At least 17 pesticides have been found in ground water in a total of 23 states as a result of agricultural practice. These results have been obtained through three different types of monitoring studies: (1) large-scale retrospective, (2) smallscale retrospective, and (3) small-scale prospective. The first two types of studies survey areas where the pesticide(s) in question has already been used. The third type of study is an intensive field study where the pesticide is applied and monitoring begins at time zero. Often, soil core data are at least as important as ground-water data. The ability to draw meaningful conclusions from large-scale studies is greatly diminished unless the studies have a statistical, stratified design. The purpose of this paper is threefold: to describe the three study types; suggest guidelines for ground-water sampling, soil sampling and well construction; and update the data summary of pesticides in ground water from agricultural practice.In 1984, Cohen et al. reviewed leaching and monitoring data on 12 different pesticides found in ground water in a total of 18 different states as a result of agricultural practice (V) · They also established criteria for predicting whether certain pesticides could leach to ground water as a result of normal use.In the less than 2 years since that paper was published, the numbers of pesticides found in ground water have increased significantly as have the number of states found to have pesticides in ground water. Seventeen pesticides have now been found in the ground water of 23 states as a result of agricultural practice (Figure 1, Table I). (As described in the "Occurrence" section at the end of this paper, many additional findings can be attributed to poor disposal practices, mixing-loading operations, etc. and are not included in this count.) This significant increase is more likely due to an increase in the quality and quantity of studies rather than an increase in This chapter not subject to U.S.

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Onsite leaching studies can be categorized into two groups-prospective and retrospective studies. The prospective study described in this report is designed to track the movement of pesticide residues from the time of application of the pesticides to a predetermined level of dissipation or length of time. The retrospective study is designed to determine if a previously applied pesticide has already reached the ground water. Onsite information is similar for both study designs and includes climatic data, hydrogeologic properties, and soil properties. The equipment used and the methods employed for the sampling of soil, soil water, and ground water can be applied to both study designs. The scheduling of sampling varies somewhat between study types, with the prospective study focusing on the unsaturated zone and the retrospective study focusing on the saturated zone. Sample collection can be economized by sample compositing, and tracer applications can provide representative samples while economizing the sampling scheme. Finally, quality-assurance methods need to be incorporated in the collection and transportation of all samples.