How frequently do nitrate concentrations exceed human-health benchmarks in samples collected from shallow wells, domestic wells, and public-supply wells?
Elevated concentrations of nitrate are more prevalent and widespread in groundwater used for drinking water than in streams, exceeding the human-health benchmark (MCL of 10 mg/L as nitrogen) in 9 percent of all 4,674 samples. Elevated concentrations of nitrate occurred throughout the U.S., but were least common in the Southeast because of aquifer and soil conditions that promote denitrification of nitrate. Eighty-five percent of all groundwater studies in agricultural areas had one or more samples (of 20 to 30 wells sampled) with a nitrate concentration greater than the MCL; and, about 40 percent of these studies had concentrations greater than the MCL in more than 20 percent of samples. Concentrations exceeded the MCL in about 7 percent of the 2,388 domestic wells sampled. Concentrations exceeded the MCL in more than 20 percent of the domestic wells in agricultural areas. Domestic wells typically are shallower than public supply wells. The shallow depth and relatively large nitrogen i
