Reconnaissance of mercury and methylmercury in the St. Croix River and selected tributaries, Minnesota and Wisconsin, July 2000 through October 2001

A reconnaissance-level assessment to characterize total mercury and methylmercury concentrations during summer lowflow conditions was conducted in the St. Croix River Basin during July 2000 through October 2001. Samples were collected at 6 main stem and 16 tributary sites. Loads of total mercury and methylmercury increased in the St. Croix River main stem between Nevers Dam and Franconia. Total mercury and methylmercury concentrations were greatest during July in the Namekagon River. Methylmercury yields in the Namekagon River and Rush Creek were greater than the yield for other tributary streams. Methylmercury concentrations and yields were greater in tributaries draining wetland/forest watersheds than in tributaries draining agricultural/forest watersheds.