Groundwater supplies nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project checks the quality of groundwater in aquifers that are important drinking-water sources — among them the Edwards-Trinity aquifer system.
The aquifer system
The system underlies more than 78,000 square miles, from west Texas into Oklahoma and Arkansas, home to about 11.5 million people; its main cities are San Antonio, Austin, Dallas and Fort Worth. It is used mainly for public supply and irrigation:
- 11th in the nation for public supply — about 411 million gallons a day in 2000;
- 22nd for agriculture — about 282 million gallons a day.
The land above it is mostly natural cover, including rangeland (81 percent), with farmland (12 percent) and a little urban and developed land (7 percent). Rainfall falls tenfold from east to west, and average temperature rises from north to south.
It is made of three complexly linked aquifers in Cretaceous carbonate and clastic rock:
| Aquifer | Area (mi²) | Share of the system's public supply | Notes |
|---|---|---|---|
| Edwards-Trinity (Plateau) | 35,425 | about 9% | across much of southwest Texas; Edwards Group carbonates over Trinity Group sands and carbonates; mainly irrigation; recharged by rain where unconfined; flows southeast |
| Edwards (Balcones Fault Zone) | 4,052 | about 66% | a narrow band of partly dissolved, highly permeable limestone along the Balcones Fault Zone; the smallest (about 5 percent of the area) but most productive — one of the most productive in the nation and San Antonio's main supply; recharged mostly by streams losing water, plus infiltration and leakage from the Trinity; big springs shelter several threatened and endangered species |
| Trinity | 38,663 | about 25% | an arc across central and northeast Texas into Oklahoma (where it is called the Antlers aquifer) and Arkansas; interbedded sandstone, sand, limestone and shale; increasingly important north of Austin; large water-level declines in places; drains to many small springs |
The Edwards lies over, and partly connects with, the Trinity; the Trinity and Edwards-Trinity are partly the same rock layers and connect in places.
The three aquifers of the Edwards-Trinity aquifer system and the wells sampled.
How it was sampled
In 2016 and 2017 the USGS sampled 74 public-supply wells, spread on a 25-cell equal-area grid in each aquifer (only 24 samples came from the Edwards-Trinity aquifer). The study covers the depth used for public supply. Results are weighted by aquifer area — 45.3 percent Edwards-Trinity, 5.2 percent Edwards, 49.5 percent Trinity — so they represent the whole study area and give the percentage of the area with high, moderate or low concentrations. Accuracy depends on the number and spread of wells, not on the size of the area.
Wells were 25 to 3,214 feet deep, averaging 846 feet. Edwards-Trinity wells were generally shallower (average 425 feet) than Edwards and Trinity wells (averages of 952 and 1,126 feet). Water was tested before any treatment for many substances from natural and human sources.
How "high" and "moderate" are defined
Concentrations are compared with drinking-water benchmarks — EPA's regulatory maximum contaminant levels (MCLs), non-regulatory health-based screening levels (HBSLs), and EPA's non-regulatory, aesthetic secondary levels (SMCLs). Human-health benchmarks exist for 28 of 55 inorganic constituents and properties and 170 of 310 organic ones.
- High: above a human-health benchmark or an SMCL.
- Moderate: above half a benchmark for inorganic constituents, or above one-tenth for organic ones, which are generally rarer and lower relative to benchmarks.
Water that reaches consumers may differ, because it may be treated after pumping.
Overview
Water from 84 percent of the wells had no contaminant at a high concentration.
Share of the study area with high (yellow), moderate (green) or low or undetected (blue) concentrations.
- Inorganic constituents with human-health benchmarks: high in about 16 percent of the study area, moderate in about 33 percent.
- Organic constituents with human-health benchmarks: moderate in about 2 percent; none high.
Inorganic constituents
- Trace elements and major and minor ions (34 analysed, 22 with health benchmarks): high in about 10 percent, moderate in about 16. Strontium was the most common at both levels (10 and 12 percent); molybdenum was moderate in about 4 percent, arsenic and thallium in about 2.
- Radioactivity — mostly from the decay of uranium and thorium naturally present in aquifer minerals (8 analysed, 4 with benchmarks): high in about 12 percent, mainly gross-alpha radioactivity; moderate in about 25, mainly gross-alpha and combined radium (Ra-226 plus Ra-228).
- Nutrients (5 analysed; nitrate and nitrite have benchmarks) — usually from fertiliser, septic systems and human and animal waste: nitrate was never high, and moderate in about 2 percent, all in the Edwards-Trinity aquifer.
Taste, colour and scaling
Eleven constituents with SMCLs — which concern taste, colour, odour, staining and scaling — were measured. At least one was high in about 48 percent of the study area and moderate in about 49.
- Total dissolved solids (TDS), a measure of salinity: high in 47 percent, moderate in 49. High levels were most common in the Trinity (68 percent) and Edwards-Trinity (29 percent) aquifers and did not occur in the Edwards.
- Chloride, fluoride and sulfate: high in about 8, 8 and 9 percent; moderate in about 8, 33 and 16 percent.
- Iron, released where groundwater has little oxygen: high in 6 percent, moderate in 2 — only in the Trinity and Edwards-Trinity aquifers.
- pH above 8.5 (alkaline, outside the 6.5–8.5 range) in about 22 percent, all in the Trinity aquifer.
Organic constituents
- Volatile organic compounds (85 analysed, 51 with benchmarks): moderate in 2 percent of the study area, from tetrachloroethylene (PCE) in the Trinity aquifer.
- Pesticides (225 compounds and breakdown products analysed, 119 with benchmarks): none at high or moderate concentrations.
Differences between the aquifers
For each aquifer, the proportion of high, moderate and low concentrations for inorganic constituents with health-based (top) and non-health-based (bottom) benchmarks, with bars for the most common constituents.
- Moderate and high concentrations of constituents with health benchmarks were most common in the Edwards-Trinity (54 percent) and Trinity (48 percent) aquifers.
- Strontium was high to a similar degree in all three (8 to 13 percent); radioactive constituents were less common in the Edwards.
- Chloride, sulfate and fluoride were each high or moderate in more than 15 percent of the Edwards-Trinity and Trinity aquifers; in the Edwards only fluoride was elevated.
These differences likely reflect the aquifers' rock composition, flow paths, geochemistry, groundwater age and climate.
About these studies
NAWQA has assessed groundwater since 1991 with three kinds of well network: Land Use Studies (shallow observation wells), Major Aquifer Studies (mid-depth domestic wells) and Principal Aquifer Studies (deeper public-supply wells), like this one. They compare untreated groundwater with benchmarks and between principal aquifers, and include analyses not usually part of regulatory monitoring — at far lower concentrations than benchmarks, and of substances that trace where groundwater comes from and how it moves.
Sources
- Musgrove, M., 2021, Groundwater quality in the Edwards-Trinity aquifer system: U.S. Geological Survey Fact Sheet 2021–3010. https://pubs.usgs.gov/publication/fs20213010
- Data: Kingsbury and others, 2020, U.S. Geological Survey data release, https://doi.org/10.5066/P9XATXV1
- The results for each constituent group, the benchmark definitions and the figures come from the fact sheet's PDF; the web version has only the background and summary.
- The original compares Edwards-Trinity wells with those "in the Edwards or Edwards-Trinity aquifers (averages of 952 and 1,126 feet)"; the second is the Trinity aquifer.
- Rewritten in hubnx's own words.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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