EFFECTS OF AGRICULTURE ON GROUNDWATER QUALITY IN 5 REGIONS OF THE UNITED-STATES

Water-quality conditions in surficial unconsolidated aquifers were ass essed in five agricultural regions in the United States. The assessmen t covers the Delmarva Peninsula, and parts of Long Island, Connecticut , Kansas, and Nebraska, and is based on water-quality and ancillary da ta collected during the 1980s. Concentrations of nitrate in ground wat er in these areas have increased because of applications of commercial fertilizers and manure. Nitrate concentrations exceed the maximum con taminant level (MCL) for drinking water of 10 milligrams per liter as nitrogen established by the U.S. Environmental Protection Agency in 12 to 46 percent of the wells sampled in the agricultural regions. Conce ntrations of nitrate are elevated within the upper 100 to 200 feet of the surficial aquifers. Permeable and sandy deposits that generally un derlie the agricultural areas provide favorable conditions for vertica l leaching of nitrate to relatively deep parts of the aquifers, The pe rsistence of nitrate at such depths is attributed to aerobic condition s along ground-water-now paths. Concentrations of nitrate are greatest in areas that are heavily irrigated or areas that are underlain by we ll-drained sediments; more fertilizer is typically applied on land wit h well-drained sediments than on poorly drained sediments because web- drained sediments have a low organic-matter content and low moisture c apacity. Concentrations of other inorganic constituents related to agr iculture, such as potassium and chloride from potash fertilizers, and calcium and magnesium from liming, also are significantly elevated in ground water beneath the agricultural areas. These constituents togeth er impart a distinctive agricultural-chemical trademark to the ground water, different from natural water.