Soil-drainage characteristics and the amount of nitrogen contributed by fertilizer, manure, and atmospheric sources were found to markedly influence the concentration of nitrate in ground water. Ground-water nitrate data generally verified the risk patterns shown on the national map. The verification analysis is based on over 10,000 samples collected through 1992. In 1990, ground water accounted for 39 percent of water withdrawn for public supply for cities and towns and 96 percent of water withdrawn by self-supplied systems for domestic use.
Ground water is the sole source of drinking water for many rural communities and some larger cities.
A variety of chemicals, including nitrate, can pass through the soil and potentially contaminate ground water. Nitrate comes from nitrogen, a plant nutrient supplied by inorganic fertilizer and animal manure. Additionally, airborne nitrogen compounds emitted by industry and automobiles are deposited on the land. Other nonagricultural sources of nitrate, such as septic systems, generally are less significant and are not shown on the national map. Beneath agricultural lands, nitrate is the primary form of nitrogen.