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t �k _UTCMOT=- GASOL=M 65-13 aydrocarbons was selectad from the daminaat hydrocarbon c' . . s, i.e. . alkaaas. cycloa1 kanes. and aromatics- glass specific were E r chosen an the .basis of their relative concentrations. ar.; mere tended :o span tha boiling point average of the gasoline ..ydr _Mons. :able 65-3 identifies the selected hydrocarbons and � y pres the ?redicted partitioning of low soil concentrations o£ those hydrocarbons k among soil particles. soil water. and soil air. The portions associated with the wacer and air phases of the soil are expected to naveahigher mobility than the adsorbed portion. : 1 � . Estimates for the unsaturated topsoil indicate that sorption is expected to be an important process for all the dominant hydrocarbon catagaries. Partitioning to the soil-vapor phase is also expected to be important for the- lover molecular weight aliphatic hydrocarbons (C,1-Cs) which are characterized by high vapor pressure and Low watar solubility. The alkyl benzenes have higher water solubilities and cransporz with infiltrating Water may be important for these compounds: volatilization. ort the other hand. may be less important. In i saturated. deep soils (containing no soil air and negligible soil I organic .carbon) , a significant percent of both aliphatic (particularly " less than Cr) and aromatic hydrocarbons is predicted co be present in chaasoil-wacer phase and available for transport with flowing ground tater. In interpreting chase results. it must be remembered that this modal is valid only for Low soil concentrations (below aqueous + N. solubility) of the components. Targe releases of gasoline (spills. leaking underground storage tanks) may exceed the sorptive capacity of the soil. thereby filliog -the pore spaces of the soil with the fuel. In chis situation, the hydrocarbon mixture would move as a bulk fluid r and the equilibrium partitioning model would not be applicable. i Overall, ground water underlying soil concaminacad with gasoline hydrocarbons is expected to be vulnerable to contamination by ac least some of these components. The type of spill (surface vs. sub-surface) is ;.of particular importance. since volatilisation from the surface is expected to be a significant removal process for low molecular weight aliphaties. At this point. it should be mentioned that enviror%manul 4 fate/exposure/toxicology chapters for several of the components in table 65-3 Were included in Volume 1 of the IRP Toxicology Guide. The 4 gasoline components addressed in Volume 1 include: bensene. eoluene. xylenes. 'ethyl benzene, and naphthalene. Three major gasoline additives - TOCP. tatraathyL lead and ethylene dibromide were addressed in Volume 2 of the IRP Toxicology Guide, while ethylene dichloride was addressed in Volume 1. 6512.1.2 Transport Studies Hundreds of thousands of underground gasoline storage tanks are a currently used ac service stations, commercial locations, residences. w and petroleum depots,. and almost all the gasoline used for r,7 a cransporzation purposes in the U.S. is stored underground at least once s 6/87 � k ; } 7 S£