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(O�, • • Mr. Brian Taylor <br /> November 9, 2007 <br /> CONESTOGA-ROVERS <br /> &ASSOCIATES ' <br /> The inability to access impacted soil and groundwater beneath site structures and equipment precludes <br /> effective implementation of site-wide cleanup measures. Furthermore, Shell believes that previous <br /> investigative and remedial activities have demonstrated that site-wide cleanup is infeasible. However, it <br /> is possible that DPE, the proposed remedial option presented in the FS, if implemented and proven to be <br /> technically feasible, will provide site cleanup beyond the hot spot or source areas (i.e., should a large <br /> radius of influence be achieved with the extraction wells). Shell requests permission to proceed with <br /> DPE as outlined in the FS as an Interim Remedial Action or by Conditional Approval. <br /> Page 2,Item 3: "The WP-FS does not specify whether pilot testing will be conducted in one or multiple <br /> SAs. The WP-FS provided an estimate of$25,000 for the cost of conducting a pilot test in one SA with <br /> additional costs for more than one SA pilot tests. Based on the differences in soil types underlying each <br /> SA, as well as the variability in contaminant concentrations, the Regional Water Board staff believes that <br /> pilot testing in all three SAs, will be necessary to obtain accurate remedial technology data. Please either <br /> revise the WP-FS to specify pilot testing in all three SA or provide the rationale to justify that pilot <br /> testing at one SA will provide representative pilot study data." <br /> Response: As presented in the FS, "Source area delineation (if possible) would be necessary to <br /> adequately define the most significant source area "hot spots." Once each source area is defined, then <br /> pilot testing would be conducted to determine technical feasibility and effectiveness (and full-scale <br /> design criteria) of DPE. The pilot test would determine the required number of DPE wells and <br /> appropriate spacing. Pilot testing is estimated to cost $25,000 (for one 5-day test at one of the hot spot <br /> areas; if testing is required at each source area additional costs would be incurred). CRA has assumed <br /> installation of sixteen DPE wells at an estimated cost of$40,000. Figure 10 shows assumed DPE well <br /> locations in the identified source area"hot spots". The well layout assumes a 25-foot radius of influence. <br /> It also assumes use of existing wells MW-20 and MW-22 as DPE wells." <br /> Based on the discussion above, the FS acknowledged that pilot testing might be required at each source <br /> area. The findings from the proposed source area definition activities will be used for determining the <br /> need for pilot testing in each of the three source areas. If similar hydrogeologic conditions exist at each <br /> source area, pilot testing in each area should not be required. However, Shell will share these findings <br /> with RWQCB staff, and a joint decision among stakeholders will be made regarding the selection of <br /> source areas for pilot testing activities. <br /> Page 3, Item 4: "We do not concur with Shell's proposal to use Region 2's ESLs or Risk-Based <br /> Corrective Action (RBCA) to establish site-specific cleanup levels. In the Central Valley Region, as <br /> described in the Basin Plan, cleanup levels for groundwater are required to be consistent with State <br /> 1729 3 <br /> Worldwide Engineering, Environmental, Construction, and IT Services <br />