FIELD DOCUMENTS - Laserfiche WebLink

Home Browse Search

Document (USEPA 1996a,b). To address the soil-to-air pathways the PRG calculations incorporate volatilization factors (VF,) for volatile contaminants and particulate emission factors (PEF) for nonvolatile contaminants. These factors relate soil contaminant concentrations to air contaminant concentrations that may be inhaled on-site. The VF, and PEF equations can be broken into two separate models: an emission model to estimate emissions of the contaminant from the soil and a dispersion model to simulate the dispersion of the contaminant in the atmosphere. It should be noted that the box model in RAGS Part B has been replaced with a dispersion term (Q/C) -derived from a modeling exercise using meteorological data from 29 locations across the United States because the box model may not be applicable to abroad range of site types and meteorology and does not utilize state-of-the-art techniques developed for regulatory dispersion modeling. The dispersion model for both volatiles and particulates is the AREA- ST, an updated version of the Office of Air Quality Planning and Standards, Industrial Source Complex Model, ISC2. However, different Q/C terms are used in the VF and PEF equations. Los Angeles was selected as the 90th percentile data set for volatiles and Minneapolis was selected as the 90th percentile data set for fugitive dusts (USEPA 1996 ab). A default source size of 0.5 acres was chosen for the PRG calculations. This is consistent with the default exposure area over which Region 9 typically averages contaminant concentrations in soils. If unusual site conditions exist such that the area source is substantially larger than the default source size assumed here, an alternative Q/C could be applied (see USEPA 1996a,b). Volatilization Factor for Soils Volatile chemicals, defined as those chemicals having a Henry's Law constant greater than 10' (atm-m'/mol) and a molecular weight less than 200 g/mole, were screened for inhalation exposures using a volatilization factor for soils (VF,). Please note that VF;s are available in the electronic version of the PRG table. The emission terms used in the VF, are chemical-specific and were calculated from physical- chemical information obtained from a number of sources including Supetfund Exposure Assessment Manual (SEAM, EPA 1988), Subsurface Contamination Reference Guide (EPA 1990a), Fate and Exposure Data (Howard 1991), and Superfund Chemical Data Matrix (USEPA 1994c). In those cases where Diffusivity Coefficients (Di) were not provided in existing literature, Di's were calculated using Fuller's Method described in SEAM. A surrogate term was required for some chemicals that lacked physico-chemical information. In these cases, a proxy chemical of similar structure was used that may over- or under-estimate the PRG for soils. Physico-chemical information is available in the electronic version of the PRG table. To access this information, the user should display the hidden columns in the table. Equation 4-9 forms the basis for deriving generic soil PRGs for the inhalation pathway. The following parameters in the standardized equation can be replaced with specific site data to 11