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range of approximately 5 to 20 gpm,use of entrainment extraction may be appropriate based on ' site-specific factors and design goals At higher water extraction rates,vacuum pump energy requirements increase, and downhole-pump systems may be more appropriate During the extraction process,contaminant mass transfer occurs from the liquid to the vapor phase because high system vacuum,high vapor/liquid ratio,and turbulence in the suction tube and extraction piping manifold This "stripping" action results in reduced extracted groundwater contaminant concentrations and enables more efficient vapor-phase treatment of the contaminants Groundwater treatment requirements may be reduced or potentially eliminated Reported stripping efficiencies of approximately 90 percent are common Use of a drop-tube type system minimizes DPE equipment as well as instrumentation and controls requirements A common blower extracts water and vapors, thus,no downhole pump is necessary for groundwater removal Only one piping manifold is required to transport the extracted media to the ' treatment system Existing monitoring wells can be converted to drop-tube type wells Removal of free product using a drop-tube entrainment extraction system may be complicated by poor water quality or high hardness content Emulsification of free product and water can occur in the air/water separator discharge pump or in the vacuum pump if they are situated upstream of the vapor/liquid separator(Tetra Tech 1996b) Hard water can also cause scaling in extraction system piping and equipment Several patents apply to various aspects of drop-tube type entrainment extraction,some of which may have overlapping features Patent holders include Xerox Corporation, International Technologies Corporation, and James Malot of Terra Vac Incorporated 4.3.2 2 Well-Screen Entrainment In well-screen entrainment DPE systems,vacuum is applied to a well screened in the vadose and saturated zones Vapor flow aspirates groundwater at the well screen for entrainment of groundwater Generally, small diameter wells(2 inch or less) are most effective for this type of DPE (Brown and others 1994), although 4-inch well screens can be used (Tetra Tech 1996c) 4-11