Regional trends in nitrate content of Texas groundwater

The purpose of this study was to compile, map, and evaluate regional patterns of nitrate occurrence in Texas groundwater. County-median nitrate levels were calculated from 7793 wells in a Texas Water Development Board database. A geographic information system was used to map and evaluate the data. Nitrate concentrations were substantially higher in the western half of the state. There were nine counties, all located in north-central and west-central Texas, in which more than 50% of the observations exceeded the maximum contaminant level (MCL) of 44.27 mg/l (10 mg/l NO3–N). In one north-central Texas county, 77% of the observations surpassed the MCL. The highest nitrate concentrations were found in the Seymour Aquifer, which had a median value of 59.9 mg/l. This aquifer also has a substantially lower median well depth than the other eight aquifers, which likely contributes to the high nitrate values. In six of nine aquifers, there was a statistically significant, inverse association between nitrate concentration and well depth, suggesting a land-surface origin for the contamination. Agriculture is a predominant land use in the study area—soil organic nitrogen and fertilizers are probable sources of nitrate throughout west Texas. Denitrification in the wetter, more densely vegetated southeastern part of the state may account for low nitrate levels observed in that region.