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TPH from the silty clay unit generally encountered between depths of 8 <br /> and 16 feet. The lower completion zone would remove VOCs and TPH from the <br /> silty sand unit which underlies the silty clay. The two well casings are <br /> separated in the borehole by a 6-inch layer of sodium bentonite pellets <br /> and an 18-inch neat cement grout seal, which would be located in the <br /> borehole adjacent to the lower two feet of the silty clay unit. This <br /> two-foot impermeable seal would isolate the two sediment types and enable <br /> the selective extraction of VOCs and TPH from the less permeable silty <br /> clay unit and from the underlying, more permeable silty sand unit. This <br /> separation of sediment units with different transmissive properties would <br /> assure that appropriate volumes of vapors are being removed from both <br /> sediment units. The lower completion zone would extend approximately <br /> 4 feet below the estimated extraction well drawdown to assure adequate <br /> vertical coverage for vapor extraction capabilities as groundwater removal <br /> > results in additional unsaturated soils. The ten proposed soil venting <br /> casings would be manifolded together and connected to a low flow, <br /> 100 cubic feet per minute (CFM), high vacuum, Rotron blower. Valving <br /> would be provided at each well casing to allow for selective adjustment <br /> of the vapor extraction rate. <br /> A variety of options for treatment of the vapor emissions from the soil <br /> venting wells, including carbon adsorption, thermal oxidation and <br /> catalytic incineration, have been evaluated for use at the site to meet <br /> San Joaquin County Air Pollution Control District requirements for <br /> emission control. A thermal oxidizer unit with a heat exchanger has been <br /> selected as the control method for this remedial action plan. The unit <br /> would raise the temperature of the airstream to approximately <br /> 1400 degrees F and would retain the heated gases for a minimum of <br /> 0.5 seconds before exhausting to the atmosphere. <br /> R A vapor incinerator destroys vapors by oxidation to produce water vapor <br /> and carbon dioxide. A temperature of about 1200 degrees F is required to <br /> oxidize most of the organic compounds in gasoline. The residence time <br /> r (time the chemical compound is subjected to the combustion temperature) <br /> required for gasoline compounds is on the order of 0.3 to 0.5 seconds. <br /> The proposed unit easily meets both the temperature and residence time <br /> requirements for effective incineration of vapors from the organic <br /> compounds in gasoline. <br /> The thermal oxidation unit would be located at the vacuum blower discharge <br /> and would be designed to meet the 90 percent control requirement specified <br /> in the San Joaquin County Air Pollution Control District's (SJCAPCD) <br /> Rule 409.9. Sample ports would be located at the thermal oxidation unit <br /> intake and exhaust for air quality sampling. Air sampling would be <br /> conducted to assure compliance with air emissions criteria established by <br /> the SJCAPCD. Initial system startup and ongoing air emission monitoring <br /> for the soil vapor extraction system would be performed in accordance with <br /> - -----------_...—.._..--.___._..---- -SJCAPCD...permitting_req uirements.- --- - --- -- <br /> 16 <br />