Requirements of the MEPAS Chronic Exposure Module

This section provides an overall summary of the requirements for the MEPAS Chronic Exposure Module. Detailed input, output, and scientific requirements are described in the sections that follow.

The MEPAS Chronic Exposure Module will

• G1 - compute chemical and radionuclide contaminant concentrations in the exposure medium at the point of exposure or contact with receptors for all exposure pathways shown in the table following the list, except for external exposure to radionuclides in outdoor air from a passing plume. The values are averaged for each interval of a set of exposure duration intervals specified by the user. For external exposure to radionuclides in outdoor air, the module is required to take the dose equivalence values produced by the atmospheric transport module and perform the averaging.
• G2 - allow up to 25 constituents to be considered in a scenario.
• G3 - meet the module specifications for the Framework for Risk Analysis in Mulimedia Environmental Systems (FRAMES).

Input Requirements of the MEPAS Chronic Exposure Module

Data needed to compute contaminant concentrations in an exposure medium are obtained from three sources. The previous module in the scenario (i.e., a transport or source module) provides the boundary conditions (i.e., time-varying, contaminant concentrations in the source or transport medium). The boundary conditions are communicated to the module through the Water Concentration File (WCF) for groundwater and surface water transport pathways, the Atmospheric Transport Output (ATO) file for the air pathway, and the Soil Concentration File (SCF) for contaminants at a contaminated soil source.

Through the MUI, the user provides exposure pathways to consider, the time discretization and exposure duration information, and the physical characteristics of the exposure media (as needed). The input data supplied through the MUI is communicated to the model through the Global Input Data (GID) file and the model pre-processor.

The chemical property database provides contaminant chemical properties (e.g., water purification factors, bioaccumulation factors, food chain transfer factors, etc.). The GID file is also used to store the contaminant chemical property data.

Thus, the general input requirement (I1) for the Chronic Exposure Module is to accept input from the WCF, ATO, SCF, and GID files and the module pre-processor.

There are also some general requirements associated with the MUI, which are as follows:

• M1 - The MUI will operate in Windows 95, 98, 2000, NT, ME, and XP and will have a standard Windows look and feel.
• M2 - The MUI will have online help in an HTML format that provides users with an easy-to-understand description of all input parameters required by the MUI.
• M3 - The MUI will provide users with a choice of units for all input parameters having dimensions associated with them.
• M4 - The MUI will include a reference feature in which the source of the specified value for each input item can be referenced if the user desires.
• M5 - The MUI will show the range of values allowed for each input data item, when the cursor is positioned on that item, as a scrolling message at the bottom of the screen. When an out-of-range value is entered in a field, the MUI will indicate this by a red background in the input field and a scrolling error message in addition to the allowed range message. Data input values within range are indicated by a green field background.
• M6 - The MUI will allow the user to input exposure control data (start time for the exposure calculations, maximum ending time for the exposure calculations, and number of time intervals for evaluation), which will apply globally to all four source and transport media. The duration of each time interval is equal to the exposure duration, which is defined by the user for each of the four source and transport media.
• M7(a) - The MUI will allow the user to select the exposure pathways to be included in the analysis. Only those pathways that make sense for the current scenario will be available as options to the user. For example, if the groundwater medium is the only medium being considered, then dermal contact while swimming is not an available exposure pathway and should either be grayed out or not shown at all. The allowed exposure pathways for each medium are table above
• M7(b) - The MUI will allow the non‑functional text boxes to be displayed as unchecked, even if the user had checked them previously. This will make it clear to the user that only their selections will be performed by the model.

The MUI must also allow the user to define additional parameters needed in the analysis. These parameters include the following:

• M8 - When, and only when, an exposure pathway involving soils is selected, the user will be allowed to select from two methods for defining the surface soil leach rate constants. Under Option 1, the user provides leach rate constants for each contaminant (including progeny). Under Option 2, the user provides soil characterization data and distribution coefficients, and the program calculates the leach rate constant. For this option the MUI requires input of surface soil thickness (i.e., effective depth of mixing), surface soil moisture content, soil dry bulk density infiltration rate, and contaminant distribution coefficient (Kd) for each contaminant (including progeny).
• M9 - The MUI will allow specification of the exposure duration for the exposed individual in the scenario, with separate values specified for groundwater, surface water, atmospheric, and measured soil exposures.
• M10 - When atmospheric transport is the source of contamination, the MUI will display the X,Y coordinates entered by the user on the General Input screen to specify the exposure location relative to the release point. The General Input screen is the screen on which the user selects the model to be used.
• M11 - The exposure module will also provide an option to customize the exposure data specific to each exposure pathway. The MUI will access a default parameter file and populate the data fields under a separate menu item.
• M12 - The user will be allowed to access the parameters under the Customize menu and change the values for the current exposure scenario. The values will be saved for retrieval and display when the MUI is next entered.

The user will have access (M13) to the following parameter sets:

• soil aerial density, plant retention fraction, and crop yields
• animal feed, water, and soil intake rates
• translocation factors to edible parts of plants
• fraction of animal feed and water that is contaminated
• delay times between harvest and consumption for farm products
• crop growing periods for four sources of contamination (groundwater, surface water, atmospheric deposition, and initial soil contamination)
• resuspension factors and mass loading factor
• indoor air factors to relate domestic water concentration to indoor air concentration for volatile chemicals, radon, and all other constituents.

Addition input requirements are as follows:

• I2 - obtain data from the WCF whenever a groundwater or surface water exposure pathway is selected. These data will include time-varying, instantaneous, aqueous concentrations for each contaminant (including progeny).
• I3 - The MEPAS Chronic Exposure Module will obtain data from the SCF whenever a soil exposure pathway is selected. These data will include the initial, instantaneous, total soil concentration on a bulk-volume basis (i.e., total mass of contaminant per bulk volume of soil) for each contaminant (including progeny).
• I4 - The MEPAS Chronic Exposure Module will obtain data from the ATO file whenever an atmospheric exposure pathway is selected.

