National Water-Quality Assessment (NAWQA) Program
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Concentrations of nitrate were greater than the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 milligrams per liter (mg/L) as N in about 4 percent of the wells. Elevated concentrations of nitrate occurred throughout the Unites States, but least commonly in the Southeast. Concentrations greater than 1 mg/L as N, which is usually considered indicative of human activities in many areas (Nolan and Hitt, 2003), were found in about 40 percent of wells located throughout the sampled areas. >, greater than; <, less than or equal to.