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3 CONCEPTUAL SITE MODEL <br />This section describes the exposure scenarios and receptors considered in the development of the soil <br />screening levels. Soil screening levels were developed for two different soil horizons. A schematic of <br />the conceptual site model for the two soil horizons is shown in Figure 1. <br />3.1 Exposure Pathways <br />The soil screening levels consider four exposure pathways simultaneously: <br />incidental ingestion of soil; <br />dermal contact with soil; <br />inhalation of volatile soil emissions; and <br />inhalation of particulate emissions. <br />Incidental ingestion of and dermal contact with soil are direct exposure pathways, i.e., the receptor is <br />assumed to contact the soil directly and, therefore, the exposure point concentration is the actual <br />concentration in soil. For the inhalation exposure pathways, the exposure medium is outdoor air; the <br />outdoor air concentration must be estimated using volatilization and particulate emission factors. <br />3.2 Receptors Considered <br />Soil screening levels were calculated for three exposure scenarios. The exposure scenarios considered <br />were: <br />residential; <br />commercial/industrial worker; and <br />a worker in a utility trench or similar construction project (utility worker). <br />It is assumed that all four of the exposure pathways (discussed in section 3.1) are potentially complete <br />for each scenario. However, the input parameter values are different for each receptor. <br />For the residential exposure scenario, it is assumed that the receptor is a child for 6 years and then an <br />adult for 24 years. When calculating carcinogenic risk, the total intake of a chemical over a lifetime is <br />used; therefore, the carcinogenic residential screening levels are protective of the combined child plus <br />adult scenario. For benzo(a)pyrene (PAHs), the mutagenic exposure equations are used for calculating <br />the screening level. In this case, the early life exposures (i.e., 0 to 2 years, 2 to 6 years and 6 to 16 years) <br />are weighted more than they are in the non-mutagenic equations (Table 1). For noncarcinogenic health <br />effects, the intake is not added over the exposure period. In this case, the child is the more sensitive <br />5