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r „ G t 04 March 1996 I AGE-NC Project No 95-0144 Page 8 of 18 In thermal oxidation systems, hydrocarbons are destroyed by open-flame combustion Natural gas 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 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 They 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 apprommately 700°F, requiring less supplemental fuel than ether thermal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal-combustion and thermal oxidizer systems 1 6 1 2 EFFECTIVE SOIL CONDITIONS Operation of the blower in a vapor extraction system creates a vacuum in the subsurface, inducing res aces Vapors move by convection toward the area of lowered air air flow through the soil pore p p , pressure (the extraction point) The efficiency of this pzocess is proportional to soil porosity and permeability Soil in the upper 18 feet at this site has a high clay content, making vapor extraction a more difficult However, soil below 18 feet and in the saturated zone consists of sands, where vapor extraction would have a greater effect Figures 4 and 5 are cross sections showing soil types at the site 1 6 1 3 MONITORING ACTIVITIES Monitoring ofvapor extraction systems involves weekly measurement of hydrocarbon concentrations In the vapor stream entering the destruction utiit (influent stream) and ex�t�ng the system (effluent stream) and periodic service and repair Systems can be equipped with automatic telecommunication system to ales t the opei ator of system malfunction,or failure Vapor samples should be collected monthly to-monitor the efficiency of the remediation program When the concentration of extracted vapors decreases and stabilizes after several months of operation, confirmatory borings are drilled to collect soil samples for quantitative analysis in the laboratory to document the status of the remediation effort 4 FEASIBILITY 61 w Generally, a vacuum of 0 10 inches of water is requij ed to successfully volatilize subsurface I