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.S S EC ® R Mr Harlan Knoll January 14, 2004 Page 2 PREVIOUS ASSESSMENTS AND REMEDIAL ACTION In May 1998, product lines and dispensers were upgraded to meet 1998 EPA standards Laboratory analytical results from pea gravel samples collected from above the USTs, and soil samples collected from the former product line and dispenser excavations indicated the presence of petroleum hydrocarbons in soil beneath the site During product line and dispenser upgrade activities, approximately 21 tons of petroleum-impacted soil were over-excavated from the site and disposed of at Forward Landfill The results of this work are documented in SECOR's report titled Removal and Replacement of Product Lines and Dispensers, dated July 22, 1998 On January 27 and 28, 1999, four on-site groundwater monitoring wells (MW-1 through MW-4) were installed by SECOR The results of this work are documented in SECOR's report titled Well Installation Report, dated March 16, 1999 On December 12 through 14, 2001, SECOR advanced one deep soil boring (SB-1) to an approximate depth of 64 feet below ground surface (bgs), and installed one on-site groundwater monitoring well (MW-5) and two off-site groundwater monitoring wells (MW-6 and MW-7) to a depth of 44 5 feet bgs MtBE (EPA Method 8260) was reported in soil samples collected between 28 feet and 35 feet bgs from SB-1 at concentrations up to 0 94 milligrams per kilogram (mg/kg) TBA was detected in the 30-foot sample from the boring for well MW-5 at 0 86 mg/kg Benzene and MtBE were detected in a grab groundwater sample from boring SB-1 at concentrations of 0 52 µg/L and 130 µg/L, respectively The results of this investigation are detailed in SECOR's report entitled Site Assessment Report, dated June 21, 2002 As part of the June 21, 2003 Site Assessment Report, SECOR performed a sensitive receptor survey to identify the location of permitted water supply wells within a 2,000-foot radius of the site The sensitive receptor survey consisted of obtaining water well drillers' reports on file with the California Department of Water Resources (DWR), 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-foot search radius According to the water well drillers' reports, no wells were identified within a 2,000-foot radius of the site However, in a conversation with Mr Ron Rall of the San Joaquin County Utilities Division (SJCUD), Mr Rall indicated that an active municipal water supply well (Colonial Heights Municipal Well #3) is located in the vicinity of the site Designated as Well DW-3 on SECOR maps, the location was confirmed at a location approximately 260 feet south of the site on the south bank of Mosher Slough, which borders the intersection of Thornton Road and Davis Road According to Mr Rall, the well is classified as a lag well, and is used to augment water supply when the other municipal wells cannot meet consumer demand This well is seldom used as a drinking water source, as another well identified as Colonial Heights Well #1 serves as the primary back-up well to the City of Stockton water supply Well DW-3, however, continues to be sampled on a monthly basis to monitor groundwater quality According to documentation received from the SJCUD, Well DW-3 has a 14-inch diameter casing extending to a depth of approximately 225 feet bgs Well DW-3 is also equipped with a 20-horsepower pump, which is capable of pumping at a rate of 312 gallons per minute (gpm) Results of the door-to-door survey confirmed the existence of three additional water supply wells in the area One well was identified at Bear Creek Veterinarian Hospital (Well DW-2), located at 8728 Thornton Road, approximately 444 feet north of the site The employees at the veterinarian hospital confirmed that the well is an active well, however, on-site personnel would not disclose the purpose a I IConocoftdhps%Tosco Marketing186711Reports\Assessment Rpt doc SECOR Intemational Incorporated