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two, r.rr <br /> Residential populations were evaluated along with occupational employees to be <br /> conservative in the risk estimates, although EPA guidance indicates that it is not <br /> necessary to assume that future hypothetical residential receptors will be located directly <br /> � on the site (EPA 1989b). <br /> 4.2 CONCEPTUAL SITE MODEL <br /> A Conceptual Site Model (CSM) shows all potentially complete exposure pathways for <br /> a source (Figure 4-1). The CSM identifies potential chemical sources, release <br /> c <br /> mechanisms, transport media, routes of chemical migration through the environment, <br /> exposure media, and potential receptors. Those exposure pathways that are significant <br /> are indicated as being complete; incomplete indicates those pathways in which <br /> 'V contaminant intakes are considered to be relatively insignificant in comparison to other <br /> exposure pathways. EPA guidance defines an insignificant pathway as one that has an <br /> exposure estimated to be two or more orders of magnitude less than by other pathways <br /> (for the same receptor); a pathway is also considered insignificant if the risks are much <br /> less for that pathway, or if the likelihood of exposure by that pathway is very small (EPA <br /> 1989b). <br /> For the YFS site, the complete exposure routes relative to soil are from the residual <br /> contaminants in the subsurface which may volatilize to the surface and pose an <br /> inhalation risk. Because the contaminants are below 15-feet bgs, ingestion and dermal <br /> exposure to contaminated soil are not complete exposure pathways. Typically, the <br /> --4 potential for exposures to contaminants in soil is greatest for soil layers comprising the <br /> 0 to 6-inch soil horizon (i.e., surface soil). Additionally the soil concentrations are <br /> significantly lower than EPA Region IX's health based Preliminary Remediation Goals <br /> (PRGs) which evaluate soil ingestion, dermal exposure to soil, and inhalation (Section <br /> 3.3). Storm water erosion, sediment transport, and fugitive dust pathways are not <br /> complete due to the depth of contamination and the 6 to 8 inches of paving at the site. <br /> The residual contamination is between 34 to 55 bgs. The residual contamination may <br /> �. potentially leach to the groundwater and pose a groundwater ingestion, shower <br /> inhalation, and dermal exposure risk to groundwater from bathing. <br /> ,y <br /> S:\LDC\YELLO.Rn May 4,1995 4-2 <br /> a.+ <br />