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65-I8 AUTOHOT= GAS OLIN E Although the mi.crobioca of most non-contaminated soils include many naturally occurring hydrocarbon-degrading populations, the ' -addition of petroleum selectively enriches that sector able to adapt { and ut: . ::e the new substrate. other environmental factors shown to have a -.,jor -effect• on biodegradability are availability of oxygen and moderate temperatures. In summary, biodegradation of the petroleum hydrocarbons comprising automotive gasolines is expected to be rapid under conditions favorable for microbial activity and when fuel components are freely available to the microorganisms. Degradation may be Limited and/or slow in environments with few degrading organisms, low pd, low temperatures, and high salinity (e.g. , arctic environments) . It should 4 be mentioned that Walker 11 al. (2257) state that even under optimum conditions. total and complete biodegradation is not expected to occur _ except possibly over an extremely long time period. 65.2.3 Primary Routes of Exposure from Soil/Ground-water Systems f The above discussion of fate pathways suggests that the major components of gasoline are highly volatile but vary in chair potential for bioaccumulation and tendency to sorb to soil. They range from e moderately to strongly sorbed to soil, and their bioaccumulation potential ranges from low to high. The variability in the properties of the components suggests they may have somewhat different potential exposure pathways. Spills of gasoline would result in the evaporative loss of the more highly volatile components leaving chose of lesser volatility in the -sail. The fraction remaining in the soil is expected to be rela- tively mobile and will be carried by gravity to the saturated zone of the soil. There, the more soluble components will dissolve into the ground water or form emulsions with it. These components are primarily aromatic and lower molecular weight aliphatic compounds; in one study using unleaded gasoline. approximately 95• of the water soluble frac- tion was benzene and substituted benzenes (2318) . The insoluble fraction of gasoline floats as a separate phase on top of the water table. The movement of gasoline dissolved in ground water is especially important because of its relatively high solubility (173-200 t mg/L (2287.2297)) . Furthermore. the movement of dissolved hydrocarbons + in ground water is much greater than that of the separate liquid phase. reaching distances of hundreds to thousands of meters compared to tens of meters for the movement of the separate phase. In the presence of cracks and fissures, however. the flow of the separate hydrocarbon phase is greatly enhanced. The movement of gasoline in ground water may contaminate drinking water supplies, resulting in ingestion exposures. Ground-water dis- charges to surface water or the movement of contaminated sail particles # to surface water drinking water supplies may also result in ingestion exposure and in dermal exposures from the recreational use of these waters. The potential also exists for the uptake of some gasoline F 6/87 J