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. For systems in which well depth exceeds applied vacuum (expressed in feet of water),priming may be necessary to induce vapor flow on startup Priming is achieved by inserting a tube into the extraction well below the water surface to introduce air flow into the well Reduced fluid column density resulting from introduction of air enables two-phase flow from the well Groundwater and soil vapors are extracted in the annular space between the primer tube and well casing After the well is pruned,vapor flow from the formation provides the air lift necessary to entrain water in the extracted stream Low permeability soils may require continued use of a pruner to maintain two-phase flow System hydraulics may facilitate the use of ambient air for pruning or may dictate the use of an air compressor to initiate air flow Injection of air into the well enhances formation of liquid droplets,which become entrained in the extracted ' soil vapor Priming may also be used to enhance mass transfer of DNAPLs by injecting air near the —confining layer(EPA 1994) _Well-screen entramment systems benefit from-the stripping action of ' high-extraction vacuum and turbulence in the extraction well and manifold piping Similar to drop-tube type extraction,water contaminant reductions of approximately 90 percent have been reported This type of DPE is the simplest to implement,however,it may have limited effectiveness for water removal from deep wells Extraction-well entrainment is most effective at sites with shallow groundwater (less than 10 feet bgs)(Brown and others 1994),but it has been used to depths of approximately 27 feet (Tetra Tech 1996c) ' Advantages of extraction-well entramment include simplicity of design and construction Because a common blower removes both soil vapor and groundwater,downhole pumps and associated controls and instrumentation are not necessary Systems that do not incorporate continuous priming require only one piping manifold (for extraction) Systems incorporating pruning,however,do require installation of an additional piping manifold as well as use of a compressor for air injection Cloggmg of the well screen can decrease extraction effectiveness (Brown and others 1994) Entrainment of rsilts and solids may occur in wells that are screened too coarsely or do not have properly sized or graded gravel pack Systems incorporating existing monitoring wells often require regular maintenance to remove accumulated fines from collection points,primarily the air/water separator • 4-12