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Tosco Corporation 3 Tosco Service Station 8671 8606 Thornton Road Stockton CA June 21,2002 • laboratory detection limits Groundwater analytical data is presented in Table 3 and shown on Figure 2 Groundwater Monitoring Well Development, Sampling, and Surveying On January 7, 2002, the groundwater monitoring wells were developed by rigorously surging over the length of the screen interval, and by purging between 10 and 12 casing volumes of water Following development, groundwater samples were collected from the newly installed groundwater monitoring wells Groundwater samples were analyzed for TPHg by EPA Method 8015, BTEX and MtBE by EPA Method 8020, and fuel oxygenates (MtBE, DIPE, TAME, EtBE, TBA, EDB, 1,2-DCA, and ethanol) by EPA Method 8260A Laboratory analysis of groundwater samples detected toluene in MW-6 and MW-7 at concentrations of 0 77 ug/1 and 0 71 ug/l, respectively Total xylenes were detected in MW-6 at a concentration of 0 58 ugll MtBE, benzene, TPHg and all other analytes were not detected above laboratory limits Groundwater analytical data is presented in Table 3 and shown on Figure 2 Field and laboratory procedures are presented as Attachment C Well development and groundwater sampling parameters are included on the field sheets in Attachment E The groundwater monitoring wells were surveyed relative to mean sea level by Morrow Surveying, a licensed surveyor Attachment F contains the surveyor's site plan showing the surveyed locations and elevations of the wells at the site Groundwater Flow Direction Depth to groundwater at the site ranged from 30 34 feet bgs to 32 84 feet bgs and the direction of groundwater flow was to the south-southeast, at an approximate gradient of 0 013 ft/ft A groundwater elevation contour map is included as Figure 3 Sensitive Receptor Survey SECOR performed a sensitive receptor survey to identify the location of permitted water supply wells within a 2,000-foot radius of the site (Figure 4) The sensitive receptor survey consisted of reviewing the California Department of Water Resources (DWR) files for locating water supply wells in the area, contacting the local water purveyor (San Joaquin County), and a conducting a door-to-door field survey in search of any wells or other sensitive receptors within 1,000 feet of the site Based on water well drillers reports obtained from the DWR, no water wells were located within a 2,000-foot radius of the site In a conversation with Mr Ron Rall, Utility District Superintendent of the San Joaquin County Utilities Division (SJCUD), Mr Rall indicated that an active municipal water supply well identified as Colonial Heights municipal water supply well #3 (Well DW-3) is located in the vicinity of the site (Figure 4) The location of Well DW-3 was verified in the field and surveyed, and is situated 260 feet south of the site, on the south bank of Mosher Slough, which borders the intersection of Thorton Road and Davis Road According to Mr Rall, the well is classified as a lag well, and is used 077 43791 222/Assessment