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r <br /> r <br /> sufficient diameter and adequate design characteristics (appropnately-sized screen slots)can be converted <br /> rfor use as extraction wells Downhole pump systems generally require larger diameter wells than either <br /> well-screen or drop-tube entrainment systems <br /> r <br /> Full-scale DPE systems have been mstalled to approximately 100 feet below ground surface (bgs) <br />' Specific limits on well installation depth have not been reported <br /> Extraction well spacing is determined by results of pilot testing and by remedial objectives For sites with <br /> dissolved-phase contamination,well spacing is largely based on the groundwater capture radius,or the <br /> distance from a well where drawdown is sufficient to overcome the regional water table gradient <br /> ' (Hackenberg and others 1993) Extraction well spacing must provide for adequate dewatering of the <br /> contaminated area—Well spacing in areas with free product is generally based on the product capture zone <br /> rof influence(Tetra Tech 1996b)or the distance from the well where the slope of the free water surface <br /> approaches zero (LNAPL will theoretically not flow toward the well beyond this distance [Hackenberg <br /> ' and others 1993] ) For highly contaminated vadose zone source areas,well spacing may be based on an <br /> SVE design zone of influence The SVE design zone of influence is generally more conservative than the <br /> actual zone of influence obtained during pilot testing and is selected to achieve accelerated remediation of <br /> vadose zone soils <br /> 1 <br /> High vacuums associated with DPE systems may promote short circuiting of air flow at the wellhead from <br /> ' ground surface,particularly in shallow formations This problem can be circumvented by use of a surface <br /> seal Surface seals are typically constructed by placing an impermeable liner over the extraction area <br /> 43.1.3 Extraction Equipment Design <br /> ' Typical components of the extraction system include an extraction blower,vapor/liquid separator,vapor <br /> phase treatment, and liquid phase treatment Design of extraction system equipment is generally based on <br /> ' desired extraction vacuum, anticipated vapor and groundwater extraction rates, and anticipated vapor and <br /> groundwater concentrations and compositions <br /> The vapor extraction rate from each extraction well is dictated by local soil pneumatic characteristics,well <br /> design and screen length, and applied system vacuum Overall vapor extraction system capacity is <br /> 4-7 <br /> 1 <br />