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• on site wells, and 150 and 500,ug/L for benzene and total BTEX,respectively,for off-site wells <br /> Approximately 2,500 pounds of contaminant(334 gallons as gasoline) and 1,051,700 gallons of <br /> groundwater were removed and treated <br /> Water discharged from the system vapor/liquid separator contained total BTEX concentrations ranging <br /> ' from 7 to 1,300µg/L Discharge criteria to a publicly owned treatment works was 3,000,ug/L <br /> ' <br /> 441.5 Release from a Gasoline Underground Storage Tank for a Vehicle Fueling Station at <br /> a Hospital in Madison,Wisconsin <br /> ' A downhole-pump extraction system was implemented for remediation of gasohne contamination resulting <br /> from leaking USTs at a hospital (Miller and Gan 1995) The system consisted of one 6-mch diameter <br /> vertical extraction well screened from 5 to 30 feet bgs with a 3-foot sump at the bottom to trap sediment <br /> ' Vapors were extracted from the well using a blower operated at 30 cfm at a vacuum of 40 inches of water <br /> column Contaminated groundwater was recovered using a submersible centrifugal pump with a design <br /> ' flow rate of 10 gallons per minute (gpm)and treated by an air stripper before discharge to an on-site storm <br /> sewer � <br /> Site soils consisted of sandy fill from ground surface to 10 to 19 feet bgs The fill was underlain by a 3-to <br /> 4-foot layer of organic silt and peat Depth to groundwater was approximately 13 to 20 feet <br /> ' The system began operation m June 1994 Approximately 8,500,000 gallons of groundwater and <br /> 120 pounds of contammant were removed during the first 15 years of operation Groundwater benzene <br /> ' concentrations dropped from 276 ug/L to 8 ug/L after 6 months of operation and to 2 Ezg/L after 15 years <br /> of operation <br /> The system was shut down in January 1996 Benzene concentrations at the extraction well were found to <br /> fluctuate around the cleanup standard of 5 mg/L and had risen to 8 ug/L approximately 6 months after <br /> shutdown The concentration increase was attributed to the presence of residual contamination in the <br /> ' capillary fringe In spite of a 10 gpm pumping rate from the well, drawdown 5 feet from the recovery well <br /> was less than 1 foot Future plans for the site include increasing the groundwater extraction rate to 20 gpm <br /> ' to enhance dewatering of the capillary fringe <br /> • 4-18 <br /> 1 <br />