Arsenic, Nitrate, Chloride And Bromide Contamination In The Gulf Coast Aquifer, South-Central Texas, USA

Abstract

Arsenic, nitrate, chloride, and bromide concentrations in the Gulf Coast Aquifer of south-central Texas, USA, were compiled, mapped, and evaluated in the context of local land use and geology. Agriculture and oil production are predominant land uses and potential sources of groundwater contamination in the study area. Data were compiled from 69 wells with a median depth of 160.5 v m. Eight observations surpassed the 44.3 v mg/L standard for nitrate (10 v mg/L NO 3 -N), and 24 observations exceeded the 10 v µg/L standard for arsenic. There was a statistically significant, inverse correlation between nitrate and well depth, and a direct correlation between nitrate and arsenic. Arsenic concentrations were significantly higher in a uranium-bearing sand formation compared to other formations in the study area. Chloride concentrations were also high relative to the (secondary) drinking water standard (250 v mg/L), with a median of 342 v mg/L and maximum of 6840 v mg/L. Most chloride/bromide ratios were near 300, but there were four significantly lower values, consistent with oilfield brine or evaporite dissolution. Results of this study suggest that (1) geology exerts a major control on arsenic concentrations in groundwater, (2) agricultural activity contributes substantially to nitrate and chloride and, to a lesser extent, arsenic concentrations in groundwater, and (3) oilfield brine has locally impacted groundwater in the study area.