Impacts of lindane usage in the Canadian Prairies on the Great Lakes ecosystem. 2. Modeled fluxes and loadings to the Great Lakes.

Atmospheric loadings of gamma-hexachlorocyclohexane (gamma-HCH) from May 1, 1998, to April 30, 1999, to the Great Lakes simulated by a coupled soil-air and water-air atmospheric transport model are presented. Modeling results on an annual basis indicate that Lake Superior received the highest dry deposition load of 2.17 kg yr(-1). Dry deposition to the lower Great Lakes (Lakes Erie and Ontario) was 2-6 times lower as compared to the upper Great Lakes. Greater deposition in the upper Great Lakes is due mainly to their larger surface area and greater proximity to sources of gamma-HCH. Dry deposition fluxes (pg m(-2)) to lake surfaces were much lower than to land as a result of lower deposition velocities and lower air concentrations of gamma-HCH over the lakes. The highest gamma-HCH loading (kg yr(-1), in 1998-99) due to wet deposition occurred for Lake Ontario. This was mainly attributed to greater annual rainfall over Lake Ontario. An investigation of average seasonal fluxes predicted by the model shows that deposition fluxes to the Great Lakes are considerably higher in the summer than that in the autumn and winter seasons. The net direction of gas exchange also exhibits a seasonal dependence. Lakes Michigan, Huron, and Ontario show net absorption in the summer 1998 whereas at all other times net outgassing occurred at all of the lakes. Overall, gas exchange was the dominant process affecting loadings to the Great Lakes. Model-derived loadings and total deposition flows across the Great Lakes basin due to dry and wet depositions and net gas exchange agree reasonably well with the summer estimates compiled by the Integrated Atmospheric Deposition Network whereas autumn values show greater discrepancies. Better agreement was also observed for dry deposition as compared to wet deposition. Specifically, to improve short-term loading estimates (e.g., over days to months), the model results indicate the need for better spatially and temporally resolved information on concentrations in air and surface water and better estimates of precipitation and deposition velocities over the lakes.