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RISK BASE CORRECTIVE ACTION—RBCA TIER 2 In order to help determine the risks associated with this site a Risk Base Corrective Action (RSCA) Tier 2 Risk Assessment was performed, see Appendix G. During the Tier 2 Assessment, Site-Specific Target Levels (SSTLs) were calculated, -using the RBCA Spread Sheet System, for the following compounds of concern (COCs): • Benzene in groundwater and subsurface soils using the current EPA Cancer SF for Benzene. • Toluene in groundwater and subsurface soils • Ethylbenzene in groundwater and subsurface soils • Xylenes in groundwater and subsurface soils • MTBE in groundwater and subsurface soils • TPHg as Hexane in groundwater and subsurface soils i Two potential transport pathways were considered during the Tier Two investigation of this site: Vapor Inhalation and Groundwater Ingestion. The third major pathway, soil ingestion, was not considered because there is no documented contamination in the upper three feet of soil and the site is paved over limiting the probability of the ingestion of contaminated soil. Vapor Inhalation was divided into two categories: 1. Volatilization to onsite inside air, this was not considered for this study because the groundwater and soil plumes are not currently located beneath the station building and the modeling for inside air does not take this into account.. 2. Volatilization to onsite outside air. Groundwater ingestion was calculated for: 1. Onsite groundwater ingestion, commercial. 2. Off site ingestion, Domestic. The closest domestic well is 780 feet from the site and is cross gradient to the groundwater plume, and the closest down gradient wells is 3390 feet from the site. Because the down gradient well is the only well likely to be impacted by the plume, a well distance of 3000 feet was used to calculate off-site ingestion. 12 R603 Update#52/CLOSURE RV 7/98