Tributary loading of mercury to Lake Michigan: Importance of seasonal events and phase partitioning

As a component of a lakewide mass balance study for Lake Michigan, we measured total mercury (HgT) concentrations and fluxes in 11 selected tributaries. Unfiltered HgT concentrations ranged from 0.56 ng l−1 at the Pere Marquette River to 182 ng l−1 at the Fox River. Highest mean HgT concentrations were observed in the Fox R., Indiana Harbor Ship Canal, Grand R. and the Kalamazoo R. Mean particulate matter HgT content ranged from about 0.1 to 1.5 μg g−1, with highest levels from the industrialized basins of the Indiana Harbor and Fox River. Highest tributary loading rates (g day−1) were observed from the Fox, Grand, Kalamazoo and St. Joseph Rivers. Increased loading rates during spring melt and summer/fall storm events in these tributaries were generally associated with particulate loading from either sediment resuspension or erosional processes. In contrast, filtered HgT represented 80% of the HgT flux in the Manistique R., whose watershed is comprised almost entirely of wetlands and forest.