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I+ T��--*�H-�DRDR � L1 u�.L 1 VAC I 3.5.4 Saturated Zone Terra Vac only considers soil samples collected from below a depth of 22 feet representative of the saturated zone. Each of the eight soil samples collected from the saturated zone contained detectable gasoline hydrocarbon concentrations ranging from 1.8 to2,200 ppm with benzene concentrations ranging from 0.1 to 18 ppm. Figure 8 illustrates saturated zone soils at depths below 22 feet impacted by benzene. ! 3.5.5 Groundwater Groundwater sampling has been conducted on a quarterly basis since June 1992. Dissolved phase benzene is present in all onsite wells. Figure 9A and 9B present the . lateral extent of the dissolved benzene plume in November 1997 and February 1998. 4. Risk-Based Corrective Action Model The purpose of this section is to describe the Groundwater Services Inc., Risk-Based Corrective Action (GSI/RBCA) model. The GSI/RBCA model is a Microsoft Excel 5.0 that consists of a series of linked worksheets that calculate Risk Baseline Screening Levels (RBSL) and/or Site-Specific Target Levels (SSTLs) for soil and groundwater remediation, based upon information provided by the user. The GSI/RBCA model is "designed to complete calculations required for Tier 1 and 2 of the. RBCA planning process, as defined in ASTM E-1739 'Standard for Risk-Based Corrective Action at Petroleum Release Sites'. Risk assessment procedures employed in the RBCA Spreadsheet System are consistent with current U.S. EPA guidelines." (Appendix A, Guidance Manual for Risk-Based Corrective Action, Groundwater Services, Inc., Houston, Texas). This risk assessment incorporates current site conditions and uses. 4.1 Contaminants of Concern The GSI/RBCA evaluation begins with a defined target risk that is protective of public health. This risk is the result of a receptor's exposure to a contaminant of concern (COC). The exposure is the result of transport of the COC from its source (impacted soil or groundwater beneath the site) to the receptor via the exposure pathway. The GSIIRBCA works backwards from the acceptable risk at the receptor to determine an allowable maximum concentration at the source. The calculated maximum concentration is the SSTL for each COC in soil and groundwater that pertains to a given exposure scenario. As previously stated benzene, toluene, ethlybenzene, and total xylenes are the CDCs. MtBE does not sorb well onto soil, sand, or clay and tends to stay in the liquid phase. MtBE is not a contaminant of concern because it was not detected in the vadose zone. Project 30-0212 7 c:lalton130-02121caplrev5lcap-rev5.doc July 15, 1998 9