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1 <br /> f i � <br /> I ` <br /> 15 November 1995 <br /> AGE-NC Project No 95-0142 <br /> Page 8 of 19 <br />' adsorption of hydrocarbons takes place, removing them from the vapor stream These systems work <br /> best for low concentrations of hydrocarbons, but become expensive when high concentrations of <br /> hydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon <br /> IIn thermal oxidation systems, hydrocarbons are destroyed by open-flame combustion Natural gas is <br /> commonly used as supplemental fuel, heating the extracted vapors to a combustion temperature of <br />' approximately 1400°F Relative to internal combustion systems, these systems may be slightly more <br /> expensive to purchase and install However, they can operate at higher air flow rates, and therefore <br /> remediate sites at a faster rate For this system to be effective, hydrocarbon concentrations should <br />' range from 5,000 ppmv to 30,000 ppmv They also operate at much lower noise levels than internal <br /> combustion systems <br />' Catalytic oxidation units provide another option for treating vadose-zone contamination, particularly i <br /> after other extraction systems have reached their effective limits due to lowered hydrocarbon <br />' concentrations These systems operate at temperatures of approximately 700°F, requiring less <br /> supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements <br /> and limitations are similar to those for internal-'combustion and thermal oxidizer systems <br /> 6 12 REQUIRED SOIL CONDITIONS <br />' Operation of the blower in a vapor extraction system creates a vacuum in the subsurface, inducing <br /> air "currents" through pore spaces Vapors move by convection toward the area of lowered air <br />' pressure (the extraction point) The efficiency of this process is proportional to soil porosity and <br /> permeability Qualitative information has been gained through examination of soil samples and the <br /> performance of a vapor extraction pilot test Soil at this site has a lugh clay content, which would <br />' make vapor extraction a difficult and slow process <br /> L r <br />' 6 1 3 MONITORING ACTIVITIES <br /> Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at the <br />' inlet to the destruction unit and periodic service and repair Systems can be equipped with an <br /> automatic telecommunication system to alert the operator of system malfunction or failure Vapor <br /> samples should be collected monthly for laboratory analysis to monitor the efficiency of the <br />' remediation program When the concentration of extracted vapors decreases and stabilizes after <br /> several months of operation, confirmation borings are drilled to collect soil samples for quantitative <br />' analysis to the laboratory to document the degree of clean up obtained <br />