate Contamination in Public Water Systems and Major and Minor Aquifers in Texas, Final Contract Report prepared by the Bureau of Economic Geology for the Texas Commission on Environmental Quality, 64 p. [PDF]
Study on Characteristics and Impacts of Groundwater Planning in the Carrizo-Wilcox Aquifer
Bridget R. Scanlon, Principal Investigator
The Bureau of Economic Geology (BEG) conducted a study to collect and review a wide variety of information, develop datasets and conduct a series of analyses regarding current activities related to groundwater management of the Carrizo-Wilcox Aquifer in Texas. The study was based on a research contract from the Texas Commission on Environmental Quality in response to the 81st Texas Legislature directive to "conduct a study of the characteristics and impacts on groundwater planning in the Carrizo-Wilcox Aquifer." (General Appropriations Act, Article VI, Texas Commission on Environmental Quality, Rider 36). The report provides results for the following themes:
- Analysis and results from stakeholder surveys developed to solicit input from interested parties, including groundwater conservation districts (GCDs) with jurisdictional responsibilities over the Carrizo-Wilcox Aquifer,
- Summary of the adequacy of science utilized by GCDs during development and adoption of desired future conditions, management plans, rules, and formal procedures,
- Evaluation of desired future conditions, management plans, rules, regional water plans, and the potential for conflict,
- An evaluation and critique of the State's Groundwater Availability Models for the Carrizo-Wilcox Aquifer, and
- An assessment of whether the presence of anthropogenic contaminants in the recharge area of the Carrizo-Wilcox Aquifer and potential pollution of the aquifer are issues that should be addressed.
- Rules and management plans provided by the 21 groundwater conservation districts
- Carrizo-Wilcox report documents [zip file]
- Carrizo-Wilcox survey documents [zip file]
Final Report: Study on Characteristics and Impacts of Groundwater Planning in the Carrizo-Wilcox Aquifer [PDF]
TCEQ Compliance Feasibility Studies for Small Public Water Systems
TCEQ sponsored a project team led by UT BEG to identify, evaluate, and rank localized compliance strategies for selected small public water systems (PWS) that were noncompliant with one or more of the federal/state chemical maximum contaminant levels (MCLs). The following includes the reports of feasibility studies to bring small Public Water Systems into compliance with the noted regulations. The reports are organized by System Name, TCEQ PWS ID #, study year, county, study region, population, water production(mgd) and contaminant type. The reports are reflective of conditions on the ground at the time the respective report was developed. [VISIT SITE]
Inclusion of Queen City & Sparta Aquifers into Existing Carrizo-Wilcox Groundwater Availability Models
Jean-Philippe Nicot, Principal Investigator; Alan Dutton (now at The University of Texas at San Antonio), H. Seay Nance, Bridget R. Scanlon, and Robert C. Reedy; assisted by Katherine Kier and Thandar Phyu.
The Queen City and Sparta aquifers are locally important water resources in Texas and are classified as minor aquifers. They are part of several large depositional systems of Paleocene-Eocene age prograding on the Gulf coast. The Queen City and Sparta Formations overlie the Carrizo Formation, separated by a leaky aquitard. The Carrizo Formation represents the upper part of a major Texas aquifer system, the Carrizo-Wilcox aquifer. Three overlapping, quasi-three-dimensional numerical models of the Carrizo-Wilcox aquifer were previously constructed and calibrated. The project involved adding three layers over the five main layers already represented in the three Carrizo-Wilcox Groundwater Availability Modeling programs (GAMs), the Queen City and Sparta Formations, as well as the intermediate aquitard.
The first step of the project consisted of determining the structure of the added layers, mainly the top and bottom of the formations, collecting data to infer hydrologic properties, and obtaining insight into recharge to the aquifers. Structure work was done by correlating stratigraphic boundaries throughout the study area at approximately 220 well locations. Collecting more than 1,000 specific-capacity data helped determine hydrologic properties. A reasonable range of recharge was estimated by relating chloride concentrations in precipitation water to those of shallow reaches of the aquifers. Water-quality analysis confirmed the current conceptual model of the aquifers.
