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11 February 2003 AGE-NC Project No. 98-0520 ' Page 8 of 21 concentrations approach the 40,000 parts per million - volume (ppmv) required for peak r perfoi=mance 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 do 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- of hydrocarbon 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 ' 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. ' 6.2.2. 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 above 20 feet and in the vadoze zone consists of sand,rendering vapor extraction as an effective remediation method. Soil between 20 and 35 feet has a relatively ' high clay and silt content, potentially making vapor extraction less effective for this zone. Figures 3A and 4A show subsurface soil types at the site. Advanced GeoEnvironmentat,Inc.