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TERRA VAC 5. Representative Contaminant Concentrations The representative contaminant concentrations are calculated by the GSI/RBCA model from actual soil and groundwater sample analytical results collected over a period of time. These concentrations represent mean contaminant concentrations beneath the site. Hydrocarbon concentration data from soil samples collected from exploratory boxings at the site were used to establish a representative hydrocarbon concentrations for the risk assessment modeling. For the purposes of arriving at conservative representative hydrocarbon concentrations, analytical results less than reporting limits were omitted. The { remaining soil sampling data were entered into the model to calculate representative +' hydrocarbon concentrations in soil beneath the site. If soil or groundwater representative contaminant concentrations are below RBSL or SSIL concentrations calculated by the GSI/RSCA model, no significant health risk is present. 5.1 Exposure Pathway Characterization and Evaluation To pose a risk to human health or the environment three components must be present at the site: an affected source medium, mechanism of contaminant transport, and a current or potential receptor. To develop risk-based cleanup goals, reasonable mechanisms for contaminant transport from the source zone to a point .of contact with a human population or ecological resource must first be identified. Tier One and Tier Two RSCA evaluations consider several standard exposure pathways for contaminant migration from the source to the receptor via air, soil, groundwater, or surface water transport mechanisms. These exposure pathways can be grouped according to the medium of exposure. Figures 10 and 11 illustrate a Baseline Exposure Flowchart and Typical Exposure Pathways For Tier 1 and Tier 2 Evaluations. ' Potential exposure pathways at the site are 1) Ingestion of impacted soil, 2) Dermal contact with impacted soil, 3) ingestion of impacted groundwater, and 4) inhalation of hydrocarbons volatilized from vadose zone soils and/or groundwater to outdoor and indoor air from sources beneath the site. Surface soils (<3 feet bgs) do not contain detectable concentrations of benzene, and so ingestion of impacted soil and dermal contact with impacted soil are unlikely exposure pathways. As there were no water production wells identifiedand all residents within the survey area were found to be on the municipal water supply, ingestion of groundwater is eliminated as a potential exposure pathway. Available data indicate that vadose zone soils may be a source for the volatilization of hydrocarbons to outdoor or indoor air. Available data also indicate that groundwater is a potential source of volatilization to outdoor or indoor air. Inhalation. of volatilized hydrocarbons from soil and groundwater, therefore, is a potential exposure pathway. 6. GSIIRBCA TIER ONE MODEL EVALUATION Terra Vac applied the GSIIRBCA Tier One evaluation using default parameters to develop site specific RBSLs that apply to future onsite commercial usage and current adjacent site residential scenarios. Project 30-0212 8 coaltonl30-02121capVev51cap-rev5Am July 15, 1998 _�,.