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-U+'TCHDT= CA.SOLI:TE 65-13 <br />' zyarocarmons was selected from c� <br /> ..e GGmitZaIIt hpdrocarcon c s ­a-e_ , <br /> alkaaes, cycloaikanes, and aromatics. These specific were <br /> cnosen on the basis of their re}.azive concentrations. arm _ere renaea <br /> -a span the boiling point average oz the gasou na rdr _sons. <br /> ;able 65-3 Ldentifies the selected ':ydrocarbons and pries, ._s the <br />' oredicted partitioning of Low soil concen=aztons of those hvdrocarbons <br /> among soil particles, soil -later, and soil air. The portions <br /> associated with the water ane air pnases of the SOLI are exvaczed to <br /> nave higher mobility=than the adsoraed portion. - - <br /> Estimates for the unsaturated topsoil indicate that sorption is <br /> expected to be an Lmportanc process for all the domxnanc hydrocarbon <br /> categories. Partitioning co the soil-vapor phase is also expected to <br /> be L=porzanz for the Lower molecular weight aliphatic hydrocarbons <br /> (C,-Cs) which are characterzzed by high vapor pressure and low water <br /> soLubility. The alkyl benzenes have higher water solubilities and <br /> transport with infiltrating water may be important for these compounds; <br /> volatilization, on the other hand. may be less Lmportanc. In <br /> saturated deep soils (containing no so-LI air and negligible soil <br /> organic carbon) , a significant percent of both aliphatic (particularly <br /> Less than C..) and aromatic hydrocarbons LS predicted to be present in <br /> the soil-water phase and avaxlabrle for transport with flowing ground <br /> -later. <br /> Iwo In interpreting these results. Lt must be remembered chat this <br /> model is valid only for low soil concentrations (below aquemm <br /> solubility) of the components. Large releases of gasoline (spills. <br /> lamking undarground storage tanks) may exceed the sorptive capacity of <br /> t - the soil. thereby filling the pore spaces of the soil with the fuol. <br /> In this situation. the hydrocarbon mixture would move as a bulk fluid <br /> and the equilibrium partitioning model would not be applicable. <br /> i <br /> Overall.. ground water underlying soil contaminated with gasoline <br /> hydrocarbons is expected to be vulnerable to contamination by at least <br />'s some of theca components The Type of spill (surface vs sub-surface) <br /> Ls of particular importance, since volatilization from the surface Ls <br /> expected to be a significant removal process for Low molecular weight <br /> aliphatxcs. At this point. Lt should be mentioned that environmental <br /> fate/exposure/toxicology chapters for several of the components in <br /> Table 65-3 were included in Volume L of the IRP ToxicoLogy Guide. The <br /> gasoline components addressed in Volume 1 include. benzene, toluene. <br /> xylenes. ethyl benzene, and naphthalene. Three major gasoline <br /> additives - TOCP, tatraethyl lead and ethylene dibromide were addressed <br /> La Volume 2 of the IRP Toxicology Guide, while ethylene dichloride was <br /> addressed in Volume 1. <br /> 65.2.1.2 Transport Studies <br /> Hundreds of thousands of undergrotmd gasoline storage tanks are <br /> currently used at service stations, commercial locations. residences, <br /> and petroleum depots; and al,mosc all the gasoline used for <br /> transportation purposes in the U.S. is stored underground at least once <br /> 6/87 <br />