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3. RISK ASSESSMENT <br /> As described in Section 1.3, a risk assessment (RA) ' for a fuel- <br /> contaminated site has six steps. The Stockton site has been <br /> c;iaracterized in Chapter 2. The four subsections below present <br /> the refraining components of the RA for the site. In this <br /> analysis, the hazard evaluation and toxicity assessment have been <br /> combined. , <br /> 3.1 HAZARD EVALUATION AND TOXICITY ASSESSMENT <br /> # Gasoline and other fuels are complex mixtures of hydrocarbons. <br /> Once gasoline has been released from a tank, the composition of <br /> the mixturo changes becaube the components have different <br /> physical and chemical properties (e.g. , solubility in water, <br /> volatility, soil adsorption efficiencies) which dictate the <br /> ability of each component to migrate through the environment. <br /> . Consequently, a human receptor will not be exposed to gasoline, <br /> but to var:uus gasoline components. It is therefore appropriate <br /> to evaluate the hazards and risks associated with gasoline <br /> components rather than the complete gasoline mixture. { <br /> One of the most frequently used measurements of gasoli.,e contami- <br /> nation of soils is the concentration of total petroleum h)•dro- <br /> carbons (TPH) . TPH measurements can be used to assess the <br /> spatial extent of hydrocarbon contamination, and therefore to <br /> help determine the source of the contamination. However, the <br /> human health and environmental significance of TPH cannot be <br /> determined, because of its variable composition. <br /> For risk-assessment purposes, gasoline constituents can be <br /> divided into four major groups: alkanes, alkenes, cycloal.kenes, <br /> ,and aromatics . . Table 5 gives several important..environmental <br /> properties of these compounds. The structural similarity within <br /> each of the groups results in similar environmental behavior and <br /> . 13 <br />