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ExxonMobil Der4 Rolaie <br /> Refining& supply Company Territory Manager <br /> 2300 Clayton Road,Suite 1250 (� [��tt/7r;�D Global Remediation—U.S.Retail <br /> Concord,CA 94520 <br /> (925)246-8768 Telephone <br /> (925)24"M FacsirnNe JUN 0 7 2002 <br /> dadn.i.rotw@ exwnmobl.com <br /> ENVIRONMENT HEALTHO11MObII <br /> PERMIT/SERVICES <br /> Refining & Supply <br /> May 30,2002 <br /> Mr.Michael J.Infurna <br /> San Joaquin County Public Tlealth Services <br /> Environmental Health Division <br /> 304 East Weber Avenue,Third Floor <br /> Stockton,California 95202 <br /> Subject: Former Exxon RAS#7-3708,2705 Country Club Boulevard,Stockton,California <br /> Dear Mr.Infama: <br /> Attached for your review and comment is a copy of the Report of Groundwater Monitoring, First <br /> Quarter 2002 for the above-referenced site. The report, prepared by EMC Engineering, Inc. of <br /> Pleasant Hill,California,details the results of the March 2002 sampling event. <br /> Methyl tertiary butyl ether (MTBE) was detected during this monitoring event in well VEW3 at a <br /> concentration of 30,300 micrograms per liter (µg/L). MTBE was not detected at or above EPA <br /> Method 8260 reporting limits in groundwater samples collected from this well in either February or <br /> March 2001. As VEW3 is located within approximately 15 feet of the existing underground storage <br /> tanks (USTs), this recent dramatic increase in MTBE concentrations from non-detectable to 30,300 <br /> µg/L may indicate that a new, unauthorized release associated with the current USTs has occurred. <br /> ExxonMobil has not operated the station since December 1995. <br /> ExxonMobil installed MW7 in 1994 in a location that is upgradient of onsite source areas and <br /> approximately 60 feet downgradient of the neighboring 7-Eleven facility No. 14117. MW7 did not <br /> contain detectable concentrations of hydrocarbons until August 2000,when MTBE was first detected. <br /> MTBE has been detected in MW7 at concentrations as high as 260 ug/L. Tertiary butyl alcohol <br /> (TBA)was detected in MW7 at a concentration of 16.1 µg/L during this most recent sampling event, <br /> which is more than six years after ExxonMobil transferred the property to New West Petroleum. As <br /> presented in the Report on Additional Site Assessment and Recommendations, prepared by The IT <br /> Group in December 2001 for the neighboring 7-Eleven Store #14117, MTBE and TBA have been <br /> detected at concentrations of up to 36,000 and 98,000µg/L,respectively, at this upgradient 7-Eleven <br /> facility. These data suggest the MTBE and TBA detected in MW7 are from offsite sources. <br />