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E C 0 R • QUARTERLY SUMMARY REPORT THIRD QUARTER 2003 Circle K Store#08671 8606 Thornton Road, Stockton, California City/County ID # Stockton County San Joaquin BACKGROUND The site is located at 8606 Thornton Road, Stockton, California The site is currently an operating gasoline service station that contains two gasoline USTs, associated product lines, and dispensers In May 1998, product lines and dispensers were upgraded to meet 1998 EPA standards 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 The results of this work are documented in SECOR's report titled Removal and Replacement of Product Lines and Dispensers, dated July 22, 1998 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 On January 27 and 28, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed onsite by SECOR The results of this work are documented in SECOR's report titled Well Installation Report, dated March 16, 1999 Since their installation, the four wells have been monitored and sampled quarterly 5 On December 12th through 14th, 2001, one deep soil boring (SB-1) was drilled to an approximate depth of 63 feet below ground surface (bgs) and three additional groundwater monitoring wells (MW- 5, MW-6, and MW-7) were installed onsite by SECOR The results of this work are documented in SECOR's report titled Site Assessment Reporf, dated June 21, 2002 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 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 E survey in search of any wells or other sensitive receptors within 1,000 feet of the site Four wells 3 were identified within the surveyed area, one municipal well and three domestic wells The domestic well was identified as Colonial Heights municipal water supply well #3 (Well DW-3) 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 Thornton Road and Davis Road Three additional water supply wells in the area were identified One well was identified at Bear Creek Veterinarian Hospital (Well DW-2), located at 8728 Thornton Road Well DW-2 is located 444 k feet north of the site Another well is located at a residence (Well DW-4), located at 8732 Davis Road (180 feet north of the site) SECOR field staff located what appeared to be a domestic water supply well (Well DW-1) on the property located at 8855 Thornton Road and is 281 feet northwest of the site The resident was not available to confirm the existence of the well, and a well survey form was not completed by the property owner, however, the possible domestic well was housed in a similar enclosure as the previously identified wells in the field The site is currently monitored and sampled on a quarterly basis During the last quarterly groundwater monitoring event conducted on August 13, 2003, maximum concentrations of TPHg and MtBE were reported at 300 ppb and 3,300 ppb, respectively Depth to groundwater at the site f f