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Mr. Maurice Benson - 3 - 1 July 2009 Defense Logistics Agency Key to the opposing views is the mobility of dieldrin under pumping conditions. DLA has relied on dieldrin concentration data from CPT-collected samples of groundwater from fine-grained materials before and after the pumping test and on assumptions that published behavioral characteristics will control the mobility-of dieldrin beneath the NWC area. As a result, DLA concludes that at least 40 groundwater extraction wells would be necessary to implement pump and treat. The State concludes that 40 wells would be excessive. The State's conclusion is based on groundwater extraction and monitoring well data collected during the 30-day pumping test performed on the upper aquifer zone beneath the NWC area. Also, the State notes that the plume behavior does not suggest that published behavior characteristics govern this plume's behavior. Specifically, dieldrin concentrations do not seem to decline as a function of distance according to a sorption isotherm. Instead, there is a sharp front where the plume just stops. This, together with evidence from the aquifer test showing the plume can be readily remobilized by pumping, justifies reconsidering the pump-and-treat alternative. Increasing the gradient by pumping at the toe of the plume may increase recovery at the leading edge of the plume. DLA continues to argue that the pump and treat remedy for dieldrin at the DDJC Tracy site is not feasible. Remediation of the NWC Area In this section, the State presents a path forward for remedy of the dieldrin plume in groundwater beneath the NWC Area. Details of this approach could be discussed during informal dispute resolution. The pump and treat alternative selected in the ROD applies to the NWC area. DLA is responsible for implementing the groundwater pump and treat alternative in the NWC area and proposing a system design in a revised Operable Unit 1 Pesticide Remedial Design document that requires an adequate number of extraction wells; the State believes that the number of wells proposed in the Draft and Draft Final FS is excessive. DLA will propose the number and placement of groundwater extraction wells and corrective action monitoring wells in the design document. Following acceptance of the design document by the State and the U.S. EPA, DLA will construct the system. DLA is responsible for operating the system until the aquifer cleanup level of 0.05 micrograms per liter(µg/L) has been met throughout the NWC area or until such time as all of the following conditions are met: 1) All feasible optimization efforts on the system, as designed and built, have been implemented such as pulsing, 2) The aquifer cleanup level for dieldrin of 0.05 µg/L has been achieved in the groundwater extraction wells, 3) Dieldrin mass removal rates in the extraction wells have decreased to asymptotically low levels, and 4) Dieldrin concentrations in the corrective action monitoring wells have not been reduced or have decreased to asymptotically low concentrations as a result of pumping groundwater extraction wells.