SITE INFORMATION AND CORRESPONDENCE

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r Mr. Maurice Benson • - 2 - 24 November 2008 Defense Logistics Agency Comments Regional Water Board staff believes that rebound testing at this time is premature. Continued operation of the groundwater extraction wells and treatment system are necessary. Staff comments are presented below. 1. The Work Plan presents an evaluation of the plume extents and COC concentrations between 2004 and 2007 at the Site. Regional Water Board staff concludes that, with two exceptions, the evaluation shows that the COC plumes have decreased in size and concentration: 1) the dieldrin plume at the northwest corner of the Site and 2) the TCE plume east of Banta Road. These two exceptions are plumes outside of the capture zones of the groundwater extraction wells at the Site. The dieldrin, TCE, and PCE plumes inside of the capture zones have decreased. The apparent changes in plumes sizes between 2006 and 2007 presented in the Work Plan evaluation are less than earlier years. However, a decision to evaluate shutdown and rebound of the entire Site groundwater extraction and treatment system must be based on longer-term trends; trends with a period of one year are not sufficient to conclude that continued operations would not result in worthwhile COC reductions. Regional Water Board staff notes that the apparent changes between 2006 and 2007 coincide with an optimization effort that resulted in a significant reduction in the number of extraction wells that were operated and in the gallons of groundwater that were pumped. 2. Groundwater modeling efforts summarized in the Work Plan were presented to argue that continued extraction well operation would not be of value. However, the data trends presented in the Work Plan do not corroborate this argument. Use of groundwater modeling as a decision tool must occur after concentration trends have shown that residual CDCs have stabilized. 3. DESJC has not completed characterization of the COC plume east of Banta Road, outside of groundwater extraction well influence. DESJC must characterize the down- gradient edge northeast of monitoring well LM157A and also the middle of the plume between extraction well EW019A and the down-gradient edge. DESJC is currently implementing the October 2008 Groundwater Remedy Enhancement Cone Penetrometer Testing and Groundwater Characterization Work Plan. This effort should provide further characterization of the COC plume east of Banta Road. Furthermore, an objective of this work is to evaluate potential on and off-Site source areas for groundwater contamination in the Banta Road area. This evaluation is another task that DESJC must compete before shutdown of all groundwater extraction wells and rebound testing at the Site. 4. The cost of groundwater treatment is one factor to consider when evaluating the effectiveness of a selected remedial action. However, increasing cost was already anticipated and is expected to continue as the concentrations of COCs in the groundwater continue to decline toward the ACL concentrations.