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► <br /> liquid will reach ground water, at which point it will float and <br /> ► may begin to vaporize and solubilize. <br /> Like most hydrocarbon liquids, gasoline is a complex mixture of <br /> many compounds, each with its own physicochemical properties. <br /> The contaminants found in ground water located beneath a layer of <br /> ► floating hydrocarbon are generally less hydrophobic and are <br /> generally found in concentrations proportional to the hydro- <br /> carbon/water partition coefficient ( i.e. , the relative solubility <br /> of a given compound in the bulk hydrocarbon to its solubility in <br /> ► <br /> water ) and to their percent composition in the gasoline. Hydro- <br /> carbons will also volatilize into the air- or gas-filled soil <br /> interstices. Volatilization is largely a function of vapor <br /> ► <br /> pressure. The natures of the contaminant mixtures, in terms of <br /> specific component mixtures, in either the aqueous or vapor <br /> ► phase, are distinctly different from each other and from the <br /> gasoline. That is, the more hydrophilic hydrocarbons will be <br /> ► more likely to move into ground water, while the more volatile <br /> compounds are more likely to move into the vapor phase, and the <br /> compounds that are both less volatile and more hydrophobic are <br /> more likely to remain in the free product or be adsorbed to soils <br /> (Hinchee and Reisinger 1987 ) . <br /> ► <br /> Hydrocarbons not remaining in the free product will partition <br /> ► into either ground water or soil vapor and migrate as the result <br /> of a variety of interacting forces. In ground water, contami- <br /> nants will migrate with the ground-water flow, interacting with <br /> the rock or soil geological medium. As the contaminants pass <br /> through a medium, organic constituents in the medium interact <br /> with the contaminants, and some are adsorbed or bound to particle <br /> surfaces ( Bruell and Hoag 1986 ) . The result is a net retardation <br /> in the velocity of movement of those compounds relative to that <br /> of the ground water in which they are dissolved. The process is <br /> ► analogous to laboratory chromatography. The compound with the <br /> least affinity for the porous medium is least retarded and there- <br /> 7 <br />