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AUI0H0TI'._ ;;ASOLUIE Some residual gasoline is left behind in the area through which - the gasoline has percolated; the residua tends cc be more concentrated In fixe sand than in the coarser materials (2329) . Solubilized gasolir.^ components may leach from residually contaminated soils for J long F . :ods .of time. Induced soil venting has been demonstrated to be a rapid and efficient method for removal of gasoline trapped in soils following a spill or leak (2320) . The importance of subsurface volatilization of gasoline components has also been demonstrated in an _ article by Yaniga (2330) . Volatilization of gasoline components from residual contamination and contamination accumulated at the ground-vater interface resulted in detection of gasoline vapors in nearby basements. The organic layer floating on the ground water is carried in the general direction of ground water flow. At the oil-water interface, some hydrocarbons are leached according to their aqueous solubility. As discussed in Chapter 64, the pollution caused by the hydrocarbon phase is much less extensive (10s-1008 of meters) than pollution caused by hydrocarbons dissolved in ground water (100s-10008 of meters) (1811) . Furthermore, the pattern of migration of the hydrocarbon phase may be very different from chat of the ground Water. Due to fluctuation in ground-water elevation over time, the organic layer on top of the aquifer may be transported into several zones where the t components occur in the gaseous phase (able to diffuse in all - directions, including upward) , liquid phase (adsorbed onto rock particles or sealed under water) or dissolved/emulsified in water r (1811.2329) . t Migration through soils may be retarded to a minor extent by sorption. Migration is expected to be fastest through previously ! contaminated soils where the sorptive sites may be unavailable; on the other hand, soil-water content increases sorption and slows migration of hydrocarbons. In fissured rock, the migration of hydrocarbons is much less uniform than in porous soils. Preferential spreading through _. crevices. sometimes changing the direction of flow. may occur. Determination of the potential ground-wacer contamination in fissured rock is thus very difficult (1811) . The water-soluble portion of gasoline was shown eo be almost entirely aromatic (87-94%) even though the product itself was almost 50% aliphatic; the aliphatic hydrocarbons either volatilized or were �+ essentially not water-soluble (1849) . In deep, saturated soils with no soil air, some low molecular weight aliphatics may be dissolved in and transported with ground water; however, the light aromatics represent the greatest threat of contamination to ground-water supplies. In summary, the physical distribution of gasoline contamination affects its impact on, and removal from, the soil environment. Lateral -� spreading along the surface increases the initial contaminated area while facilitating evaporative removal of the low molecular weight hydrocarbons. Subsurface release or vertical penetracion mediated by t 6/87 `p k..