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RI/FS Report Review -5- 5 July 1991 DORW, Tracy alternative for the final ground water remedial action was Alternative 8. This alternative proposes extraction of the contaminated ground water at 500 gpm, treatment by air stripping, the addition of oxygen and methane to a portion of the treated water followed by reinjection for the in-situ biological treatment in the source area and disposal of the remaining water by shallow injection wells. My concern with the evaluation of the eight alternatives is that the supporting documentation used in the evaluation of the effectiveness of each alternative was not presented. The effectiveness of the alternatives were frequently evaluated using a model . The model selected for evaluation was not consistent between alternatives. For example, the numerical models "PLASM" and "WELLS" were used in the evaluation of Alternatives 3 and 4, respectively, but an analytical model , the Theis equation, was used in the evaluation for Alternative 5. The FS should have included the supporting graphics and rationale used in the evaluation of the effectiveness of each alternative and should have justified why different methods of evaluation were used for the different alternatives. In addition, a formal submittal of the modeling results (ground water flow and contaminant transport) used in the RI/FS needs to be submitted. Of the four alternatives selected for the detailed analysis, the no-action and the IRM at 340 gpm were not acceptable. The no action was selected only for the purposes of comparison. The IRM at 340 gpm (Alternative 3) is not acceptable because complete plume capture off-site is not possible with this alternative. My overall concern with the selected alternative of 500 gpm pump and treat with in-situ bioremediation (Alternative 8) is that this alternative may not maximize the use of the existing IRM. The existing IRM was not adequately described and support documentation was not presented in the FS Report. The IRM should be incorporated into the final remedial design to the fullest extent possible. Alternative 8 includes the installation of two extraction wells off-site at the LM-55, LM-56 and LM-57 cluster and at LM-76. The rationale for the placement of these off- site extraction wells needs to be expanded. The extraction well at LM-76 may ultimately need to be relocated as the leading edge of the plume has yet to be defined. Discussions on Alternative 8 did not include sufficient information on well design. Schematics for the design treatment system were provided however, schematics for the design of the extraction and injection wells were not provided. Because there is TCE contamination above the 5 µg/l level in the upper, middle and lower horizons, the extraction wells should be designed to maximize plume capture vertically as well as horizontally. The final remedial design should include information on well design including the well depth, screen length, filter pack design, casing type and diameter and information on the pump, as well as adequate discussion in support of the designs. The discussion in the FS Report on the different disposal options was generally good. However, my concern with the disposal portion of the FS is that all of the alternatives but shallow reinjection were dismissed from further consideration. My concern is that the performance of the injection wells in the IRM has not been fully evaluated and that this method of disposal has already been selected. Tracy should consider use of another disposal alternative either as a back-up system or a parallel disposal system in case the performance of the injection wells in the IRM is less than desired and for backup if the injection wells are down.