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+ r Potential drinking water source and several city and private f wells exist within a mile radius of the site. These wells are reportedly in use daily (Cox 1988) . 1.4 PRINCIPLES AND APPLICATION OF SOIL VAPOR CONTAMINANT ASSESSMENT The soil vapor survey, or SVCA, technique takes advantage of the behavior of hydrocarbon mixtures and the physicochemical proper- ties of the individual components in the subsurface. Following a subsurface gasoline release, free product will migrate downwards towards the ground water, some of the gasoline will volatilize, and some will adsorb to the soils. In the case of a spill of sufficient volume to exceed the soil binding capacity, free liquid will reach ground water, at which point it will float and *nay begin to vaporize and solubilize. Like most hydrocarbon liquids, gasoline is a complex mixture of many compounds, each with its own physicochemical properties. sThe .contaminants found in ground water located beneath a layer of floating hydrocarbon are generally less hydrophobic and are generally found in concentrations proportional to the hydro- ca.rbon/water partition coefficient (i.e.,. the relative solubility of a given compound in the bulk hydrocarbon to its solubility in water) and to their percent composition in the gasoline. Itmay be noted that concentration of total benzene, toluene, and xylenes in product-saturated water may exceed 10-20 mg/h .(API r. 1985a) . 3 - s , i Hydrocarbons will also volatilize into the .air- or gas-filled r soil interstices. Volatilization is largely a function of vapor pressure. The natures of the contaminant mixtures, in terms of i Specific component mixtures, in either the aqueous or vapor Phase, are distinctly different from each. other and from the F gasoline. That is, the more .hydrophilic hydrocarbons will be more likely to move into ground water, while the more volatile EXXON9:84L g i