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M. Scott Mansholt - 4 - 26• March 2008 990 Beechnut-Tracy Using the tier-2 evaluation, the cumulative risks and hazard indexes were calculated on a sample-by-sample basis. Cumulative risks and hazard indexes were estimated for all samples to assess whether there are discrete areas of the site where screening criteria are exceeded. According to the report, several soil samples from the six boring locations coincide in part with the same sampling intervals identified by the tier-one evaluation. Based on our review, Regional Water Board staff has the following comments: • The elevated concentrations of TPH compounds were identified at onsite boring locations SB-2, GMX-02, GMX-09, GMX-11 , GMX-13, and GMX-25 exceed unacceptable levels of cancer risk and hazard; • We concur with the risk assessment findings that site-wide cumulative theoretical excess lifetime cancer risk and hazard indexes (for the six onsite boring/sample locations) were above target levels of 1 X 10-6 and 1.0, respectively, for both future residential and commercial/industrial worker scenarios; and, • The affected soil should be removed and post-excavation confirmation soil sampling and analysis completed; following remedial excavation and verification sampling that the affected soil has been adequateiy removed, an amended screening level risk assessment should be submitted. By 15 August 2008, please submit a workplan and time schedule for removal of the affected soil and post-excavation confirmation sampling at the locations discussed above. If you have any questions or comments, please contact Dean Hubbard at (559) 445-5179. SHELTON R. GRAY C. DEAN HUBBARD Senior Engineering Geologist Engineering Geologist P.G. No. 6357 cc: Michael Infurna, San Joaquin County EHD, Stockton Gary Foote, Geomatrix, Oakland Robert Cheung, Geomatrix, Oakland R/common/CDH/Chevron/990Tracy/risk assessreview/Apr08