Boron and selenium contamination in south Texas groundwater.

Boron and selenium concentrations from 112 water wells in an irrigated agricultural region of south Texas were compiled, mapped, and statistically analyzed. Wells in the study area produce water from the Gulf Coast Aquifer System, comprising coastward sloping beds of clay, silt, sand, and gravel. Nearly 84 percent of boron observations exceeded the 600 ug/L advisory level for drinking water, 70% exceeded the 1250 ug/L level for sensitive crops, and 24% exceeded the 3750 ug/L level for tolerant crops. Additionally, 21% of selenium observations exceeded the 20 ug/L advisory level for irrigation water, and five percent surpassed the 50 ug/L standard for drinking water. Many wells with high boron concentrations also had high selenium concentrations, and several clusters of high concentrations were in irrigated parts of the study area. However, there was no association between selenium and well depth, and a direct rather than inverse association between boron and well depth. Progressively brackish water in downdip reaches of the aquifer largely controls observed boron patterns. Both irrigation practices and prevailing groundwater chemistry significantly influence selenium concentrations in the study area.