SITE INFORMATION AND CORRESPONDENCE FILE 1

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CLEARWATER G R O U P, I N C. Fn Bi rnn meita! Serairrs Clearwater agrees that a complete understanding of contamination distribution and mass is an integral element of remedial system design; however, continued debate over the exact mass of contaminants is not beneficial. It is not disputed that significant concentrations of residual petroleum hydrocarbons remain in the subsurface. However, pilot test results provide the most reliable data for design of an operational remedial system, as groundwater/soil vapor flows rates, influent hydrocarbon concentrations, and resulting zones of influence are empirically derived. Once the system is installed, it will likely operate until contaminant recovery rates are reduced to low levels regardless of what exact contaminant mass was estimatedduring the assessment phase. Suffice it to say that the actual mass of contamination is likely somewhere between the Clearwater estimate and PHS/EHD estimate, and that the remedial recommendation is appropriate for such site conditions. ` Remedial Action Plan Clearwater conducted a dual-phase vapor extraction test on RW-1 and soil vapor extraction tests on VW-1, VW-2, and VW-3. The tests conducted using these four wells provided empirical. data concerning achievable vapor flow and radii of influence. Based on these data, Clearwater predicted that an additional dual-phase i extraction well, located near MW-1, used in conjunction with those wells noted above, would be sufficient to exert vapor recovery influence across the lateral extent of soil contamination beneath the site (Figure 12 of Remedial Action Plan). The PHS/EHD noted that systems do not always perform according to theory and suggested that an additional vapor extraction well be incorporated into the conceptual design. It should be noted that empirical data is available for 80% of the proposed extraction points (RW-1, VW-1, VW-2, and VW-3). Although flow rate and radius of influence for proposed RW-2 are estimates, they are based on sufficient site specific data. If data collected during operation of the system as designed indicate an additional well appears necessary in the future, it would not be difficult to plumb it to remedial trench hardware. This sequential approach ensures against inefficient system overdesign. The PHS/EHD also expressed; concern [regarding "smearing" of separate-phase hydrocarbons (SPH) in the cone of depression resulting from groundwater extraction. It should be noted that this smeared interval will be aggressively targeted by coeval soil vapor extraction, and that Clearwater has proposed no sole groundwater extraction wells. All groundwater extraction points will also serve as vapor extraction points. Concern was also expressed regarding smearing of soluble- phase contamination as the water table is lowered within the pumping well. It should be noted that maximum drawdown within each pumping well is anticipated to be approximately 10 to 11 feet. It should be noted that groundwater beneath the site naturally fluctuates up to 7 feet annually, from 10 to 17 feet below grade. Considering that the vertical extent of groundwater contamination is probably at Ruiz.4491.W.DurhaniFerry.Tracy.CA 3 September 17,1996 r