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[lievron <br /> Chevron U.S.A. Products Company <br /> 2410 Camino Ramon,San Ramon,California•Phone (510) 842-9500 <br /> Mail Address: P.O.Box 5004,San Ramon,CA 94583-0804 <br /> December 21, 1992 <br /> Ms. Mary Meays a ` <br /> San Joaquin County Environmental Health M! 0 B ISM <br /> P.O. Box CA EIV1RONMB,41AL HEALTH <br /> Stockton, CA 95201 PERMIT/SERVICES <br /> Re: Former Chevron Station#9-1918.45 East Harding Way, Stockton CA <br /> Attached work plan for offsite assessment(GTI, 12/15/92) <br /> Dear Ms.Meays: <br /> Attached is a work plan dated December 15, 1992, which was prepared by Chevron's consultant, <br /> Groundwater Technology Inc. (GTI), to describe the proposed installation of one downgradient <br /> offsite groundwater monitoring well at the subject site. This work was requested in your October <br /> 22, 1992 letter to Chevron. <br /> The vapor extraction and treatment system which had operated at the site since November 1990 <br /> was recently removed as a result of the significant decrease in soil vapor hydrocarbon <br /> concentrations which had been measured monthly since the start-up of the system. The vapors <br /> decreased from over 6500 parts per million (ppm) to less than 100 ppm over the two years of <br /> system operation. In addition to the decrease in hydrocarbon concentrations in the soil vapor, the <br /> levels of hydrocarbons detected in soil samples collected from the former tankpit area had shown a <br /> significant decrease as well. The historically highest concentration of benzene, 160 ppm, was <br /> observed in boring V-3 at a depth of 26.5 feet in January 1989,before the vapor extraction system <br /> began operating. In July 1991, eight months after the vapor extraction system began operating, the <br /> benzene concentration had decreased to 1.7 ppm as indicated by a soil sample from boring MW-9 <br /> at a depth of 26 feet. In May 1992, eighteen months after the vapor extraction system began <br /> operating, the benzene concentration had decreased to non-detectable levels at 25 feet depth and <br /> 0.12 ppm at 27.5 feet in boring V-6. With such a low concentration of benzene in the soil at such a <br /> depth, the practicality of further treatment no longer exists. The oxygenation of the subsurface <br /> which has occurred as a result of the vapor extraction system's past operation will provide <br /> sufficient oxygen for natural biological organisms to complete the degradation of any remaining <br /> hydrocarbons. <br /> I look forward to your approval of the attached work plan. Upon issuance of the well permit by <br /> your office, GTI can schedule the proposed fieldwork. If you have any questions or comments, I <br /> can be reached at(510) 842-8658. <br /> Sincerely, <br /> Clint B. Rogers <br /> Environmental Engineer <br />