SITE INFORMATION AND CORRESPONDENCE_2001-2018

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• • Mr.Brian Taylor ( November 9, 2007 CONESTO3A-ROVERS &ASSOCIATES Page 3, Item 6: "We concur that additional groundwater data are needed to assess the extent of pollution in the deeper zone and to determine whether the downward vertical gradient is resulting in the downward migration of the plume. However, if pollutants are detected in the deeper zone, an accurate assessment of their extents will require monitoring well installation. The WP-FS needs to be revised to specify that monitoring wells will be installed where grab groundwater samples show that pollution is present in the deep zone. To prevent cross-contamination of deeper zone, dual wall geoprobe borings should be used to complete the exploratory borings, and the wells should be completed using conductor casing." Response: Shell concurs with RWQCB staff's comment regarding installing wells at locations where grab groundwater samples identify petroleum impacts. The drilling methods used for both borings and wells will employ methods and equipment which minimize the risk of possible cross contamination of the deeper zone. Page 3,Item 7: "We do not concur that the southwestern edge of the plume is an appropriate location for the four proposed deep borings and/or monitoring wells. The stated purpose of these borings is "to assess the presence and extent of TPHd, TBA, and DIPE" in the deeper zone near DW-2, but the proposed borings are distant from DW-2. A single boring advanced downgradient of DW-2, will be sufficient to address this data gap. If pollution is encountered, this boring should be converted into a monitoring well. Additional wells may be needed to further define the extent of pollution." "The other three borings should be located at the three SAs to (1) determine the vertical extent of pollution in these areas and (2) evaluate whether a DPE system designed for the shallow zone could effectively address pollution in the deep zones. These borings should be converted to monitor wells and screened below the maximum depth of pollution to provide monitoring data at the vertical extent of pollution." Response: Shell agrees with the RWQCB staff comments regarding a more proximal boring placement near well DW-2, and evaluating the vertical extent of petroleum impacts beneath the three source areas. The investigation approach for the source areas, however, should be changed from the proposed direct push drilling methods to methods which minimize the possibility of cross contamination of the deeper zone. Consequently a work plan addendum describing this, and the modifications to the downgradient investigation near well DW-2 will be submitted under separate cover. 1729 5 Worldwide Engineering, Environmental, Construction, and IT Services