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These factors, and the design basis established to address these factors, are <br /> discussed in following subsections. <br /> Soil Vapor Extraction Rate and Expected Radius of Vacuum Influence <br /> Site soil, to a depth of approximately 25 feet, consists predominantly of <br /> silty and clayey soils of low permeability. To induce a subsurface flow in <br /> this low permeability soil and to achieve effective soil vapor extraction, a <br /> high vacuum of approximately 20 inches Hg will be applied to each dual <br /> phase extraction well. Pilot testing conducted at the site indicates that <br /> application of 20 inches Hg vacuum will establish a radius of vacuum <br /> influence (RVI) of approximately 70 feet within the vadose zone. <br /> To provide comprehensive coverage of the vadose zone soil VOC plume, <br /> eight dual phase extraction wells and three soil vapor extraction (SVE) <br /> wells will be installed. The locations of these extraction wells, along with <br /> the expected cumulative RVI is shown in Drawing G-2. As shown in <br /> Drawing G-2, the dual phase extraction wells are located such that the <br /> associated RVIs overlap. This overlapping prevents the formation of <br /> subsurface regions at the site which could be unaffected by the applied <br /> vacuum. <br /> SVE wells will be installed within two isolated areas of vadose zone soil <br /> VOC impacts, as shown in Drawing G-2. Two SVE wells will be located in <br /> the area of soil borings 15, 23, 24 and TYB1. One SVE well will be located <br /> in the vicinity of MW-13A. <br /> For an applied vacuum of 20 inches Hg to the extraction wells, the <br /> anticipated soil vapor flow rate is expected to be at least 15 standard cubic <br /> feet per minute (scfm) from an extraction well, based on pilot test data. <br /> The expected operating scheme for vapor extraction is for four to five <br /> wells to operate at any time, with non-operational vapor extraction wells <br /> serving as passive air induction points. Therefore, the dual phase <br /> extraction system has been designed for a total soil vapor influent <br /> flowrate of 75 scfm. <br /> Each dual phase extraction well and soil vapor extraction well will be <br /> screened, starting at approximately 5 feet bgs. Screened intervals for dual <br /> phase extraction wells will extend to 5 feet below the static water table, <br /> while the screened intervals for the SVE wells will extend to <br /> approximately 10 feet bgs. To prevent short-circuiting from the ground <br /> surface (especially in the area of dual phase extraction), due to the shallow <br /> 4-2 <br />