Groundwater Quality in Texas's Gulf Coast Aquifer System, 2013–15

The USGS National Water-Quality Assessment (NAWQA) Project assesses groundwater in aquifers that supply drinking water — including the coastal lowlands aquifer system, known in Texas as the Gulf Coast aquifer system. It runs from Louisiana to Mexico and supplies cities including Houston.

Well networks sampled in the Gulf Coast aquifer system, 2013–15. USGS.

Three kinds of study

The shallower networks sit within the PAS network, giving a fuller picture for regional and national models.

The aquifer

• Sediments: mostly loose to poorly consolidated deposits of Miocene age and younger, laid down in continental (alluvial plain), transitional (delta, lagoon, beach) and marine (continental shelf) settings.
• Shape: a fluvial-deltaic wedge of discontinuous sand, silt and clay beds in bands parallel to the coast, thickening and deepening toward the Gulf; coarser land-laid deposits inland grade into finer marine ones seaward.
• Units: the Chicot aquifer, Evangeline aquifer, Burkeville confining unit, Jasper aquifer and Catahoula Sandstone. The Chicot and Evangeline are connected, told apart only by finer grain with depth, and are Houston's main drinking-water sources.

What was measured

All three networks were tested for the same things: basic properties (dissolved oxygen, pH, specific conductance, temperature, turbidity), major ions, nutrients, organic carbon, trace elements, age-dating tracers, radionuclides, and a broad suite of pesticides, volatile organic compounds (VOCs), pharmaceuticals and hormones. Results for this untreated groundwater were compared with human-health benchmarks. NAWQA's data go beyond regulatory monitoring: much lower concentrations, and tracers of where groundwater comes from and how it moves.

How results are rated against a human-health benchmark. USGS.

Results

• Dissolved solids — a measure of salinity, mostly from major ions, compared with a nonregulatory secondary standard that concerns taste. All water carries some from weathering minerals; human activity such as fertilizer adds more, and high levels can reflect older water from deeper wells.
• Nutrients: five analyzed, two (nitrate and nitrite) with health benchmarks. Low levels are natural; moderate or high levels usually come from fertilizer, septic systems, or human and animal waste. Most nitrite was below detection (0.04 mg/L), so the nitrogen is mostly nitrate.
• Trace elements: 34 analyzed, 19 with benchmarks. Only arsenic and manganese exceeded benchmarks. Chromium was never moderate or high but was often detected — a concern where industry is heavy.
• Radionuclides: eight analyzed, four with benchmarks. Radioactivity in groundwater mostly comes from natural uranium and thorium in aquifer minerals. Only gross alpha and gross beta activity exceeded benchmarks.
• Organic compounds: VOCs, pesticides, pharmaceuticals and hormones — from household, business, industrial and farm products, via normal use, spills or improper disposal. Many lack benchmarks, but none of the measured concentrations exceeded any benchmark.

(A) Dissolved solids and (B) well depth, by study. USGS.

(A) Arsenic, (B) manganese and (C) chromium, by study. USGS.

Organic compounds: the share of 217 compounds detected at least once, and the share of samples with at least one detection — from 27 public-supply, 8 domestic and 7 urban monitoring wells. USGS.

Sources

Based on Water-Quality Comparison of the Gulf Coast Aquifer System at Various Scales in Texas From National Water-Quality Assessment Groundwater Studies, 2013–15, by Patricia B. Ging, U.S. Geological Survey Fact Sheet 2020–3010 (publication page), citing DeSimone and others (2014), Ryder (1996), Oden (2011), Oden and Szabo (2015), Norman and others (2018), Hem (1985) and Arnold and others (2016–2018). Figures restored from the fact sheet's PDF. A work of the United States government in the public domain.