other, by a fundamental principle that is stated here as the First Law of Ground Water Vulnerability:

All ground water is vulnerable.

Vulnerability is not an absolute property, but a relative indication of where contamination is likely to occur; no ground water, with possible exceptions such as deep sedimentary basin brines, is invulnerable. Furthermore, it may be necessary to consider long term effects on ground water quality, perhaps over decades, in carrying out vulnerability assessments.

Ground water vulnerability is an amorphous concept, not a measurable property. It is a probability (i.e., "the tendency or likelihood") of contamination occurring in the future, and thus must be inferred from surrogate information that is measurable. In this sense, a ground water vulnerability assessment is a predictive statement much like a weather forecast, but for processes that take place underground and over much longer time scales.

The potential for contaminants to leach to ground water depends on many factors, including the composition of soils and geologic materials in the unsaturated zone, the depth to the water table, the recharge rate, and environmental factors influencing the potential for biodegradation. The composition of the unsaturated zone can greatly influence transformations and reactions. For example, high organic matter or clay content increases sorption and thus lessens the potential for contamination. The depth to the water table can be an important factor because short flow paths decrease the opportunity for sorption and biodegradation, thus increasing the potential for many contaminants to reach the ground water. Conversely, longer flow paths from land surface to the water table can lessen the potential for contamination for chemicals that sorb or degrade along the flow path. Recharge rates affect the extent and rate of transport of contaminants through the saturated zone. Finally, environmental factors, such as temperature and water content, can significantly influence the degradation of contaminants by microbial transformations.

An array of approaches for predicting ground water vulnerability has been developed from an understanding of the factors that affect the transport of contaminants introduced at or near the land surface. These methods fall into three major classes: (1) overlay and index methods that combine specific physical characteristics that affect vulnerability, often giving a numerical score,  (2) process-based methods consisting of mathematical models that approximate the behavior of substances in the subsurface environment, and (3) statistical methods that draw associations with areas where contamination is known to have occurred.

Each of these methods requires that adequate data be available on factors that affect ground water vulnerability, such as soil properties, hydraulic properties, precipitation patterns, depth to ground water, land use and land



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