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CLEARWATER <br /> G R O U P, I N C. <br /> Ennrron Wren ta! serz,"s <br /> least equal in thickness to this drawdown, it is not thought that groundwater <br /> extraction will result in smearing a deeper, clean saturated interval. <br /> Clearwater appended articles to June 5, 1996 correspondence, which discussed <br /> advantages of dual-phase extraction via bioslurping and enhanced vapor recovery <br /> technologies. The intent of this was not to propose these remedial technologies for <br /> the referenced site, but to demonstrate the general efficacy of a dual-phase extraction <br /> approach, which was questioned in earlier correspondence by the PHS/EHD. <br /> Although these are proven technologies, they are inappropriate for this site. A brief <br /> discussion of these technologies and how they relate to the is provided below. <br /> Bioslurping is a preferred remedial option at sites with relatively thick layers of SPIE <br /> where the remedial goal is to quickly and efficiently remove the SPH. A vacuum <br /> stinger is placed downwell, just above the SPH surface, and a mixture of SPH, water, <br /> and vapor is vacuumed from the well. Bioslurping is advantageous because it <br /> produces less water than conventional dual-phase extraction, thus reducing the <br /> volume of water for disposal. Bioslurping is not appropriate for the site because <br /> there is not a thick layer (i.e. >1 foot) of SPH beneath the site and remedial goals go <br /> beyond simply removing SPH. Application of bioslurping would in effect equate <br /> with conventional soil vapor extraction at this site. This would lead to an <br /> upwelling cone of product and/or water within the extraction well, thus occluding <br /> available screen for vapor extraction and creating a local hydraulic gradient away <br /> from the well, which could spread contamination across the site. <br /> The difference between bioslurping and enhanced vapor recovery, is that in <br /> enhanced vapor recovery the stinger placed below the static water surface. The well <br /> is then nearly vacuumed dry. This technology is suited to sites with very tight soils <br /> and used to enhance groundwater recovery rates for wells which yield little water <br /> (i.e. <0.5 gpm). Enhanced groundwater recovery is not warranted at the subject site, <br /> and would only increase volumes of water for disposal. <br /> The discussion above indicates that although these two technologies are useful, they <br /> are not well suited to the subsurface conditions at the site. Clearwater still believes a <br /> dual-phase groundwater and soil vapor extraction system is the appropriate <br /> remedial approach. Pilot testing indicated that groundwater pumping during soil <br /> vapor extraction, allowed for greater soil vapor flow rates and contaminant <br /> recovery. Sole vapor extraction may result in upwelling of water in the silts and <br /> occlusion of well screen, thus reducing vapor flow, radius of influence, and rates of <br /> contaminant removal. Use of total fluids pumps in the dual-phase wells will also <br /> aid the capture and removal of SPH, along with soluble-phase contaminants at the <br /> site. <br /> Ruiz.449 1.W.DurhamFerry.Tracy.CA 4 September 17, 1996 <br />