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Dispersion in air is modeled from a ground-level source. It is assumed that the air in the outdoor air "box" is well-mixed; The receptor is located onsite, directly over the impacted soil, 24 hours/day for the entire exposure duration; and A long-term average exposure point concentration is estimated for the entire exposure duration. The conceptual model for volatile emissions and inhalation of outdoor air is shown in Figure 2. The assumed receptor location at the edge of the downwind side of the source is the most conservative location that could be used. The dispersion of contaminant in the air, or mixing, is limited to the height of the breathing zone; that is, upward vertical dispersion (i.e., dilution), as the air blows towards the receptor, is not considered in the model. The ASTM VF is actually composed of two equations as presented in Table 5: one equation assumes an infinite source, and the other one equation includes a mass balance check to limit the volatilization term so that the amount volatilized cannot exceed the total amount of mass in the soil initially. The VF is calculated using both equations and the lower of the two volatilization rates is used for the VF in the exposure equations. The default input values are presented in Table 6. Unless there are site-specific conditions, reasonable estimates for the length and width of the source are 25 meters each (approximately 82 by 82 feet). The thickness of impacted soil is assumed to equal 3.05 meters (10 feet). 4.1.3 Particulate Emission Factor A particulate emission factor (PEE) is used to estimate the outdoor air concentrations due to chemicals airborne on particulates (dust). The default value used for the PEF for the residential and commercial/industrial scenarios is 1.3 x 109 [(mg/kg)/(mg/m 3 )] (DISC 2011). For the utility trench (construction) worker, a PEE value of 1 x 106 [(mg/kg)/(mg/m 3 )] was used (DISC 2011). 4.2 Exposure Parameter Values Used All of the default exposure parameters for the receptors were obtained from DTSC's "Human Health Risk Assessment (HHRA) Note Number 1" (DTSC 2011). Table 4 presents the default values for each parameter and provides the reference document where each parameter value was obtained. 4.2.1 Ingestion of Soil Receptors working or playing outdoors may ingest soil through incidental contact of the mouth with hands and clothing. For the residential and commercial exposure scenarios, one of the very conservative assumptions made is that the chemical concentrations remain constant over time in the soil. In reality, this would not be the case, especially for volatile chemicals in the top few feet of soil, where most of the direct contact would occur. Benzene, ethylbenzene and naphthalene are highly fugitive in surface soil, quickly depleting the upper soil depths. 7