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27 December 1996 <br /> AGE-NC Project No 95-0143 <br /> f Page 6 of 19 <br /> supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements <br /> and limitations are similar to those for internal-combustion and thermal oxidizer systems <br />' 5 1 2 REQUIRED SOIL CONDITIONS <br />' Operation of the blower in a vapor extraction system creates a partial vacuum in the subsurface, <br /> inducing air"currents"through the soil pore spaces Vapors move by convection toward the area of <br /> lowered air pressure (the extraction point) The efficiency of this process is proportional to soil <br /> porosity and permeability Qualitative information has been gained through the examination of soil <br /> samples collected at the site Soil in the upper 18 feet has a relatively high clay content, potentially <br /> making vapor extraction less effective for this zone However, soil below 18 feet and in the saturated <br />' zone consists of sand, rendering vapor extraction as an effective remediation method for this zone <br /> Figures 3 and 4 depict cross sections, showing subsurface soil types at the site <br />' 513 MONITORING ACTIVITIES <br /> Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at <br /> the inlet to the destruction unit and periodic service and repair Systems can be equipped with an <br />' automatic telecommunication system to alert the operator of system malfunction or failure Vapor <br /> samples should be collected monthly for laboratory analysis to monitor the efficiency of the <br /> remediation program When the concentrations of extracted vapor have decreased and stabilized after <br /> a certain period of operation, confirmation borings are drilled to collect soil samples for quantitative <br /> analysis in the laboratory to document the degree of cleanup obtained <br />' 514 FEASIBILITY <br /> Generally, a vacuum of at least 0 10 inches of water is required to successfully volatilize subsurface <br /> hydrocarbons The radius of influence is often considered to be the distance from the extraction well <br /> at which a vacuum of at least 0 1 inches of water is observed (EPA - A guide for Corrective Action <br /> Plan Reviewers) A vapor extraction feasibility test has not been performed at the site, however, data <br /> obtained during the installation of soil borings and monitoring wells may be useful in determining <br /> a theoretical effective radius of influence Installation of extraction wells would be required at the <br /> site to implement an SVE system <br /> One drawback of SVE is that not all contaminated zones of soil are remediated effectively or at the <br />. same rate For instance, a sandy soil zone will cleanup more quickly than a silty one SVE is not <br /> usually effective in soil with high clay content Other drawbacks of SVE include ineffective <br />