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receptor; therefore the noncarcinogenic screening levels are developed for a child receptor and are <br />protective for the adult resident. <br />The commercial/industrial exposure scenario assumes that the receptor is an adult and works in an <br />office or outdoors at a site. In this scenario, it is assumed that the receptor works for a total of 25 years <br />at 250 days/year at the same location. <br />For the utility or construction worker, the exposure duration is assumed to be much shorter than in the <br />other two scenarios (1 year); however, the chemical intake per day is assumed to be higher due to <br />increased incidental ingestion and/or increased inhalation rates. <br />3.3 Depths to Which the Screening Levels Apply <br />Two sets of screening levels were developed for the residential and commercial/industrial scenarios <br />based on depth of impacted soil: one set applies to 0 to 5 feet bgs and the other set applies to 5 to 10 <br />feet bgs. The full depth of 0 to 10 feet is assumed to contribute to outdoor air concentrations for all <br />scenarios. <br />For the residential and commercial/industrial exposure scenarios, it is assumed that residents and <br />commercial workers could contact soil at depths between ground surface and 5 feet bgs. For the utility <br />or construction worker, it is assumed that direct contact with soil could occur at depths between ground <br />surface and 10 feet bgs. <br />4 DERIVATION OF SCREENING LEVELS <br />This section describes how the soil screening levels were calculated. Except the volatilization term, <br />standard equations from the USEPA RSLs were used. A target risk level of 1x10-6 for carcinogens and a <br />target hazard index of 1.0 for noncarcinogens were used. <br />4.1 Equations Used <br />4.1.1 Exposure Equations <br />The equations used to develop the soil screening levels are presented in Tables 1 through 3 for each <br />receptor. The variable definitions are presented in Table 4. USEPA considers the carcinogenic PAHs to <br />be "mutagens" and as such, has unique equations to calculate screening levels. The mutagenic <br />equations are for "early life exposures" and therefore only apply to the residential scenario. <br />4.1.2 Volatilization Factor <br />The volatilization factor (VF) used to predict an outdoor air concentration due to volatilization from the <br />soil is based on the ASTM guidance (1995). The assumptions in the ASTM volatilization factor algorithm <br />(ASTM 1995) are: <br />6