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14 November 1995 `r AGE-NC Project No 95-0121 ' Page 8 of 20 , concentrations decrease below levels normally required for combustion Typically, internal ' combustion engines work effectively where m-situ hydrocarbon concentrations approach the 40,000 ppmv required for peak performance ofthe engine As concentrations decrease below 20,000 ppmv, ' supplemental fuel requirements become burdensome Another disadvantage of these systems is the noise generated by the engine, however, tins can be mitigated by placing the unit inside an enclosure, such as a building T In a carbon-canister system, hydrocarbon vapors are routed through activated carbon filters, where adsorption of the hydrocarbons takes place, removing them from the vapor stream These systems ' work best for low concentrations of hydrocarbons, but become expensive when high concentrations „ of hydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon ' 1 r ! ' In thermal oxidation systems, hydrocarbons are destroyed by open-flame combustion Natural gas is commonly used as supplemental fuel, heating the extracted vapors to a combustion temperature of r 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 They also operate at much lower noise levels than internal combustion systems r 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 r supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal-combustion and thermal oxidizer systems ' 6 12, EFFECTIVE SOIL CONDITIONS t 1 Operation of the blower in a vapor extraction system creates a vacuum in the subsurface, inducing air flow through the soil,pore spaces Vapors move by convection toward the area of lowered air 1 } pressure (the extraction point) The efficiency of this process is proportional to soil porosity and rpermeability Qualitative information has been gained through examination of soil samples and the perforance of a vapor extraction pilot test Soil at this msite has a high clay content, making vapor extraction a more difficult and slow process i r y4 i 6 1 3 MONITORING ACTIVITIES Monitoring of vapor extraction systems involves weekly measurement of vapor hydrocarbon r r