This document describes the requirements for the Multimedia Environmental Pollutant Assessment System (MEPAS) chronic exposure module. The module is specifically designed for inclusion in the Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES), which is a platform that allows the linking of various multimedia modules into complete source/transport/exposure assessment systems (Whelan et al., 1997). These requirements can be used by software engineers and testers to ensure this module meets the needs of the clients and users.

2.0 Purpose of MEPAS Chronic Exposure Module

The MEPAS chronic exposure module is designed to take time-varying, contaminant concentrations in environmental media (i.e., groundwater, surface water, air, and soil) and generate average contaminant concentrations in exposure media at the point of exposure or contact with receptors. The total time frame over which the environmental media concentrations occur is divided into discrete exposure intervals (which may or may not overlap) as determined by the user, and an average concentration is determined for each of these intervals. Table 1 shows the exposure pathways addressed by this module as a function of the source or transport media. The exposure module receives input data from source and transport modules, the chemical properties database, as well as data supplied by the user through a module user interface (MUI). The average contaminant concentrations in the exposure media are supplied to receptor intake modules or can serve as the endpoint of the simulation.

3.0 Summary of Requirements for 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 sections 4.0, 5.0, and 6.0.

1. 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 Table 1, 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 a user. For external exposure to radionuclides in outdoor air, the module shall take the dose equivalence values produced by the air transport module and perform the averaging.
2.  Allow up to 25 constituents to be considered in a scenario.
3. Operate under Windows 95, and have a user-friendly MUI with a standard Windows look and feel.
4. Meet the module specifications for FRAMES.

Table 1. MEPAS Chronic Exposure Module Exposure Pathways by Transport or Source Medium
 
 

4.0 Input Requirements for the MEPAS Chronic Exposure Module

Data needed to compute contaminant concentrations in an exposure medium are obtained from three sources. The boundary conditions (i.e., time-varying, contaminant concentrations in the source or transport medium) are obtained from the previous module in the scenario (i.e., a transport or source module). The exposure pathways to consider, the time discretization and exposure duration information, and the physical characteristics of the exposure media (as needed) are obtained from the user through the MUI. The contaminant chemical properties (e.g., water purification factors, bioaccumulation factors, food chain transfer factors, etc.) are obtained from a chemical property database. 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. 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 GID file is also used to store the contaminant chemical property data.

1. The MUI will operate in Windows 95 and will have a standard Windows look and feel.
2. The MUI will have on-line help in an HTML format that provides users with an easy-to-understand description of all input parameters required by the MUI.
3. The MUI will provide users with a choice of units for all input parameters having dimensions associated with them, .
4. 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.
5. 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 shown with a green field background.

1. start time for the exposure calculations
2. maximum ending time for the exposure calculations
3. number of time intervals for evaluation.

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. Also, the MUI allows the user to select the exposure pathways to be included in the analysis. Only those pathways that make sense for the current scenario are 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 given in Table 1.

1. When, and only when, an exposure pathway involving soils is selected, the user is allowed to select from two methods for defining the surface soil leach rate constants.  The options are

• Option 1 - The user provides leach rate constants for each contaminant (including progeny) .
• 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 the following parameters:
• surface soil thickness (i.e., effective depth of mixing)
• surface soil moisture content
• soil dry bulk density, infiltration rate
• contaminant distribution coefficient (Kd) for each contaminant (including progeny).

1. The MUI allows 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.
2. 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 where the user selects the model to be used.

1. The MUI accesses a default parameter file and populates the data fields under a separate menu item.
2. The user has access 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.

The MEPAS chronic exposure module will obtain data from the WCF file whenever a ground water or surface water exposure pathway is selected.  This data will include time-varying, instantaneous, aqueous concentrations for each contaminant (including progeny).

The MEPAS chronic exposure module will obtain data from the SCF file whenever a soil exposure pathway is selected.  This 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).

