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Y N r gasoline. That is, the more hydrophilic hydrocarbons will be. more likely to move into ground water, while the more volatile compounds are more likely to move into the vapor phase, and the compounds that are both less volatile and more hydrophobic are more likely to remain in the free product or be adsorbed to soils (Hinchee and Reisinger 1987) . Hydrocarbons not remaining in the free product will partition into either ground water or soil vapor and migrate as the result of a variety of interacting forces. In ground water, contami- _ Rants. will._migrate .with the ground-water flow, interacting_with the^rock or soil geological medium. As the contaminants pass through a medium, organic constituents in the medium interact with the contaminants, and some are adsorbed or bound to particle surfaces (Bruell and Hoag 1986) . The result is a net retardation in the velocity of movement of those compounds relative to that of the ground water in which they are dissolved. The process is analogous to laboratory chromatography. The compound with the least affinity for the porous medium is least retarded and there- fore moves most rapidly. This compound, then, is present at the leading edge of a contaminant plume. The affinity of a compound for the soil porous medium is partly a function of the compound's hydrophobicity--that is, the mora hydrophobic a compound the more likely it is to adsorb to the solid medium. Aqueous solubility is a good indicator of hydro- phobicity: the more soluble a compound is, the less hydrophobic and more hydrophilic it is, and vice versa. Vapor pressure is a good indicator of volatility; compounds with higher vapor pressures are more volatile. In determining the environmental fate of various. hsdrocarbon compounds in a hydrocarbon mixture such as gasoline, those which have a high vapor pressure are more likely to move into the vapor phase, or evaporate. Compounds with high solubility are more CHV1B:81J 3