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' r , 28 February 1997 '16 AGE-NC Project No 95-0144 Page b of 20 1 f ` In a carbon canister system, hydrocarbon vapor is routed through activated carbon filters where ' adsorption of hydrocarbons onto carbon takes place, removing the hydrocarbons from the vapor a stream This type of system works best for low concentrations of hydrocarbons, but becomes expensive when high concentrations of hydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon r ' Thermal oxidation' systems destroy hydrocarbons by open-flame combustion Natural gas is commonly used as supplemental fuel, heating the extracted vapor stream to a combustion temperature of approximately 1400°F Relative to internal combustion systems, these systems may be slightly more expensive to'purchase and install However, they can operate at higher air flow ' rates, and therefore remediate sites at a faster rate For this system to be effective, hydrocarbon concentrations should range from 5,000 ppmv to 30,000 ppmv Thermal oxidation systems operate ' at much lower noise levels than internal combustion systems u , Catalytic oxidation units provide another option for treating vadose-zone contamination,particularly ' after other extraction systems have reached their effective limits due to lowered hydrocarbon concentrations These systems operate at temperatures of approximately 700°F, requiring less supplemental fuel than either thenal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal-combustion and thermal oxidizer systems ' 5 1 2 REQUIRED SOIL CONDITIONS ' Operation_of the blower in a vapor extraction system creates a partial vacuum in the subsurface, inducing`air"currents"through the soil pore spaces Vapors move by convection toward the area of lowered air pressure (the extraction point) The efficiency of this process is proportional to soil ' porosity and permeability Qualitative information has been gained through the examination of soil ; 'samples collected at the site Soil in the upper 19 feet has a relatively'high clay and silt content, potentially making vapor extraction less effective for this zone Soil below 19 feet and in the , saturated zone consists of sand, rendering vapor extraction as an effective remediation method However, due to a recent rise in ground water at the site,this zone is below the current ground water table Figures 3 and 4 depict cross sections, showing subsurface soil types at the site 5 l 3 MONITORING ACTIVITIES Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at ' the inlet to the destruction unit and periodic service and repair Systems can be equipped with an automatic telecommunication system to alert the operator of system malfunction or failure Vapor samples should be collected monthly for laboratory analysis to monitor the efficiency of the r