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Mr. Don Pratt Re: Arco Station No. 6100 25775 Patterson Pass Road Page 2 of 3 migrating down-gradient along the bottoms of some sand units, as is the LNAPL where present. Currently available data indicate that, if laterally continuous, the plume of LNAPL may be more than 22 feet wide in the sand unit monitored by VW-1 and VW-4, and more than 45 feet wide in the sand unit monitored by MW-1 and MW-15. With moderate permeability and reported gradients of 0.108 and 0.175, a very large mass of LNAPL may be migrating down-gradient along the base of the impacted sand units. An estimate of the capture zone for monitoring well MW-1 has not been presented to the EHD, but without continuous pumping from the well, intuitively it would not seem likely to be large. This presents a potential for significant volumes of LNAPL to bypass the impacted wells and migrate unimpeded further down-gradient. The long-term continual presence of LNAPL in MW-1 and the large mass of hydrocarbons removed during a relatively short period of continuous SVE operation are consistent with the potential for the presence of a large LNAPL mass. The initial monitoring results from new monitoring wells MW-16, MW-17 and MW-18 appear to demonstrate the down-gradient extent of impacted groundwater — if the sand units are correctly correlated and the plume did not drop to a lower interval between the monitoring wells on the south side of Patterson Pass Road and the monitoring wells in the field south of the road. Groundwater monitoring wells MW-19 and MW-20 have apparently been dry. Due to the uncertainties associated with the LNAPL plume and the potential for a significant effect on the groundwater resource of the area, the EHD is directing you to prepare a report on the status of the LNAPL plume for submission to the EHD by 15 November 2006. Please address the following issues in the report: • An evaluation of the current hydrogeological model (and sand unit correlations) and potential LNAPL migration routes; • The current status of the LNAPL plume delineation; • An estimate of the LNAPL mass that may have bypassed the line between well MW-1 and MW-15; • The environmental fate of LNAPL that has bypassed the line between MW-1 and MW-15; • An estimate of the LNAPL mass that may currently be upgradient of the line between wells MW-1 and MW-15; and • An interim remediation plan to maximize LNAPL recovery. If additional information is required to fully address the issues noted above, the report should be accompanied by a work plan to acquire such data.