SITE INFORMATION AND CORRESPONDENCE_1992

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3-D Model/RD Report Memorandum -4- 14 December 1992 DDRW, Sharpe Sharpe should consider the intermittent pumping of wells like these (i .e. , with concentrations slightly greater than the aquifer cleanup level ) in order to reduce the volume of ground water with low contaminant concentrations needing treatment. Sharpe should also consider increased pumping rates of other extraction well in order to maintain the desired hydraulic control of the plume. However, all five of these wells may need to be used for the remedial action in order to achieve the proposed aquifer cleanup level of 0.5 µg/1 for several of the VOC ground water contaminants. We concur with the proposal to eliminate A-4 from the extraction system at this time, as we had originally made this proposal in our 26 June 1992 letter. However, this well should not be abandoned because it may be needed in the future if the configuration of the plume changes so that the TCE concentrations exceed 5 Ag/l in A-4. Inorganics The Modeling Report indicates that the weighted average concentrations of arsenic are less than the Primary Maximum Contaminant Level (MCL) of 50 Ag/1 (Table 9-4, page 9-7) . This estimate differs from the estimated 56 µg/1 influent concentration for arsenic presented in the Feasibility Study (FS) Report. The Modeling Report proposes to sample the extraction wells after development in the areas where monitor well samples have had elevated inorganic concentrations of contaminants that exceed the MCL (455B, 460AB and 475A) . The results of these extraction well samples would be used to calculate the individual and combined effluent levels. If the calculated combined effluent levels for inorganics are similar to the B and C Zones, then with blending of the waste streams, injection into any zone may be possible (page 9-6) . This proposal is acceptable. As stated in the Modeling Report, the reinjection of the treated ground water must be of adequate quality to assure that the receiving water zones are not degraded. We commend Sharpe for adhering to the State Water Resources Control Board Resolution No. 68-16 (Anti-Degradation Policy) . However, the original proposal to design separate plants may now not be necessary if this requirement can be met in a more cost effective manner. Whether a separate treatment plant is constructed, whether only a portion of the effluent is treated for inorganics, or whether all of the waste streams are blended, the treated ground water must meet the effluent limits listed in Table 9 of the Draft Final ROD for inorganics. Estimated Duration of Remedial Action Figures 9-2 to 9-4 present the projected TCE concentration in the A, B and C Zones during the life of the remedial action. The Modeling Report states that the TCE concentrations will remain above the proposed aquifer cleanup level (5 µg/l) for approximately 30 years in the A and C Zones and 40 years in the B Zone (page 9-10) . These estimates far exceed the estimate of 16 years provided in the FS Report and the Proposed Plan. Because of the significant discrepancies in estimates of the duration of the ground water remedial action, it apparent that the effectiveness of the remedial action must be thoroughly examined as part of the five year review process. If contaminant concentrations are not reducing in particular areas and the cleanup follows the model prediction that remediation may continue up to or beyond