Fertilizers. During the 1960s and 1970s, nitrogen, phosphorus and potassium fertilizer use steadily increased to a peak of 23 million tons in 1981. By 1987, however, fertilizer use had declined to 19.2 million tons, reflecting the large number of acres withdrawn from production as part of the Conservation Reserve Program and other government programs.

If nitrogen supply exceeds nitrogen uptake by crops, excess nitrogen can be leached to groundwater. In such areas, local nitrate-nitrogen concentrations may exceed the federal drinking water standard of 10 mg/L.

Pesticides. Pesticides have been used since the 1940s to combat a variety of agricultural pests. Between 1964 and 1982, the amount of active ingredients applied to croplands increased 170 percent. Herbicide usage peaked in 1982, and since then has declined from about 500 million pounds of active ingredients per year to about 430 million pounds in 1987.

Spilled pesticides can cause contamination in locations where pesticides are stored, and where sprayers and other equipment used to apply pesticides are loaded and washed. Pesticides most frequently detected in groundwater are the fumigants ethylene dibromide (EDB) and 1,2-dichloropropane; the insecticides aldicarb, carbofuran and chlordane; and the herbicides alachlor and atrazine.

Although 44 state summaries in the U.S. Geological Survey's l986 National Water Summary on groundwater quality mention detection of pesticides in groundwater, data are insufficient to draw conclusions about the extent of contamination. The state summaries do, however, express widespread concern that the frequency of detections and the concentrations of pesticides will increase over time.

The U.S. EPA has compiled reports on the occurrence of 46 pesticides in groundwater. In 26 states, one or more pesticides have been detected in groundwater that can be attributed to normal agricultural use. EPA currently is conducting its National Pesticide Survey to provide a nationwide estimate of the occurrence of pesticides in drinking water wells. The survey includes the collection of water samples from a statistically representative sample of community wells.

Irrigation. Percolation of irrigation water into soils dissolves soil salts and transports them downward. Evapotranspiration of applied water from the root zone concentrates salts in the soil and increases the salt load to the groundwater. Chemigation, the practice of mixing and distributing pesticides and fertilizers with irrigation water, may cause contamination if more chemicals are applied than plants can use. It may also cause local contamination if chemicals back-siphon from the holding tank directly into the aquifer through an irrigation well.