Texas Groundwater: Gulf Coast and Carrizo-Wilcox Aquifers Compared

By Patricia B. Ging, U.S. Geological Survey. Fact Sheet 2020–3009, February 2020.

Two aquifers, many users

The USGS National Water-Quality Assessment (NAWQA) Project samples aquifers that supply much of the nation's drinking water, through Principal Aquifer Surveys of untreated water from domestic and public-supply wells.

Figure 1: the sampled public-supply well networks in the Gulf Coast aquifer system and Carrizo-Wilcox aquifer. USGS.

The rocks

• Gulf Coast aquifer system: a wedge of river and delta sand, silt and clay laid down in bands parallel to the Gulf coast, Miocene to Holocene in age, thickening toward the Gulf. From youngest to oldest: the Chicot and Evangeline aquifers, the Burkeville confining unit, the Jasper aquifer and the Catahoula Sandstone. The Chicot and Evangeline once supplied most of Houston's water; as of 2019, only 14 percent.
• Carrizo-Wilcox aquifer: mostly sand, with local gravel, silt, clay and lignite.
• Between them: the Yegua-Jackson, Sparta and Queen City aquifers, supplying many cities and farms.

What was measured

The wells sampled were deeper than domestic wells. The water was tested for basic properties (oxygen, pH, conductance, temperature, turbidity), major ions, nutrients, organic carbon, trace elements, age tracers, radionuclides, and many pesticides, volatile organic compounds, pharmaceuticals and hormones — many of them not part of routine compliance testing.

Benchmarks: results were compared with drinking-water standards for context — though treated tap water often differs from untreated groundwater.

Salt

Dissolved solids measure salinity; with chloride, they're judged against non-regulatory limits for taste and nuisance problems such as staining and scaling. All groundwater picks up dissolved solids from rocks and sediments; some come from human activity such as fertilizer. Higher levels may reflect older water in deeper wells.

Figure 3: dissolved solids (A) and chloride (B), in milligrams per liter, and well depth (C). USGS.

Nutrients

Nitrate, nitrite and phosphorus occur naturally at low levels; moderate or high levels usually mean human sources — fertilizer, septic systems, human and animal waste. Five nutrients were measured; nitrate and nitrite have health benchmarks. Nitrite was mostly below detection (0.04 mg/L), so the nitrogen is mostly nitrate, which may be tied to dissolved oxygen levels.

Trace elements

34 were measured, 19 with health benchmarks. They occur naturally in rocks and soils. Only arsenic and manganese exceeded a health benchmark.

Figure 5: arsenic (A) and manganese (B), in micrograms per liter. USGS.

Radioactivity

Most radioactivity in groundwater comes from the natural decay of uranium and thorium in aquifer minerals, and people are always exposed to a little. Eight radionuclides were measured, four with benchmarks; only gross alpha, gross beta and radon-222 exceeded them.

Figure 6: gross alpha (A), gross beta (B) and radon-222 (C), in picocuries per liter. USGS.

Organic compounds

Man-made organic compounds — from household, business, industrial and farm products — reach the environment through use, spills or improper disposal, and are common in streams and sediments. The surveys tested VOCs, pesticides, pharmaceuticals and hormones. Many have no health benchmark, but none measured in either aquifer exceeded one that exists.

Sources

Based on Water-Quality Comparison of the Gulf Coast Aquifer System and Carrizo-Wilcox Aquifer in Texas From National Water-Quality Assessment Project Principal Aquifer Surveys, 2013 and 2015, by Patricia B. Ging, USGS Fact Sheet 2020–3009, U.S. Geological Survey; a work of the United States government in the public domain. The import had left out the sections on nutrients, trace elements and radionuclides, the benchmark definitions and the figures; they are taken from the fact sheet's PDF.