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Environmental Modeling Community of Practice
This portion of the results section gives the computed effective diffusion coefficients for the unsaturated zone and the combined unsaturated and capillary zone, the A, B, and C parameters defined by Paul Johnson (2002) and the Johnson & Ettinger attenuation factors computed using the contaminant, soil and building parameters entered in the upper section of the webpage.
The effective diffusion coefficient for either the unsaturated zone (for soil gas media) or for the combined unsaturated and capillary zones (for ground water contamination) controls diffusion through the subsurface.
The attenuation factor is a proportionality constant relating indoor air concentrations to soil or groundwater concentrations:
Cindoor air=αSG x Csoil gas or Cindoor air=αGW x Cgroundwater x H
The larger the attenuation factor, the less the vapor concentration will attenuate between the subsurface and indoor air. There are two attenuation factors computed: one for soil gas (αSG) and one for groundwater (αGW). The groundwater attenuation factor is less than the soil gas attenuation factor because of the additional impedance to diffusion caused by the very wet capillary fringe.
Also presented here are the parameters A, B and C and a short description of the transport mechanisms implied by analysis of these parameters. The user is directed to Johnson, P.C. (2002) for further explanation of these parameters and their usefulness.
This portion of the results section computes the risk-based indoor air concentration. This concentration is computed differently depending on whether the contaminant of concern has carcinogenic or non-carcinogenic health effects. For contaminants that are carcinogens (i.e., have URF greater than zero), the target indoor air concentration is:
Ccancer = [(Target Cancer Risk x Averaging Time)/(Exposure Frequency x Exposure Duration x Unit Risk Factor)]
The default Target Cancer Risk is set to 1 x 10-6, but users may select from Target Risks of 1 x 10-5 and 1 x 10-4 as well.
If the contaminant has non-carcinogenic health effects (i.e., RfC greater than zero), then the target indoor air concentration is:
Cnon-cancer = [(Target Hazard Quotient x Averaging Time x Compound's RfC)/(Exposure Frequency x Exposure Duration)]
If the contaminant has both carcinogenic and non-carcinogenic health effects, then the lower of the target indoor air concentrations produced by the two methods above is used. The basis of the target indoor air concentration is presented on the output screen. Note that a Risk Factor is not used in calculating the non-carcinogenic indoor air concentration. If target indoor air (IA) concentrations are based on non-cancer risk, no changes in IA concentrations will be produced when selecting a different Risk Factor.
This portion of the results section presents the target media concentrations. Target soil-gas concentrations are produced by dividing the target indoor air concentration (explained above) by the soil gas attenuation factor. Target soil-gas concentrations are provided in units of μg/m3 and parts-per-billion-volume (ppbv). Target groundwater concentrations are produced by dividing the target indoor air concentration by both the groundwater attenuation factor and the compound's Henry's Law Constant. Target groundwater concentrations are provided only in units of μg/L.
A range of soil gas and groundwater target concentrations are presented including "Less Protective", "Best Estimate" and "More Protective" categories. The "Best Estimate" concentrations are based on the best guesses of depth to the contamination source and residual moisture content for the chosen soil type. The "Less Protective" and "More Protective" range of values is computed based on user-specified uncertainty in both depth to the contamination source and unsaturated zone moisture content. The "bounds" on the best estimate concentration allows users to see what effects the uncertainty in these two values has on resulting target media concentrations. A display below the table indicates which combination of the two parameters produced the upper and lower bounds on the best estimate concentrations.
If the computed target groundwater concentration is greater than the solubility of the given compound in water, then the word "Solubility" is displayed in the target groundwater concentration location of the table. Similarly, if the computed target soil-gas concentration is greater than the Henry's Law Constant partitioning of the given compound at solubility, then the word "Saturation" is displayed in the target soil-gas concentration location of the table.
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