SITE INFORMATION AND CORRESPONDENCE_1992

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3-D Model/RD Report Memorandum -5- 14 December 1992 DDRW, Sharpe 30 to 40 years, then additional extraction wells may need to be added to expedite the cleanup of the aquifer. However, if the remedial action proceeds more quickly (i .e. , within the 16 year estimate) , then Sharpe must provide an evaluation that it would be technically and economically infeasible to continue with the remediation to achieve in-situ TCE concentrations of less than 5 14g/1 . Resolution No. 92-49 requires cleanup to background if technically and economically feasible. This Resolution was adopted in June 1992. Our review of the Draft ROD was dated 21 May 1992. Therefore, this applicable or relevant and appropriate requirement (ARAR) was not identified in the Draft Final ROD. It seems reasonable to request that Sharpe submit this evaluation at the five year reviews to determine if the in-situ TCE concentrations may be reduced lower than the proposed aquifer cleanup level of 5 µg/1 . Figures 9-2 to 9-4 also depict the estimated residual TCE remaining in the aquifer 100 years after implementation of the remedial action. These figures show that an additional 60 to 70 years may be needed to reduce the TCE concentrations in the aquifer to background or the detection limit of 0.5 µg/1 . This would be cost prohibitive. We commend Sharpe for including these projections because this analysis partially addresses the technical and economical feasibility of treating to background as required by Resolution No. 92-49. Because numerical model predictions are one of the best means to assess the duration of the remedial action, the 3-D model should continue to be used in the five year review process as a means to evaluate, or to demonstrate, the technical and economic feasibility (or infeasibility) of reducing contaminant concentrations to less than 5 µg/1 for TCE. If the rate of cleanup follows the model predictions and as verified by actual performance monitoring data over time, then a cleanup level of 5 µg/l is acceptable and would meet the requirement in Resolution No. 92-49. Extraction and Injection Well Designs The Modeling Report includes several proposal for the design of the extraction well field and extraction wells (page 9-1) . These proposals include: 1 . The installation of one extraction well in the area of monitor wells 453A and 453B which would be fully penetrating by screening across all of the transmissive portions of the A and B Zones. The installation of one extraction well in the area of monitor well 460AB where a distinct aquitard is not present. If any significant aquitards are encountered during the installation of these extraction wells, blank casing and grout in the annular space adjacent to the aquitard would be used to prevent an undue amount of silt and clay from entering the well bore. 2. A test drilling program should be performed prior to the final design and construction of the injection system. This program would include split spoon sampling and geophysical logging of the boreholes. The results of the investigation would be summarized by isopach maps of the zones in the injection well field. These proposals are acceptable and should be used by Sharpe as part of the construction of the remedial action. We have previously discussed similar concepts in our letters dated 17 December 1991 and 16 September 1991 .