Groundwater & Pesticides -- Section 3.3 Pesticide Impact on Groundwater

The potential for rainwater to "run off" of farm fields or suburban lawns --carrying with it eroded soils, pesticides and excess fertilizers -- has long been understood if not reckoned with. In contrast, the prospect of groundwater contamination by these same contaminants was for many years dismissed. While Rachel Carson in Silent Spring raised the specter of groundwater contamination early on, the federal government commission that responded to her warnings did not dwell on the issue.

Many water quality problems are the result of inefficiencies in chemical use, and these losses can clearly be viewed as an economic loss to farm income as well as a threat to water supplies.

George R. Hallberg ²

The quick dismissal of the possibility of groundwater contamination from normal agricultural use was not unusual for the time -- or for many years following. Well into the 1970s, the general wisdom espoused by chemical makers as well as some water supply experts and regulators was that agricultural chemicals applied according to directions would break down into non-toxic compounds before reaching water-yielding aquifers.

The "not a problem" logic began to unravel, however, with the discovery in 1979 of the soil fumigant DBCP and the insecticide aldicarb in the groundwater in several states. The initial response to these findings downplayed the implications -- the applicators may have erred, the chemicals involved were somehow unusual, the local environment and hydrogeology was particularly sensitive to contamination. Soon, however, it was clear that these seeming aberrations were far too common.

The EPA began to seek out information from states and researchers and to look behind the detections to determine the circumstances of pesticide use that led to contamination. By 1984, EPA documented findings of 12 pesticides in groundwater from 18 states believed to be the result of normal field applications. The count rose to 17 pesticides in 23 states in 1986,³ and 46 pesticides in 26 states in 1988⁴ -- all associated with legal pesticide application practices.

Perhaps the most serious case of pesticide contamination of ground water began with the discovery in 1982 of the pesticide ethylene dibromide (EDB) in two California wells and three wells in Georgia. By the end of the following year, EDB contamination of ground water had been discovered in 16 different counties in California, Florida, Georgia, and Hawaii. The EDB levels reported varied between 0.02 ppb and 300 ppb, but typically were found between 0.05 and 5 ppb. EDB has been described by the National Cancer Institute as "the most potent cancer-causing substance ever found in [their] animal test program." Finding EDB in numerous wells caused EPA to issue an immediate emergency suspension of all EDB use as a soil fumigant in September 1983.

U.S.EPA ⁵

Today, environmental researchers report that pesticides or their chemical breakdown products -- often called "metabolites" -- have been found in the groundwaters of 43 states.⁶ At least 143 pesticides and 21 metabolites have been detected.⁶ (As noted earlier, there is no guarantee that a pesticide's metabolites will be less toxic than the "parent" compound. In fact, some metabolites are as toxic or more toxic than the original chemical.) The geographic coverage of groundwater monitoring remains spotty, but localized studies and several nationwide investigations have laid to rest any doubt that the common, legal application of pesticides can and does affect groundwater quality.

1. Wargo, John, Our Children's Toxic Legacy: How Science and Law Fail to Protect Us from Pesticides, 1996.

2. Hallberg, George R., "When Agrichemicals and Groundwater Meet: Understanding the Connection," in Freshwater Foundation Journal, vol 11, 1988.

3. Hallberg, George, et al, "Pesticides, Groundwater, and Rural Drinking Water Quality in Iowa, in Pesticides and Groundwater: A Health Concern for the Midwest, proceedings of a conference held October 16-17, 1986, published by the Freshwater Foundation, 1987.

4. Kellogg, Robert, et al, Agricultural Chemical Use and Ground Water Quality: Where Are the Potential Problem Areas?, USDA, 1992.

5. US EPA, Office of Ground-Water Protection, Pesticides in Ground Water: Background Document, 1986.

6. US Geological Survey, Pesticide National Synthesis Project, "Pesticides in Ground Water," Factsheet FS-244-95, 1995, available at <http://water.wr.usgs.gov/pnsp/gw/>.