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F" WA PRINCIPLES Or 5501L VAPOR C01ttTAMINANT ASSCSSKENT The Doll vapor survey, or GVCAf technique takes advantage of the behavior of hydrocarbon mixtures and the phynicochemical proper- tied of the Individual components In the subsurfaces. Following a subsurface gasoltne relcase, free product will migrate downwards towards the ground water, Dom* of the gasoline will volatilize# and none will adsorb to the aoila. In the case of a *pill of sufficient volume to exceed the soil binding capacity, free liquid will reach ground waterf at which point It will float and may begin to vaporize and Bolubilize. Like most hydrocarbon liquids, 9AA01ine in a cortylex mixture cl. many compounds, each with its own phyalcochemical properties. The Contaminanta found in ground water located beneath a layer of floating hydrocarbon arcs generally lot* hydrophobic and are generally found In concentrations proportional to the hydro- carbon/water partition coefficient (i.e., the relative solubility Of A given compound In the bulk hydrocarbon to its anlubility in water) and to their percent composition in the gasoline. It may be noted that concentration Of total benzene, toluene, and xylenes in product-saturated water may exceed 10-20 mg/L (API 1955) . Hydrocarbons will also volatilize Into the air- or gas-fllleo soil interstices. Volatilization is largely a function of vapor pressure. The natures Of the Contaminant aUtureaf in terms of APtCIfic component tLixtures# in either the aqueous or vapor phase, are distinctly different from-"Ch other and-from tivo-- gasoline. That is, the mora hydrophilic hydxocarbons will be *sorer likulY to mOY* Into ground water, while the more volatile 4-%mPCuAds Or* More likely to 00ve Into the vapor phase, and the c*vpmmdv that are both less volatile anal more hydrophobic are more likely to re in In tha free product or be adsorbed to soils (Hinch** and W-vinger 1987) . CAV82A.&PA