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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 /> 2.0 ENVIRONMENTAL FATE AND TRANSPORT OF PETROLEUM <br /> HYDROCARBONS <br /> 2.1 BACKGROUND <br /> The saturated zone is a dynamic interdependent system of abiotic and biotic factors that are <br /> linked by physical, chemical, and biological processes. Chemicals inrroduced into this system <br /> ' use these linkages to migrate within and between the various media, and as they migrate are <br /> transformed and degraded. The processes of importance can be categorized as transport, <br /> ' sorption, transformation/degradation, and volatilization. <br /> For chemicals which travel in the dissolved-phase, parameters that affef,t migration include water <br /> ' solubility, Henry's Law constant, and molecular weight. Because of its high water solubility, <br /> Henry's Law constant, and low molecular weight, benzene may be selected as the indicator <br /> ' chemical for diesel/gasoline range fuels. Using benzene as a surrogate for gasoline and diesel <br /> fuel hydrocarbons results in a conservative estimate of the migration potential of gasoline. <br /> 2.2 PETROLEUM HYDROCARBON PERSISTENCE <br /> Petroleum hydrocarbons are subject to a number of fates in the environment. The fate of the <br /> individual hydrocarbons are determined by different processes. For example, volatile organic <br /> ' components are likely to volatilize from soil or ground water. Aromatic ;ompounds, particularly <br /> those with lower molecular weights, are subject to dissolution in soil moisture and subsequent <br /> ' migration within the saturated and unsaturated zones. Some of the heavier, longer hydrocarbon <br /> chains will adsorb to soil and are not likely to migrate significantly in the saturated zone. Many <br /> petroleum constituents are readily biodegradable. In general, bio&,gradation is favored by <br /> ' aerobic conditions, and a variety of oxygenated metabolic intermediates are fonned <br /> (Botta et al., 1984). Of the more significant components of gasoline, benzene is readily <br /> ' biodegradable and has an atmospheric half-life of less than 1 day (Kori,, and Klein, 1982). <br /> Biodegradation is a primary mechanism for the removal of gasoline and diesel fuels in the <br /> ' terrestrial environments. The actual rate of degradation in soil is affected by temperature, <br /> oxygen, moisture content. DH. and the presence, of mineral m1teianta <br /> 30-0136-11 <br /> !�`. 2-1 <br />