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• Draft Ground Water Modeling Work Plan -2- 22 September 1993 DDRW, Tracy document be prepared. In addition, we request that DDRW, Tracy present status reports on model development during the Project Manager's Meetings in order to solicit agency input prior to finalization of the ground water modeling report. 2. The models will be used to simulate the current extent and to predict future migration of TCE and PCE. DDRW, Tracy should evaluate the cost-effectiveness of modeling other potential contaminant plumes from heavy metals or pesticides, should the site-wide remedial investigation verify the presence of these contaminants. 3. The Draft Ground Water Modeling Work Plan did not indicate whether a sensitivity analysis would be performed. DDRW, Tracy should consider performing a sensitivity analysis to quantify the uncertainty in the calibrated ground water flow and transport models. The normal uncertainties associated with the model parameters include hydraulic conductivity, vertical leakance, dispersivity and retardation factor. The sensitivity analysis will help to determine the adequacy of the proposed remedial design with respect to TCE and PCE plume containment and removal efficiency despite the uncertainties in model parameters. 4. We request that DDRW, Tracy summarize the calibration history of the ground water flow and contaminant transport models in tables. This summary format will help DDRW, Tracy to track development of the models and will help the reviewer to understand the procedures taken during calibration and verification of the models and should expedite the review process. 5. The Draft Ground Water Modeling Work Plan proposes to use a variable grid map for development of the models of which we approve. The Plan indicates that the finer grid will be used across the plume areas. It is unclear if constant grid spacing will be used both on- and off-site in plume areas. We recommend that the variable grid spacing be used in all areas of the plume, particularly off-site where the contamination is more extensive, in order to reduce the time needed for data manipulation and computer computation. 6. It is unclear if the ground water models will be used to evaluate the affects of the disposal system, including the injection well field and pond areas, on plume capture and overall aquifer cleanup. Model applications should include these analyses and specifically, should evaluate the location and pumping rates of the injection wells for the ground water remedial action.