to all assessments of ground water vulnerability. These laws should caution users of vulnerability assessments that ignoring uncertainty can lead to considerable error. Careful attention should be paid to the effects of these potential errors on decisions that will be informed by a vulnerability assessment. If the decisions would not change if the uncertainty were considered, then users of the assessment should have increased confidence in using its results. If the decision would change in the face of uncertainty, then the use of the assessment in making decisions would have to be viewed with caution.
This pervasive, inherent uncertainty led the committee to a probabilistic, rather than deterministic, definition of ground water vulnerability: The tendency or likelihood for contaminants to reach a specified position in the ground water system after introduction at some location above the uppermost aquifer. Ground water vulnerability is not a measurable property, but a probability statement about future contamination that must be inferred from surrogate measurements. Such information, in its simplest form, may be a single parameter, such as depth to ground water. Like a weather forecast, vulnerability to contamination is best expressed as a probability of an event (e.g., 30 percent chance of rain). Yet very few of the vulnerability assessment methods discussed in Chapter 3 produce results in the form of probabilities. This report distinguishes between two types of ground water vulnerability: intrinsic vulnerability, which reflects properties that are a function of the natural setting and does not consider the attributes and behavior of particular contaminants, and specific vulnerability, which reflects factors that relate to the properties of the specific constituent(s) of concern, and possibly specific circumstances of land and chemical use (Chapter 1).
Using vulnerability assessments currently available, it is fairly easy to delineate many areas of high vulnerability, difficult to say for certain that an area has very low vulnerability, and not possible to make fine gradations in between.
Ground water vulnerability assessment is a dynamic, iterative, and interactive process that must involve the cooperative efforts of policy makers, resource managers, and technical experts. Figure 1.3, Chapter 1, illustrates the dynamic interactions among the four major components of an assessment: intended purpose, approaches, required data, and management actions. Chapter 2 describes the uses of vulnerability assessments and the technical and institutional considerations that should be addressed in planning a vulnerability assessment as a tool for management. The case studies (Chapter 5) illustrate how the vulnerability assessment process is being