SITE INFORMATION AND CORRESPONDENCE

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SOnt Byi BASELINE; 510 420 1707; NGV-9-99 17:01.; Page 2113 is currently present on-site and no vegetation will be grown on the site in the future for human consumption. The spreadsheet, including all exposure parameters, is included in Appendix D. RISK CHARACTERIZATION AND SUMMARY TABLES Results for Chemicals of Potential Concern Sites with risk estimates greater than 1 x 10-4 or a hazard index greater than unity(1.0)are generally considered by DTSC as sites where the presence of contamination may pose a significant threat to residential site users. Other regulatory agencies consider risk estimates from f x 10'd (one-in-ten thousand) to 1 x 10 as posing a significant threat to public health; for example, chemicals listed under Proposition 65, the Safe Drinking Water and Toxic Enforcement Act, are regulated at an excess lifetime cancer risk of l x 10-5(one-in-one hundred thousand). A one-in-one million excess lifetime cancer risk (1 x 10-') is often misconstrued as an expectation that one out of one million people exposed will be stricken with cancer. In actuality, the carcinogenic risk is not an actual risk but,rather a mathematical probability that an individual may develop cancer over a 70-year life span, as a result of the exposure conditions evaluated. The non-cancer hazard index is expressed as the ratio of the estimated exposure over a specified time period to a modified toxicity value (called a reference dose) for a similar exposure period; if the exposure exceeds the reference dose (i.e., the Hazard >1), there may be concern for non-cancer adverse health cffccts undcr the exposure conditions evaluated. The calculated excess lifetime cancer risk from soil ingestion and dermal contact for the Weber Block property was 2.3 x 10" (cane-in-one hundred thousand), which exceeds the I x'10' DISC cancer risk level. The largest contributor to the risk from this exposure pathway was from benzo(a)pyrene; the rnaximum 'concentration of this chemical was found at 8 feet bgs. The hazard associated with soil ingestion and dermal contact was calculated as 0.0083,which is below a hazard index of 1.0. The excess lifetime cancer risk from groundwater ingestion and showering/bathing with volatile organic compounds and non-volatile organic compounds, as calculated using on-site groundwater data, was 1.1 x 10-5 and 0.66. The largest contributor to the risk from this exposure pathway was from benzene. By contrast, the excess lifetime cancer risk from groundwater ingestion and showering/bathing with volatile organic compounds and non-volatile organic compounds, as calculated using off-site groundwater data collected at the ARCO site was 2.4 x 10', and a hazard of 151.7. Groundwater from the ARCO site is hydrologically upgradient from the site and could impact the duality of groundwater at the Weber Black site. Both calculated excess lifetime cancer risks exceed the DTSC threshold of 1 x 10-6 for residential sites.The hazard index for the ARCO site exceeded unity(1.0). The excess lifetime cancer risk from exposure tb volatiles and non-volatiles in ambient air was 2:0 x 10`and the hazard index was 0.0012. These risks and hazards would be greater if the calculations took into account potential volatilization into ambient air from contaminated groundwater underlying the site. This calculated excess l.ifetirne cancer risk and hazard were below the applicable DTSC thresholds. 97369stk,pA.wpd-1 1/9199 24