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.. 27 June 1996 AGE-NC Project No. 95-0118 Page l I of 23 expensive when high concentrations of hydrocarbon vapors necessitate frequent canister replacement ,., and disposal of spent carbon. f Thermal oxidation systems destroy hydrocarbons by open-flame combustion. Natural gas is +1 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 1;=� be slightly more expensive to purchase and install. However, they can operate at higher air flow rates, and can 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. These systems also 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. 7.1.2. REQUIRED SOIL CONDITIONS Operation of the blower in a vapor extraction system creates a partial vacuum in the subsurface, inducing air "currents" through 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 infonnation has been gained through examination of soil =� samples and the performance of a vapor extraction pilot test. Soil at this.site has a high sand content, which should make vapor extraction an effective and rapid process. I 7.1,3. MONITORING ACTIVITIES t.�a Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at the inlet to the destruction unit and periodic service and repair. Systems can be equipped with an t .? automatic telecommunication system to alert the operator of system malfunction or failure. Vapor samples should be collected monthly for laboratory analysis to monitor the efficiency of the remediation program. When the concentration of extracted vapors decreases and stabilizes after .-J _. several months of operation, confirmation borings are drilled to collect soil samples for quantitative analysis in the laboratory to document the degree of clean-up obtained. 4