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a y Advanced GeoEnvironmental, Inc. 10 January 2008 U-1 ECE AGE-NC Project No. 95-0130 JAM 1 6 2008 ENVIRONMENT HEALTH Mr. Nuel Henderson PERMITSERV!CES San Joaquin County Environmental Health Department 600 East Main Street Stockton, California 95202 Subject: Proposal for Additional Soil Investigation TOP FILLING STATION 101 South Wilson Way, Stockton, California Dear Mr. Henderson: In accordance with our telephone conversation on 20 November 2007,Advanced GeoEnvironmental, Inc. (AGE) is submitting this letter to the San Joaquin County Environmental Health Department (SJCEHD)to recommend advancement ofthree additional soil borings near the former underground storage tank (UST) excavation area at 101 South Wilson Way, Stockton, California (site). The purpose for the additional soil borings are (1) to confirm interim soil vapor extraction (SVE) progress at the site, and(2)to further assess hydrocarbon-impacted soil near the east and west ends of the former UST area. The site setting is illustrated on Figure 1. Site structures, proposed soil boring locations and monitoring well locations are depicted on Figure 2. In September 1998,two 4,000-gallon gasoline underground storage tanks(UST Nos.2 and 3)were removed from the site.Laboratory analysis of soil samples collected from the east and west ends of the tanks detected total petroleum hydrocarbons quantified as gasoline(TPH-g)at concentrations as high as 8,200 milligrams per kilogram(see Table 1). To date,soil samples have not been collected near the southeast or southwest ends of the former UST area to fix-cher assess the vertical extent of hydrocarbon-impact.Therefore,AGE recommends at least two 40-foot soil borings be advanced near the southeast and southwest corners of the former UST area. Additionally,between September 2004 and present day,interim SVE has been performed at the site from SVE wells VW-lA(8 to 23 feet),VW-113(28 to 43 feet),VW-213 and VW-313(19 to 39 feet). During this time,AGE has calculated that a total mass of 19,869 pounds of petroleum hydrocarbons have been removed(Quarterly Report-2nd Quarter 2007,dated 06 November 2007).Furthermore, during the above SVE interim remediation period, average influent SVE concentrations (TPH-g) have steadily declined from approximately 71,333 micrograms per liter (µg/l) in September 2004 to 180µg/1 in June 2007(see Table 2). Therefore,AGE recommends that at least one soil boring be advanced to a depth of at least 40 feet bsg and centrally located between existing SVE wells VW-1, 837 Shaw Road, Stockton, California 95215 Telephone(209)467-1006 Fax(209)467-1118