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A.R, WATER & HAZARDOUS WA� LABORATORY CERTIFIED by CAL:FOnNIA DEPT of HEALTH SE-P% :-�cS ..r CHANGES IN FUEL COMPOSITION UPON RELEASE TO THE ENVIRONMENT The relative proportions of fuel components discharged to the environment will be changed due to volatilization, differential solubility In water, differences in biodegradation rates and relative affinities for soil components. Volatile components such as propones, butones, pentones and hexanes will hove a stronger tendency to be lost from surface spills because they have a higher vapor pressure than other fuel components. Lighter fuel components as shown in TABLE I hove a greater solubility in water than do heavier components. Accordingly, these will be preferentially stripped away as rainwater moves through hydrocarbons spilled in the vodose zone on its way to become groundwater. Thus the vodose zone will be depleted in lighter fractions relative to the original •� fuel while the groundwater will be enriched. Similarly, unsaturated and cyclic components are more soluble than their straight chain and saturated counterparts. For example, the solubility of butene is more than three times larger than that of butane while the solubility of both hexene and cyclohexone are more than four times larger than that of hexone. Aromatics exhibit even larger increases in solubility relative to the corresponding aliphatics. For example, benzene is more than 100 times as soluble as hexane. Thus gasoline fractions remaining in the vodose zone lose their light ends and aromatics and begin to resemble diesel #2. Diesel #2 fractions dissolving in groundwater become enriched in light ends and aromatics and therefore begin to resemble gasoline. In order to properly identify fuel types in the environment it is necessary to take these factors into account. The issue of changes in fuel composition upon release to the environment is further complicated by the differences in affinity which fuel components have for soils. In general, organic-rich soils and finer soils will retard the movement of fuel components more strongly than orgonic-poor and coarser soils. Retardation of dissolved fuel components by fine soils is not to be confused with the tendency of free product to gather in coarse soil lenses which is a function of free volume rather than the tendency of the product to adhere to the soil. Since the affinities of fuel components for soil and soil organics .r are often the reverse of their solubilities, the selective differentiation of these components according to solubility is further accentuated by their relative retardation by the soils into which they have been released. Biodegradation rates are also strongly influenced by molecular structure. In general, straight-choin saturated hydrocarbons are degraded more readily than aromatics which, in turn, are degraded more readily than alicyclics and highly branched aliphatic hydrocarbons. Since much of this biodegradation tales place in the vodose zone, contaminated soils are often enriched in alicyclics and highly branched aliphatics relative to the hydrocarbon product which was the source of the spill. Alkylaromotics such as toluene can be biochemically degraded either by attack on the ring to produce 3-methylcotechol or by successive oxidation of the side chain to produce benzyl alcohol , benzoldehyde, r benzoic ocid and then cotechol. p-Xylene undergoes a similar stepwise biodegradation to produce 4-methylbenzyl alcohol, 4-methylbenzoldehyde, p-toluic ocid and 4-methylcotechol. Aerobic biodegradation can often be enhanced when sampling exposes oxygen-poor groundwaters to the atmosphere. It is therefore very important to properly preserve groundwater samples containing hydrocarbons. Pogo 3 •� CENTRAL C-14ST ANALYTICAL SERVICES Son Luis a forni Obispo. California_ ('25) 543-253