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 ofnitrate 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 sampledin 2007. nitrate concentrations in three wells exceeded the maximum contaminant level (44 mg/l) for drinkingwater. the highest nitrate concentration was 149 mg/l, and concentrations were generally higher in shallowerwells. chloride concentrations exceeded the 250 mg/l secondary drinking water standard in two wells, with nosignificant association between chloride concentration and well depth. results of this study suggest localizedhuman impacts, especially for nitrate, and identify areas warranting future monitoring.