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vacuum to desiccate previously saturated and partially saturated soils in the capillary fringe and smear <br /> zone As a result of exposure to soil vapor flow,capillary fringe and smear zone contamination can be <br /> volatilized and removed by the extraction system DPE can also expedite removal of saturated soil <br /> contaminants in the dewatered zone VOCs with limited water solubility and high affinity for soil carbon <br /> can be more effectively removed by exposure to soil venting and volatilization than by desorption and <br />' recovery in a groundwater extraction system <br /> DPE can accelerate treatment of dissolved groundwater contamination and free-phase product <br /> Groundwater and free product recovery rates are enhanced by the additive effects of hydraulic and <br /> pneumatic gradients generated by concurrent extraction of groundwater and soil vapors from the extraction <br /> well Thus,more rapid removal and treatment of contaminants is possible Vacuum also tends to <br /> counteract capillary forces holding LNAPL in soil pore spaces, enabling recovery of free-phase product <br /> that would not otherwise be extractable (Baker and Bierschenk 1995) <br /> 423 Soil Characteristics <br /> DPE is most effectively implemented in areas with saturated soils exhibiting moderate to low hydraulic <br /> conductivity(silty sands, silts, and clayey silts) Lower permeability soils enable formation of deeper water <br /> 1 table cones of depression,exposing more saturated soils and residual contamination to extraction system <br /> vapor flow <br /> I <br /> DPE systems installed in soils with higher hydraulic conductivities generally require higher equipment and <br /> operating costs for effective implementation due to higher water extraction rates and the resulting treatment <br /> and disposal requirements The more broad, shallow cones of depression formed in permeable soils may <br />' not adequately expose capillary fringe soils to soil venting Thus,soils remaining below the water table <br /> may act as a continued source of groundwater contamination until the slower process of desorption and <br />' removal by groundwater extraction is complete <br /> As with conventional groundwater extraction systems,depth of saturated soils to a confining medium <br /> 1 affects the ability of a DPE system to capture and remediate a groundwater plume <br /> 1 <br /> 4-4 <br />