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Geosyntec1340 Treat Boulevard,Suite 208 <br /> Walnut Creek,California 94597 <br /> PH: <br /> consultants 27&8688 <br /> www.geosgeosyntec.com <br /> July 1, 2022 <br /> Mr. Jeff Tierney, E.I.T. <br /> Federal/Private Sites Cleanup Unit <br /> Central Valley Regional Water Quality Control Board <br /> 11020 Sun Center Drive, #200 <br /> Rancho Cordova, California 95670 <br /> Re: Work Plan for Vapor Intrusion Investigation Addendum <br /> Former Agricultural Chemical Facility <br /> 1904 West Charter Way, Stockton, California <br /> Dear Mr. Tierney: <br /> On behalf of Glenn Springs Holdings, Inc., a subsidiary of Occidental Chemical Corporation, and <br /> J.R. Simplot Company, Geosyntec Consultants, Inc. (Geosyntec) has prepared this Work Plan for <br /> Vapor Intrusion Investigation Addendum (Addendum) for the former agricultural chemical <br /> distribution facility located at 1904 West Charter Way in Stockton, California(the site, Figure 1). <br /> This Addendum describes the steps proposed to be performed in Q3-2022 to further evaluate <br /> concentrations of selected volatile organic compounds (VOCs) in soil gas at the off-site area east <br /> of the site. <br /> BACKGROUND <br /> This Addendum has been prepared to update and revise the July 30, 2021, Work Plan for Vapor <br /> Intrusion Investigation' (Work Plan [Attachment 1]; Geosyntec, 2021), which was approved by <br /> the Central Valley Regional Water Quality Control Board (CRWQCB) in its letter dated <br /> September 9,202 12. The Work Plan was prepared to describe the steps that were performed in Q4- <br /> 2021 to evaluate potential for vapor intrusion at the adjacent residential area east of the site, to <br /> assess potential health risks to residents. In summary, a total of four semi-permanent soil vapor <br /> probes (SVPs) were installed in Q4-2021 at two on-site locations near monitoring wells MW-5B <br /> and MW-7B (Figure 2). At each of these two monitoring wells an SVP was installed at 5 feet <br /> below ground surface(ft bgs)and at 10 ft bgs. Subsequently, soil gas samples were collected from <br /> each of the four SVPs and, concurrently, groundwater samples were collected from monitoring <br /> wells MW-5B and MW-7B. An assessment of the potential for vapor intrusion to an off-site <br /> resident was then performed based on the correlation observed between the average on-site <br /> groundwater and average soil vapor concentrations at 10 ft bgs. The correlation factor developed <br /> from average on-site concentrations was then extrapolated to estimate the off-site soil vapor <br /> 1 Geosyntec Consultants,Inc.2021. Work Plan for Vapor Intrusion Investigation,July 30,2021. <br /> 2 Central Valley Regional Water Quality Control Board.2021.REVIEW OF WORK PLANFOR SOIL VAPOR <br /> INTR USION INVESTIGA TION, 1904 CHARTER WAY, STOCKTON, SAN JOA Q UIN CO UNTY,September 9,2 02 1. <br /> engineers I scientists I innovators <br />