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PDDR • 4- • 31 March 1995 DDRW, Tracy purpose of determining operating costs, without applying a similar cost basis to the other alternatives. The PDDR must provide the rationale for limiting this approach to only one remedial alternative. 5. Concentration versus Tyne Graphs. Plume maps and graphs depicting estimates of cleanup times for the TCE and PCE plumes within each water bearing zone (horizon) should have been provided in the PDDR. Specifically, we request that graphs be presented which indicate VOC concentrations at specific well locations over time (cost versus concentration over time graphs). DDRW Tracy previously committed to providing these maps and graphs in Section 3.0.0.2 of the Ground Water ITM " in order to demonstrate compliance with Chapter 15 and Resolution No. 92-49 as required in the ROD for OU-1." As discussed in our 22 September 1993 comment letter, these graphs should be developed to evaluate the time that may be needed to clean up the plumes, in each water bearing zone to 5.0, 3.0, and 0.5 micrograms per liter. 6. Capture Zone Analyses. We have reviewed the remedial alternatives in Section 5.0 which were developed to provide cleanup of the VOC plume. Although the model projections for these scenarios appear to be discussed in some detail, we have been unable to determine if the capture zones which would be provided by the proposed extraction well fields would meet the remedial objectives. The figures presented in Appendix H, do not show the anticipated capture zone of the proposed extraction well networks. The PDDR should include maps which show the water elevation contours relative to the VOC plumes for each water bearing zone over the model simulation period. A further improvement which would help our evaluation of the various remedial alternatives would be to provide larger maps which also show the maximum anticipated concentrations of the VOCs in each water bearing zone. 7. Design Concept. The proposed 10 percent design concept includes only on-site remedial wells. We believe that this approach is limited, because it may not include the most optimal design for providing hydraulic capture of the VOC plumes given the plume configurations. The plume maps provided in the PDDR and recent investigations appear to indicate that the plume has migrated downgradient of the influence of the proposed infiltration and extraction well network. Additional monitoring wells have been proposed in these areas, and DDRW Tracy has indicated that these wells will not be installed prior to completion of the RD phase. Results from ground water sampling and analyses of these future wells may confirm previous CPT results. MCLS may be exceeded in these wells. We believe that since the lateral and vertical extent of the VOC plume is uncertain, at a minimum, DDRW Tracy should consider design contingencies for providing additional remedial wells in these areas. If you have any questions, please call me at (916) 255-3065. KAREN A. BESSETTE Project Engineer KAB:kb