National Water-Quality Assessment Program
Quality of Water
in the Trinity and Edwards
Aquifers, South-Central Texas, 1996–98
By Lynne Fahlquist and Ann F. Ardis
U.S. Geological Survey
Scientific Investigations Report 2004–5201
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Contents
Abstract
Introduction
Purpose and Scope
Environmental Setting
Approach
Data Evaluation
Water Quality
Major Ions
Trace Elements
Nutrients
Pesticides
Volatile Organic Compounds
Radon
Tritium
Stable Isotopes
Summary
References Cited
Figures
| 1. | Map showing physiographic setting and location of wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs that were sampled for water quality in south-central Texas, 1996–98 | |
| 2. | Generalized hydrogeologic section of the Trinity and Edwards aquifers in south-central Texas | |
| 3. | Hydrograph showing change in water-level altitude after October 1998 storm in an urban unconfined Edwards aquifer monitor well | |
| 4. | Trilinear diagrams showing water quality is more uniform in the Edwards aquifer than in the Trinity aquifer as indicated by relations between major cations and anions in water samples collected in south-central Texas, 1996–98, from the (a) Trinity aquifer; (b) Edwards aquifer, unconfined; and (c) Edwards aquifer, confined | |
| 5. | Boxplots showing distribution of concentrations of ammonia and nitrate nitrogen in water samples collected from wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs, south-central Texas, 1996–98 | |
| 6–9. | Maps showing: | |
| 6. | Pesticide detections in water samples collected from wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs, south-central Texas, 1996–98 | |
| 7. | Volatile organic compound detections in water samples collected from wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs, south-central Texas, 1996–98 | |
| 8. | Radon concentrations in water samples collected from wells completed in the Trinity and Edwards aquifers, south-central Texas, 1996–98 | |
| 9. | Tritium concentrations and estimated ground-water ages of water samples collected from wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs, south-central Texas, 1996–98 | |
| 10. | Graph showing relation between hydrogen and oxygen isotopes in water samples from wells completed in the Trinity and Edwards aquifers and Edwards aquifer springs, south-central Texas, 1996–98 | |
Abstract
During 1996–98, the U.S. Geological Survey studied surface- and ground-water quality in south-central Texas. The ground-water components included the upper and middle zones (undifferentiated) of the Trinity aquifer in the Hill Country and the unconfined part (recharge zone) and confined part (artesian zone) of the Edwards aquifer in the Balcones fault zone of the San Antonio region. The study was supplemented by information compiled from four ground-water-quality studies done during 1996–98.
Trinity aquifer waters are more mineralized and contain larger dissolved solids, sulfate, and chloride concentrations compared to Edwards aquifer waters. Greater variability in water chemistry in the Trinity aquifer likely reflects the more variable lithology of the host rock. Trace elements were widely detected, mostly at small concentrations. Median total nitrogen was larger in the Edwards aquifer than in the Trinity aquifer. Ammonia nitrogen was detected more frequently and at larger concentrations in the Trinity aquifer than in the Edwards aquifer. Although some nitrate nitrogen concentrations in the Edwards aquifer exceeded a U.S. Geological Survey national background threshold concentration, no concentrations exceeded the U.S. Environmental Protection Agency public drinking-water standard.
Synthetic organic compounds, such as pesticides and volatile organic compounds, were detected in the Edwards aquifer and less frequently in the Trinity aquifer, mostly at very small concentrations (less than 1 microgram per liter). These compounds were detected most frequently in urban unconfined Edwards aquifer samples. Atrazine and its breakdown product deethylatrazine were the most frequently detected pesticides, and trihalomethanes were the most frequently detected volatile organic compounds. Widespread detections of these compounds, although at small concentrations, indicate that anthropogenic activities affect ground-water quality.
Radon gas was detected throughout the Trinity aquifer but not throughout the Edwards aquifer. Fourteen samples from the Trinity aquifer and 10 samples from the Edwards aquifer exceeded a proposed U.S. Environmental Protection Agency public drinking-water standard. Sources of radon in the study area might be granitic sediments underlying the Trinity aquifer and igneous intrusions in and below the Edwards aquifer.
The presence of tritium in nearly all Edwards aquifer samples indicates that some component of sampled water is young (less than about 50 years), even for long flow paths in the confined zone. About one-half of the Trinity aquifer samples contained tritium, indicating that only part of the aquifer contains young water.
Hydrogen and oxygen isotopes of water provide indicators of recharge sources to the Trinity and Edwards aquifers. Most ground-water samples have a meteorological isotopic signature indicating recharge as direct infiltration of water with little residence time on the land surface. Isotopic data from some samples collected from the unconfined Edwards aquifer indicate the water has undergone evaporation. At the time that ground-water samples were collected (during a drought), nearby streams were the likely sources of recharge to these wells.
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