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• 4.41 Performance <br /> 1 <br /> DPE has been implemented at a vanety of sites contaminated with gasohne-range petroleum hydrocarbons <br />' and VOCs The following case studies describe the design and performance of five full-scale DPE systems <br /> Three of the case studies involve drop-tube entrainment type systems,one involves well-screen entramment, <br />' and one involves downhole-pump extraction <br />' 4.4.1.1 Underground Storage Tank Release from a Gasoline Station in Houston,Texas <br />' Vacuum enhanced pumping(VEP), a form of drop-tube cntramment extraction,was implemented for <br /> remediation of a groundwater contaminant plume at a gasoline station (Mastroianni and others 1994) The <br /> -VEP system design incorporated a self-priming drop tube in each extraction well and included nine <br /> 1 extraction wells, a vacuum blower, a vapor/liquid separator, and an oil separation and water treatment <br /> system Vapor treatment was accomplished using a thermal oxidation system equipped with auto dilution <br />' The vacuum blower was operated at approximately 300 scfm at 12 inches of mercury <br /> Site soils were overlain by asphalt as well as concrete and consisted of clay to a depth of approximately <br /> 16 feet,becoming silty below 13 feet and interbedded silts and sands between 16 and 25 feet Silty clay <br />' extended between 25 and at least 27 feet below grade Contamination of concern consisted of a <br /> groundwater benzene,toluene,ethylbenzene,and xylene (BTEX)plume and an associated free-product <br />' plume The aenal extent of the groundwater plume was approximately 50,000 square feet BTEX <br /> concentrations in a majority of the plume exceeded 30 milligrams per liter(mg/L) The maximum free <br /> ' product thickness was approximately 3 feet <br />' The system used both new and existing monitoring wells for extraction The wells were installed to depths <br /> of approximately 30 feet with spacings generally between 30 and 50 feet Initially,recovery and treatment <br />' operations for soil vapor, LNAPL, and groundwater were conducted from one extraction well to avoid <br /> overloading treatment capacity of the thermal oxidizer used for vapor treatment As the hydrocarbon <br /> content of the process stream from the initial extraction well decreased, additional extraction wells were <br /> 1 brought on line All wells were brought on line within the first 500 hours of system operation <br /> • 4-14 <br /> 1 <br />