The MEPAS chronic exposure module will obtain data from the ATO file whenever an atmospheric exposure pathway is selected.  The following data will be obtained:

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

1. water treatment purification factor
2. degradation/decay half-life
3. transfer factors for animal feed to meat and milk
4. soil to plant transfer factors
5. deposition velocity from air
6. bioaccumulation factors (for fin fish and shellfish)
7. octanol-water partition coefficient (Kow)
8. Henry’s law constant.

5.0 Output Requirements for the MEPAS Chronic Exposure Module

The MEPAS exposure module is required to produce an Exposure Pathway File (EPF) that contains time-varying, average contaminant concentrations in the exposure medium at the defined exposure point in accordance with the FRAMES data file specifications. The module also is required to produce a listing file (*.ELS file) in ASCII format that documents the data actually read in by the model and provides a summary of intermediate calculation results (e.g., computed leach rate constants for agricultural soil, etc.). The following data is output to the EPF:

1. The exposure module will write 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. Multiple exposure locations are allowed only for the atmospheric transport analysis.
2. The chronic exposure module will write the average radiation dose (Sv) to the EPF file in the specified format, if the ATO file data is in Sv, for the external radiation exposure to outdoor air pathway, specified time interval, contaminant (including progeny), and exposure location. Otherwise the chronic exposure module will write results in intake concentration units (Bq/m^3).

6.0 Scientific Requirements for 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, etc.), and by the selections the user makes in the MUI. The mathematical formulations for this module are provided in Strenge and Chamberlain (1995).

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 one-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 for irrigation of 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.

1. The reduction of contaminant concentrations due to processing at a water supply treatment plant is an option for all exposure pathways involving the domestic use of contaminated water.
2. The loss of contaminants, due to degradation/decay or other processes, from the water distribution system during transport to a domestic use or irrigation location are considered for all exposure pathways involving the domestic or agricultural use of contaminated water.
3. The loss of contaminants, due to degradation/decay or other processes, from plants after harvest but before consumption by animals or exposed individuals is considered for all pathways involving agricultural crops.
4. The accumulation of contaminants in agricultural soil over time is considered for all pathways involving agricultural crops.
5. The processes of deposition to plant surfaces, deposition to agricultural soil with root uptake by plants and animal feed, and deposition to agricultural soil with animal ingestion of soil (in conjunction with feed intake) is considered for all pathways involving atmospheric transport.
6. 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) is considered for all pathways involving atmospheric transport.
7. Air concentrations resulting from atmospheric deposition to soil followed by resuspension, are evaluated using the resuspension factor method (because the contaminant soil concentrations have dimensions of mass of contaminant per unit area). The resuspension of measured contaminant concentrations in soil is evaluated using a mass loading factor because contaminant soil concentrations have dimensions of mass of contaminant per unit mass of soil.
8. Radioactive decay is evaluated explicitly for all processes modeled that involve periods of time, provided the decay has not been previously evaluated by an earlier component. The decay includes production and decay of progeny radionuclides when chain decay is involved.
9. The use of irrigation water for food and animal feed production, and use of water as an animal drinking water source is considered for all groundwater and surface water analyses.
10. 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), is considered for all groundwater and surface water analyses.
11. 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. Therefore, special models more appropriate for tritium and carbon-14 accumulation and transfer to agricultural products are used for these constituents.
12. The MEPAS exposure module evaluates 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.

7.0 References

Strenge, D. L. and P. J. Chamberlain. 1995. Multimedia Environmental Pollutant Assessment System (MEPAS): Exposure Pathway and Human Health Impact Assessment Models. PNL-10523. Pacific Northwest Laboratory, Richland, Washington.

Whelan G., K. J. Castleton, J. W. Buck, G. M. Gelston, B. L. Hoopes, M. A. Pelton, D. L. Strenge, and R. N Kickert. 1997. Concepts of a Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES). PNNL-11748, Pacific Northwest National Laboratory, Richland, Washington.