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SECTIONF®UR RKPosumAss®ssmeng Residential populations were evaluated along with occupational employees to be conservative in the risk estimates, although EPA guidance indicates that it is not necessary to assume that future hypothetical residential receptors are located on the site or in the vicinity (EPA, 1989b) Residential ' receptors were assumed to be located 500 feet and 1,000 feet downgradient of the site (both in the wind and groundwater flow direction) 4.2 CONCEPTUAL SITE MODEL A Conceptual Site Model (CSM) shows all potentially complete exposure pathways for a source (Figure 4-1) The CSM identifies potential chemical sources, release mechanisms, transport media, routes of cherrucal migration through the environment, exposure media, and potential receptors Those ' exposure pathways that are significant are indicated as being complete, incomplete indicates those pathways in which contaminant intakes are considered to be relatively insignificant in comparison to ' other exposure pathways EPA guidance defines an insignificant pathway as one that has an exposure estimated to be two or more orders of magnitude less than by other pathways (for the same receptor), a pathway is also considered insignificant if the risks are much less for that pathway, or if the likelihood of exposure by that pathway is very small(EPA 1989b) For the Shop 10 site, the complete exposure routes relative to soil are from the residual contaminants in the subsurface which may volatilize to the surface and pose an inhalation risk Soil ingestion and dermal exposure to surface soils were also included in the risk calculations even though the site is paved where COCs in sort are located (Table 3-1) Additionally, the soil concentrations are significantly lower than EPA Region ]Xs health based PRGs for industrial soil PRGs take into account soil ingestion, dermal exposure to soil, and inhalation of volatiles and particulates assuming that there is no paving at the site (Section 3 3) Storm water erosion, sediment transport, and fugitive dust pathways are not consider kccomplete exposure pathways due to the depth of contamination and the 6 to 8 inches of paving at the site The potentially complete exposure pathways relative to groundwater include groundwater ingestion and inhalation of volatiles from groundwater The residual soil contamination may continue to leach to the groundwater and posel a groundwater ingestion risk if the water was used for beneficial purposes we. The closest groundwatef is orated approximately 1,000 feet cross gradient of the site and is screened ' at 500 feet bgs (Section 2 2) The District Manager of the California Water Service Company, Eric Mar, indicated that he is not aware of any hydraulic communication between the shallow aquifer and the deeper aquifer m the area of Shop 10 (E Mar Personal Communication, 1219198) i Wbo4We T%1997W79009NAWOCKTOMREPORT%SNQREPOI Docw*- &991873009NALSNA 4-2