SITE INFORMATION AND CORRESPONDENCE

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Greg K. Vaughn • -4- • 11 May 1988 impoundment. At the O/LDC facility, only the uppermost portion of the uppermost water-bearing zone has been investigated, under the assumption that deeper aquifers could not have been affected by leachate. The HAR states (page 3 .8-12) , "The usual method for determining the nature and characteristics of hydraulic interconnection between aquifers is to perform a pumping test. " This is followed by a discussion of the performance of aquifer tests. I agree that a pumping test is one method for obtaining this information. The demonstration of knowledge of how a test should be run, however, without an actual test, is insufficient. A pumping test designed to characterize aquifer interconnection should be run at O/LDC. The HAR makes an argument that the upper aquifer is not hydraulically interconnected to deeper aquifers, based on drillers reports from offsite wells. Referring to lithologic data from these logs, the HAR states, "These clay layers may be continuous, providing an aquiclude between the near surface aquifer . . . and aquifers greater than 80 feet below the ground surface" , and , "One aquitard may exist (emphasis added) from 60 to 85 feet below the ground surface beneath the 0/LDC facility. " Alluvial materials in this region are highly heterogeneous, as evidenced in the drillers logs and in boring logs of onsite wells. Offsite wells were drilled by several methods, some of which are not conducive to accurate •lithologic logging, and were not logged by qualified geologists. While some stratigraphic evidence has been derived from the drillers logs, nothing is known about the hydraulic properties or water bearing characteristics of materials logged as clay on the drillers logs. In addition, no conceptual geologic model of deposition in this area is proposed which would support the existence of a thick, continuous and extensive low permeability zone. To date, no onsite borings have encountered the proposed aquitard beneath the site, making its presence there purely conjectural. A series of deep borings must be completed at the facility to investigate the full vertical extent of the upper aquifer, to characterize any confining layers that may exist beneath the site, and to determine the extent to which deeper water-bearing zones may have been affected by leachate. Lithologic evidence alone is inadequate to characterize the interconnection between aquifers. Hydraulic and chemical evidence must also be developed to support the proposed model . In short, while it is true that a condition of separation between shallow and deeper water-bearing zones may exist , the evidence cited is inadequate to demonstrate the protection of water quality in deeper aquifers. The extrapolation of drillers log evidence over large areas to make such a demonstration, without supporting evidence, is beyond the realm of good hydrogeological judgement.