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S E C 0 R • QUARTERLY SUMMARY AND MONITORING REPORT Fourth Quarter 2005 t Former 76 Service Station #4409 1502 North EI Dorado Street, Stockton, California p y�4 1 Y City/County ID # Stockton _ v xLv1Nr County _San Joaquin �'x ry�� SITE DESCRIPTION The site is located on the northeast corner of North El Dorado Street and East Harding Way in Stockton, California The site is a former 76 Unocal service station that dispensed F' gasoline and diesel fuel beginning in 1982 No fuel has been dispensed since the underground storage tanks (USTs) were removed in 1999 The surrounding land is a mix of commercial and residential properties There is a co-mingled plume study being done with the neighboring ARCO station located southwest of the site at 16 East Harding Ways 2,YI GROUNDWATER MONITORING ZONE CLASSIFICATION i e Prior to the vertical assessments conducted in November 2004 and April 2005, 'shallow' ' and 'deep' were used to identify the groundwater monitoring zones for wells completed to depths ranging from 45 to 50 feet bgs (MW-1 through MW-4) and 79 to 83 feet bgs (MW-5 7`' through MW-14), respectively However, the three wells installed during November 2004 and April 2005 were each completed to a depth of approximately 120 feet bgs, which is below the previously termed 'deep' groundwater monitoring zone Therefore, 'shallow' and 'deep' will no longer be used to describe the groundwater monitoring zones and will be replaced by the A monitoring zone and the B monitoring zone, respectively The three new wells (MW-15 through MW-17) installed to approximately 120 feet bgs are in the third groundwater monitoring zone, which is referred to herein as the C monitoring zone PREVIOUS INVESTIGATION ' On April 7, 1999, three 10,000-gallon capacity underground storage tanks (USTs), which were used for 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 collected from the bottom and sidewalls of the excavated area rA xv qr 4,E t 4409 4Q2005 QSR(CR)doc SECOR International Incorporated a — - Uj