Diffuse and point source mercury inputs to the Mississippi, Minnesota, and St. Croix Rivers

Total mercury concentrations and loadings in three major rivers in the Minneapolis—St. Paul, Minnesota (USA) metropolitan area were determined using clean sampling and analytical techniques. Variations of total mercury concentrations over time showed strong seasonal influences. Low baseline concentrations in winter reflected primarily groundwater inputs. Climate-driven runoff events resulted in elevated mercury concentrations and loadings throughout the remainder of the year. Over 90% of the annual mercury loading was carried during the 9 months from March through November. These results indicate that diffuse inputs throughout the watersheds dominate mercury loadings in these rivers. Measured mercury inputs from wastewater treatment plants represented less than 4% of the annual river mercury loading. The percentage of downstream river mercury loading attributable to treatment plant inputs varied from less than 1% during high river loadings in the spring and summer, to over 50% during wintertime baseflow.