of these uses or functions exhibits dramatically different tolerances for contamination that must be accounted for in identifying ground water impacts.
Zoning and screening functions ultimately involve decisions affecting individual sites. Regional vulnerability assessments do not provide site-specific resolution, but may appear to contain information relevant to site-level decisions. Limitations in the ability to predict the effect of land uses and activities on ground water include scale constraints, errors, and uncertainty. While it might be tempting to construct a hypothetical ground water zoning map based on a vulnerability map, such a zoning map could not of course, be more detailed or more certain than the underlying vulnerability assessment. Unfounded application of regional assessments at the site level could result in misinformed decisions with unfortunate and even tragic consequences for the land user, land regulator, and the ground water resource itself.
In issuing permits for activities that may contaminate ground water, regulators often establish specific requirements that relate to the characteristics of the activity or site. Conditions or required mitigation may include treatment to acceptable levels of contaminant concentration, limits on discharge volumes, prohibition of discharge, and required containment to deal with accidental spills. Vulnerability assessments also can guide the establishment of sampling routines for compliance monitoring. Land uses in comparatively more vulnerable areas may, therefore, require more monitoring for compliance assurance than uses in less vulnerable areas.
As before, the utility of vulnerability assessments to the manager in setting conditions or establishing mitigation will vary with the specificity of the assessment. Vulnerability assessments conceivably could be used to establish more stringent requirements in areas of high vulnerability, including more frequent or intensive monitoring to demonstrate compliance with permit conditions.
The availability of a vulnerability assessment can give both land users (e.g., farmers) and managers (e.g., water supply superintendents) a proper sense of caution and some information on how to avoid excessively risky actions. A vulnerability assessment prepared by the Soil Conservation Service, for example, may be linked to a set of alternative land uses and conservation practices that would minimize contamination of the water system. Farmers would select from the acceptable alternatives based on resource protection factors as well as social and economic factors. Other responses