The second step of the project consisted of calibrating the groundwater model. Modeled heads and stream-flow values were adjusted to match field measurements by varying and fine-tuning hydrologic parameters. Hydrologic parameters were regionally varied according to our conceptual models but never only locally in order to avoid the trap of overcalibration. After calibration, the numerical model was used as an evaluative and predictive tool, and simulations were made to help us understand future water-level changes with assumed periods of normal and drought-of-record precipitation. The project was funded by the Texas Water Development Board as part of its GAM program, and the Bureau was a subcontractor to Intera.
Please Pass the Salt: Using Oil Fields for the Disposal of Concentrate From Desalination Plants
Jean-Philippe Nicot, Principal Investigator, Ali D. Chowdhury (Texas Water Development Board), Robert E. Mace (Texas Water Development Board), and Alan R. Dutton (now at The University of Texas at San Antonio)
The Texas Water Development Board (TWDB) and BEG, funded by the Bureau of Reclamation, investigated the feasibility of injecting concentrates from desalination plants into depleted oil or gas fields. The objective of this study was twofold: (1) to evaluate the use of depleted fields as sites for injection wells to dispose of desalination concentrates and (2) to demonstrate to the regulatory community that deep-well disposal of concentrates in oil and gas fields is safe and reliable. The State of Texas is interested in diversifying its water resources in the face of a growing demand. Large volumes of practically untapped brackish water relatively well distributed across the state are available. However, communities interested in desalination are concerned about what to do with their (nontoxic) concentrate that is chemically enriched relative to initial brackish water composition and that accounts for one-third to one-tenth of the input water stream. One possibility is to inject it into oil fields, along with produced waters. To show through physical and chemical modeling that oil fields can readily accept the waste, fields within specific basins were investigated. Basins were chosen so that their characteristics would cover the range of variability in the state: Permian, East Texas, Gulf Coast, Anadarko, Dallas-Fort Worth, and Maverick Basins.
Despite some differences, the six analysis areas show a consistent picture. All areas have a history of fresh water flooding, especially during early production in the first half of the twentieth century, suggesting a favorable outlook for concentrate injection. Achievable injection rates are not on average historically high, which is confirmed by low permeability values of Paleozoic formations. The East Texas and Gulf Coast reservoirs have higher permeability and subsequent maximum potential injection rates. Multiple wells will be needed to accommodate the desalination concentrate stream of a typical plant. Scaling tendency by calcite and gypsum is not outside that typically encountered and dealt with by the oil and gas industry. The greatest risk for formation damage may be changing the ionic ratio of formation water or the selectivity of ion exchange between water and clay minerals, although water sensitivity of the clayey material can be accommodated using operational solutions such as pretreatments with appropriate chemicals or buffer solutions. Technical challenges of injecting desalination concentrates into oil-producing formations are not unlike those of injecting water from a source different from that of formation water. The oil industry has a long history of dealing with such issues. This work suggests that injection of desalination concentrates in formation water will likely not be a problem if the injection water and the formation are appropriately pretreated, as is done routinely in the oil industry.
Presentations and Reports
Please Pass the Salt: Using Oil Fields for the Disposal of Concentrate from Desalination Plants
- National Ground Water Association's 2005 Ground Water Summit, San Antonio, April 19, 2005 [PDF]
- Final Report, June 2005 [PDF]
- Nicot, J. P., and Chowdhury, A. H., 2005, Disposal of brackish water concentrate into depleted oil and gas fields: a Texas study: Desalination, 181, 61-74 [PDF]
Please Pass the Salt: Using Oil Fields for the Disposal of Concentrate
- Presentation of final report to the U.S. Bureau of Reclamation, September 29, 2004 [PDF]
Please Pass the Salt: Can the Oil Industry Benefit from Desalination Wastes?
