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I 28 April 1995 AGE-NC-95-0103 Page G of 1 I 1 In a carbon-canister system, hydrocarbon vapors are routed through activated carbon filters, %%,here adsorption of hydrocarbons takes place, removing them from the vapor stream These ' systems .%ork- best for low concentrations of hydrocarbons, but become expensive when high concentrations of hydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon In thermal oxidation ,systems, hydrocarbons are destroyed by open-flame combustion Natural gas Y is commonly used as supplemental fuel, heating the extracted vapors to a combustion temperature ' of approximately 1400 °F Relative to internal combustion systems, these systems may be slightly more expensive to purchase and install Ho�%e%er, they can operate at higher air flow rates, and I therefor remediate sites at a faster rate For this system to be effecti"e, hydrocarbon concentrations should range f3 om 5,000 ppniv to 30,000 ppmv They also operate at much lower noise lexels than internal combustion systems Catalytic oxidation units provide another option for treating vadose-zone contamination, particularly af3er other extraction systems have reached their effective limits due to lowered I h)di ocai bon concentrations These systems operate at temperatures of approximately 700 °F, requii in�tZ less supplemental fuel than either thermal oxidizers or internal combustion engines Other iequiiements and limitations are similar to those for internal-combustion and thermal oxidizer sN stems 5 1 2 REQUIRED SOIL CONDITIONS Opei anon of the blower in a vapoi extraction system ci eates a vacuum in the subsurface, inducing Iair "cui i eats" through pore spaces Vapors move by convection toward the area of lowered air pressuie (the extraction point) The eflicienc\ of this process is proportiopal to soil porosity and pei meability Qualitative information has been gained through examination of soil samples and the pei formance of a vapor extraction pilot test The information indicates that soil conditions at the site ai e adequate for operation of a vapor extraction s)stem I 5 1 3 EXTRACTION WELLS At least one extraction well is required in a tiapor extraction system Well VW-1 was installed for this pui pose This well was sci eened through the contamination plume (see section 2 2) The I results of the vapor extraction test indicate that one well should be adequate to reach the pei imeter of the hydi ocarbon plume (see section 5 1 1 )