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SECTIONFOUR Upoeure Asseesmeag Guidance for Superfund (EPA, 1989b) and DTSCs Supplemental Guidance for Risk Assessment ' (DTSC, 1992) The RME was obtained by selecting chemical concentrations at the exposure Point and the characteristics of the receptor population and their activity patterns so that the 1 combination of values would result in a maximum exposure reasonably expected to occur at the site The intent of the RME is to conservatively estimate a chemical intake that is well above ' average, but still within the range of possible exposures For example, EPA guidance recommends that the 95% UCL on the anthmetsc mean concentration be used for characterizing the chemical concentration for the RMIE scenario (EPA, 1989b) The 95% UCL is a conservative restimate of concentrations to which a receptor may be exposed over time 4.4.2 Exposure Point Concentrations Exposure point concentrations are the chemical concentration in environmental media (soil, groundwater, and air) at the point of contact Exposure point concentrations were estimated for air (on site and off site) and groundwater (at the hypothetical well 1,000 feet downgradient) by fate and transport modeling Measured soil and groundwater concentrations at the site were used to calculate 95% UCL concentrations for chemicals in soil and groundwater (Tables 3-1 through Table 3-7) A more detailed description of the calculation of exposure point concentrations for the groundwater and air pathways are presented below Fate and transport models were selected ' from the GSI Tier 2 RBCA software program (1996)to estimate exposure point concentrations in groundwater and outdoor air Physical-chemical properties of the COCs used in the fate and transport modeling in the RBCA assessment are provided in Appendix A 1 4.4.3.9 Groundwater Concentrations at Hypothetical Well ' To calculate the potential groundwater concentrations of the CDCs at a hypothetical groundwater well located 1,000 feet downgradient of the site, a saturated zone transport model, the Domenico analytical solute transport model described in ASTM RBCA, was utilized to calculate the lateral transport within the groundwater zone ' The Domenico analytical solute transport model accounts for the attenuation of groundwater concentrations between the on-site wells and the hypothetical well The model uses a penetrating vertical plane source, perpendicular to groundwater flow, to simulate the release of organics from the ming zone to the moving groundwater Within the groundwater flow regime, the model accounts for the effects of advection, dispersion, sorption, and biodegradation Given a representative concentration, the Domenico transport model predicts centerline plume concentrations at any downgradient distance based on one-dimensional advectrve flow and three-dimensional dispersion 1 Wmm7S�•Clyd@ ® T11997187900®MA%grOGKMN%REPORT%SHOREP#1 DOC18-FEB4XM73DO9 FA%SNA 4-6