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The DATENTER worksheet of each of the soil gas models is different than those of the soil <br /> and groundwater models. Figure 10 shows the DATA ENTER worksheet of the SG-ADV model. <br /> Note that there is no option for running the model to calculate a risk-based media concentration. As <br /> with the other models, the user enters the CAS number of the chemical of interest. This <br /> automatically retrieves the chemical and toxicological data for that chemical. The CAS number must <br /> match one of the chemicals listed in the VLOOKUP worksheet, or the message "CAS No. not found" <br /> will appear in the. "Chemical" box. The user also has the opportunity to add new chemicals to the <br /> data base. Next, the user must enter a value for the soil gas concentration of the chemical of interest. <br /> The user may enter this value in units of ug/m3 or parts-per-million by volume(ppmv). If the soil <br /> gas concentration is entered in units of ppmv, the concentration is converted to units of pg/m3 by: <br /> C _ Cg x MW (33) <br /> g RxTS <br /> where Cg' = Soil gas concentration, pg/m3 <br /> Cg = Soil gas concentration, ppmv <br /> MW = Molecular weight, g/mol <br /> R = Gas constant(= 8.205 E-05 atm-m'/mol-OK) <br /> Ts = System (soil)temperature, 'K. <br /> In the soil gas models, the steady-state indoor air concentration is calculated by Equation 19 <br /> (i.e., Cbuilding= a -source). The value of the vapor concentration at the source of emissions (Csource) <br /> is assigned the value of the user-defined soil gas concentration. The value of the steady-state <br /> attenuation coefficient(a) in Equation 19 is calculated by Equation 13. Because no evaluation has <br /> been made of the extent of the source of emissions, steady-state conditions (i.e., a non-diminishing <br /> source) must be assumed. <br /> The SG-SCREEN model operates under the assumption of homogeneously distributed soil <br /> properties and isotropic conditions with respect to soil vapor permeability from the soil surface to <br /> an infinite depth. The SG-ADV model, on the other hand, allows the user to specify up to three <br /> different soil strata between the building floor in contact with the soil and the soil gas sampling <br /> depth. Soil properties within these three strata may be varied to allow for different diffusion <br /> resistances to vapor transport. <br /> 4.2 SOIL GAS SAMPLING <br /> In order to use the soil gas models, soil gas concentrations must be measured at one or more <br /> depths below ground surface (bgs). The user is advised to take samples directly under building slabs <br /> 61 <br />