SITE INFORMATION AND CORRESPONDENCE_FILE 1

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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 more _ 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 hydrocarbon 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 likely to move into ground water from the free product and, once in ground water, tend to move more rapidly. Compounds of low vapor pressure and low solubility tend to remain in the free product or be adsorbed to the solid matrix and remain relatively immobile. — Dissolved compounds will tend to volatilize from the aqueous phase. The Henry ' s Law constant is the equilibrium ratio of a compound' s concentration in the vapor phase to its concentration in the aqueous phase. The higher a compound' s Henry' s Law con- stant, the greater its tendency to volatilize from water into air. — Figure 3 graphically illustrates the vapor pressure, aqueous — solubility, and Henry' s Law constants, and their relationships, for selected hydrocarbons typically found in gasoline. The — Henry ' s Law constant is approximated here as the ratio of vapor pressure to solubility. The Henry ' s Law constant is directly related to the tendency of compounds to volatilize, as opposed to solubilizing. Compounds with Henry' s Law constants greater than 8