SITE INFORMATION AND CORRESPONDENCE FILE 2

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76/former BP# 11192 page 2 1403 Country Club Blvd, Stockton July 6, 2009 The concentration of TBA in the deep well, MW-11 (onsite) ground water samples collected on February 2, 2009, was 16,000 micrograms per liter (ug/1). No current TBA ground water data exists onsite at depths greater than this well's screened interval of 68' to 78' bgs. Ground water data from the 76/BP borings B-2 and CPT-3 and from MW-13, all north to northeast of MW-11, do not provide valid data to support delineation conclusions for the TBA plume,.as the data from all of these points are either from shallower depths or predate August 2004, when the TBA concentrations began to dramatically increase in MW-11. As such, the TBA ground water plume is not fully characterized (vertically defined) on site. Additional investigation is warranted, and the EHD recommends a deeper monitoring well be installed in the area of MW-11. No ground water data is available to characterize the TBA plume laterally from MW-11 on the 76/BP site, immediately down gradient onsite, or offsite toward the northeast. The only monitoring point for investigating the lateral extent of this contamination is Shell's deepest monitoring well S-17, which was impacted by TBA at 550 ug/l during the November 2008 sampling event but has been "non-detect" during four other sampling events. During the February 2009 sampling event (the most current event to date), Shell's intermediate depth monitoring well S-15 continued to contain TBA. As such, the TBA plume is also not laterally delineated at depths monitored by well S-15. A site conceptual model (SCM) should be prepared to demonstrate the TBA migration pathways and the adequacy of the plume delineation. Among Shell's onsite wells, TBA has been detected only in ground water samples from two monitoring wells, S-2 and S-3, near the western property line, both located down gradient from the 76/BP site and obliquely upgradient to the Shell contamination source. The EHD believes that the historically elevated TBA ground water contamination noted in the 76/BP wells, MW-3 and MW-7, (240,000 and 100,000 ug/I respectively) has migrated offsite, across the Shell site, and at least as far northeast as Elmwood Avenue, as shown in Shell's well, S-15. The extent of this TBA plume is not defined and must be further investigated by 76/BP. The EHD recommends that you revise the Plan and include additional measures to complete the delineation of TBA impacted ground water. The proposal in the Plan to install only the one shallow well, MW-14, is approved, [H&S Code 25296.1 0(c)(3)], but may be revised as noted above. To comply with the recently adopted Resolution #2009-0042 by the State Water Resources Control Board, the sampling requirements for your site has been reduced to the following: • Semi-annual — MW-3, MW-7, and MW-11 • Annual — MW-6 and MW-12 • Biennial — MW-1, MW-4, MW-5, MW-8, MW-9, MW-10, and MW-13. You may propose an alternative sampling schedule with a technical justification.