ARCHIVED REPORTS_XR0011572 - Laserfiche WebLink

Home Browse Search

6 Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California ' The general population of Northern California is exposed to potentially carcinogenic chemicals from vehicles, industrial operations, and consumer products. In 1y87, the California Aix Resources Board (CARB) published a study which quantified the magnitude of population 't exposure from existing sources of 20 air toxics, including benzene. The reported concentration of benzene in ambient air ranges from 1.0 to 4.9 parts per billion (ppb) (CARB, 1987). The ' highest benzene concentrations estimated in this risk assessment are approximately 2 to 8 times lower than the ambient levels measured by CARB. Therefore, the incremental cancer risk associated with background ambient levels of benzene is greater than the hypothetical cancer risk for benzene associated with this risk assessment. A series of pie charts depicting the relationship between allowable ambient concentrations of benzene (as defined by the ACGIH) and estimated ambient concentrations of benzene (based on hypothetical current and future site conditions) are presented in Figures 6a and 6b. 6.7 MONTE CARLO SIMULATION ' Monte Carlo simulation is a technique for simulating a complex real world situation that involves uncertainty. A random number generator is used to generate values for.assumptions which have been defined by the operator. These assumptions have specific be undary and probability distributions (i.e., Gaussian, Lognormal, etc.). The generated numbers used in the calculation ' of forecast values. The calculations are repeated using newly generated values, within the confines of the boundaries and probabilities. This iterative procedure produces a distribution ' of values for the forecasted quantity and the probability with which it is likely to occur. Certainty level shows the certainty of achieving the values in the forecasted range. ' Monte Carlo simulations were run on the future site use as a retail facility, using Crystal Ball 2.0, to produce a range of cancer risks (for benzene) and hazard indices (for toluene, ethylbenzene, and xylenes). The exposure likely to occur at the onsite facility is subject to fewer confounding variables than the exposures at the nearby retail facilities The assumptions and ' output for the simulation are listed in Appendix D. Output from the simulation is intended to provide an approximation of the most likely risk or hazard index for the site. Point estimates of risks and hazard indices (such as those presented in sections 6.3 and 6.4)produce a numerical estimate of the desired value. However, the utility of the point estimate is limited to the 1nU1V1UUc11 jLLU11�eiicai 'vaiu%5 aJJignvta tv .aCh pa u.'::eter of the ri�l; nr ila?arrl 1nrIP.X alum-thm 30-0136-11 6-7 1 LW