SITE INFORMATION AND CORRESPONDENCE_1992

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• MEMORANDUM . CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD - CENTRAL VALLEY REGION 3443 Routier Road, Suite A Phone: (916) 255-3000 Sacramento, CA 95827-3098 CALNET: 8-494-3000 TO: Antonia K. J. Vorster FROM: Camilla Williams Senior WRC Engineer Engineering Geologist DATE: 14 December 1992 SIGNATURE: �ry> iGG�2� SUBJECT: REVIEW OF REMEDIAL WELL-FIELD DESIGN USING THREE DIMENSIONAL GROUND WATER FLOW AND TRANSPORT MODELING REPORT, DEFENSE DISTRIBUTION REGION WEST (DDRW), SHARPE, SAN JOAQUIN COUNTY I have reviewed the Remedial Well-Field Design Using Three Dimensional (3-D) Ground Water Flow and Transport Modeling (Modeling) Report for DDRW, Sharpe submitted on 14 October 1992. The Modeling Report was prepared by Engineering Technologies Associates, Inc. (ETA) for the U. S. Army Toxic and Hazardous Materials Agency (USATHAMA) . The Report summarizes the results of the 3-D modeling effort including the design, calibration, verification and the sensitivity analysis of the model and provides recommendations for the remedial design (RD) of the injection and extraction well fields. Overall , the Modeling Report is thorough, well prepared and documented, but more importantly, the rationale for decisions and professional judgements were generally provided. The proposed injection and extraction well fields appear to be reasonable and the model appears to be an acceptable tool for use in the RD. However, clarification of the design for the treatment and disposal system is needed because the RD Report proposes changes in these systems that are different than that proposed in the Draft Record of Decision (ROD) . In addition, as with any model , the results must be verified with actual field data because a model is a predictive tool and is not a replacement for verification monitoring. Therefore, verification monitoring of the extraction and injection systems must be performed throughout the duration of the ground water remedial action. Sharpe must commit to modifications of the extraction and injection systems (specifically, the installation of additional wells) should the performance monitoring data not support model predictions. Below are discussions on my major concerns with the Modeling Report which must be adequately addressed by Sharpe. Grid Map The grid map for the model is presented in Figure 4-1. A constant grid spacing of 125 feet was used over the area covering the site and the agricultural area immediately to the west (page 4-1) . The grid spacing across this area is relatively dense. Justification for using this dense of spacing was not provided. It appears that a less dense spacing (perhaps 250 feet) could have been used with less manipulation and associated computer time, but with similar results. Recharge A portion of the Brown and Caldwell (BC) (1985) two dimensional flow model preprocessor was used to assign annual recharge values over the model domain and to