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BETR North America: A Multimedia Contaminant Fate Model

Progress in understanding contaminant concentrations observed in remote locations requires the development of a computer simulation model that can link these concentrations with long-range transport potential at a continental scale. Researchers at Trent University's Canadian Environmental Modeling Center and Berkeley Lab's Environmental Energy Technologies Division are now developing such a model, the Berkeley-Trent North American contaminant fate model (BETR North America).

BETR is a regionally segmented multi-compartment, continental-scale, mass balance chemical fate model. The model's framework links contaminant fate models of individual regions that encompass a larger, spatially heterogeneous area. It models North America's environment as a group of 24 ecological regions with boundaries based on geographic features mainly waterways and soil types. (See Figure 1.)

Regional segmentation of the BETR North American model

Figure 1. Regional segmentation of the BETR North American model.

The environment within each region is modeled as a system of seven compartments: upper atmosphere, lower atmosphere, vegetation, soil, fresh water, coastal water, and freshwater sediment. A database of hydrological and meteorological data, organized using geographic information systems software, provides the basis for modeling transport between regions in the atmosphere, freshwater, and coastal water media. The model's environment is flexible enough to be adapted to other environmental settings.

Seven equations describe the contaminant fate in each region of the model, so 168 mass balance equations are needed to constitute the model for the 24 regions of North America.

Relation to other models

BETR North America is the first model that can track the movement of persistent organic pollutants on a continental scale. Most existing multimedia models describe the transport and fate of contaminants on a smaller, regional scale, and make the assumption that the region is homogeneous. Examples of these multimedia models include ChemCAN, CalTOX, and SimpleBOX, the model used by the European Union. Like these other models, BETR uses the fugacity-based mass balance concept first proposed by Don Mackay in 1979. Fugacity is a way of representing chemical activity at low concentrations. Fugacity-based multimedia models have been used extensively for modeling the transport and transformation of chemical contaminants in complex environmental systems. Other models focus on air-transport patterns, without accounting for the interaction of the contaminant with water and soil.

Calculated steady-state inventory of toxaphene in fresh water due to a 10,000 kg-year release to the lower air compartment of the Mississippi Delta region.

Figure 2. Calculated steady-state inventory of toxaphene in fresh water due to a 10,000 kg-year release to the lower air compartment of the Mississippi Delta region.

Toxaphene example

An example of the model's results is provided in Figure 2, showing the distribution of the now-banned pesticide toxaphene, which was used extensively in the southeastern United States. Toxaphene is a widely studied persistent organic pollutant and is suspected to be accumulating in the waters of the Great Lakes and their aquatic food chains as the pesticide volatilizes from the soils in which it was deposited. The figure shows the steady-state distribution of toxaphene throughout the 24 regions as a result of a hypothetical release of 10,000 kg/year to the lower air compartment of the Mississippi Delta region.

Conclusions

The BETR North America model can be a valuable tool for assessing long-range transport potential of persistent organic pollutants and other contaminants. Once the model has been fully parametrized, its results can be compared with measured concentrations of contaminants in remote locations, to validate the model's results and deduce continental-scale mass balances. Researchers are currently working to complete its parameterization, including the incorporation of human exposure calculations.

—Thomas McKone and Randy Maddalena

For more information, contact:

  • Thomas McKone
  • (510) 486-4924; fax (510) 486-6658
  • Randy Maddalena
  • (510) 486-6163; fax (510) 486-6658
  • Matthew McLeod

The model was developed by Matthew MacLeod, David Woodfine, and Don Mackay (Trent University), Thomas McKone (Berkeley Lab), Deborah Bennett (formerly Berkeley Lab, now Harvard School of Public Health), and Randy Maddalena (Berkeley Lab).

This research was funded by Canada's Toxic Substances Research Initiative, and the Natural Sciences and Engineering Research Council (NSERC).

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