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EPA Comments on <br /> Review of Scenario 3: TCE Plume Capture and <br /> Containment Issues, Appendix D, <br /> Development of a Comprehensive Groundwater <br /> Model of the Defense Distribution Depot San Joaquin (DDJC), <br /> Sharpe Site, Lathrop, California, <br /> May 2003 <br /> GENERAL COMMENTS <br /> 1. In general, the modeling effort described in the Scenario 3: TCE Plume Capture and <br /> Containment Issues, Appendix D (herein referred to as Appendix D) appears <br /> conservative. However, there are a number of data gaps in the conceptual site model that <br /> likely increase the uncertainty in the modeling effort. These include the following: <br /> • The effect of agricultural pumping. Appendix D acknowledges that there are <br /> agricultural wells in the area that are not included in the modeling simulations <br /> because information regarding their pumping rates, screen intervals and mode of <br /> operation is unknown. An attempt should be made to collect more data regarding <br /> these wells so their potential influence on plume capture estimates can be <br /> quantified. <br /> Actual surface water levels from the San Joaquin Irrigation District Canal were <br /> not collected. Overall, this canal appears to have a measurable effect on <br /> groundwater elevations at the site. As attempt should be made to measure surface <br /> water elevations directly from this canal rather than guess at what the elevation <br /> might be based on nearby groundwater monitoring wells. <br /> The highest degree of error in the modeling simulations for A-zone, B-zone and <br /> C-zone occurred near basins 1 and 2. In Appendix D, it is hypothesized that the <br /> reason for the high model error in this area is the potential for perched water at <br /> two locations (MW486A and MW48713) in the A-zone. However, this explanation <br /> does not account for the knowledge that the highest measured error in the model <br /> consistently occurred near these basins regardless of the zone. This type of model <br /> error usually indicates a flaw in the conceptual model and/or the assumptions <br /> used. It appears that this area may be receiving more recharge than is expected <br /> even though flow to these basins has been discontinued. This area needs to be <br /> investigated further to determine the cause of the discrepancy between measured <br /> and modeled results and if possible, correct the discrepancy. <br /> 1 <br />