Mercury and suspended sediment loadings in the lower Minnesota River

Mercury (Hg) enters rivers from both diffuse and point sources; control strategies should be based on reliable estimates of loadings from both source types. In this study, diffuse a nd point source components of total Hg loadings in the Lower Minnesota River (Minnesota) were characterized using clean sampling and analytical techniques. Large seasonal differences in river Hg concentrations and loadings were observed, showing dramatic increases in response to spring snowmelt and precipitation events. Approximately 98% of the annual Hg load in the river was carried during the 9 months from March through November. Total Hg concentrations were highly correlated with total suspended sediment (TSS) concentrations, and filtration experiments confirmed the strong association of Hg with suspended sediments. Measured point source Hg loadings accounted for only 1% of the total annual river Hg loading. These results indicate that Hg inputs to the Lower Minnesota River are driven by sediment input processes in the basin and suggest that efforts to control sediment inputs are necessary to reduce Hg loadings.