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5 SUMMARY OF ASSUMPTIONS AND RESULTS OF RBCA ANALYSIS <br /> In further support of case closure, and to assess possible risks associated with site closure, a risk- <br /> based corrective action (RBCA) analysis was performed and is presented as Appendix H and <br /> summarized below <br /> ' The RSCA process is the integration of site assessment, remedial action selection, and <br /> monitoring with USEPA-recommended nsk and exposure assessment practices This creates a <br /> process by which corrective action decisions are made in a consistent manner that is protective of <br /> ' human health and the environment The American Society for Testing and Materials (ASTM) <br /> RBCA process is implemented in a tiered approach, involving increasingly sophisticated levels <br /> of data collection and analysis The assumptions of earlier tiers are replaced with site-specific <br /> ' data and information Upon evaluation of each tier, decisions are made with respect to the need, <br /> if any, for additional site-specific analysis and/or remediation <br /> The following paragraphs document the approach to performing this RBCA analysis, including <br /> development of the conceptual site model for risk assessment, Tier I RBCA analysis, uncertainty <br /> analysis, and associated conclusions <br /> 5.1 CONCEPTUAL SITE MODEL <br /> ' As the initial step in evaluation of health risks due to potential chemical exposure, a conceptual <br /> site model (CSM) of chemical occurrence, fate, transport, and potential exposure was developed <br /> Specifically, the CSM documents sources of chemicals, affected media and transport <br /> mechanisms, and potential exposure pathways and receptors Development of the CSM was <br /> based on available information characterizing hydrogeologic conditions, soil and groundwater <br /> quality, and land use at the site Figure 2 of Appendix H is a graphical representation of the <br /> CSM <br /> ' 5.2 AFFECTED MEDIA AND TRANSPORT MECHANISMS <br /> Review of historical investigation results indicates the presence of petroleum hydrocarbons and <br /> ' MTBE in soil and/or groundwater Presence of impacted groundwater is the likely result of <br /> chemical dissolution from soil and subsequent migration to the water table <br /> ' Due to the volatile nature of various petroleum hydrocarbon compounds, soil vapor underlying <br /> the site may also be impacted As a result, volatilization of hydrocarbons from soil and <br /> groundwater, and subsequent transport in soil vapor toward the ground surface is considered a <br /> ' possible transport mechanism for hydrocarbons and MTBE <br /> Dissolved constituents at the water table may be subject to offsite migration via groundwater <br /> transport However, given the relatively low concentrations of hydrocarbons and MTBE in past <br /> groundwater samples underlying the site, the potential for offsite migration of chemicals at <br /> measurable levels is considered minimal <br /> 1 <br /> F SPio�ccss1701371M aster/W plClosw+e 1100101 37cile[l do 13 <br />