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EarthTech 695 River Oaks Parkway P 408.232.2800 San lose,CA F 408.232.2801 A tgCO International ltd.Company 95134 www.earthtech.com January 24, 2006 Earth Tech Project No. 90080 Mary E. Serra California Regional Water Quality Control Board-Central Valley Region 3443 Routier Road, Suite A Sacramento, CA 95827-3003 Re: Additional Soil Vapor Sampling Work Plan Delta View Apartments,Stockton Parcel 2A Stockton,CA Dear Ms. Serra: This Work Plan was prepared in response to a request by the Central Valley Regional Water Quality Control Board(RWQCB)during discussions held on November 10, 2005 and in correspondence dated November 28, 2005 to verify soil vapor analytical results at previous sampling location SV-25,and characterize the soil vapor in the area immediately surrounding this sampling location, at the Delta View Apartments, Stockton, California(site)(Figure 1). Soil vapor sample SV-25 was collected as part of the site-wide sampling program for use in the Human Health Risk Assessment(HHRA)(Figure 2). The results of this sampling program were reported in the Soil Vapor Investigation Report, Delta View Apartments, Stockton Parcel 2A. (Earth Tech,2005). Sample SV-25,located in the western portion of the site,contained hydrogen sulfide(H2S)at 3,500 parts per billion by volume(ppbv),and methane at 3.1 percent. Based on these results,the RWQCB requested that verification sampling be performed to further characterize this location regarding H2S and methane.The L&M OU has agreed to perform this sampling event as part of the effort to revise the HHRA. It should be noted that this soil vapor sampling location was first sampled as SV-10 in October 2003 for use in the HHRH. These results were reported in the HHRA(Earth Tech,July,2005)and showed that H2S was not detected at this location and methane was detected at 3.9 percent. Both H2S and methane are naturally occurring, and often result from the anaerobic decomposition of plant or animal organic matter. Although both chemicals can have non-biological origins,their presence at this site is most likely to be of biologic origin because the site was historically a swamp and former wetlands environment that was subsequently filled in. Although crude oil can also be a source of H2S and methane,crude oil was not used or stored at this site in significant quantities, and thus is likely a minor source at most. Similarly, the decomposition of more refined petroleum products is generally not considered to be a significant source of these chemicals compared to submerged swamps or wetlands.