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1 ' 01 dune 1999 AGE-NC Project No 95-0137 Page 8 of 26 Thermal xidation system 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 1450°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 in extracted soil gas should range from 5,000 ppmv to 35,000 ppmv Thermal toxidation systems operate at lower noise levels than internal combustion systems catalytic oxidation sy, tems provide another option for treating hydrocarbon impacted soil gas, ' particularly after other extraction systems have reached their effective limits due to lowered hydrocarbon concentrations These systems operate at temperatures of approximately 650°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 ' 5 12 Suitabilityof Site Soil Conditions s Operation of the blower in a vapor extraction system creates a partial vacuum in the subsurface, inducing air"currents"through the soil pore spaces Soil gas moves toward the area of lowered air ' pressure (the extraction point) The efficiency of this process is proportional to soil porosity and permeability ' Information gained through the examination of soil samples collected at the site indicates that soil in the vadose zone (upper 15 feet) has a relatively high sand content, making vapor extraction effective for this zone Soil deeper than 15 feet bsg and in the saturated zone predominantly consists ' of sand and coarse sand to gravel, rendering vapor extraction an effective remediation method for the vadose portion impacted by hydrocarbons at the site Due to a recent rise of ground water at the site, the vertical range of the vadose zone has been reduced, although a portion of the impacted soil ' remains in sandy vadose soil, making soil vapor extraction suitable for remediation of that portion of the site Figures 3 and 4 depict cross sections, showing vertical and lateral distribution of subsurface soil types at the site and the top of the saturated zone ' 5 13 Monitoring Activities Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations in ' the extracted soil gas at the inlet to the vapor 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 Soil gas samples should be collected monthly for laboratory analysis for hydrocarbon vapor concentrations to monitor the efficiency of the remediation program When Advanced GeoEnvironmental,Inc 1