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%P, McDowell&Davis Towing Geoprobe Soil Boring&Revised Risk Assessment and Conceptual Site Model Report Page: 7 %.ftw the vapor infiltration through the building foundation / floor and mixing with the indoor air. Cracks in the foundation/ flooring are estimated at I percent with a 20 air " volumes exchanges per day. Subsurface soil contaminant concentrations are assumed to remain constant without biodegradation with a constant rate of volatilization over the exposure period. These assumptions have the effect of making the exposure estimation for indoor air exposure extremely conservative. The facility is composed of a fenced compacted fill yard cover with an asphalt surface for use as parking lot. A small cement slab on grade cinder block building is present at the rear of the property. The property is currently used as an auto storage yard for a local towing company. The building is used as a vehicle maintenance shop and an office for the towing company where dispatch orders and vehicle impoundment and release documents are processed. V 5.0 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 fto 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 4 purposes will have a lesser daily exposure than residential locals. Model site-specific parameters are shown in Appendix D. 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 I in 1,000,000 (LOE-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 onsite indoor air commercial exposures to contaminated vapors from subsurface soil and contaminated groundwater were calculated as 7.4E-8, carcinogenic risk / 4.2E-3 hazard quotient for benzene, 1.2E-1 hazard quotient for ethyl benzene, 2.6 +i E-2 hazard quotient for MTBE, 5.2E-3 hazard quotient for Toluene, and 6.5E-4 hazard quotient for xylenes. It should be noted that the estimated cancer risk for benzene (7.4E- 8) and the hazard quotient for xylenes (6.5E-4) do not exceed the acceptable onsite +- indoor air commercial exposures to contaminated vapors from subsurface soil and contaminated groundwater for benzene (8.3E-6) and the hazard quotient for xylenes (LOE-0).