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What do these results mean?


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 airSG x Csoil gas or Cindoor airGW 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.


How do these results differ from other EPA Johnson & Ettinger Models?

Results produced by this OnSite Johnson and Ettinger model may differ from ones obtained from other EPA J&E sources in the following ways:

1. Temperature - Both the Office of Emergency Response (OERR) and this OnSite Johnson and Ettinger model have user-specified system temperatures. Neither the OSWER Draft Guidance main text nor Appendix G provides the temperatures used in producing the "Figure 3" attenuation factors, though an OSWER representative has stated that a temperature of 10oC was used. Further, page D-6 of the draft guidance says 25oC is used in back-calculating groundwater concentrations.

Usage of Temperature in the Calculations - Temperature is used in computing a temperature-dependent Henry's Law Constant. The temperature-dependent Henry's law constant is then used in computing the effective diffusion coefficient. Diffusivity in air and water has temperature dependence that is not included in any of the three models. The Henry's Law Coefficient is also used in back-calculating acceptable ground water concentrations from a given indoor-air concentration and alpha value.
For example, the Henry's Law Constant for benzene ranges from 0.1152 at 10oC to 0.2263 at 25oC. For a 5-m deep system in loamy-sand, this temperature difference produces a 2% difference in the overall computed effective diffusion coefficient (and essentially no change in the target soil gas concentration) but produces a 93% increase in target ground water concentration for the 10oC system over the 25oC system. Thus, the temperature-dependence of the effective diffusion coefficient is small, while the dependence of the ground water concentration on temperature is large. The OERR model also includes temperature-dependence of dynamic viscosity of air and water in its optional calculation of the Qsoil parameter. In the OnSite model, Qsoil is not calculated but rather entered as an input value. The OSWER draft guidance uses a Qsoil value of 5 L/min in developing their target values.

2. Building Parameters - The "Figure 3" attenuation factors and target media concentrations in the OSWER Draft Guidance were produced assuming a basement foundation structure. Both the OERR spreadsheets and this OnSite model allow for either basement or slab-on-grade structures.

Usage of Building Parameters in the Calculations - Several building-related parameters are used directly in calculating the Johnson & Ettinger attenuation factor including subsurface building area, building crack ratio, and airflow into the building. The OERR spreadsheets, OSWER Draft Guidance and the OnSite Johnson & Ettinger model all include the same suggested parameter values for basement structures. The OnSite model includes the OERR suggested parameter values for the slab-on-grade system.

3. Soil Types - The OSWER Draft Guidance "Figure 3" attenuation factors are only calculated for the following soil types: sand, sandy loam, loamy sand and loam. This OnSite calculator also has only choices of these soil types. The OERR spreadsheets allows for these additional 8 SCS soil textural classifications: sandy clay loam, sandy clay, clay loam, silt loam, clay, silty clay loam, silt, silty clay.

Usage of Soil Types in the Calculations - Soil types are used to pick database values for soil-type-dependent parameters of total and water-filled porosities and capillary zone rise - all of which are used in calculating the overall effective diffusion coefficient for the system. Additionally, soil-type-dependent parameters are used in calculating Qsoil in the OERR spreadsheets.

4. Contaminant Parameters - The OSWER Draft Guidance lists target media concentrations for 114 contaminants of concern, while the OERR spreadsheets and this OnSite Model include only 108 compounds. The additional compounds included in the OSWER guidance are beta-Chloronaphthalene (CAS # 91587); Bis(2-chloroisopropyl)ether (CAS#108601); Bis(chloromethyl)ether (CAS#542881); 1,2-Dibromo-3-chloropropane (CAS#96128); Epichlorohydrin (CAS#106898); and N-Nitro-di-n-butylamine (CAS#924163). Additionally, the OSWER Draft Guidance uses composite, compound-independent values for the diffusivities in air and water of all chemicals. Both the OERR spreadsheets and this OnSite calculator utilize compound-specific values for these parameters.

Usage of Contaminant Parameters in the Calculations - Compound diffusivities in air and water are used in calculating the effective diffusion coefficient for the system. The values of these parameters for most compounds of interest fall within a narrow range. When considering the entire suite of chemicals the values of diffusivity in water do range over an order of magnitude with values of diffusivity in air ranging over 2 orders of magnitude.
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