Herbicides, though applied in greater amounts and detected more frequently than other types of pesticides, were not detected at concentrations that exceed MCLs. Samples from five public supply wells contained only p,p' -DDE, a breakdown product of DDT, which was discontinued for sale in the United States in the early 1970s. More modern pesticides were not detected, suggesting that these deeper wells are probably withdrawing older ground water that predates the use of newer pesticides. Low concentrations of pesticides and VOCs were detected in 60 percent of samples from 117 shallow domestic and monitoring wells and in 46 percent of samples from 138 deeper public supply wells. Thirty-one pesticides and VOCs were detected in samples from shallow domestic and monitoring wells, whereas 18 pesticides and VOCs were detected in samples from public supply wells.
Shallow wells have a greater number and percentage of pesticide detections.
Health effects of combinations of pesticides in drinking water are not clearly understood. Most detections of more than one pesticide occurred in samples from very shallow monitoring wells. Pesticides were frequently detected in wells with elevated nitrate Many chemicals, including nitrate and pesticides, can leach through the soil to ground water. Because of this consistent correlation, sampling for nitrate may be cost effective for identifying wells that may be at risk of pesticide contamination. Nitrate testing is far less expensive than pesticide testing, and the latter could then be targeted more carefully.