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1 <br /> 10 After 7,000 hours (approximately 290 days)of operation, two small BTEX plumes with concentrations <br />' below 2 and 5 mg/L remained Free product had been completely removed Cumulative contaminant mass <br /> removed from the site was approximately 36,000 pounds (approximately 5,400 gallons) Approximately <br />' 162 million gallons of groundwater were removed and treated Following system shutdown,monitoring <br /> was conducted at the site until its closure in 1996 <br />' Remediation goals of the system were 50 mg/L TPH, 1 mg/L total BTEX, and 0 5 mg/L benzene <br /> 4.4.1.2 Underground Storage Tank Release from a Former Car Rental Lot in Los Angeles, <br /> California <br /> A drop-tube entrainment system was installed to remediate hydrocarbon contamination resulting from <br /> leaking underground storage tanks (UST)at a former car rental lot(Trowbridge and Ott 1991) The <br /> extraction well network initially consisted of 29 extraction wells incorporating drop tubes,but was later <br />' expanded twice to a total of 46 wells to address migration of the contaminant plume The treatment system <br /> consisted of a vapor/liquid separator,vacuum blowers, and catalytic oxidation for vapor treatment The <br /> vacuum blowers were capable of a combined flow of 1,000 scfm at an inlet vacuum of 15 inches of <br /> mercury Water from the separator was treated using liquid-phase GAC Photographs 4-1,4-2,and 4-3 <br /> (provided courtesy of Terra Vac Incorporated)show an extraction wellhead and the extraction treatment <br />' system for this site <br />' Site soils consisted of brown silty clay to approximately 50 feet bgs A perched groundwater table was <br /> present at depths of approximately 25 to 30 feet bgs Gasolme-range hydrocarbon contamination at the <br /> site ranged in depth from 10 to 35 feet below the surface and covered an area of approximately 280 feet <br /> by 450 feet The highest contaminant concentration detected was 1,400 mg/kg, with an average <br />' concentration of 100 mg/kg Monitonng wells at the site contained up to 3 feet of floating product <br />' Extraction wells were typically screened from approximately 20 to 35 feet bgs, although some screens <br /> extended up to 10 feet bgs,and others extended down to 50 feet bgs Well spacing was approximately <br />' 40 feet,with closer spacings used in areas with higher contaminant concentrations An average of 20 scf n <br /> was obtained from each well at a wellhead vacuum of 10 inches of mercury After 10 weeks of operation, <br />' measured groundwater levels were an average of 5 feet lower than before operations began <br /> 4-15 <br /> 1 <br /> 1 <br />