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J least. affinity for the porous medium is least retarded and there- fore moves most rapidly ) . The affinity of a compound for the soil medium is partly a function of a compound for the soil medium is partly a function of the compound ' s hydrophobicity. That is, the more hydrophobic a compound is ,the more likey it is to adsorb to the solid medium. Aqueous solubility is a good indicator of hydrophobicity: the more soluble a comopund is, the less hydrophobic and more hydrophilic it is, and vice versa. Compounds dissolved in ground water will tend to volatilize from the aqueous phase. The Henry ' s Law constant is the equilibrium ratio of a compound ' s concentraton in the vapor phase to its concentration in the aqueous phase. The higher a compound ' s Henry ' s Law constant, the greater its tendency to volatilize from water into air . Figure 1 -3 graphically illustrates the vapor pressure, aqueous . solublity, 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. Compounds with Henry' s Law constants greater than 0 . 001 atm.m3/mole volatilize from water into air very rapidly (Lyman et al. 1982 ) ; those with Henry' s Law con- stants greater than 0 . 01 atm.m3/mole are generally volatilized so rapidly that they are seldom found in ground water near a gaso- line release but are observed in close proximity to free product. It should be noted that tetra ethyl lead (TEL ) has an extremely low solubility and a relatively low vapor pressure. As a result, it would not be expected to solubilizy and migrate in ground water, and although its low vapor pressure would indicate slow volatilization, its Henry ' s Law constant indicates that it may be more rapidly volatilized than solubilized. The fate of TEL would be expected to be long-term binding to the soil . Frequently at sites which had a hydrocarbon release two distinct soil vapor plumes may be detected. The vapor plume arising from 3