4.5	FINDINGS AND CONCLUSIONS

PRELIMINARY FINDINGS

This and earlier work confirm that:

• significant emissions of persistent toxic substances originate from a variety of different source types and a broad geographical range of locations;
• individual persistent toxic substances exhibit distinct behaviour in the atmosphere, with a wide range of estimated lifetimes/transport distances/re-emission rates, all of which must be accounted for in model development;
• for the contaminants under review in this report, dispersion, transformation, and deposition mechanisms generally serve to reduce quantities deposited as distance from the source increases; [For some other Binational Toxics Strategy pollutants, re-emission or re-volatilization (grasshopper effect) can significantly lengthen the effective transport distance and even cause an eventual accumulation at distances far from the source, in locales such as the Arctic.]
• on a source-by-source basis, all things being equal, nearby sources will generally have a greater depositional impact than more distant sources; [However, the number of sources will increase with increased distance from the receptor, offsetting the lowered depositional impact per source with the presence of more sources.]
• comprehensive models appear to have the capability to identify sources of specific persistent toxic substances to individual lakes in the Great Lakes basin and to quantify their associated deposition.

This project to date has revealed the following:

• For dioxin, it is estimated that approximately 75 percent of deposition to the five Great Lakes from air pathways originates from within the Great Lakes states and provinces.
• In considering sources of atmospheric deposition of dioxin to Lake Ontario, approximately 50 percent appears to originate from sources in close proximity to the lake, while the balance occurs from sources at a much greater distance (400-1500 km [250-930 miles] ); a similar pattern occurs in deposition to Lake Erie and Lake Michigan.
• Regarding deposition to Lake Superior, transport of dioxin from outside the region is relatively more important (40 percent of deposition is from sources between 400 -- 700 km (250 and 435 miles) distant, since there are few immediately adjacent upwind sources. This finding is also applicable to Lake Huron.
• The HYSPLIT model has demonstrated a capability to credibly link specific sources and source regions of a particular persistent toxic substance to quantified deposition in a particular Great Lake. The methodology used appears to yield determinations of reasonable accuracy with relatively moderate computational resource requirements.

To ensure more certainty in the application of this tool, useful additional activities would include:

• a further examination of the quality of the associated emissions inventories with subsequent appropriate upgrading;
• further testing and refinement of the fate and transport modules within the model;
• a more extensive ambient monitoring program in both air and water media to yield additional data for model evaluation purposes; and
• some further comparison with the methodologies, algorithms and outputs of other efforts, such as RELMAP and Models-3, as applied to some of these Binational Toxics Strategy (BTS) contaminants.

PRELIMINARY CONCLUSIONS

Specific to the dioxin example outlined in this work, the board has shown that:

• a binational 'seamless boundary' base map of emissions inventories for dioxin can be prepared and presented in electronic and printed form;
• based on detailed work with dioxin and ongoing efforts for atrazine and cadmium, the NOAA HYSPLIT model can be used to develop transfer coefficients estimating the portion of emissions of those substances from specific locations across Canada and the United States that would be theoretically deposited in individual basins of the Great Lakes;
• applying the best available emissions inventories for these contaminants to the transfer coefficients so developed, quantities deposited in particular Great Lakes from specific point sources, source categories and discrete area sources can be estimated; and
• the overall modeling results for specific contaminants can be verified through comparison to concentration measurements in ambient air and precipitation.

THE IMPLICATIONS OF THE WORK TO DATE

The IAQAB, through its multiyear effort to enhance the understanding of atmospheric deposition to the Great Lakes, has supported development and demonstration of a powerful and efficient methodology for identifying specific sources and source regions of selected persistent toxic air emissions which are deposited in the Great Lakes. The methodology:

• utilizes emissions inventory data and an atmospheric fate and transport model to estimate the atmospheric deposition to the lakes from each emission source. The HYSPLIT model has been used in this work, although other models could also perform the required tasks.
• reinforces the need for a continental, systematic, and sustained approach to improved annual or biennial estimation of emissions, in order to allow modeling that will determine associated trends in deposition reductions and demonstrate progress achieved by control strategies. For successful application to all of the substances of concern in the Great Lakes, more accessible and comprehensive emission information, including the removal, where necessary, of current inventory confidentiality restrictions, is essential.
• systematic measurements of ambient air, precipitation, and water concentrations must be extended to provide information on long term trends of contaminant loadings to and concentrations in Great Lakes waters. These measurements would also facilitate additional model development and model evaluation and validation.
• although there is a large number of mandatory and voluntary control programs for persistent toxic substances on both sides of the border, it is difficult to assess their effectiveness in reducing emissions. This deficiency could be rectified by the Parties producing regular updates of emission inventories in a coordinated binational manner.

RECOMMENDATION

The IAQAB recommends that the International Joint Commission actively advocate, to the U.S. and Canadian governments, the coordinated use of this or a similar methodology to reexamine current control programs and to identify additional actions necessary to address the goals contained in the Great Lakes Water Quality Agreement and the associated Binational Toxic Strategy for the reduction of inputs of persistent toxic substances, particularly those transported via the atmospheric pathway.