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1 <br /> reducing turbulent conditions within the extraction tube, and thus <br /> minimizing VOC stripping from ground water. If the extraction tube is <br /> placed at too great a distance above the water table, a lower rate of <br /> ground water extraction will occur, thus decreasing the ground water <br /> capture zone. <br /> Therefore, during extraction tube positioning, measurements for air <br /> velocity and pressure will be collected. These measurements will be <br /> converted to flow measurements, based on the extraction tube diameter. <br /> Vacuum measurements will also be collected at surrounding soil gas <br /> monitoring points to ensure that the necessary RVIs to vacuum are <br /> achieved. <br /> Dual Phase Extraction Well and Soil Vapor Extraction Well <br /> Operation <br /> It is anticipated that extraction wells will be operated in a pulsed fashion <br /> to maximize soil vapor extraction efficiency. The pulsed operation will <br /> consist of operating 4 to 5 dual phase extraction wells at 20 inches Hg, and <br /> monitoring concentrations of VOCs in extracted soil vapor. As the <br /> measured VOC concentrations at an extraction well stabilize over time (as <br /> defined by VOC concentration reductions in soil vapor to 30 percent of the <br /> initial concentration), the extraction well will be shut-down, or the draw <br /> tube will be lowered into the water table such that only ground water is <br /> extracted for a certain recovery time. The recovery time will allow VOCs <br /> adsorbed to soil particles to desorb from the soil as the subsurface <br /> achieves an equilibrium condition. Following the recovery time, the dual <br /> phase extraction well will be returned to a soil vapor extraction operating <br /> condition. <br /> During the initial period of dual phase extraction well operation, it is <br /> anticipated that four to five dual phase extraction wells will be operated in <br /> a vapor recovery mode for one month, followed by a two week downtime <br /> period. During this downtime period, the previously non-operational <br /> dual phase extraction wells may be operated in the vapor recovery mode, <br /> and additionally, one or more of the SVE wells may be operated in <br /> conjunction with the dual phase extraction wells. It is expected that as soil <br /> vapor extraction continues, the duration of vapor recovery mode <br /> operation will decrease. <br /> To establish subsurface soil vapor flow pathways, non-operational dual <br /> phase extraction wells may be used as passive air induction wells. This <br /> 6-2 <br />