A coupled atmospheric transport and soil-air, water-air exchange model is developed to investigate organochlorine pesticides transport on the regional scale. The soil-air exchange module is a dynamic, three-soil layer, fugacity-based model. While the water-air exchange module is a simple two-film mass transfer coefficient dependent model. Processes of re-emission from soil, volatilization and absorption via water are included in the exchange modules. The boundary layer physics package of our 3-D regional scale atmospheric transport, dispersion and deposition model, has a parameterization scheme that accounts for the sub-grid scale heterogeneous terrain effects on the surface layer, boundary layer height and dry deposition variables. Processes of dry and wet depositions are integral parts of the transport and dispersion model.

In the first instance, some preliminary results of g-HCH concentration over Great Lakes and St. Lawrence regions, estimated from the coupled model, are presented and discussed. Using an g-HCH emission inventory, constructed from the knowledge of g-HCH usage data in Canadian prairie provinces (the largest g-HCH usage in the North America) and Ontario and Quebec, the coupled model is implemented for May-June, 1998 to investigate activities of g-HCH over the Great Lakes and St. Lawrence valley. It is found that the upper Great Lakes receive much higher concentration than the lower Great Lakes. This indicates that even though there are no emission sources near the lakes Superior, Huron and Michigan, long distance transport of g-HCH from Canadian Prairie Provinces where g-HCH is applied heavily in canola field, make a major contribution. On the other hand, this preliminary study indicates the long-range transport of g-HCH is not a significant contributor to the Lakes Ontario and Erie, and St. Lawrence valley. However the local sources make a significant contribution to these regions, but overall concentrations are considerably smaller than over the upper Great Lakes region. The total loading due to dry and wet deposition reveals the similar distribution of g-HCH over the Great Lakes and St. Lawrence ecosystem. Modeled concentrations and loadings are compared with the data from the IADN (integrated atmospheric deposition network) monitoring stations and also from some field studies. The comparison suggests that the model-simulated parameters and their distribution are in general agreement.