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Environmental Modeling Community of Practice
This portion of the results section gives the Johnson & Ettinger attenuation factor computed using the contaminant, soil and building parameters entered in the upper section of the webpage. The attenuation factor is a proportionality constant relating indoor air concentrations to soil or groundwater concentrations:
αSG = (Cindoor air)/(Csoil gas) or αGW = (Cindoor air)/(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 presents the J&E-model predicted indoor air concentrations. Indoor air concentrations are computed by one of following relations, depending on whether the sample is from soil gas or ground water:
Cindoor air = αSG x Csoil gas or Cindoor air = αGW x Cgroundwater x H
Indoor air concentrations are provided in units of μg/m3 and parts-per-billion-volume (ppbv).
A range of computed indoor air concentrations are presented including "Low Prediction", "Best Estimate" and "High Prediction" categories. The "Best Estimate" concentrations are based on the best guesses of depth to the sample location (or water table, for ground water sample) and residual moisture content for the chosen soil type. The "Low Prediction" and "High Prediction" range of values is computed based on user-specified uncertainty in both depth to the sample and unsaturated zone moisture content. This "bounds" on the best estimate concentration allows users to see what effects the uncertainty in these two values has on resulting indoor air concentrations. A display below the table indicates which combination of the two parameters produced the upper and lower bounds on the best estimate concentrations.
Presented directly below the model-predicted indoor air concentrations are the Cancer Risk levels (for carcinogenic contaminants) and Hazard Quotients (for contaminants that have non-carcinogenic health effects). Cancer risk levels produced by model-predicted indoor air concentrations are computed by the following relation:
Cancer Risk = (Exposure Frequency x Exposure Duration x Unit Risk Factor x Cindoor air) / (Averaging Time x 365 days/year)
If the contaminant has non-carcinogenic health effects (i.e., RfC greater than zero), then the Hazard Quotient produced by the model-predicted indoor air concentration is:
Hazard Quotient = (Cindoor air x Exposure Frequency x Exposure Duration) / (Compound's RfC value x Averaging Time)
Note that some compounds will have both carcinogenic and non-carcinogenic health effects and will, therefore, have computed values for both Cancer Risk and Hazard Quotient.
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