currently available for assessing aquifer sensitivity or ground water vulnerability to pesticide contamination. Their categorization includes three broad classes of approaches depending on the factors included in the assessment method. Each class is broken down further into specific types of approaches, such as aquifer sensitivity assessment methods which consider only hydrogeologic factors; hybrid methods, which consider hydrogeologic and pesticide factors; and ground water vulnerability assessment methods, which consider hydrogeologic, pesticide, and agronomic factors. Statistical tools are also noted for their usefulness in validating methods or providing hydrogeologic setting information.

Our alternative classification scheme places assessment methods in three general categories: (1) overlay and index methods, (2) methods employing process-based simulation models, and (3) statistical methods.

Assessment methods in the first category, overlay and index methods, are based on combining maps of various physiographic attributes (e.g., geology, soils, depth to water table) of the region by assigning a numerical index or score to each attribute. In the simplest of these methods, all attributes are assigned equal weights, with no judgment being made on their relative importance. Thus, areas where simple confluence of the specified attributes occurs (e.g., sandy soils and shallow ground water) are deemed vulnerable. Such methods were the earliest to be used and are still favored by many state and local regulatory and planning agencies. Overlay and index methods that attempt to be more quantitative assign different numerical scores and weights to the attributes in developing a range of vulnerability classes, which are then displayed on a map. Popularization of GIS technology has made it increasingly easy to adopt map overlay and index methods.

The assessment methods in the second category, methods employing process-based simulation models, require analytical or numerical solutions to mathematical equations that represent coupled processes governing contaminant transport. Methods in this category range from indices based on simple transport models to analytical solutions for one-dimensional transport of contaminants through the unsaturated zone to coupled, unsaturated-saturated, multiple phase, two- or three-dimensional models.

Statistical methods having a contaminant concentration or a probability of contamination as the dependent variable form the basis for the third category. These methods incorporate data on known areal contaminant distributions and provide characterizations of contamination potential for the specific geographic area from which data were drawn. Statistical methods are sometimes used by regulatory agencies that have the regional databases on ground water contamination needed to develop models.

Some characteristics of selected vulnerability assessment methods used in the United States are listed in Table 3.3. Comparative details on these



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