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1. H. KLEINFELDER &ASSOCIATES <br /> drawn off into the vapor phase, where thAy are induced to migrate to a <br /> single collection point by an engineered, subsurface pressure gradient. <br /> Vapor phase extraction techniques remove volatile organic chemicals from <br /> sail by continuously exhausting these substances from the gaseous phase, <br /> thereby naintaining a large negative chemical gradient between the gaseous <br /> Phase and the liquid and solid phases. This large negative gradient <br /> accelerates the transfer of volatile organic chemicals from liquid and <br /> solid phases into the gaseous phase (Hoag, 1982). The continuous <br /> replacement of these contaminated gases with clean air results in a net <br /> reduction of volatile organic chemical concentrations in the soil matrix. <br /> Vapor extraction is estimated to be relatively insensitive to preferential <br /> air flow patterns through the zone of contaminated soil. If preferential '. <br /> air flow occurs and some zones are treated more effectively than others, <br /> these areas will have lower contaminant concentrations and the adjacent �r <br /> untreated subsurface contamination will tend to. be drawn toward these I _ <br /> areas by diffusion, in response to the concentration gradient. <br /> 2.2 EFFECTIVENESS OF THE TECHNOLOGY JJ <br /> i <br /> Vapor extraction is most effective for the !dost volatile constituents and <br /> least effective, in a relative sense, for the less volatile constituents <br /> (Dunlap, 1984). However, vapor extraction is judged to be an appropriate <br /> response to hydrocarbon contamination or volatile constituents, for the <br /> following reasons: <br /> a The most volatile constituents are generally those that pose the <br /> greatest environmental risk; <br /> f <br /> --- ------- .._.-... __--. _o -Secondary---biolog Kcal processes are 3ikely beinduced <br /> by <br /> subsurface soil aeration; <br /> J1. <br /> 41-87-137 4 <br />