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N. s h direction. At depth there is a migration from the west of poor- quality waters which underlie the Delta. GTI provided well locations in the report dated May 6, 1997. These locations have been reproduced in Figure 3. 1.4 PRINCIPLES AND APPLICATION OF SOIL VAPOR CONTAMINANT ASSESSMENT The soil vapor survey, or SVGA, technique takes advantage of the behavior of hydrocarbonhm _ mixtures and the physicoceical proper- .__• . _h _ - - ---ties of the •ind�+tdual 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 may begin to vaporize and solubilize. Like most hydrocarbon liquids, gasoline is a complex mixture of many compounds, each with its own physicochemical properties. The 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- carbon/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. It may be noted that concentration of total benzene, toluene, and xylenes in product-saturated water may exceed 14-24 mg/L (API 1985a) . Hydrocarbons will also volatilize into the air- or gas-filled soil interstices. Volatilization is largely a function of vapor pressure. The natures of the contaminant mixtures, in terms of specific component mixtures, in either the aqueous or vapor phase, are distinctly different from each other and from the CHV13:813 2