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EarthTech 695 River Oaks Parkway P 408.232.2800 <br /> San Jose,CA F 408.232.2801 <br /> A tYCO International Ltd.Company 95134 www.earthtech.com <br /> December 13,2005t ��`` <br /> Earth Tech Project No. 90080.01 <br /> DEC 2 17 <br /> Mary E. Sena <br /> California Regional Water Quality Control Board-Central Valley Region �M�T� L'rV wL <br /> 3443 Routier Road, Suite A <br /> Sacramento, CA 95827-3003 <br /> Re: Confirmation Soil Vapor Sampling Work Plan <br /> Delta View Apartments,Stockton Parcel 2A <br /> Stockton,CA <br /> Dear Ms. Serra: <br /> This Work Plan was prepared in response to a request by the Central Valley Regional Water <br /> Quality Control Board(RWQCB) during discussions held on November 10, 2005 and in <br /> correspondence dated November 28, 2005 to verify soil vapor analytical results at a sampling <br /> location at the Delta View Apartments, Stockton,California(site) (Figure 1). <br /> Soil vapor sample SV-25 was collected as part of the site-wide sampling program for use in the <br /> Human Health Risk Assessment(HHRH) (Figure 2). The results of this sampling program were <br /> reported in the Soil vapor Investigation Report, Delta View Apartments, Stockton Parcel 2A. <br /> (Earth Tech,2005). Sample SV-25, located in the western portion of the site,contained hydrogen <br /> sulfide(HZS) at 3,500 parts per billion by volume(ppbv), and methane at 3.1 percent. <br /> Based on these results,the RWQCB requested that verification sampling be performed to further <br /> characterize this location regarding HZS and methane. The L&M OU has agreed to perform this <br /> sampling event. It should be noted that this soil vapor sampling location was first sampled as SV- <br /> 10 in October 2003 for use in the HHRA. These results were reported in the HHRA(Earth Tech, <br /> July, 2005) and showed that HZS was not detected at this location and methane was detected at <br /> 3.9 percent. <br /> Both HZS and methane are naturally occurring, and often result from the anaerobic decomposition <br /> of plant or animal organic matter. Although both chemicals can have non-biological origins,their <br /> presence at this site is most likely to be of biologic origin because the site was historically a <br /> swamp and former wetlands environment that was subsequently filled in. Although crude oil can <br /> also be a source of HZS and methane, crude oil was not used or stored at this site in significant <br /> quantities, and thus is likely a minor source at most. Similarly,the decomposition of more <br /> refined petroleum products is generally not considered to be a significant source of these <br /> chemicals compared to submerged swamps or wetlands. <br />