Quality of groundwater used for public supply in the aquifers of the Edwards-Trinity aquifer system

The Edwards-Trinity aquifer system is an important source of groundwater for public supply. The aquifer system underlies about 77,000 square miles, largely in Texas and also in small parts of Oklahoma and Arkansas. In 2016 and 2017, the National Water-Quality Assessment (NAWQA) project of the U.S. Geological Survey collected raw-water samples from 75 public-supply wells in the Edwards-Trinity aquifer system as part of a sampling effort focused on the quality of groundwater used for public supply in principal aquifers across the Nation. The Edwards-Trinity aquifer system, which ranks 11^th among the Nation’s principal aquifers for groundwater withdrawal for public supply, is characterized as three related aquifers in carbonate and clastic Cretaceous-age rocks: the Trinity aquifer, the Edwards aquifer, and the undifferentiated Edwards-Trinity aquifer. Twenty-five public-supply wells were sampled in each of the three aquifers, which allows for comparison across the aquifers. The wells were selected for sampling under a nationally consistent design that uses equal-area grids to achieve a spatially unbiased dataset, and were sampled for a comprehensive suite of analytes (major and trace elements, nutrients, pesticides, volatile organic compounds, radionuclides, microbial indicators, pharmaceuticals, hormones, and selected isotopes and groundwater age tracers). Results of this sampling effort are being incorporated into ongoing efforts by NAWQA to characterize the occurrence of contaminants of concern for human health, to identify the primary factors controlling that occurrence, and to improve understanding of groundwater processes that are important to management of water resources.