HISTORICAL TRACE METAL LOADING TO A LARGE RIVER RECORDED IN THE SEDIMENTS OF LAKE ST. CROIX, USA

Steven J. Balogh, Metropolitan Council Environmental Services, St. Paul, MN*
Laura D. Triplett, St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, MN**
Daniel R. Engstrom,St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, MN
Yabing H. Nollet, Metropolitan Council Environmental Services, St. Paul, MN

*Corresponding author. Tel.: (651) 602 8367; fax: (651) 602 8215, steve.balogh@metc.state.mn.us
**Current address: Department of Geology, Gustavus Adolphus College, St. Peter, MN

The global cycling of anthropogenic trace metals intensified during the 20th century, impacting aquatic systems throughout the world. There are, however, few quantitative records showing the history of this contamination in large rivers. Here we present a well-dated sedimentary record of trace metal accumulation in Lake St. Croix, a natural riverine lake on the St. Croix River (Minnesota and Wisconsin, USA), revealing the history of heavy metal inputs to the river over the past 200 years. Concentrations of Hg, Pb, Ag, Cd, Cr and Zn and stable Pb isotopes were measured in eight ²¹⁰Pb-dated sediment cores collected from profundal depositional areas throughout the lake. Time trends of trace metal concentrations and accumulation rates differed greatly between the upper lake (above Valley Creek) and the lower lake, reflecting contrasting sediment sources along the flow axis of the lake. For most of the study period (1800-2000), sediment deposited throughout the lake derived almost exclusively from the suspended sediment load carried by the main-stem river into the lake. From 1910 through 1970, however, large inputs of eroded soils and stream channel sediments from side-valley tributaries resulted in greatly increased sediment and trace metal accumulation in the lower lake. Anthropogenic accumulation rates of Hg, Pb, Cd, Zn, and Ag in the upper lake correlate well with those from Square Lake, a small, relatively undisturbed nearby lake that has received trace metal inputs almost exclusively via atmospheric deposition. The similarity of these records suggests that atmospheric deposition was primarily responsible for trace metal accumulation trends in upper Lake St. Croix. Trace metal accumulation in the lower lake was also strongly influenced by atmospherically-derived inputs, but metal contributions from native soils were important, as well, during the period of elevated sediment inputs from side-valley tributaries.