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• frequently determined by multiplying the vapor extraction rate for a single well (as determined through pilot testing or software modeling)by the total number of wells to be installed 1 The groundwater extraction rate is affected by water drawdown within the well itself and soil hydraulic characteristics as well as the applied system vacuum Lowering the water table at the well creates a ' hydraulic gradient,which induces groundwater(and free product, if any)flow into the well Vacuum applied at the point of water extraction introduces an additional pneumatic gradient,which can enhance the ' overall rate of groundwater and free product recovery ' The system groundwater treatment capacity is generally determined by multiplying the groundwater extraction rate for a single well by the total number of wells to be installed Data from additional aquifer ' testing or existing operating extraction wells within the treatment system area may also be incorporated into assessment of system groundwater treatment capacity requirements The free water surface in the vicinity of a DPE well is a combination of the cone of depression resulting from groundwater extraction and the upwellmg caused by vacuum extraction The shape of the free water surface is critical at sites requiring remediation of capillary fringe soils Vapor and groundwater extraction rates must be balanced to ensure that the free water surface elevation at any distance from the well does not ' rise above static water levels as a result of excessive vapor extraction system influence(Hackenberg and others 1993) Vacuum requirements largely dictate the type of vacuum blower or pump incorporated into the extraction system Applied DPE vacuums can range up to 28 inches of mercury(approximately 32 feet of water) Types of vacuum pumps commonly used at DPE sites include liquid nng pumps,rotary lobe compressors, ' and regenerative blowers Vacuum pumps are selected based on desired operating characteristics (inlet flow rate and achievable vacuum)and desired efficiency Lower vacuums tend to be associated with downhole pump type systems,which are more common at sites with higher yielding aquifers High vacuums are more common at sites using well-screen entrainment and drop tube entrainment ' Vapor/hqurd separation is generally accomplished upstream of the vacuum blower or pump but can be accomplished downstream of a liquid rung vacuum pump,which can use extracted water as seal fluid if ' generated in sufficient quantities Use of extracted water for seal fluid generally requires close monitoring 4-8