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McDowell&Davis Towing ` Risk Assessment and Conceptual Site Model 1) Onsite outdoor inhalation of contaminant vapor sourced from subsurface -- contaminated soil 2) Onsite outdoor inhalation of contaminant vapor sourced from contaminated L groundwater 3) Offsite outdoor inhalation of contaminant vapor sourced from subsurface contaminated soil 4) Offsite outdoor inhalation of contaminant vapor sourced from contaminated groundwater 5) Onsite indoor inhalation of contaminant vapor sourced from subsurface contaminated soil 6) Onsite indoor inhalation of contaminant vapor sourced from contaminated groundwater 7) Offsite ingestion of contaminated groundwater. 4.0 ASTM - RBCA RISK BASED CORRECTIVE ACTION EVALUATION The ASTM — RBCA model calculates exposures to selected contaminates based on the route of exposure and site conditions. Factors included in the model include attenuation due to vapor or groundwater migration or outdoor exposures, contaminant concentration due to indoor exposures, and duration of exposure controlled by the use of or proposed use of the property or location of the identified receptor. Locals used for commercial purposes will have a lesser daily exposure than residential locals. Model site-specific parameters are shown in Appendix A. Lifetime cancer risks are estimated through the calculation of a chronic daily intake and the determination of the added probability of incurring cancer through long-term exposures. Acceptable risk levels for exposures to carcinogens have been established at 1 in 1,000,000 (1.0E-6). System toxicants are evaluated through a hazard quotient, which compares the calculated chemical intake against an established reference dose. Hazard quotients greater than 1 indicate possible' non-carcinogenic effects in sensitive `' populations. }. Potential risk from exposures to offsite ingestion of contaminated groundwater at'th� nearest identified water well were calculated as 3.6 E-9, carcinogenic risk for benzene, 3.3E-5 hazard quotient for ethyl benzene, 7.4E-3 hazard quotient for MTBE, 5.6E-9 hazard quotient for Toluene, and 8.9-E5 hazard quotient for xylenes. Out run sheets from the modeling are presented in Appendix Potential risk from offsite outdoor air residential exposures to contaminated vapors from subsurface soil and contaminated groundwater were calculated as 6.5 E-9, carcinogenic risk/3.1E-4 hazard quotient for benzene, 3.1E-4 hazard quotient for ethyl benzene, 6.2E- �, 5 hazard quotient for MTBE, 6.4E-4 hazard quotient for Toluene, and 1.2E-3 hazard quotient for xylenes. Potential risk from onsite outdoor air commercial exposures to contaminated vapors from subsurface soil and contaminated groundwater were calculated as 4.75 E-9, carcinogenic risk/2.7E-4 hazard quotient for benzene, 9.3E-5 hazard quotient for ethyl benzene, 5.3E- 5