and other methods were published recently by EPA (1992a). Inspection of Table 3.3 reveals some general similarities within the broad classes of methods. Overlay and index methods tend to be applied at small map scales (large study areas), typically greater than 1:50,000, whereas most current process-based models apply to problems at much larger map scales (smaller study areas). Most overlay and index methods and most statistical methods refer to the saturated zone (the ground water resource) or water table as the reference location. In contrast, most process-based models have a floating reference location depending on the extent to which contamination is investigated through the vadose zone (for example, the reference location may be the bottom of the crop root zone for agricultural scenarios). Most overlay and index methods are designed to evaluate intrinsic vulnerability or have mixed specific and intrinsic utility. In contrast, most process-based models and statistical methods are designed for specific classes of contaminants such as pesticides or nitrate.
Overlay and index methods rely primarily on qualitative or semiquantitative compilations and interpretations of mapped data. Selected overlay and index methods are listed in Table 3.4 together with the parameters used in their application. Additional methods are summarized by the U.S. Environmental Protection Agency (1992a). Variables used in the overlay and index methods typically include approximate depth to the water table, ground water recharge rate, and soil and aquifer material properties.
The shorter the distance to ground water, the less soil and underlying unsaturated zone material is there to act as a filter or adsorbent. Depth to ground water also affects the transit time available for various abiotic and biotic processes to degrade the chemical. Depth to ground water corresponds to the depth to water table in unconfined aquifers or to the depth to the bottom of a confining geologic unit when the uppermost aquifer is confined. Varying degrees of confinement over an area are common. Overlay and index methods use a single depth to ground water at each location. However, large seasonal fluctuations in water levels in unconfined aquifers can complicate the estimate of single representative values. Seasonally high water table depths may be used to provide conservative estimates. Information on the depth to ground water is available from many sources, including well logs, federal and state agency computer files, and water-level maps published by federal and state agencies, universities, and consulting firms.