- Presented at the TIPRO mid-winter policy meeting, January 13, 2004, Fort Worth, Texas [PDF]
- Presentation Notes [PDF]
Bridget R. Scanlon, Principal Investigator
The purpose of this study is to determine the distribution of arsenic in Texas groundwater; assess the potential of past application of arsenical pesticides as a source of arsenic in groundwater in the Southern High Plains and Southern Gulf Coast, evaluate the role of phosphate fertilizers in mobilizing arsenic in areas of arsenical pesticide application and high groundwater arsenic concentrations, evaluate geologic sources of arsenic in Texas, and target geographic areas of domestic drinking-water wells potentially affected by high arsenic levels. This study began in November 2004 and will continue through August 2005. BEG will conduct a number of tasks to accomplish this objective. (1) This study will include a review of arsenic levels in surrounding states and an evaluation of research related to elevated arsenic in groundwater nationwide. (2) Potential anthropogenic sources of arsenic, such as arsenical pesticides in the Southern High Plains and the Southern Gulf Coast, will be examined using GIS overlay analyses and soil sampling. The ability of phosphate fertilizers to mobilize arsenic from arsenical pesticide applications will also be evaluated, when information on phosphate fertilizer application can be obtained. (3) Potential geologic sources of elevated arsenic concentrations in groundwater in Texas will be evaluated using relationships between arsenic concentrations and different geologic units. Relationships between arsenic concentrations and other ions, particularly uranium and oxyanions, will be evaluated using existing databases to determine geologic rather than anthropogenic sources of arsenic. The impact of different redox conditions on the distribution of arsenic will also be examined. Limited additional groundwater sampling will be conducted where necessary and feasible. Materials will be developed for technology transfer for interested agency staff.
Groundwater/Surface Water Interactions in Texas: Implications for Water Resources and Contaminant Transport (TCEQ)
Bridget R. Scanlon, Principal Investigator
The objective of this study is to assess impacts of groundwater/surface water interactions on the quantity and quality of water in Texas. The assessment will be accomplished through a review of existing studies, an evaluation of the potential impact of groundwater/surface water interactions on both water quality and water quantity, and technology transfer of results to interested agency staff.
BEG will conduct a number of tasks to accomplish the objective. (1) The study will include a review of studies assessing groundwater/surface water (gw/sw) interactions in the U.S. relative to water quality and water quantity issues. (2) Evaluation of impacts of gw/sw interactions on water quality will include comparison of stream water quality with adjacent groundwater quality to assess connectivity. Potential impacts of groundwater discharge on river segments identified as impaired in the Total Maximum Daily Load program will also be examined. The status of knowledge on distribution of riparian vegetation will also be examined because riparian vegetation can markedly affect contamination of surface water from groundwater discharge. (3) Assessing impacts of gw/sw interactions on water quantity will include evaluation of different approaches for hydrograph separation to quantify the component of surface water that is groundwater. Previous programs to model water quantity of surface water (Water Availability Models [WAM]) and groundwater (Groundwater Availability Models [GAM]) will be examined to determine the feasibility of using output from GAM for a selected aquifer as input to WAM. Materials will be developed for technology transfer for interested agency staff. This study began in November 2004 and will continue through August 2005.
Field Validation of Geologic Assessment of Features Sensitive to Pollution in Karst and Development of Best Management Practices
Susan D. Hovorka, Principal Investigator; Adrien Lindley, and Mike Barrett
In this multiyear study funded by Texas Commission on Environmental Quality, the current geomorphic method for assessment of sensitive features is quantitatively evaluated using hydrologic methods. Typical upland karst features in the Edwards aquifer recharge zone, specifically sinkholes, are studied to determine their connectedness to the subsurface. Because sinkholes may allow water or contaminants to be transported rapidly to the subsurface via fractures or conduits, they have the potential to be sensitive features during development. Eight pairs of constant-head, single-ring infiltrometer experiments at sinkholes and control plots, as well as another excavated solution cavity, have been conducted at two sites located in the Edwards aquifer recharge zone in southern Travis and northern Hays Counties. Initial results indicate that infiltration rates at soil-lined sinkholes are not significantly higher, in fact somewhat lower, than the background. Both rates are within the range of infiltration for soils in the area, as described by county soil surveys. Although these features have slightly lower infiltration rates than average, owing to the 30 to 40 percent clay in the soil, these features most likely recharge volumetrically more than the background as a result of microtopography. The excavated solution cavity and a sinkhole with a cobble-filled drain had infiltration rates as much as 30 times that of the background. More tests are planned for the San Antonio area to determine possible differences between Barton Springs and San Antonio segments of the Edwards aquifer. Future testing using GPR will explore geometry of the soil/rock contact, and ponding tests with dyed water will allow us to compare the area of wetting and preferential flowpaths within sinkholes and control plots. Subsequent excavation of features will enable us to determine the hydrologic function of epikarst.
