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• MEMORANDUM <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VALLEY REGION <br /> 3443 Routier Road, Suite A Phone: (916) 255-3000 <br /> Sacramento, CA 95827-3098 CALNET: 8-494-3000 <br /> TO: Antonia K. J. Vorster FROM: Camilla Williams <br /> Senior WRC Engineer Engineering Geologist <br /> DATE: 22 September 1993 SIGNATURE: 417xeLLr{/ LtGL &41)1_ iP/ <br /> SUBJECT: REVIEW OF THE DRAFT GROUND WATER MODELING WORK PLAN, <br /> DEFENSE DISTRIBUTION REGION WEST (DDRW), TRACY, SAN JOAQUIN <br /> COUNTY <br /> I have reviewed the Draft Ground Water Modeling Work Plan for DDRW, Tracy submitted on 7 <br /> July 1993. The Draft Work Plan was prepared by Montgomery Watson for the Corps of Engineers. <br /> Overall, the Draft Work Plan contains sufficient information on general modeling procedures <br /> (Section 3.4) which are applicable to any site and indicate that adequate measures will be taken to <br /> calibrate and verify the model. In addition, the Plan had some site specific information indicating <br /> that a preliminary effort has been taken to develop a conceptual model. <br /> My general and specific comments are discussed below: <br /> General Comment <br /> The Draft Work Plan did not indicate how modeling projections will be used to comply with the <br /> substantive requirements of Chapter 15 and Resolution No. 92-49. In the Record of Decision <br /> (ROD) for Operable Unit No. 1 (OU-1), DDRW, Tracy committed to comply with these <br /> requirements. Chapter 15 and Resolution No. 92-49 require that DDRW, Tracy demonstrate <br /> whether or not it technically and economically feasible to cleanup to background concentrations, <br /> specifically to non-detect (ND) levels for trichloroethylene (TCE) and tetrachloroethylene (PCE). <br /> The demonstration of compliance with Chapter 15 and Resolution No. 92-49 may be easily <br /> demonstrated by preparation of graphs depicting estimates of the TCE and PCE concentrations <br /> within each horizon at specific well locations versus time. The TCE and PCE concentrations should <br /> be calculated for these wells at specific intervals during the transport simulations. Construction of <br /> these graphs will indicate how much time may be needed to achieve the aquifer cleanup levels in <br /> each horizon, as wells as the additional time needed to cleanup to lower concentrations. We are <br /> specifically interested in estimating the time required to cleanup the aquifer to 5.0 micrograms per <br /> liter (µg/1), 3 1Ag/1 and 0.5 µg/l for TCE and PCE so that differences in remediation costs may also <br /> be estimated. We request that the report on the ground water modeling effort contain these graphs. <br /> Specific Comments <br /> 1. The Draft Ground Water Modeling Work Plan does not indicate if a stand-alone document <br /> reporting the results of the modeling effort will be prepared or if the modeling results will be <br /> presented as part of the Remedial Design (RD) documents. We prefer that a separate <br />