cover, and other characteristics of the area to be assessed. Different types and amounts of data are necessary depending on the specific assessment method used. The product of most vulnerability assessments to date has been a map depicting areas of relative vulnerability. Some researchers have chosen instead to express results as probabilities with the associated uncertainties displayed in tabular form.
It is infeasible and perhaps impossible to formulate a universal technique for predicting vulnerability, one that considers all of the ways in which contamination occurs or that is appropriate for all situations. Key elements to consider in a vulnerability assessment for a particular application include the reference location, the degree of contaminant specificity, the contaminant pathways considered, and the time and spatial scales of the vulnerability assessment. The reference location is the position in the ground water system specified to be of interest. The ground water table is the reference location used in most existing techniques. However, managers may determine that another reference location is more useful for their purposes. Vulnerability assessments may or may not account for the different behavior of different contaminants in the environment. Thus, there are two general types of vulnerability assessments. The first addresses specific vulnerability, and is referenced to a specific contaminant, contaminant class, or human activity. The second addresses intrinsic vulnerability and is for vulnerability assessments that do not consider the attributes and behavior of specific contaminants. In practice, a clear distinction between intrinsic and specific vulnerability cannot always be made. Contaminants can enter aquifers by a variety of pathways. Most existing assessment techniques address only transport that occurs by simple percolation and ignore preferential flow paths such as biochannels, cracks, joints, and solution channels in the vadose zone. The omission of preferential flow paths is likely a significant limitation of vulnerability assessments in many environments. Some overlay and index methods have attempted to address contamination that might occur by wells and boreholes by mapping those features in combination with the results derived from other assessment methods. The overall utility of a vulnerability assessment is highly dependent on the scale at which it is conducted, the scale at which data are available, the scale used to display results, and the spatial resolution of mapping.
The combination of these elements makes up a vulnerability assessment method. Inherent in any such combination will be scientific uncertainties associated with errors in data, errors in method, and potential misapplication of an approach to a given area. The prediction of ground water vulnerability is an imprecise exercise, as stated in the Second Law of Ground Water Vulnerability:
Uncertainty is inherent in all vulnerability assessments.