Global and regional contributions to total mercury concentrations in Lake Michigan water

The Lake Michigan mercury mass balance model, LM2-Mercury, which was calibrated to a comprehensive data set collected from Lake Michigan during 1994-1995, was applied to predict long-term total mercury concentrations in lake water for different mercury loading and air concentration scenarios. The model predictions (volume-weighted, lakewide average total mercury concentrations) appear to be comparable to the available independent measurements from 2005 through 2013. The forecast based on the constant condition scenario, where conditions representative of 1994-1995 were held constant, shows that total mercury concentrations in the lake are near steady-state. The model was used to investigate the relative importance of different global versus regional impact scenarios on total mercury concentrations in Lake Michigan. The results suggest that mercury from global sources could contribute between 30% and 70% of total mercury water concentrations in Lake Michigan. Results for declining global emission scenarios modeled, based on both high and low global contribution estimates and information on mercury emission trends from current observations and the literature, indicate that total mercury concentrations in the water column in Lake Michigan will continue to decrease.