Nitrate and Chloride Concentrations in Groundwater beneath a Portion of the Trinity Group Outcrop Zone, Texas

Using a geographic information system and statistics, we evaluated spatial distributions of nitrate and chloride concentrations in groundwater in an area of north-central Texas with agricultural activity, in addition to oil and natural gas exploration and production. Data were compiled from 40 water wells sampled in 2007. Nitrate concentrations in three wells exceeded the maximum contaminant level (44 mg/L) for drinking water. The highest nitrate concentration was 149 mg/L, and concentrations were generally higher in shallower wells. Chloride concentrations exceeded the 250 mg/L secondary drinking water standard in two wells, with no significant association between chloride concentration and well depth. Results of this study suggest localized human impacts, especially for nitrate, and identify areas warranting future monitoring.