A national look at nitrate contamination of ground water

Ground water provides drinking water for more than one-half of the Nation's population (Solley and others, 1993), and is the sole source of drinking water for many rural communities and some large cities. In 1990, ground water accounted for 39 percent of water withdrawn for public supply for cities and towns and 96 percent of water withdrawn by self-supplied systems for domestic use.

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 given off by industry and automobiles are deposited on the land in precipitation and dry particles. Other nonagricultural sources of nitrate include lawn fertilizers, septic systems, and domestic animals in residential areas.

Beneath agricultural lands, nitrate is the primary form of nitrogen. It is soluble in water and can easily pass through soil to the ground-water table. Nitrate can persist in ground water for decades and accumulate to high levels as more nitrogen is applied to the land surface every year.

Knowing where and what type of risks to ground water exist can alert water-resource managers and private users of the need to protect water supplies. Although nitrate generally is not an adult public-health threat, ingestion in drinking water by infants can cause low oxygen levels in the blood, a potentially fatal condition (Spalding and Exner, 1993). For this reason, the U.S. Environmental Protection Agency (EPA) has established a drinking-water standard of 10 milligrams per liter (mg/L) nitrate as nitrogen (U.S. Environmental Protection Agency, 1995). Nitrate concentrations in natural ground waters are usually less than 2 mg/L (Mueller and others, 1995).