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SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 6 7-Eleven Store No 14117, 2725 Country Club Blvd Stockton,CA June 20 2002 ICarbon adsorption using granular activated carbon (GAC) can be used to treat extracted soil vapors at a reduction efficiency of up to 99 percent Cost-effectiveness is generally greatest at low vapor concentrations of less than 5% of the lower explosive limit (LEL), due to the cost of exchanging fresh carbon for organic-saturated carbon once saturation has been achieved ' Catalytic oxidizers are also used to destroy organic vapors during SVE, and operate most cost-efficiently at moderate vapor concentrations of 5—25% LEL (700 - 3,500 parts per million by volume [ppm(v)]) Catalytic oxidizers are capable of destroying at least 95% of extracted hydrocarbon vapors within their ' optimum concentration operating range At concentrations higher than the maximum capacity of the equipment, dilution is required At concentrations of less than 5% LEL, auxiliary electricity is required ' Thermal oxidizers are used to destroy organic vapors during soil vapor extraction, and operate most cost- efficiently at concentrations ranging from 25—60% LEL (3,500 - 8,400 ppm[v]) Thermal oxidizers have a I destruction efficiency of at least 95% At influent concentrations of more than 60% LEL, dilution is required At concentrations of less than 60% LEL, auxiliary fuel (natural gas) is required in varying amounts (more is required as concentrations decrease) Both catalytic and thermal oxidizers require ' electricity to operate, and can also require natural gas for combustion 43 Dual-Phase Extraction/Intrinsic Biodegradation Dual-phase extraction (DPE) consists of simultaneous groundwater extraction and SVE Equipment used ' for DPE includes either a combination of submersible pumps to extract the groundwater operated simultaneously with SVE using the same well, or insertion of a drop tube into the well for water extraction combined with SVE In the second scenario, a high-vacuum positive displacement(PD) blower is used to ' extract the groundwater combined with any vapors entrained in the extracted media, combined with SVE applied to the vadose-zone portion of the well The PD blower is connected to a"knockout'tank, where, after separation, the groundwater is routed through GAC prior to discharge to either the storm drain or ' sanitary sewer Vapors are routed through the SVE part of the system for destruction Successful plume capture depends on the size of the capture zone relative to the lateral plume distribution, if the downgradient edge of the plume is contained within the capture zone radius, full plume capture is inferred The size of the capture zone depends on inherent aquifer properties, number and placement of wells, and rate of groundwater withdrawal Intrinsic biodegradation can be successful for remediation of MTBE and TBA, in the proper site setting For aerobic degradation (observed at dissolved oxygen [DO] levels of more than 3 milligrams per liter ' [mg/Q, MTBE biodegrades to TBA and/or tert-butyl formate (TBF) TBA then degrades to 2-propanol, which converts to acetone, then pyruvic acid, with the end product being biomass plus carbon dioxide and water From TBF, the degradation products are formaldehyde, and then biomass plus carbon dioxide and ' water(Steffan et al 1997) Anaerobic degradation is also thought to be prevalent under certain conditions, but the degradation pathways are less well documented 14117Scm2 doc