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3.0 RISK-BASED CORRECTIVE ACTION ANALYSIS The RBCA process is the integration of site assessment,remedial action selection, and monitoring with USEPA-recommended risk and exposure assessment practices This creates a process by which corrective ' action decisions are made in a consistent manner that is protective of human health and the environment The ASTM RBCA process is implemented in a tiered approach, involving increasingly sophisticated levels of data collection and analysis The assumptions of earlier tiers are replaced with site-specific data and ' information Upon evaluation of each tier, decisions are made with respect to the need, if any, for additional site-specific analysis and/or remediation The following paragraphs document the approach to performing this RBCA analysis, including development of the conceptual site model for risk assessment, Tier I RBCA analysis, uncertainty analysis, and associated conclusions 3.1 Conceptual Site Model ' As the initial step in evaluation of health risks due to potential chemical exposure, a conceptual site model (CSM) of chemical occurrence, fate, transport, and potential exposure was developed Specifically, the CSM documents sources of chemicals, affected media and transport mechanisms, and potential exposure pathways and receptors Development of the CSM was based on available information characterizing hydrogeologic conditions, soil and groundwater quality, and land use at the site Figure 2 is a graphical representation of the CSM 31.1 Sources of Chemicals Historical site investigations have identified the subsurface presence of chemicals consistent with those used at the site The highest chemical concentrations have been detected at ' and in the immediate vicinity of the former tank basin Past remediation efforts have resulted in reduction of chemical concentrations in soils and groundwater to residual levels 3.1.2 Affected Media and Transport Mechanisms Review of historical investigation results indicate ' the subsurface presence of petroleum hydrocarbons and MTBE in soil and/or groundwater Presence of impacted groundwater is the likely result of chemical dissolution from soil and subsequent migration to the ' water table Due to the volatile nature of various petroleum hydrocarbon compounds, soil vapor underlying the site may also be impacted As a result, volatilization of hydrocarbons from soil and groundwater, and subsequent ' transport in soil vapor toward the ground surface is considered a possible transport mechanism for hydrocarbons and MTBE Dissolved constituents at the water table may be subject to offsite migration via groundwater transport However, given the relatively low concentrations of hydrocarbons and MTBE in past groundwater samples underlying the site,the potential for offsite migration of chemicals at measurable levels is considered ' minimal 3.1.3 Potential Exposure Pathways and Receptors Potential exposure pathways and receptors (exposure scenarios)were evaluated based on the previously defined sources of chemicals, affected media, transport mechanisms, and land use at and in the vicinity of the site ' 5