SITE INFORMATION AND CORRESPONDENCE_1992

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3-D Model/RD Report Memorandum -3- 14 December 1992 DDRW, Sharpe 1 . That Sharpe anticipates that the TCE concentrations will increase to 5 µg/l or greater at these points by the time the extraction system is installed, or 2. That the plume will continue to migrate and increase the contaminant concentrations from below the TCE aquifer cleanup level to concentrations greater than the aquifer cleanup level . If the latter assumption was used in selection of the design objective then this objective is too liberal because current contaminant concentrations should represent worst case for the extent of the ground water contamination. This is of particular concern with respect to the design objective for the west of the site (well cluster 510) . The aquifer should not be allowed to degrade any further than is found to exist at start up of the ground water remedial action. The lateral limits used for the plume containment design objective may be inadequate because it does not address the complete remediation of ground water contamination by VOCs other than TCE. Sharpe has proposed 0.5 µg/l as an aquifer cleanup level for many of the VOCs, such as 1, 1, 1-trichloroethane (TCA) . The 1, 1,1-TCA concentrations in monitor wells 505D, 5108 and 510C slightly exceed the proposed aquifer cleanup level . Total VOC concentrations must be considered for determining cleanup compliance. Data Gap The solute transport model assumed the sources to be present in the aquifer as depicted on the Third Quarter 1991 capture zone maps (page 8-3) . The Third Quarter maps depict Plumes 1 and 3 as separate plumes. We do not concur with this assumption and identified this area as a data gap in our letter dated 21 March 1991. Because the model assumes that Plumes 1 and 3 are separate, the model simulations do not account for this data gap. Therefore, model predictions with respect to cleanup of the aquifer in the area between Plumes 1 and 3 may not be as credible as model predictions in other areas. Verification of cleanup in this area should be addressed in the Remedial Design/Remedial Action (RD/RA) as the area between Plumes 1 and 3 remains a data gap. Optimal Design Scenario The Optimal Design Scenario includes the proposal to eliminate five wells from the extraction system in the South Balloon Area (A-1, A-3, A-4, B-3 and C-2) . The temporary elimination of some of these wells is acceptable where the TCE concentrations in these wells are below the proposed aquifer cleanup level (5 µg/1 ) and because continual use of these wells will cause excessive ground water to be treated. However, some of the five wells have TCE concentrations greater than the proposed aquifer cleanup level . Based on the Second Quarter 1992 ground water monitoring results, A-3 and C-2 had TCE concentrations greater than 5 Ag/1 (9.64 and 16.62 µg/l , respectively) . One of the purposes of the ground water remedial action is to maintain hydraulic control of the plume with concentrations greater than the aquifer cleanup level (5 µg/1 for TCE) . Therefore, elimination of A-3 and C-2 from the extraction system is premature and is not acceptable at this time because these areas must be remediated to the proposed aquifer cleanup level .