Guidance documents for best engineering practices for protecting water quality during development near features found to be sensitive were revised and updated and are in review.
Study to Evaluate Electronic Access to Geologic Data and Surface Casing Depths Necessary to Protect Usable Groundwater in the State
Edward W. Collins, Principal Investigator; Ian Duncan, Thomas A. Tremblay, and Aaron Averett
This study is a collaboration with TCEQ surface casing staff to develop a prototype Website for electronic access to geologic data and surface casing depths necessary to protect usable groundwater in Texas. The result, the Surface Casing Information Site, provides a digital database for use by TCEQ, well operators, and the public and enables 24-hour electronic access to geologic data necessary for Texas' oil and gas operators and TCEQ. Users can generate location maps of proposed well locations required by TCEQ, obtain estimates of probable surface casing requirements, and review well logs and other relevant information. The pilot website, with data from Brazos County, became available in 2004. This study has continued and will include data for 14 counties by August, 2008. The project goal is to develop a digital database of statewide information that will benefit oil and gas operators and the public and that will reduce the time and effort required for the TCEQ surface casing review.
Geological Support for Development of a Groundwater Flow Model for the Edwards Aquifer in the San Antonio Area
Susan D. Hovorka, Principal Investigator; Alan R. Dutton, Joseph S. Yeh, and John R. Andrews
This study is developing an aquifer database for an improved computer model of groundwater flow in the San Antonio segment of the Edwards aquifer. The Edwards aquifer, the major source of water for more than 1.5 million people in the San Antonio area, provides nearly all of the water used in the region for industrial, military, irrigational, and public supplies. If withdrawals of groundwater are accelerated, furthermore, spring flow at Comal and San Marcos Springs will be threatened. Both springs supply water to meet downstream needs, sustain Federally listed endangered species, and support local economies by attracting tourists. The U.S. Geological Survey and the Bureau are collaborating to develop the database and computer model on behalf of the Edwards Aquifer Authority. The study, which began late in 2000, will be completed in 2003. The model being developed will enable water managers to test the effects of alternative, potentially costly management scenarios before enactment. Input simplifications and output enhancements will make the model user-friendly for trained personnel, as well as ensure that the graphics-rich output is understandable to nonscientists.
Edwards Aquifer Fracture/Conduit Study
Susan D. Hovorka, Principal Investigator; Bridget R. Scanlon, and Robert C. Reedy
The Edwards aquifer of South Texas has a complex and highly heterogeneous flow system. Integration of multiple data sets has the best chance of providing an adequate glimpse of the nature of the heterogeneities and their impact on aquifer performance. Karst has developed in this carbonate aquifer in response to the interaction of structure and gradient. Karst capture, favored by fractures of the Balcones Fault Zone, has diverted surface-water flow from toward the Gulf of Mexico into the subsurface and caused it to flow eastward and discharge at Comal and San Marcos Springs. We mapped large troughs in the potentiometric surface by grouping the large volume of historic-water-level data according to aquifer stage. Cave orientations confirm a history of karst capture at a smaller scale.
We are examining the implications of these karst trends for interpreting natural chemistry and introduced-contaminant distribution, as well as high-frequency water-level records.
Groundwater Availability Modeling of the Barton Springs Segment of the Edwards Aquifer
Bridget Scanlon, Principal Investigator
The Bureau of Economic Geology and subcontractor Barton Springs Edwards Aquifer Conservation District developed a groundwater flow model for the Barton Springs segment of the Edwards Aquifer to evaluate the effects of future pumpage and potential future droughts on groundwater availability (Fig. 1). The model covers an area of ~ 260 square miles and uses a 14,400 node grid (7,043 active nodes) with rectangular cells (500 ft x 1000 ft) to simulate the potentiometric surface and spring discharge in response to past, present, and future pumpage and potential future droughts. Dr. Robert Mace of the Texas Water Development Board ((TWDB) was involved in the early development of this model. The model was calibrated using a combination of trial and error and an inverse optimization code (UCODE).
