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12 55 2694653433 FIFTH FLOOR PAGE 02 <br /> ENVIRONMENTAL HEALTH DEPARTMENT <br /> SAN JOAQUIN COUNTY <br /> �o�i co UrsuSupenuon <br /> Donna K.Heran,RE H.S. Carl Borgman,RE H 5 <br /> Director 304 East Weber Avenue, Thud Floor Mikc Huggins,R E x S,R D 1 <br /> Al Olsen,R EM 5 Stockton, California 95202-2709 Douglas W Wilson,R E H S <br /> - P Program Manager Tele hone (209) 465-3420 Margaret l,agono,RL li S <br /> Laurie A Cotulla,RE H S p Rabcrt McClellon,R E H S <br /> Program Manager Fax' (209} 464-0138 Mark Barcellns,PLE H S <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 /> STOCK-TON CA 95209 <br /> San Joaquitl County, Environmental Health Division (EHD) is the lead agency providing the <br /> oversight of the mvestl tion and cleanup of releases from underground storage tanks (UST) <br /> vvitbm San jaaqu,,, Counter RHD has a contract with Cahfomia. State Water Resources <br /> Control Board (SWRCB) to conduct an UST corxect>.ve action program. Tkus 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 acttvlttes and findings, and conclusions and recommendations <br /> In addition to the Secor report we have also reviewed the December 16, 2002, Fourth <br /> Quartcr Groundwater Mowtonng Report prepared by Gecder-Ryan Inc on behalf of Tosca <br /> Groundwater contamirianon has been documented in samples collected from the on-site <br /> Monitoring Wells pM 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 m December 2001 <br /> The MTBE result of the grab water sample was posiuve, 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 Secoi s recommendation to mstall a sentry well The sentry well wsll be <br /> located between the know source area on site and the San]oaquirl 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 petforatlons, and is gravel packed <br /> The sentry well is anticipated to be screened at 110 feet bgs Soil samples must be collected <br /> m the target zone to ensure the sentry well screen corresponds wsth an interval likely to have <br /> 0 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 />