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McDowell&Davis Towing Risk Assessment and Conceptual Site Model 5 hazard quotient for MTBE, 5.5E-4 hazard quotient for Toluene, and 1.0E-3 hazard quotient for xylenes. Potential risk from onsite indoor air commercial exposures to contaminated vapors from subsurface soil and contaminated groundwater were calculated as 8.3E-6, carcinogenic risk/4.7E-4 hazard quotient for benzene, 1.7E-1 hazard quotient for ethyl benzene, 9.5E- `' 2 hazard quotient for MTBE, 9.8E-1 hazard quotient for Toluene, and 1.0E+0 hazard quotient for xylenes. It should be noted that the estimated cancer risk for benzene (LOE- 6) and the hazard quotient for xylenes (LLE-1) 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). 5.0 CONCLUSIONS AND RECOMMENDATIONS The soil and groundwater contamination at the McDowell and Davis underground storage tank site has been evaluated through an ASTM — Risk Based Corrective Action model. _Groundwater mpacts at the site do not pose a risk to offsite ingestion at the closest identified water approximately 750 feet from the site. Onsite groundwater impacts were below water quality goalslin the May 2002 monitoring event. Individual contaminant concentrations do not pose a greater cancer and toxicant risk; however the cumulative effect of indoor air exposure of multiple contaminants does pose a greater than acceptable cancer and toxicant risk. In many cases contaminant effects are magnified by the presence of other contaminants causing an elevated risk. The cancer risk estimated for benzene is 8.3E-6 slightly greater than the established acceptable minimum additional risk of i.OE-6. The cumulative toxicant risk estimated for toluene is 1.9E+0 and the cumulative toxicant risk for all contaminants of concern is estimated at 3.6E+0 slightly greater than the established acceptable minimum additional risk of 1.0E+0. Additional remediation of the contamination does not appear warranted at this time. The standard parameters for commercial indoor air exposures are based on a postulated inhalation of 20 cubic meters /day for 250 days a year for 25 years by a 70 kg individual. The model assumes a cross media transfer, organic vapor flux from the impacted soil and the vapor infiltration through the building foundation / floor and mixing with the indoor air. Cracks in the foundation / flooring are estimated at 1 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. T 0 The facility is composed of a fenced compacted fill yard,cover with an asphalt surfacc 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 .w 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. The model exposure parameters described above exceed the G_ actual conditions and provide a very conservative risk analysis. 6