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1-4asibility Study—Enhancement of Existing Remediation Activities 8200 N. Highway 99, Stockton, CA Page 3 of 10 effort by installing an additional groundwater extraction well upgradient of MW 12. 1.1 Groundwater Extraction and Treatment System The groundwater extraction and treatment system (GETS) was put into operation on November 5, 2003. Treated groundwater is discharged to the ground surface pursuant to WDR General Order No. R5-2003-004412. The GETS is connected to two extraction wells EW1 and EW2. No water was treated between April 1 and June 30, 2007, because the groundwater treatment system was idle pending repairs to the air stripper_ The system has been repaired and has been back in operation since August 2, 2007. A total of approximately 2,536,898 gallons of groundwater has been treated through June 30, 2007. Approximately 117 pounds of gasoline constituents, including approximately 38 pounds of MTBE, were removed through groundwater extraction and treatment between November 2003 and June 30, 2007. MtBE concentrations in monitoring wells MW1, MW3, MW4, and MW5 have shown a steady decline since groundwater extraction began in November 2003. No gasoline constituents were detected in any of the five nearby domestic wells sampled since September 2003. 1.2 Soil Vapor Extraction System The soil vapor extraction system (VES) was placed in operation on August 8, 2005. The VES operated in thermal oxidizer mode until November 4, 2005 and was then modified and restarted in catalytic oxidizer mode on November 22, 2005. The VES is connected by sub-grade piping to two vapor extraction wells (VW1 and VW2), a dual-phase extraction well (EW1), and two monitoring wells (MW3 and MW4). Since March 7, 2006, the vapor extraction system has drawn vapors almost exclusively from well MW4 in an effort to extract groundwater from that well. A totalizing flow meter was installed on the transfer pump to record the volume of water removed from the VES. Between March 7, 2006, and June 30, 2007, approximately 120,851 gallons of water was extracted from well M. and transferred to the GETS. Due to declining mass removal rates (currently <1 pound per day), GZA requested and received authorization from the San Joaquin Valley Air Pollution Control District (SJVAPCD) to discharge extracted vapors directly to the atmosphere. The vapor extraction system was modified and direct discharge began on March 21, 2006. Vapor samples were not collected between April 1 and June 30, 2007, due to pending repairs to the air stripper. As of June 30, 2007, the vapor extraction system had operated for a total of 7,669 hours and had removed an estimated 1,894 pounds of gasoline hydrocarbons from the subsurface. G:SGROUNDZEITULEBURGIRepons10709 FS rem enliancement.doc