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i u .. 22 November 2006 AGE-NC Project No. 95-0167 F Page 7 of 15 5 . 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 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 the examination of soil 1 samples collected at the site. Soil in the upper 15 feet has a relatively high clay and silt content, potentially making vapor extraction less effective for this zone. Soil below 15 feet and in the saturated zone consists of sand, rendering vapor extraction as an effective remediation method. Figures 3 depicts a cross section, showing subsurface soil types at the site. 5 . 1 .3 . MONITORING ACTIVITIES _ Monitoring of vapor extraction systems involves weekly measurement ofvapor concentrations at the f inlet to the 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. Vapor samples should be collected monthly for laboratory analysis to monitor the efficiency of the lremediation program. When the concentrations of extracted vapor have decreased and stabilized after a certain period of operation, confirmation borings are drilled to collect soil samples for quantitative analysis in the laboratory to document the degree of cleanup obtained. � J 5 . 1 .4. FEASIBILITY ' I Generally, a vacuum ofat least 0. 10 inches ofwater is required to successfully volatilize subsurface hydrocarbons. The radius of influence is often considered to be the distance from the extraction well J at which a vacuum of at least 0 . 1 inches of water is observed (EPA - A guide for Corrective Action Plan Reviewers). A vapor extraction feasibility test has not been performed at the site, however, data obtained during the installation of soil borings and monitoring wells may be useful in determining a theoretical effective radius of influence. Installation of extraction wells would be required at the site to implement an SVE system. Due to the lateral and vertical extent of contamination located beneath the main building on the property, SVE is the best way to access and remediate the entire plume, successfully. I One drawback of SVE is that not all contaminated zones of soil are remediated effectively or at the €- same rate. For instance, a sandy soil zone will cleanup more quickly than a silty one. SVE is not usually effective in soil with 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 (i.e., diesel, motor oil). Ground water remediation should be conducted Advanced GeoEnvironmental, Inc. Id