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FILE COPY <br /> ENVIRONMENTAL HEALTH DEPARTMENT <br /> o'Pgo. N C SAN JOAQUIN COUNTY <br /> Unit Supervisors <br /> _ ' ? Donna K. Heran, RE.H.s. <br /> 304 East Weber Avenue, Third Floor Carl Borgman, R.E.H.S. <br /> Director Mike Huggins, R.E.H.S., R.D.I. <br /> • Al Olsen, R.E.H.S. Stockton, California 95202-2708 Douglas W. Wilson, RTHS. <br /> • c'9 i;iFOR`'�P Program Manager Telephone: (209) 468-3420 Margaret Lagorio, R.E.H.S. <br /> Laurie A. Cotulla, R.E.H.S. Fax: (209) 464-0138 Robert McClellon, R.E.H.S. <br /> E Program Manager Mark Bazcellos, R.E.H.S. <br /> l <br /> --MAR-13 2003 <br /> TOSCO MARKETING COMPANY <br /> ED RALSTON <br /> 2000 CROW CANYON PLACE SUITE 400 <br /> SAN RAMON CA 94583 <br /> RE: 8606 THORNTON ROAD SITE CODE: 2261 <br /> STOCKTON CA 95209 <br /> San Joaquin County, Environmental Health Division (EHD) is the lead agency providing the <br /> oversight of the investigation and cleanup of releases from underground storage tanks (UST) <br /> within San Joaquin County. EHD has a contract with California State Water Resources <br /> Control Board (SWRCB) to conduct an UST corrective action program. This site was <br /> placed in the LOP in May 1998. <br /> EHD has reviewed the June 21 , 2002, Site Assessment Report submitted by Secor on behalf <br /> of Tosco for Station #8671 located at the above referenced address. The report contains: <br /> site background, assessment activities and findings, and conclusions and recommendations. <br /> In addition to the Secor report we have also reviewed the December 16, 2002, Fourth <br /> Quarter Groundwater Monitoring Report prepared by Gettler-Ryan Inc. on behalf of Tosco. <br /> Groundwater contamination has been documented in samples collected from the on-site <br /> Monitoring Wells (MW) . Groundwater sample results from MW-4 have MTBE <br /> concentrations that continue to increase indicating an unstable plume; MTBE has risen over <br /> time and is currently 1600 ppb. The static water level at this site is currently 37 feet below <br /> ground surface (bgs). A grab water sample was collected at 64 feet bgs in December 2001 . <br /> The MTBE result of the grab water sample was positive; MTBE was reported at <br /> concentration of 130 ppb. The vertical extent of the MTBE contamination remains <br /> undefined in the source area on site. <br /> EHD agrees with Secor's recommendation to install a sentry well. The sentry well will be <br /> located between the know source area on site and the San Joaquin County Colonial Heights <br /> Municipal Well #3 which is located approximately 260 feet south of the site. The Municipal <br /> well is 225 feet in total depth, is cased to 190 feet without perforations, and is gravel packed. <br /> The sentry well is anticipated to be screened at 110 feet bgs. Soil samples must be collected <br /> in the target zone to ensure the sentry well screen corresponds with an interval likely to have <br /> significant hydrologic conductivity. In addition to the sentry well installation and sampling, <br /> EHD also directs you to define the vertical extent of MTBE in the source area. After <br />