Pesticides have been detected at concentrations exceeding regulatory levels in groundwater samples from many parts of the world. In efforts to minimize the impact that pesticides have on groundwater resources in the United States, the U.S. Environmental Protection Agency (USEPA) proposed that every state develop a pesticide management plan. Development of an effective plan requires that planners identify areas that are most vulnerable to pesticide contamination. The purposes of this article are to present a vulnerability modeling methodology for identifying areas that are vulnerable to pesticide contamination and to evaluate the reliability of the modeling methodology using historic groundwater samples. An objective vulnerability index model was developed from the one-dimensional, advective transport equation to assess groundwater contamination potential based on spatially variable environmental and chemical properties that influence the pesticide leaching potential from the ground surface to the water table. The model was implemented and modified in a geographic information system (GIS) using spatially variable data to produce five vulnerability ratings ranging from very low to very high. Groundwater samples from areas of very high, high, moderate, low, and very low vulnerability had pesticide detection frequencies of 59, 29, 23, 12, and 3 percent, respectively, which indicate that the model is reliable. This model can be used to assess the potential impact of various organic chemicals, application rates, and intensities of irrigation, making it valuable for developing an effective pesticide management plan. This methodology can be applied to nearly any region in the conterminous United States because the required data are generally available in digital format.

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