back-siphoning of agricultural chemicals into wells, and injection wells. Ground water can also become contaminated by substances released at or near the soil surface in a more dispersed manner including pesticides, fertilizers, septic tank leachate, and contamination from other nonpoint sources.

Nitrates from fertilizers and animal wastes are the most pervasive type of ground water contamination. An estimated 20.5 million tons of fertilizers were applied to crops during 1988-1989 (USDA 1989). Between 1960 and 1985, agricultural use of nitrogen quadrupled, to 12 million tons (USDA 1987). Nitrate levels in ground water have increased concurrently with these rises in fertilizer application.

Pesticides also contribute significantly to ground water pollution. Each year about 661 million pounds of active pesticide ingredients are used in agriculture (OTA 1990, USEPA 1987). The first reported instances of ground water contamination by pesticides occurred in 1979 when dibromochloropropane (DBCP) was detected in California and aldicarb in New York. Subsequently, DBCP was detected in ground water in four additional states. By 1983, ethylene dibromide (EDB) had been found in wells in 16 counties of California, Florida, Georgia, and Hawaii (USEPA 1987). These findings prompted the suspension of EDB use in the United States. By 1988, pesticides had been detected in the ground water of more than 26 states (USEPA 1988). The largest monitoring study conducted in the United States, EPA's National Pesticide Survey (USEPA 1990), concluded that about 10.4 percent of wells in community water systems and 4.2 percent of rural domestic well water had detectable residues of one or more pesticides; fewer than 1 percent of all wells, however, were estimated to contain at least one pesticide in excess of established levels of health concern.

Once contaminated, ground water is very expensive to clean up; in many cases, cleanup may not be possible within a reasonable time (Mackay and Cherry 1989, Haley et al. 1991). In addition, ground water is the only source of drinking water for many rural areas (USGS 1990). The cost of replacing contaminated sources with bottled water or other alternatives is high relative to that of existing ground water resources (Abdalla 1990).

PROTECTING GROUND WATER FROM FUTURE CONTAMINATION

The seriousness and intractability of the problem of contaminated ground water has led resource managers to pursue a policy of prevention. Boxes 1.1 and 1.2 provide background information on the ground water protection programs of two federal agencies—the U.S. Department of Agriculture (USDA) and U.S. Environmental Protection Agency (EPA).

The factors that affect the ability of contaminants introduced at the land surface to reach ground water vary from place to place. Many extreme



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