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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty
may infiltrate downward toward the water table) and discharge zones (places where ground water moves upward toward a stream or other discharge point). For example, the vulnerability assessed using the water table as the reference location may be greater at discharge zones than at recharge zones because the water table is shallower in discharge zones, whereas the potential for contaminants to migrate farther in the ground water system once they arrive at the water table may be significantly greater at the recharge zones. In some situations, water recharging the ground water system may move essentially horizontally along the water table gradient from high to low elevation and discharge at a surface water body or land surface depression. In other situations, the recharge can move substantially downward in the aquifer below the water table. In the former case, the potential for contamination is greatest near the water table; in the latter case, contaminants can spread through a large portion of the aquifer.
Specific and Intrinsic Vulnerability
Vulnerability assessments may or may not account for the different behavior of different contaminants in assessing vulnerability. In general, two types of vulnerability assessment can be defined. The first, specific vulnerability, is used when vulnerability is referenced to a specific contaminant, contaminant class, or human activity. A second term, intrinsic vulnerability, refers to vulnerability determined without consideration of the attributes and behavior of particular contaminants. In practice, a clear distinction between intrinsic and specific vulnerability cannot always be made. Many vulnerability assessment methods do not refer to specific contaminants (and hence are intrinsic); however, many of the parameters used in assessment methods (e.g., organic carbon content) will have different influences on different contaminants.
Contaminants can enter aquifers by several different means as illustrated in Figure 1.1. In general, vulnerability assessments consider only those types of contamination that begin as downward percolation from a surface source or from sources in the shallow subsurface. Thus, for example, direct entry of contaminants into wells resulting from a spill or back-siphoning during chemigation is not a pathway considered in vulnerability assessment.
Most measures of ground water vulnerability to contamination assume simple percolation from the land surface and ignore preferential flow paths, such as biochannels (root holes and worm holes) and cracks, joints, and solution channels in the vadose zone. These pathways, however, may give