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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 /> 6.5 CONSERVATISM ASSOCIATED WITH THE RISK ASSESSMENT <br /> The duration, frequency, and intensity of potential exposures to chemicals in various <br /> environmental matrices are evaluated when quantifying the dose received by potential receptors. <br /> Attempts are made to select exposure parameter values that accurately reflect the actual site <br /> ' conditions. However, the absence of site-specific data requires that values for some exposure <br /> parameters be estimated. When it is necessary to estimate values, conservative estimates are <br /> t used in order to safeguard human and environmental health. Due to the,e conservative exposure <br /> assumptions, the corresponding risk estimates are likely to be overestimates. Table 6-1 identifies <br /> sources of conservatism associated with this assessment and their impact on the final risk <br /> ' estimates. <br /> ' The most significant of the conservative factors incorporated in this health risk assessment is the <br /> assumption of constant chemical source. The assumption of constant chemical source does not <br /> ' account for mass reduction due to volatilization or other loss mechanisms. That is, the chemical <br /> concentration is assumed to remain constant over the entire duration of the study period. In <br /> reality, concentrations of petroleum hydrocarbons in the subsurface will decrease over time. <br /> Based on the chemical flux rates used to estimate upper-bound risk estimates, the total mass of <br /> petroleum hydrocarbons would have volatilized from soil before the end of the exposure <br /> ' duration. For example, the total mass of benzene in the subsurface would be depleted within <br /> approximately four years. As a result, estimated incremental cancer risks would be at least <br /> one-fifth of the value calculated in this assessment. <br /> 6.6 RELATIVE SITE RISKS <br /> The estimated incremental cancer risks presented in this document provide the reader with <br /> ' information regarding theoretical health risks related to site-specific chemicals and conditions. <br /> However, the risks reported in this assessment could be viewed in context with other cancer <br /> ' risks. For example, the overall probability of contracting cancer over a lifetime is <br /> approximately 25 percent (250,000 per 1,000,000 individuals) (DHS, 1956; CRA, 1991). The <br /> highest theoretical cancer risk as defined for this risk assessment is !:�; 7.0 x 10-' <br /> ' (:5 0.1 per 1,000,000 individuals). Clearly, the highest cancer risk estimated in this assessment <br /> is significantly less than We background cancer risk. <br /> 30-0136-11 <br /> 6-6 <br />