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. a s <br /> f . <br /> i <br /> compounds are more likely to move into the vapor phase, and the <br /> r compounds that are both less volatile and more hydrophobic are <br /> j more likely to remain in the free product or be adsorbed to soils <br /> (Hinchee and Reisinger 1987) , <br /> i <br /> j Hydrocarbons .not remaining in the free product will partition ; <br /> into either ground water or soil vapor and migrate as the result A' <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 orj' soil geological medium. As the contaminants piss ' <br /> ;4 - through a Medium, organic constituents in the medium interact F <br /> i with the cr:intaminants, and some are adsorbed or bound to particle <br /> surfaces .(Bruell and Hoag 1986) . The result is a net retardation <br /> j in the velocity of movement of those compounds relative to that <br /> `� of the gr6und water in which they are dissolved. The process is <br /> analogous to laboratory chromatography. The compound with the' <br /> t <br /> �A least affinityfor the <br /> porous medium is least retarded and <br /> therefore moves most rapidly. This compound, then, is present at <br /> the leading edge of a contaminant plume. <br /> ,f <br /> The, aff,i.nity of a compound for the soil porous medium is partly a <br /> function of the compound's hydrophobicity----that is, the more ` <br /> hydrop�iobic a compound the more likely it is to adsorb to the <br /> solid,medium. Aqueous solubility is a good indicator of hydro- <br /> , <br /> phobicity: the more soluble a compound is, the less hydrophobic 1 <br /> and more hydrophilic it is, and vice versa. vapor pressure is a' <br /> good "indicator of volatility; compounds with higher vapor <br /> - - <br /> } <br /> -:pressures are-more volatile. } - <br /> In: determining" the environmental fate .of various hydrocarbon <br /> t t. <br /> compounds. in a hydrocarbon mixture such as gasoline., those which <br /> Ai <br /> `I have a high vapor:pressure are more -likely to .move into the:vapor F' <br /> Lphase, or evaporate. Compounds with high solubility are more <br /> likely to move into ground water f._om the free product and, once . r <br /> in ground water, tend to move more rapidly. Compounds of low <br /> R vapor pressure and low solubility tend to remain in the free <br /> EXXON9:84L 9 <br /> l <br />