Transport of mercury in three contrasting river basins

Total mercury (THg) concentrations and loadings in the Minnesota, Mississippi, and St. Croix Rivers were determined over a 2-year period. These rivers drain watersheds with greatly contrasting land use, soil, and hydrological characteristics. The Minnesota River basin is characterized by a vast artificial drainage network and fertile prairie soils that support the intensive cultivation of row crops, Mercury levels in this river are strongly correlated with total suspended solids concentrations, varying widely in response to precipitation and snowmelt runoff events. The St. Croix River and the headwater Mississippi River (above its confluence with the Minnesota River) drain watersheds characterized by more acidic and sandy soils, more forest and wetland areas, less artificial drainage, and less cultivation as compared to the Minnesota River watershed. Mercury concentrations and loadings in these rivers are much lower than those observed in the Minnesota River, vary over a much narrower range, and reflect a greater dissolved mercury component. These results illustrate the significant influence of watershed characteristics on Hg mobility.