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E 20 March 1996 AGE-NC Project No 95-0130 Page 11 of 23 E6 1 2 EFFECTIVE SOIL CONDITIONS Operation of the blower in a vapor extraction system creates a vacuum in the subsurface, inducing air flow 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 examination of soil samples and the performance of a vapor extraction pilot test Soil at this site has a high silt and clay content, making vapor extraction a more difficult and slow process 6 1 3 MONITORING ACTIVITIES Monitoring of vapor extraction systems involves weekly measurement of hydrocarbon concentrations E in the vapor stream entering the destruction unit (influent stream) and exiting the system (effluent stream) and periodic system service and repair Systems can be equipped with an automatic telecommunication system to alert the operator of a 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 several months of operation, confirmatory borings are drilled to collect soil samples for quantitative analysis in the Elaboratory to document the status of the remediation effort E614 FEASIBILITY Generally, a vacuum of 0 10 to 10 inches of water is required to successfully volatilize subsurface hydrocarbons The maximum vacuum measured at VW-1, VW-2 and VW-3 during the pilot test was plotted versus the distance of each well from the extraction well (Figure 10) The effective radius of influence(theoretical)is determined by drawing a best-fit line through these data points and dropping a line from the appropriate vacuum to the corresponding distance from the extraction well A clear correlation of induced vacuum verses distance was not observed for this site However, the results of the vapor extraction test indicate that a moderate radius of influence can be achieved Based upon data obtained during the pilot test, the theoretical radius of influence at the site appears to be approximately 30 feet However, the effective radius of influence is more likely to be around 20 feet A strong vacuum will be required to create significant flow rates at this site A drawback of SVE is that not all contaminated zones of soil are remediated effectively A second drawback is that different soil types may remediate at differing rates For instance, a sandy soil will clean-up more quickly than a silty soil, SVE is not usually as effective in soil with a high clay content Other drawbacks of SVE include ineffective treatment of soil lying below ground water or within the "smear" zone and ineffective treatment of longer-chain hydrocarbons Ground water remediation would need to be conducted concurrently with SVE to facilitate remediation of the smear zone