The Barton Springs aquifer constitutes the sole source of water to about 45,000 residents. Barton Springs pool also serves as a municipal swimming pool in Zilker Park, downtown Austin. Increased population growth and recent droughts (1996) have focused attention on groundwater resources and sustainability of spring flow. The primary management issue for this aquifer is maintaining spring flow during drought periods and assessing current and future pumpage effects on spring flow. Maintaining spring flow is a critical objective because the spring outlets are the sole habitat of the Barton Springs salamander, which is listed as an endangered species.
Numerical modeling of groundwater flow in the Barton Springs Edwards aquifer presented many challenges because of the complex geology resulting from numerous faults with large vertical offsets, and the extremely dynamic nature of the aquifer as a result of large conduits. Unique aspects of the model that made it particularly suitable for modeling include:
- stream gauge data upstream of the outcrop areas for recharge estimation
- detailed pumping records reported by individual users of large wells collected by the BSEACD since 1989
- daily spring flow records for ³ 100 yr for Barton Springs and water level hydrographs distributed throughout the aquifer for variable time periods up to 10 yr for comparison with simulated values
- detailed synoptic water level maps developed by BSEACD for low and high flow periods for comparison with model simulations
- No other aquifer in the state has such detailed records for model input and evaluation of model results. The information provided by these data was critical in developing a conceptual model of flow for this Barton Springs Edwards Aquifer.
Model calibration was particularly challenging because of the complex geologic structure in the model area and the dynamic nature of flow. Model parameterization included a trial and error approach based on the distribution of hydraulic heads followed by automated inverse modeling to further reduce the root mean square (RMS) error between simulated and measured heads. Steep head gradients in the outcrop area were assigned low values of hydraulic conductivity and shallow head gradients in the central and eastern part of the model were assigned high hydraulic conductivities. Highest hydraulic conductivities were assigned to a zone surrounding Barton Springs because this area should reflect the convergence of flow paths. This approach of parameterising hydraulic conductivity resulted in a parsimonious distribution of hydraulic conductivity and low RMS errors.
The transient model was run using data from 1989 through 1998 and generally reproduced the temporal variability in spring discharge with no calibration. The challenge with the model was to accurately simulate low spring discharges because these discharges are critical for using the model to predict the effect of increased pumpage and potential future droughts on spring discharge. By varying the specific yield in the outcrop area, the low spring discharges could be simulated fairly accurately. High spring discharges were generally overestimated because the model did not include reported ungauged springs that start flowing at high discharges.
To assess the impact of future pumpage and potential future droughts on groundwater availability, transient simulations were conducted using extrapolated pumpage for 10-yr periods (2001 through 2050) and average recharge for a 3-yr period and recharge from the 1950's drought for the remaining 7 yr. Results of these simulations were compared with those using average recharge and future pumpage. Simulated spring discharge in response to future pumpage under average recharge decreased proportionally to future pumpage (2 cfs per decade), whereas spring discharge decreased to 0 cfs in response to future pumpage under drought-of-record conditions. Management of water resources under potential future drought conditions should consider enhanced recharge and conservation measures.
In addition, simulations based on the distributed parameter modeling of the Barton Springs Edwards Aquifer using MODFLOW were compared to those based on a much simpler lumped parameter model developed by Barrett and Charbeneau, 1987. Results from this comparison indicated that both the distributed and lumped parameter models could adequately simulate spring discharge and well hydrographs, but that the distributed parameter model is required to simulate the potentiometric surface and to evaluate aquifer response to future pumpage.
The results of this study demonstrate the ability of equivalent porous media distributed and lumped parameter models to simulate regional groundwater flow, which is critical for managing water resources in karst aquifers and predicting the impact of future pumping and potential future drought conditions on spring flow.
