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` . 1"1'c EDrAUNG G.BROWN JR. <br /> �;^�� arn•ernnn <br /> MATTHF. <br /> 9,0z IA <br /> Rcn',Rmurrz <br /> cnixVnwia aeVwfrnwv h'VP <br /> r:rvRnwMtnral.PRaTcr.rv"" <br /> Mater Beards <br /> Central Valley Regional 'W'ater Quality Control Board <br /> TO: Shelton R. Gray <br /> Senior Engineering Geologis <br /> FROM: C. Dean Hubbard <br /> Engineering Geologist <br /> DATE: 13 March 2012 <br /> SUBJECT: REVIEW— ADDITIONAL INVESTIGATION WORKPLAN, 990 BEECHNUT AVENUE, <br /> TRACY, SAN JOAQUIN COUNTY <br /> Chevron Environmental Management Company (Chevron) submitted a Workplan for Additional <br /> Investigation (Workplan) dated 1 February 2012, for the 990 Beechnut Avenue project (Site). The <br /> Workplan was prepared on behalf of Chevron by AMEC Geomatrix, Inc. (AMEC) and submitted to <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board). I have reviewed the <br /> referenced Workplan and project files and my comments are provided below. <br /> According to previous reports, including a 2008 Screening Level Risk Assessment (SLRA), AMEC staff <br /> completed an excavation in November 2010 to remove polynuclear aromatic hydrocarbon (PAH) <br /> affected soil in the southwest corner of the Site. Approximately 4,024 tons of affected soil was removed <br /> and disposed of at the Class II Forward Landfill. According to AMEC's 23 December 2010 Excavation <br /> Completion Report and Updated Screening Level Risk Assessment Report, the.targeted PAH affected <br /> soil was excavated and removed. <br /> Following the excavation, the SLRA was updated to reflect the results of the soil removal action. <br /> AMEC's updated SLRA findings concluded the Site was suitable for future commercial/industrial uses <br /> with the exception of another area of the Site where ethylbenzene was previously reported in shallow <br /> soil. Ethylbenzene at a concentration of 0.06 milligrams per kilogram (mg/kg) was reported in soil from <br /> a sample depth of 12 feet below ground surface (bgs). The potential vapor intrusion of ethylbenzene <br /> from the shallow soil slightly exceeded the screening level for a hypothetical commercial/industrial <br /> worker. <br /> Additionally, the updated SLRA noted another location at the Site where total petroleum hydrocarbon <br /> as crude oil (TPHc) at a concentration of 21,000 mg/kg in a soil sample was collected at 9.5 to 10 feet <br /> bgs. This single concentration exceeds the direct contact residential screening level for a hypothetical <br /> onsite resident, possibly limiting future unrestricted onsite resident uses for the Site. The affected soil <br /> previously reporting ethylbenzene and TPHc was not excavated during the 2010 excavation but the <br /> analytical results were incorporated into AMEC's updated SLRA. <br /> Consequently, Chevron has proposed additional soil sampling to better characterize potential direct <br /> contact exposure for these two areas of the Site to address hypothetical residential use in the future. <br /> KAnt. E. I...QNGLEY ScD, P.E.• CHAIR j PAMELA C. C'i RE:i:.firJN. :xE:;U rYi:CiFf iCf::r? <br /> 1685 E Street.Fresno,CA 93706 1 www.waterboaros.ca.povicentratvatley <br /> FAPf.n <br />