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release (or surface-release with migration downward ) , a chemical <br /> will partition between these three compartments. The parti- <br /> tioning is largely based on the physicochemical properties of the <br /> chemical. Hydrocarbons and other volatile chemicals ( i.e. , those <br /> with high vapor pressures ) will have a tendency to partition into <br /> the air-filled pores. Hydrophilic chemicals ( i.e. , those with <br /> high water solubilities ) will have a tendency to move into ground <br /> water. Chemicals that are both nonvolatile and hydrophobic are <br /> likely to remain as free product or to be adsorbed to soils <br /> ( Hinchee and Reisinger 1987 ) . <br /> The Henry' s Law constant of an organic chemical is directly <br /> related to the tendency of the compound to volatilize, as opposed <br /> to solubilizing. Figure 3 graphically illustrates the vapor <br /> pressure, aqueous solubility, and Henry' s Law constants, and <br /> their relationships, for selected hydrocarbons. The Henry' s Law <br /> constant is approximated here as the ratio of vapor pressure to <br /> solubility. Compounds with Henry' s Law constants greater than <br /> 0 . 001 (atm'm3/mole ) volatilize from water into air very rapidly <br /> (Lyman et al. 1982 ) ; those with Henry' s Law constants greater <br /> than 0. 01 (atm'm3/mole ) are generally volatilized so rapidly that <br /> they are seldom found in contaminated ground water. It may be <br /> observed (Figure 3 ) that tetraethyl lead (TEL) has an extremely <br /> low solubility and a relatively low vapor pressure. As a result, <br /> this constituent would not be expected to solubilize and migrate <br /> in ground water, and although its low vapor pressure would indi- <br /> cate slow volatilization, its Henry' s Law constant indicates that <br /> it may be more rapidly volatilized than solubilized. The fate of <br /> TEL would be expected to be long-term binding to the soil. <br /> On the basis of these properties it can be seen that associated <br /> with any ground water, soil, or free-product contamination is <br /> vapor phase contamination. The SVCA technique takes advantage of <br /> this, and through the collection and analysis of soil vapor per- <br /> mits a rapid, cost-effective delineation of the extent of contam- <br /> ination. This technique is especially good as a screening tool <br /> c40/90265 3 <br />