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STATE OF CALIFORNIA -Environmental ProtecAMAgency PETE WILSON Governor CALIFORNIA REGIONAL WATER QUAMY CONTROL BOARD CENTRAL VALLEY REGION 3443 Routier Road, Suite A Sacramento, CA 95827-3098 PHONE: (916) 255-3000 DOD FAX: (916) 255-3052 29 December 1993 Mr. Jim Pinasco Department of Toxic Substances Control 10151 Croydon Way, Suite 3 Sacramento, CA 95827-2106 GROUND WATER MODELING INTERIM TECHNICAL MEMORANDUM, DEFENSE DISTRIBUTION REGION WEST (DDRW), TRACY, SAN JOAQUIN COUNTY We have reviewed the Ground Water Modeling Interim Technical Memorandum (ITM) for DDRW, Tracy submitted 1 November 1993. We approve of this document and request that DDRW, Tracy proceed with development of the ground water flow and contaminant transport models. We believe that development of these models will aid in the design of effective extraction and disposal systems for the ground water remedial action. We commend Montgomery-Watson for preparing a well organized document and producing graphics of high quality. Our specific comments to the Ground Water ITM are presented below. 1. Section 5.3 MODEL GRID, subsection 5.3.0.3, page 11: We concur with the use of a variable grid size to optimize the resolution of results in the study area while reducing the total number of model nodes. However, the base grid of 100 feet by 100 feet used to cover the DDRW, Tracy site may be too dense and extensive. A less dense fine grid spacing (perhaps 200 feet by 200 feet) covering approximately the northern one third of the DDRW, Tracy site is adequate. In addition, a larger grid (increased by a factor of 1.5) should be used across the entire model domain. The larger nodes/grid pattern should produce similar modeling results and significantly reduce model manipulation and associated computer time. 2. Section 5.4 MODEL LAYERS, subsection 5.4.0.1, page 11: This section describes the uppermost geologic horizon for model layer 1 as the relatively coarse Upper Horizon of the Upper Tulare Formation. Model layer 1 does not include the siltier saturated Above Upper Horizon in which the highest levels TCE and PCE concentrations have been recorded. The Above Upper Horizon must be considered the shallowest model layer. Inclusion of the Above Upper Horizon as a model layer is necessary to develop the optimum remedial design for the cleanup of contaminants in the Upper Tulare Formation. 3. Section 5.5 BOUNDARY CONDITIONS, pages 11 and 12: This section describes the ground water model boundary conditions as illustrated in Figure 9. We concur with the boundaries selected for the northern and eastern edges of the study area. However, data collected from monitoring wells at Triple E Produce and Georgia Pacific, located southwest of DDRW, Tracy indicate a ground water flow direction in the northwesterly direction in the shallow zone; as opposed to the northeasterly general regional flow.direction shown in Figure 16. Maps (prepared by Roy F. Weston, 1992, and Brown & Caldwell, 1991 and