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d Man jai likely to move into ground waiter from the free product ants, 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,..Eo xo.Otilize_.from_water,into .� air........_......... . _ . Figure 4 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 Lays constant is directly related to the tendency of compounds to volatilize, as opposed to solubilizing. 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 constants greater than 0.01 (atm- m3/mole) are generally volatilized so rapidly that they are seldom found in gasoline-contaminated ground water. it may be ohnerved (Figure 4) that tetraethyl lead (TEL) has an extremely low solubility ani+ a relatively low vapor pressure. As a result, this constituent would not be expected to solubilize 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. CHV18:813 4