ARCHIVED REPORTS_TIER I RISK BASED CORRECTIVE ACTION EVALUATION

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ARCADES 1381 Mr. Brian Taylor July 27,2007 Risk Characterization Using standard equations recommended by Cal-EPA and USEPA,the predicted indoor air concentrations, and the exposure assumptions and toxicity criteria described above,the potential health risks and hazards were estimated for onsite office workers. The potential health risks presented by carcinogenic chemicals, such as benzene, are described in terms of excess cancer risk, or the probability of getting cancer over a lifetime that may be associated with the particular exposures evaluated. Noncarcinogenic chemicals,such as toluene, ethylbenzene, and xylenes are expressed in terms of a hazard quotient or a ratio of the chemical exposure estimate to a"safe"dose.Assuming the risk and hazard estimates are additive, chemical-specific risk and hazard estimates were then summed to estimate the total (cumulative)carcinogenic risk and hazard index(HI). Based on indoor air concentrations predicted by the GW-SCREEN model shown in Attachment A,Table 5 presents the estimated excess lifetime cancer risk and noncancer hazard index for onsite office workers assuming maximum COPC concentrations in groundwater beneath the building. Tables A-1 to A-10 in Attachment A present the GW-SCREEN model runs for each COPC assuming slab- on-grade building foundation and maximum COPC groundwater concentrations as the source beneath the office.As shown in Table 5,the potential excess lifetime cancer risk was estimated to be 1 x 105 for an onsite adult office worker. The potential noncancer hazard index(for total exposure to BTEX,and MTBE)was estimated to be 0.05. Table 6 shows a comparison of COPC groundwater concentrations with Cal-EPA San Francisco Bay Regional Water Quality Control Board (SFB-RWQCB,2005) ESLs. Maximum and arithmetic average COPC concentrations detected in MW- 69UA and WSWA were compared with groundwater ESLs for the evaluation of potential vapor intrusion concerns based on either high or low/moderate vadose- zone permeability. None of the maximum COPC concentrations in groundwater exceeded the ESLs. Uncertainties Uncertainty,which includes natural variability and incertitude originating from a lack of understanding (i.e., relevant data)are inherent in the risk assessment process. Many of the variables used to estimate risks and hazards are standard default values and may not accurately describe future conditions or characteristics of the site.As default values tend to overestimate exposure,the impact of this uncertainty is to typically overestimate risks. Page: G,M 11D AIWINI W17111W i 1MM EnWM.tlea 4/7