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i] extraction manifold piping system to an air/water separator,from which vapors are routed to a treatment Isystem Groundwater drawn off the separator is treated (if necessary)before discharge A schematic of a drop-tube entrainment extraction well is presented in Figure 4-2 During startup of an extraction system incorporating drop tubes,it may be necessary to prize the Iextraction well with air to induce vapor flow through the drop tube if well depth exceeds the applied vacuum (expressed in feet of water) Priming involves the introduction with air into the bottom of the drop tube when it is below the water level in the well to create an au'-hftmg effect Self-pruning drop-tube designs have been developed to enable automatic pruning of the system upon startup One patented I drop-tube design incorporates single or multiple perforations,which enable vapor flow to reduce fluid column density in the well,thus allowing air lift of water from depths greater than the applied vacuum (expressed in feet of water)(Tetra Tech 1996b) -Another method involves insertion of an air-bleed tube exposed to atmospheric or compressed air inside the drop tube(Hackenberg and others 1993) Manual priming can be conducted by slowly lowering the drop tube into the extraction well,entraining water at the water level interface until the well is dewatered to design-tube extraction depth As the extraction area is dewatered during operation of a drop-tube type system,increases in saturated zone thickness and soil vapor flow are accompanied by a decrease in manifold vacuum at the vapor/hquid separator As a result,unbalanced conditions may occur in which vacuum at some extraction wells drops below that required to entrain water Water column buildup in these wells may cut off vapor flow and Iresult in short circuiting Rebalancing of system vacuums may be necessary to restore vapor flow to all extraction wells vapor Liquid and va r removal in drop-tube type systems is limited by pressure loss through the drop tube q ' Wellhead vacuum may be reduced by as much as 30 to 50 percent through the suction tube (Brown and others 1994) The ability of a drop-tube system to air lift groundwater from a given depth is a function of applied wellhead vacuum in the annulus between the drop tube and well screen,the air and groundwater flow rates, and the inner diameter of the drop tube (Stenning and Martin 1968) 1 Drop-tube type systems are generally inefficient for high flaw rate groundwater removal and are more effective in soils with low hydraulic conductivity and low groundwater yield Generally,extraction well ' yields of 5 gallons per minute (gpm)or less are considered suitable for entrainment extraction Within a 4-10 I 1