ARCHIVED REPORTS_TIER I RISK BASED CORRECTIVE ACTION EVALUATION

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ARCADES BBL Mr. Brian Taylor July 27,2007 For example, prediction of indoor air vapor concentrations for buildings is an area of uncertainty. However, the air exposure models incorporated into GW-SCREEN are based on equations recommended by Cal-EPA DTSC and USEPA which provide a very conservative overestimate of indoor air concentrations for most cases, which in turn will result in an overestimate of risks. The degree to which this may impact the estimated risks and hazards was not quantified. The degradability of volatile petroleum hydrocarbons in groundwater was not considered in this evaluation, though a significant decline in concentrations over the 25 year exposure duration is likely, suggesting that that risk and hazard estimates presented herein have been overestimated. Use of petroleum hydrocarbon groundwater data to evaluate potential onsite indoor air exposures, without considering natural attenuation mechanisms known to affect petroleum hydrocarbons, is likely to overestimate risks and hazards for onsite workers. However, the degree to which this may impact risk and hazard estimates cannot be quantitatively determined. Although detected in groundwater monitoring wells beneath the footprint of the office building, neither TBA or TAME were selected as COPCs. Inhalation toxicity criteria for these oxygenates have not been established by either Cal-EPA or USEPA, and thus it is uncertain that exposures to these chemicals in indoor air vapor may pose adverse health effects. Regardless, both Cal-EPA OEHHA(1999)and the American Petroleum Institute(API, 2005)have derived oral toxicity criteria for TBA. Cal-EPA OEHHA has derived an oral cancer slope factor(CSF)of 3 x 10-3(mg/kg-day)"' and API has derived an oral reference dose (RfD)of 0.22 mglkg-day.Assuming route-to- route extrapolation of the OEHHA-derived oral CSF results in an estimated excess cancer risk of 7 x 10-9,while use of the API-derived oral RfD results in a hazard index of 3 x 10-6.These levels would not significantly affect the overall estimated excess cancer risk and hazard index presented in this evaluation. The RWQCB also raised concerns associated with the potential inhalation hazard posed by the office shower.The shower located within the office building is expected to only be used for emergency dousing if a worker is sprayed by petroleum product. The emergency shower is supplied by WSW-1. Since the only COPCs detected in WSW-1 are xylenes, and they were detected at very low concentrations, exposure associated with emergency showering is likely to be negligible as compared to the vapor intrusion pathway. Finally, this evaluation was based on use of the maximum detected groundwater COPC concentrations as a source beneath the onsite office building.Typically,the 95 percent upper confidence limits of the mean groundwater concentrations of the COPCs are also evaluated, because they are likely to be more representative Page: c:mxnnomniro+ni roa��oo iwinaGs�.iur .mo 5/7