contaminants a more direct and rapid path to ground water than they would otherwise have. In some cases, considerable potential exists for water to move from contaminated shallow aquifers to deeper aquifers via existing or improperly sealed abandoned wells. Cross-contamination might take place in the wellbore or outside the well casing in an unsealed annulus. It is difficult to incorporate these types of contaminant pathways into quantitative measures of ground water vulnerability, yet they may be the primary control on the vulnerability of deeper aquifers to contamination. One approach is to overlay information on the potential for cross-contamination of deeper aquifers (e.g., sink holes or agricultural drainage areas) on traditional maps of shallow aquifer vulnerability as was done in Iowa (see case example in Chapter 5).
Vulnerability to contamination is commonly treated as a static property although ground water development can have a marked effect on vulnerability by changing the flow regime. Pumpage-induced movement of contaminated shallow ground water into deeper aquifers may be a significant consideration in some situations. Inclusion of these factors into a vulnerability assessment generally entails some level of computer simulation.