Importance of Ground Water

Pesticide contamination of ground water is a national issue because ground water is used for drinking water by about 50 percent of the nation's population. Concern about pesticides in ground water is especially acute in agricultural areas, where most pesticides are used, and where over 95 percent of the population relies upon ground water for drinking water.

Prior to the mid-1970's, it was generally assumed that soil provided a protective "filter" or "barrier" that stopped infiltrating contaminants before they reached ground water. The detection of pesticides and other contaminants in ground water, however, has demonstrated that this is not always the case. By 1980, contamination of ground water by the insecticide aldicarb was discovered in New York and Wisconsin, and contamination by the fumigant 1,2-dibromo-3-chloropropane (DBCP) was found in California, Arizona, South Carolina, and Maryland.

In addition to their application to targeted areas, pesticides are discharged to the environment through accidental spills and leaks, and through improper disposal. They may then reach ground water not only by leaching through soil, but by a variety of other routes as well. Some of these routes include seepage of contaminated surface waters into underlying ground-water reservoirs or "aquifers," transport down abandoned or poorly sealed wells, and injection through wells used to dispose of agricultural or urban runoff.