• time-varying, annual average, airborne concentrations for each contaminant and all pollutant types (i.e., gaseous, particle 1, etc.)
• time-varying, annual average, deposition flux rates for each contaminant and all pollutant types
• time-varying, annual average dose equivalent for external radiation exposure to outdoor air in a passing plume for each radioactive contaminant and all pollutant types.

As needed, the following data are obtained (I5) from the chemical properties database:

• water treatment purification factor
• degradation/decay half-life
• transfer factors for animal feed to meat and milk
• soil-to-plant transfer factors
• deposition velocity from air
• bioaccumulation factors (for fin fish and shellfish)
• octanol-water partition coefficient (Kow)
• Henry's law constant.

Output Requirements of the MEPAS Chronic Exposure Module

The exposure module has the following output requirements:

• O1 - produce an Exposure Pathway File (EPF) that contains the average exposure medium concentration to the EPF in the specified format for each selected exposure pathway (except for external radiation exposure to outdoor air), specified time interval, contaminant (including progeny), and exposure location. Allow multiple exposure locations only for the atmospheric transport analysis.
• O2 - produce a listing file (*.ELS file) in ASCII format that documents the data actually read in by the model and summarizes intermediate calculation results (e.g., computed leach rate constants for agricultural soil, etc.).
• O3 - write the average radiation dose (Sv) to the EPF file in the specified format, if the ATO file data are in Sv, for the external radiation exposure to outdoor air pathway, specified time interval, contaminant (including progeny), and exposure location. If the ATO file data are not in Sv, write results in intake concentration units (Bq/m^3).

Scientific Requirements of the MEPAS Chronic Exposure Module

The scope of the exposure analysis is determined by the transport or source modules connected to the exposure module (e.g., atmospheric transport, aquifer) and by the selections the user makes in the MUI. The mathematical formulations for this module are provided in Strenge and Chamberlain, 1995 (PNL-10523).

The contaminant concentration in the source or transport medium is the starting point for the exposure analysis. Except for the atmospheric transport medium, this concentration is an instantaneous value. For the atmospheric transport medium, the concentrations are averaged over a 1-year period and the deposition rates are annual average values. The transport medium may or may not be the medium of exposure. For example, the groundwater transport pathway generates estimates of contaminant concentration in the groundwater at the well. In this case, the well water is also the medium of exposure, although some modifications to the concentration are possible during transfer through the treatment plant and distribution system to the individuals exposed during domestic water uses. When the well water is used to irrigate agricultural crops, the exposure medium is not the well water, but the foods produced. For agricultural pathways, models are used to estimate the transfer of pollutants from the irrigation water to the food consumed by humans.

The processes affecting the transfer of contaminants from the transport or source medium to the exposure medium are defined in the following list of scientific requirements for the exposure module:

• S1 - include the reduction of contaminant concentrations from processing at a water supply treatment plant as an option for all exposure pathways involving the domestic use of contaminated water.
• S2 - consider the loss of contaminants, through degradation/decay or other processes, from the water distribution system during transport to a domestic use or irrigation location for all exposure pathways involving the domestic or agricultural use of contaminated water.
• S3 - consider the loss of contaminants, through degradation/decay or other processes, from plants after harvest but before consumption by animals or exposed individuals for all pathways involving agricultural crops.
• S4 - consider the accumulation of contaminants in agricultural soil over time for all pathways involving agricultural crops.
• S5 - consider the processes of deposition to plant surfaces, to agricultural soil with root uptake by plants and animal feed, and to agricultural soil with animal ingestion of soil (in conjunction with feed intake) for all pathways involving atmospheric transport.
• S6 - consider the process of atmospheric deposition to residential soil for evaluation of soil exposure pathways (external ground exposure, soil ingestion, soil dermal contact, and inhalation of resuspended contaminants) for all pathways involving atmospheric transport.
• S7 - evaluate using the resuspension factor method air concentrations resulting from atmospheric deposition to soil followed by resuspension (because the contaminant soil concentrations have dimensions of mass of contaminant per unit area). Evaluate measured contaminant concentrations in soil using a mass loading factor because contaminant soil concentrations have dimensions of mass of contaminant per unit mass of soil.
• S8 - evaluate radioactive decay explicitly for all processes modeled that involve periods of time, provided the decay has not been previously evaluated by an earlier component. Include in the decay production and decay of progeny radionuclides when chain decay is involved.
• S9 - consider the use of irrigation water for food and animal feed production, and use of water as an animal drinking water source, for all groundwater and surface water analyses.
• S10 - consider the processes of irrigation water deposited on plant surfaces and agricultural soil, with root uptake by plants and animal feed and animal ingestion of soil (in conjunction with feed intake), for all groundwater and surface water analyses.
• S11 - use special models for tritium and carbon-14. Tritium occurs in the environment and food chain as tritiated water, and carbon-14 occurs as carbon dioxide. Because water and carbon dioxide are normal constituents in biological systems, these contaminants do not behave in biological systems like trace pollutants, hence the need for special models to calculate tritium and carbon-14 accumulation and transfer to agricultural products.
• S12 - evaluate exposures for the measured soil transport with the soil concentration being defined at the start of the analysis (as opposed to being represented as an average value over a period of time). The change in initial soil concentration with time is accounted for by loss from the soil for radioactive decay, leaching, and by other processes as described by the soil loss rate constant for the constituent.

Other References

• Concepts of a Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES). (Whelan et al. 1997. PNNL-11748).

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