Section 8: Ground Water Aquifers
All sections prior to this one have covered HRS concepts that apply to all or most pathways. Beginning with Section 8, these concepts are applied to specific pathways as if documentation records were being prepared.
This section discusses issues surrounding aquifers, the basic unit of evaluation in the HRS ground water pathway. These concepts also apply to the ground water to surface water migration component of the HRS surface water pathway.
Aquifers are addressed individually in evaluating the ground water pathway. Thus, the general definition of "aquifer" for HRS purposes is crucial. In addition, precise descriptions of the aquifer of concern must be included in the HRS documentation record when evaluating the ground water pathway for a site. Such a description must include identification of the location of aquifers in relation to potential targets, the dimensions of the aquifers (deepness and thickness), and the extent of the aquifers (intervening layers, interconnections, and discontinuities). Once this description is in hand, the HRS analysis of the ground water pathway is relatively straight-forward.
The HRS evaluates each aquifer underlying the site separately. The highest scoring aquifer is selected as the aquifer of concern, and the HRS documentation record for the ground water pathway is prepared based on that aquifer: the score for that aquifer is the ground water pathway score. Identifying the highest scoring aquifer prior to scoring is usually infeasible because the likelihood of release and waste characteristics factor category values generally decrease with depth while the targets factor category value may increase with depth. Consideration should be given to including scores for each aquifer underlying a site in the HRS documentation record in order to facilitate responses to public comments.