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J� Tosco Corporation,A Subsidiary of Phillips Petroleum Company 3 Tosco Service Station No 4409, 1542 North El Dorado Street,Stockton,Cahfomm May 29,2002 • hydrocarbons were detected in soil samples collected between 60 and 70 feet bgs, at concentrations of TPHg, and benzene up to 4,780 parts per million (ppm) and 55 3 ppm (MW-6@60') Petroleum hydrocarbons were detected in groundwater samples collected from all three wells Maximum concentrations of TPHg, benzene, and MtBE were reported at 2,300 ppb (MW-6), 22 ppb (MW-6), and 16 ppb (MtBE, EPA Method 8260) Based on the results of the Investigation, IT concluded that the vertical extent of petroleum hydrocarbons in soil were generally defined to a depth of 80 feet bgs IT further recommended that additional assessment activities be performed to evaluate the lateral extent of dissolved petroleum hydrocarbon impacts between approximately 50 and 80 feet bgs SCOPE OF WORK On August 30 and 31, and September 5, 2001, IT installed three on-site deep zone groundwater monitoring wells (MW-8 through MW-10) at locations upgradient of the UST complex, in the vicinity of the pump Islands and downgradient of the pump Islands (Figure 2) Drilling activities were performed by Cascade Drilling of Rancho Cordova, California As required by the Occupational Health and Safety Administration (OSHA) Standard "Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910 120), and by California Occupational Health and Safety Administration (Cal-OSHA) "Hazardous Waste Operations and Emergency Response" guidelines (CCR Title 8, Section 5192), IT prepared a site-specific Health and Safety Plan (HASP) Prior to the commencement of fieldwork, the HASP was updated with appropriate field personal and subcontractor Information The field staff and contractors reviewed the HASP before beginning field operations at the site The completed scope of work is discussed below • Monitoring Well Installation. The exploratory borings for on-site groundwater monitoring wells MW-8 through MW-10 were drilled to a depth of 80 feet bgs, and converted to groundwater monitoring wells The wells were constructed with 2-inch diameter schedule 40 PVC casing with approximately 10 feet of 0 020-inch slotted screen material Well screen intervals extended from approximately 69 to 79 feet bgs Conductor casings were not installed in the boreholes because petroleum hydrocarbon concentrations at the water table were relatively low to non-detectable All work was conducted in accordance with stipulations of permits obtained from the SJCPHS/EHD IT's field and laboratory procedures are presented in Attachment B Boring logs and well construction details prepared by IT are included in Attachment C • Soil Analytical Results. Soil samples were collected at 5-foot intervals from each boring A total of eight groundwater-saturated soil samples were selected from the three borings for chemical analysis, based on the detection of petroleum hydrocarbon odors, elevated photo-Ionization detector (PID) readings, and the I ITosco Marketing144091Reports\AsmtRpt doe