Scanlon, B. R., Mace, R. E., Barrett, M. E., and Smith, B., 2002, Can we simulate regional groundwater flow in a karst system using equivalent porous media models? Case study, Barton Springs Edwards Aquifer, USA. J. Hydrol. 276:137–158. [PDF]
Groundwater Availability Model of the Central Part of the Carrizo-Wilcox Aquifer in Texas
Alan R. Dutton, Principal Investigator; Jean-Philippe Nicot, Bridget R. Scanlon, and Robert C. Reedy
A quasi-three-dimensional, numerical model of the occurrence and movement of groundwater in the central part of the Carrizo-Wilcox aquifer in Texas was developed to help us estimate groundwater availability and water levels in response to potential droughts and future pumping, including new well fields. Formations of the Paleocene-Eocene-age Wilcox Group, along with the overlying Carrizo Formation, make up a major aquifer system in Texas. This six-layer model is based on data on geological structure and depositional setting of the aquifer, hydrological properties, water-use survey estimates of historical groundwater withdrawals, and base flow of rivers and streams. New insights into how the downdip circulation of freshwater is affected by fault zones and a deep-basin geopressured zone are based on maps of total dissolved solids and equivalent water levels from the outcrop to depths of more than 10,000 ft. In addition, results of field studies using "environmental" tracers yielded regional estimates of recharge rates that broadly match estimates from previous models.
A steady-state model representing "predevelopment" (no pumping) conditions was calibrated against water levels measured before 1950 and historical low-flow measurements in streams. A transient version of the model was calibrated against water-level hydrographs and stream-flow data for the period from 1950 through 1990 and verified by comparison with water levels recorded between 1991 and 2000. Recharge rates, vertical hydraulic conductivity, specific storage, specific yield, and boundary-flux properties were calibrated using the model. Horizontal hydraulic conductivity is one of the better known attributes of the aquifer, given the number of pumping- and specific-capacity tests and the quality of regional mapping of the distribution and thickness of sandstones that make up the permeable architecture of the aquifer.
To demonstrate the use of the groundwater model as an evaluative and predictive tool, simulations were made of future water-level changes with assumed periods of normal and drought-of-record precipitation. Pumping rate is expected to continue to increase between 2000 and 2050, but at a rate slower than that of the past decade. Overall, total pumping from the Carrizo-Wilcox aquifer in the study area is expected to increase from 197,000 acre-feet per year in 2000 to 320,500 acre-feet per year in 2050. The simulated decline of water level related to groundwater pumping will occur mainly through a decrease in artesian storage. The model also suggests that the major rivers will continue to flow even with increased pumping and under drought conditions. The project is funded by the Texas Water Development Board as part of their Groundwater Availability Modeling (GAM) program.
Field Studies to Estimate Groundwater Recharge in the Central Carrizo-Wilcox Aquifer
Bridget Scanlon, Principal Investigator; Robert C. Reedy
Recharge was estimated using Cl concentrations in the unsaturated zone and groundwater and using 3H/3He concentrations in groundwater in 7 boreholes in the outcrop area of the Simsboro Formation in the central part of the Carrizo-Wilcox aquifer (Fig. 1). Long-term (50 yr) mean annual precipitation in the central part of the Carrizo-Wilcox aquifer ranges from 29 inches in the southwest to 48 inches in the northeast of the area.
Average chloride concentrations in the unsaturated zone ranged from 23 to 519 mg/L. Variability in mean chloride concentrations was high locally. Chloride concentrations were also highly variable within each profile. There was no systematic variation in chloride concentrations with depth. Recharge rates were calculated for the portion of the profiles that generally represented the last 50 yr where possible. In some cases recharge rates were so low that a 50 yr section corresponded to a very narrow depth interval. Recharge rates generally ranged from 0.2 to 1.4 in/yr. The time required to accumulate chloride in each profile ranged from 110 to 2815 yr. Groundwater chloride concentrations were generally lower than those in the unsaturated zone (5–180 mg/L). Representative recharge rates based on groundwater chloride concentrations range from 1 to 1.5 in/yr and are higher than those based on unsaturated zone chloride. The generally higher recharge rates based on groundwater chloride relative to unsaturated zone chloride are considered more representative of the regional system whereas the unsaturated zone data indicate that locally recharge rates are lower.
