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1 ' 18 February 1997 AGE-NC Project No 95-0104 ' Page 8 of 22 ' 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 insonnation has been gained through laboratory analysis of soil ' samples collected at the site Av -permeability at the site was approximately 6 x 10', while porosity percentage at the site was approximately 43 10 percent Generally, soil beneath the former UST area has a relatively high silty sand content in the upper thirty-feet, potentially making vapor ' extraction an effective remedial method for this zone Furthermore, soil between thirty and eighty- feet has a relatively high clay and silt content, potentially making vapor extraction less effective for this zone Figures 3 and 4 depict cross sections, showing subsurface soil types at the site An SVE pilot test was performed at the site, results of the pilot test are summarized in Section 3 0 One drawback of SVE is that not all contaminated zones of soil are remediated effectively or at the same rate For example, 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 Ground water remediation may have to be conducted concurrently with SVE to facilitate remediation of the smear zone 6 1 3 MONITORING ACTIVITIES Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at the unlet to the destruction unit and periodic service and repair Systems can be equipped with an I automatic telecommunication system to alert the operator of system malfunction or faiEuic Vapor samples should be collected monthly for laboratory analysis to monitor the efficiency of the remediation 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 I614 FEASIBILITY Generally, a vacuum of at least 0 10 niches of water is required to successfully volatilize subsurface hydrocarbons The radius of influence is often considered to be the distance from the extraction well at which a vacuum of at least 0 1 inches of water is observed (EPA - A guide for Corrective Action IPlan Reviewers) A vapor extraction feasibility test was performed at the site in April 1995 Data obtained during the SVE test has determined a theoretical effective radius of influence at I approximately 28 feet when a transmitted vacuum of 0 1 inches of water is utihzed as a cut-off point (Figure 5 - Effective Radius of Injhience) Based upon the results of the pilot test, it appears that SVE would be an effective remediation alternative for soil at the site I