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C1� lig Quarterly Summary Report Second Quarter 2002 Former Tosco (Unocal) Service Station #4409 1502 N EI Dorado Street, Stockton, California City/County ID# Stockton County San Joaquin BACKGROUND The site is located on northeast comer of El 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 Excavation dimensions were approximately 35 feet by 40 feet by 15 feet deep Initial remedial activities included source removal 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 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 collected from the bottom and sidewalls of the excavated area 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 Residual hydrocarbons remain in the soil and groundwater A total of approximately 1,707 tons of soil was transported off-site for disposal at the Forward Landfill in Manteca, California Four groundwater monitoring wells (MW-1 through MW-4) were installed in December 1999 Vertical assessment was conducted in July 2000, and soil and groundwater impacts were encountered at depths of 60 feet bgs (GP-I through GP-4) The SJCPHS/EHD verbally requested a workplan for further delineation of soil and groundwater impacts Three on-site deep groundwater monitoring wells (MW-5 through MSW-7) were installed in November 2000, which identified deep zone groundwater petroleum hydrocarbon impacts Results of this investigation indicate that the lateral and vemcal extent of petroleum hydrocarbon impacts have been defined across the site In August and September 2001, three additional deep zone groundwater monitoring wells (MW-8 through MW-10) were installed to deternune the extent of deep zone groundwater impacts RECENT QUARTER ACTIVITIES (Second Quarter 2002) 1 Quarterly groundwater monitoring and sampling performed by GRI on April 18, 2002 2 Prepared and submitted the quarterly summary report l 3 Prepared and submitted a work plan for additional off-site assessment