Pesticides in Ground Water

U.S Geological Survey Fact Sheet FS-244-95

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Effects of Agricultural Practices and Well Construction

Much effort has been directed toward evaluating the many natural and human factors that govern the transport and fate of pesticides in soil and ground water (see Figure 7). However, the nature of many of these influences is not yet well understood because of the difficulty of sorting out the confounding effects of numerous variables. Extensive research indicates that, in addition to reducing use, the most effective ways to reduce the likelihood of ground-water contamination by pesticides are to minimize the rate of water movement through the soil, especially following pesticide application; and to slow the rate at which the active ingredients are released to the soil. Efforts to minimize water movement affect not only irrigation techniques, but the timing of pesticide applications and the manner in which the soil is tilled. Field studies have shown that, other factors being equal, pesticides have a greater potential to move to ground water under "no-till" conditions than under conventional tillage, even though the effects of tillage on water and pesticide movement may be seasonal.

Figure 7. Factors associated with pesticide detections in ground water.

Of all the factors that may affect pesticide detections in wells, construction characteristics have been among the most frequently studied. Pesticide concentrations and detection frequencies usually decrease with increasing well depth. In addition, pesticides are more likely to be detected in springs, and in dug, bored, or driven wells, than in drilled wells. Drilled wells are typically deeper and provide more complete isolation of ground water from contaminant sources. Similarly, wells with proper seals at the surface or around the well casing show consistently lower frequencies of contamination than those without seals. Bedrock wells with deep casings exhibit less frequent pesticide contamination than boreholes installed without casings. Pesticide detection frequencies also increase with well age -- an indirect indicator of well integrity and well depth.

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