the state level. For example, Illinois (Berg and Kempton 1988, McKenna and Keefer 1991) and Iowa (Hoyer and Hallberg 1991) have developed GIS-based maps using overlay methods with an emphasis on geology as the key attribute for assessing vulnerability. An overlay map for the state of Wisconsin considers depth to water, geology, and soils information (Wisconsin Department of Natural Resources and Wisconsin Geological and Natural History Survey 1987).
In contrast to simple overlay methods, index methods assign a numerical value to each attribute based on its magnitude or qualitative ranking. Each attribute, in turn, is assigned a relative importance or weight compared to the other attributes. A consensus of experts may be solicited (the Delphi approach) to determine the relative weights assigned to different attributes and the numerical values assigned to different levels of each attribute. The weighted-attribute ratings are summed to obtain an overall numerical score for ground water vulnerability. These numerical scores are used to group similar areas into classes or categories of vulnerability (e.g., low, medium, and high) that are then displayed on a map. Some methods multiply the numerical scores or values assigned to the attributes together rather than adding them (c.f., Back et al. 1984).
Several types of indices have been developed for ground water vulnerability assessments. The DRASTIC index (Aller et al. 1987) is perhaps the best known of these methods. Some state regulatory agencies have developed index assessment methods similar to DRASTIC (cf., Rupert et al. 1991). Using information about pesticide leaching abilities, Kellogg et al. (1992) developed the GWVIP and GWVIN indices to generate national-scale vulnerability maps for pesticides and nitrates, respectively (see Chapter 5).
Overlay and index methods have often been developed with the availability of information keenly in mind. These methods are driven largely by data availability and expert judgment, with less emphasis on processes controlling ground water contamination. One can argue whether the factors included in the methods are the relevant ones for vulnerability assessment and whether the factor ratings are appropriate. For example, Banton and Villeneuve (1989) questioned the basis for the numerical weighting scheme used by Agricultural DRASTIC after comparing its results with those from a process-based modeling approach (PRZM). Further, Holden et al. (1992) concluded that "the complex weighting and coding procedures used in the DRASTIC scoring are self defeating," and that in the short-term, "simpler classification schemes, focusing on only a few major vulnerability factors, look to be more useful than DRASTIC." There are no quantitative criteria for evaluating the scientific basis of these methods.
Many overlay and index methods address intrinsic vulnerability, although some of them address what might be called pseudospecific vulnerability.