Sulfate and chloride concentrations in Texas aquifers.

Median sulfate and chloride concentrations in groundwater were calculated for 244 Texas counties from measurements at 8236 water wells. The data were mapped and analyzed with a geographic information system (GIS). Concentration clusters for both solutes were highest in north-central, west, and south Texas. Thirty-four counties had median sulfate levels above the secondary standard of 250 mg/L, and 31 counties registered median chloride concentrations above 250 mg/L. County median concentrations ranged from < 1.5 to 1,953 mg/L for sulfate, and from 6 to 1,275 mg/L for chloride. Various factors contribute to high sulfate and chloride levels in Texas aquifers, including mineral constitutents of aquifers, seepage of saline water from nearby formations, coastal saltwater intrusion, irrigation return flow, and oil/gas production. Ten counties in central and northeast Texas lack data and warrant additional monitoring.