Groundwater tritium concentrations ranged from 0.76 to 3.57 TU. These tritium levels were much greater than the detection limit for tritium (~ 0.2 TU) and indicate that a component of the water was recharged in the last 50 yr. Tritium/helium was also used to date the water in 3 of the 7 wells (CW3, CW4, and CW6). There were problems with analysis of 3He in water samples from CW6. 3He concentrations were low in well CW3 and much higher in well CW4. The low 3He concentrations in CW3 indicate a short residence time of the water of 2.2–6.1 yr (replicate analyses) whereas the much higher 3He concentrations in CW4 indicate a residence time of 21.4 yr. The times represent the time of 3He accumulation since it was isolated from the unsaturated zone. Water velocities were calculated by dividing the distance between the water table and the center of the well screen by the age of the water and resulted in velocities of 0.4 (CW4) to 1.4–4.0 ft/yr (CW3). Recharge rates of 1.6 (CW4) to 6.0–16.7 in/yr (CW3) were calculated by multiplying the velocities by the average porosity of 0.35. The recharge rate for CW4 of 1.6 in/yr is similar to that estimated from the groundwater chloride concentration. The recharge rate for CW3 of 6.0–16.7 in/yr is high similar to the high recharge rate calculated the groundwater chloride concentration (6.2 in/yr). Future work should consider using gas diffusion samplers for tritium-helium dating rather than copper tubes used in this study, particularly in low yield wells. This approach would help reduce the uncertainties in the age estimates.
Reedy, R. C., Scanlon, B. R., and Dutton, A. R., 2003, Collection and analysis of environmental tracers for estimation of recharge rates in the GAM model of the central Carrizo-Wilcox aquifer, in Dutton, A. R., Harden, B., Nicot, J. P., and O'Rourke, D., editors, Groundwater Availability Model for the Central Part of the Carrizo-Wilcox Aquifer in Texas. Contract Report prepared for the Texas Water Development Board.
Identification of Geographic Areas Suitable for Groundwater Banking
Bridget R. Scanlon, principal investigator; Robert C. Reedy
The Bureau of Economic Geology is a subcontractor to D. B. Stephens & Associates on a project designed to identify suitable parts of the state for storing excess surface water in groundwater systems. The population of Texas is expected to double in the next 50 years, and water demand is projected to increase by 18 percent. With the decline in development of new surface-water reservoirs because of adverse impacts on the environment, groundwater managers are turning to artificial recharge of groundwater, using? excess surface water to meet water needs during times of drought. In this project suitable sites were evaluated for artificially recharging groundwater using spreading basins adjacent to streams. A statewide screening was conducted that included water quality, regional water demand, aquifer characteristics (recharge area, depth to groundwater), distance from surface water, and slope. A total of 48 counties passed the statewide screening. Within six regions of the state, more detailed analysis permitted evaluation of water resources, water storage and conveyance systems, and infiltration rate, area, and time period for infiltration. A successful site should be high in soil permeability and located in topographically flat areas, close to a stream. The results of this study provide important information to water managers in the state on the potential for groundwater banking.
Review of Existing Data on Groundwater Recharge in Texas
Bridget Scanlon, Principal Investigator
The purpose of this study was to assess the status of data on recharge for the major aquifers in Texas to provide input to the Groundwater Availability Modeling program at the Texas Water Development Board, evaluate the reliability of the recharge estimates, develop conceptual models for recharge for each of the aquifers, review techniques for quantifying recharge, and recommend appropriate techniques for quantifying the recharge of each of the major aquifers.
Recharge rates for all major aquifers were compiled from published reports. The Edwards aquifer is the most dynamic, and recharge rates are highly variable spatially and temporally. Recharge is fairly accurately quantified using stream-gauge data. Estimates of recharge rates in the Carrizo-Wilcox aquifer range from 0.1 to 5.8 in/yr, with higher recharge in the sandy portions of the aquifer (i.e., the Carrizo and Simsboro Formations). Reported recharge rates for the Gulf Coast aquifer (0.0004 to 2 in/yr) are generally lower than those for the Carrizo-Wilcox aquifer.Regional recharge rates in the High Plains aquifer, outside irrigated areas, are generally low (0.004 to 1.7 in/yr), whereas playa-focused recharge rates are much higher (0.5 to 8.6 in/yr). Irrigated areas also have fairly high recharge rates (0.6 to 11 in/yr). Recharge rates in the Trinity and Edwards-Trinity Plateau aquifers generally range from 0.1 to 2 in/yr. The Seymour aquifer has recharge rates that range from 1 to 2.5 in/yr. Recharge rates for the Hueco-Mesilla Bolson and the Cenozoic Pecos Alluvium are represented as total recharge along mountain fronts and valley floors.
