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Human Health Risk .Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br />' Tracy, California <br /> The formulation methods of gasoline influence its physical performance specifications. For <br /> example, gasoline performance is achieved by adding low-molecular-weight hydrocarbons, such <br /> ' as butanes and pentanes (McDermott and Killiany, 1978). Antiknock ;apability is achieved in <br /> regular gasoline by adding small quantities of organic lead (tetraethyl or tetramethyl lead), while <br /> ' lead-free gasoline achieves this by blending branched-chain aliphatic or aromatic compounds <br /> (e.g., benzene). Therefore, most investigators report the compc:;?tion of gasolines as a <br /> ' percentage of broad categories, namely: 1) saturates such as paraffins, cycloparaffins or <br /> napthenes; 2) unsaturates or olefins; and 3) aromatics. The physicw and chemical properties <br /> of gasoline are summarized in Table 3-3. <br /> 3.1.2 Environmental Fate <br /> Gasoline and diesel range fuels are subject to a number of fates iz, ,'Ie environment. These <br />' include three major processes: 1) evaporation, 2) dissolution, and 3) iodegradation. Volatile <br /> organic components are likely to volatilize from soil or ground water Aromatic compounds, <br /> particularly those with lower molecular weights, are subject to dissolution in soil moisture and <br /> subsequent migration within the saturated and unsaturated zones. Maga et al., (1985 found <br /> Y ) <br /> it difficult to distinguish between the effects of evaporation and dk;olution on diesel fuel. <br />' However, the studies concluded that evaporation appeared to be the r:wre significant process. <br /> Some of the heavier, longer hydrocarbon chains will adsorb to soil. Many fuel constituents are <br />' readily biodegradable. In general, biodegradation is favored by aerobic onditions, and a variety <br /> of oxygenated metabolic intermediates are formed (Botta et al., 1980. McKay et al., (1985) <br /> estimated that 75% of the straight chain alkane components of a fresh diesel fuel spill are lost <br /> primarily by biodegradation. Of the more significant components of gasoline, benzene is readily <br /> biodegradable and has an atmospheric half-life of less than one day (Yorte and Klein, 1982). <br /> 3.1.2.1 Degradation <br />' Degradation of gasoline and diesel fuel is a i <br /> ma or fate mechanism J a n c nestnal environments. <br /> The actual rate of degradation is soil is affected by temperature, oxygen moisture content, pH, <br />' and the presence of mineral nutrients. While freezing conditions sloe& microbial activity (and <br /> therefore degradation), biodegradation will continue at temperatures as low as A-F C as ]ons <br /> 30-0136-11 <br /> 3-2 <br />