Solutes and potential sources in a portion of the Trinity aquifer, Texas, USA

Concentrations of several solutes-arsenic, nitrate, chloride, and bromide-along with total dissolved solids (TDS) in a portion of the Trinity aquifer were compiled, mapped, and evaluated relative to regional land use and geology. Agriculture and oil/gas production are significant land uses and potential sources of groundwater contamination in the study area. Data were compiled from 60 wells with a median depth of 245 ft (75 m). Only two observations surpassed the 44.3 mg/L drinking water standard for nitrate, and no observations exceeded the 10 mu g/L standard for arsenic. A statistically significant, negative correlation was apparent between nitrate and well depth. The median chloride concentration was 28.7 mg/L; however, the maximum level was more than double the secondary drinking water standard of 250 mg/L. Nearly half of the TDS observations exceeded the secondary drinking water standard of 500 mg/L. Most chloride/bromide ratios were between 100 and 300; the median value was 184. Two mapped chloride/bromide ratios above 1,000 were consistent with evaporite dissolution. Results suggest modest impacts to groundwater quality from a combination of natural and human sources, the latter potentially including agriculture and oil/gas production.