The main techniques that have been used for estimating recharge are Darcy's Law, groundwater modeling, base-flow discharge, and stream loss. Uncertainties in estimates based on Darcy's Law are considered high because of uncertainties in estimates of regional hydraulic conductivity. Recharge estimates based on groundwater model are not considered highly reliable because calibration based on hydraulic head data alone can only be used to estimate the ratio of recharge to hydraulic conductivity. This review of existing data indicates that additional studies are required to provide more quantitative estimates of recharge to the major aquifers.
Scanlon, B. R., Dutton, A. R., and Sophocleous, M. A., 2003, Groundwater Recharge in Texas: The University of Texas at Austin, Bureau of Economic Geology, Submitted to Texas Water Development Board. [PDF]
Field Studies to Estimate Groundwater Recharge beneath Irrigated and Nonirrigated Regions in the Southern High Plains, Texas
Bridget Scanlon, Principal Investigator; Robert C. Reedy
Field studies were conducted in collaboration with the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) program to estimate recharge beneath irrigated and nonirrigated regions in the Southern High Plains. Two boreholes were drilled in areas that had been irrigated since 1958 (Roberts and Maple sites) and one borehole was drilled in a non-irrigated site in the Muleshoe National Wildlife Refuge (MNWR) for comparison with irrigated sites. The drilling, sampling, and analyses were conducted by the USGS. as part of the NAWQA program. Post-bomb tritium was generally restricted to the root zone at the non-irrigated site and indicates negligible recharge whereas postbomb tritium was found throughout the unsaturated zone in the irrigated sites indicating much higher recharge.Heat dissipation sensors were installed to monitor the negative pressures in the unsaturated zone to determine the direction of water movement and to evaluate drainage beneath the irrigated plots. Heat dissipation sensors were installed in shallow boreholes beneath the pivot irrigation system and in the deep boreholes drilled by the USGS. The instruments are logged daily and data are telemetered to the Bureau using a cell phone system. Matric potential profiles in the non-irrigated site were generally much lower (more negative) than those in the irrigated sites in the upper 10 ft in the spring and summer indicating generally drier conditions in the non-irrigated site. The vertical matric potential profile in the non-irrigated site indicates matric potentials as low as -20 to -25 bars in the shallow subsurface and increasing to close to zero at a depth of ~ 38 ft. The increase in matric potentials with depth indicates an upward driving force for water movement and suggests upward flow. The vertical matric potential profiles in the irrigated plots are close to zero throughout the profile indicating fairly wet conditions as a result of irrigation. The time series plots of matric potentials provide information on infiltration of water as a result of precipitation and irrigation and indicate very little water movement over time beneath the irrigated and nonirrigated sites. These data suggest that recharge from irrigation return flow is negligible beneath the current very efficient center pivot irrigation systems but may have been much higher in the past when flood irrigation was used.
Reedy, R. C., Scanlon, B. R., Bruce, B. W., McMahon, P. B., Dennehy, K. F., and Ellett, K., 2003, Groundwater recharge in the southern high plains, in Blanford, T. N., Blazer, D. J., Dutton, A. R., and Scanlon, B. R., editors, Groundwater Availability of the Southern Ogallala Aquifer in Texas and New Mexico Numerical Simulations through 2050. Contract Report Submitted to Texas Water Development Board.
Reedy, R. C., and Scanlon, B. R., 2002, Comparison of different approaches for estimating recharge in the High Plains Aquifer, Texas (abs.), in Eos, v. 83, no. 47, Fall Meeting Supplement, American Geophysical Union, Abstract H61B-0777. [PDF]