(with possible exceptions such as deep sedimentary basin brines) is invulnerable.
An important consequence of the First Law is that the time a potential contaminant would take to travel from the point of introduction to the specified position in the ground water system must be either an implicit or explicit part of any attempt to identify vulnerable areas. A long travel time by itself, however, does not guarantee that an aquifer has low vulnerability. Rather, a key issue is the extent to which processes such as dispersion, sorption, and biochemical transformations are likely to reduce concentrations of the contaminants of interest and/or transform the contaminants to benign products. Thus, it may be misleading to assign low vulnerability to a setting simply because the unsaturated zone is very thick. Dependent on the unsaturated zone materials, if the contaminants of interest are sufficiently persistent and mobile to reach ground water, then they will eventually reach the aquifer. For example, several investigators (e.g., Pratt et al. 1972) have shown that nitrates can take decades to reach ground water. By extension, pesticides that are persistent, but less mobile than nitrate due to sorption, could take even longer.
Ground water vulnerability is an amorphous concept, not a measurable property. It is a probability (i.e., "the tendency or likelihood") that contamination will occur, and thus must be inferred from surrogate information that is measurable. In this sense, a vulnerability assessment is a predictive statement much like a weather forecast, but for processes that take place underground and on much longer time scales.
An array of methods for predicting ground water vulnerability has been developed. Many of them are based on mathematical models using equations that approximate the behavior of substances in the subsurface environment. These methods are called process-based methods. Another set of methods combine physical characteristics that affect vulnerability in a weighted index or numerical score. A third approach uses statistical methods to draw associations with areas in which contamination is known to have occurred. Generally, the more complex and detailed methods require more complex and detailed knowledge of the system being assessed. Simpler methods incorporate more approximations and are less precise, but require less detailed information about the system being assessed. Although complex methods may describe transport mechanisms more precisely, the data required are often unavailable and must be approximated from limited existing information.