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' 20 January 2005 AGE-NC Project No 97-0343 ' Page 10 of 21 ' (oxides of carbon and hydrogen) are further broken down in a catalytic converter before being emitted to the atmosphere These systems require supplemental fuel, such as propane or natural gas, which can become expensive as vapor concentrations decrease below levels normally required for ' combustion Typically, Internal combustion engines work effectively where in-situ hydrocarbon concentrations approach the 40,000 parts per million-volume(ppmv)required for peak performance of the 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, this can be mitigated by placing the unit Inside an enclosure, such as a building ' 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 stream This type of system works best for low concentrations of hydrocarbons, but becomes ' expensive when high concentrations ofhydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon ' 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 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 I supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements and Iimitations are similar to those for internal-combustion and thermal oxidizer systems 9 12 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 above 50 feet and in the vadoze zone consists of sand and silts with some clay, rendering vapor extraction as an effective remediation method Soil between 20 and 35 feet(three extraction wells exist a this depth)has a relatively high sand content,potentially making vapor extraction more effective for this zone Figures 4 and 5 show subsurface soil types at the site IAdvanced GeoEnviron mental,Inc I