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Chevron Products Company 2 } Chevron Station#9-4275 2905 Ben dlinR Holt Drive Stockton California February 23,2001 the northern product lines reported 310 mg/kg TPHg and 0.63 mg/kg of benzene. Soil samples from the waste oil UST were non-detect for TPHg and benzene, however, total petroleum hydrocarbons calculated as diesel (TPHd) were reported at a concentration of 0.0011 mg/kg. A soil sample from beneath the former waste oil UST was analyzed for total oil and grease, halogenated volatile organic compounds (HVOCs), poly chlorinated biphenyls (PCBs), and pesticides. None of these constituents were detected. A new tank pit was excavated and five double-walled, fiberglass USTs were installed (4 gasoline and 1 methanol). Nine groundwater monitoring wells (MW-1 through MW-9), and ten soil vapor extraction/air sparge (SVE/AS) wells (GM-1 through GM-9 and VW-1) have been installed onsite. In addition, four soil borings (SB-1 through SB-4) have also been drilled. Soil samples collected and analyzed from the installation of monitoring wells MW-3, 4, 6, 7, and 8 did not contain report any detections of TPHg and benzene. The station was renovated to include additional pump islands and product lines in the winter of late 1996 to early 1997. Soil samples taken from beneath the product lines reported a maximum concentration of 8.1 mg/kg TPHg and 0.29 mg/kg of benzene. Corrective action is currently Y underwa which includes remediation by SVE/AS. Only the AS system is currently operating; the SVE well screens are submerged. An ARCO service station located south u radient of the site across Benjamin Holt ( pg ) � 1 Drive, had a spill of approximately 2,000 gallons of gasoline in May of 1989. Another release of gasoline was reported in 1995. ARCO's hydrocarbon plume may be co-mingled with the Chevron hydrocarbon plume, evidenced by elevated methyl tertiary butyl ether (MtBE) detections in MW-9 on September 24, 1996, which was previously unaffected. Historical MtBE concentrations in ARCO wells near Benjamin Holt Drive have been an order of magnitude greater than the Chevron wells. Fourth quarter 2000 monitoring results are included in Attachment A. The City of Stockton maintains a nearby inactive municipal well No. 10, which is routinely sampled. Fourth quarter 2000 laboratory results reported MtBE at a concentration of 11.0 ug/L by EPA Method 8021B. This result could not be validated as the accompanying trip blank was reported to contain MtBE at a concentration of 30 ug/L by EPA Method 8021B. The well is scheduled to be sampled March 7, 2001, During the fourth quarter, groundwater was measured at approximately 10 feet bgs. Groundwater flow was to the north at a gradient of 0.0007 to 0.002 feet per foot (ft/ft). PROPOSED SCOPE OF WORK The objective of this assessment is to determine the potential mass discharge (Md) of MtBE from the Chevron facility and to assess the potential for impact on Municipal Well No. 10. The scope of work proposed to meet this objective includes performing 10 cone I:IChevron1942751REPQRTS12001 Assessment_RevB.doc