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develop a simple site-specific PRG Source area • Average soil moisture content • Average fraction organic carbon content • Dry soil bulk density The basic principle of the VF, model is applicable only if the soil contaminant concentration is at or below soil saturation. Above this level, the model cannot predict an accurate VFX. If the PRG calculated using VF, was greater than the calculated "sat" (Equation 4-10), the PRG was set equal to "sat" in accordance with Soil Screening Guidance (USEPA 1996 ab). Volatilization Factor for Tan Water For tap water, an upperbound volatilization constant (VF.) is used that is based on all uses of household water (e.g showering, laundering, and dish washing). Certain assumptions were made. For example, it is assumed that the volume of water used in a residence for a family of four is 720 Uday, the volume of the dwelling is 150,000 L and the air exchange rate is 0.25 air changes/hour (Andelman in RAGS Part B). Furthermore, it is assumed that the average transfer efficiency weighted by water use is 50 percent (i.e. half of the concentration of each chemical in water will be transferred into air by all water uses). Note: the range of transfer efficiencies extends from 30% for toilets to 90% for dishwashers. Particulate Emission Factor for Soils Inhalation of chemicals adsorbed to respirable particles (PMIO) were assessed using a default PEF equal to 1. 316 x 10' m'/kg that relates the contaminant concentration in soil with the concentration of respirable particles in the air due to fugitive dust emissions from contaminated soils. The generic PEF was derived using default values in Equation 4-11, which corresponds to a receptor point concentration of approximately 0.76 ug/mi. The relationship is derived by Cowherd (1985) for a rapid assessment procedure applicable to a typical hazardous waste site where the surface contamination provides a relatively continuous and constant potential for emission over an extended period of time (e.g. years). This represents an annual average emission rate based on wind erosion that should be compared with chronic health criteria; it is not appropriate for evaluating the potential for more acute exposures. The impact of the PEF on the resultant PRG concentration (that combines soil exposure pathways for ingestion, skin contact, and inhalation) can be assessed by downloading the PRG tables and displaying the hidden columns. With the exception of specific heavy metals, the PEF does not appear to significantly affect most soil PRGs. Equation 4-11 forms the basis for deriving a generic PEF for the inhalation pathway. For more details regarding specific parameters used in the PEF model, the reader is referred to Soil Screening Guidance: Technical Background Document (USEPA 1996a). 12