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I S EC ® R Mr Mike Infurna October 29, 2004 Page 3 Quarterly Summary Report Third Quarter 2004 Former 76 Service Station #4409 1502 N EI Dorado Street, Stockton, California City/County ID # Stockton County San Joaquin BACKGROUND The site is located on northeast corner of EI Dorado Street and Harding Way in Stockton, California, and is a former operating Unocal service station On April 7, 1999, three 10,000-gallon capacity underground storage tanks (USTs), which contained gasoline and diesel fuel, were removed from the site The tank pit area was overexcavated Excavation dimensions were approximately 35 feet by 40 feet by 15 feet deep Initial remedial activities included source removal by excavation of obviously impacted soil to a depth of approximately 18 feet below ground surface (bgs) beneath the center of the western fuel tank Additional remedial activities included source removal by excavation of obviously impacted soil in the area beneath the former dispenser and product lines A concrete pad restricted the lateral extent of excavation During excavation of impacted soil, unidentified fiberglass piping was found Additional excavation and sampling was completed on April 8, 1999 to remove the unidentified piping Samples were 4 ? collected from the bottom and sidewalls of the excavated area �Y i Following removal of the former station canopy, canopy footings and kiosk, additional over- excavation was completed in the dispenser area Over-excavation was terminated when obviously impacted soil was removed, the structural integrity of the sidewalk was endangered, or groundwater was encountered A total of approximately 1,707 tons of soil was transported off-site for disposal at E the Forward Landfill in Manteca, California . Four shallow groundwater-monitoring wells (MW-1 through MW-4)were installed in December 1999, ; to assess the potential for hydrocarbon impact to groundwater beneath the site (Attachment A) Vertical assessment was conducted in July 2000, and soil and groundwater impacts were ; encountered at depths of 60 feet bgs (GP-1 through GP-4) r The San Joaquin County Environmental Health Department (SJCEHD) subsequently requested a work plan for further delineation of soil and groundwater impacts Three on-site deep groundwater- monitoring wells (MW-5 through MW-7) (Attachment A) were installed in November 2000, which identified deep zone groundwater petroleum hydrocarbon impacts In August and September 2001, three additional deep zone groundwater monitoring wells (MW-8 through MW-10) (Attachment A) were installed on-site to determine the extent of deep zone groundwater impacts In June and July 2003, four off-site deep zone groundwater monitoring wells (MW-11 through MW-14) (Attachment A) . were installed to further define the lateral extent of dissolved petroleum hydrocarbon plume within the deeper water-bearing zone 11ConocoRhi1hps\Tosco Marketing144091Qssrs1200413Q2004_QSR doc SECOR International Incorporated