Recent influxes of metals into Lake Pepin, a natural lake on the Upper Mississippi River

Abstract

The recent chronology of metal deposition was examined in Lake Pepin, a large natural lake on the Upper Mississippi River about 75 km downstream from the Minneapolis-St. Paul, Minnesota, metropolitan area. The lake, which has a high trapping efficiency for suspended sediments, serves as a sink for metals from industrial and domestic effluents discharged into the river. Sediment cores collected in 1981 from seven locations in the lake were analyzed for Cd, Cr, Cu, Ni, Pb, Zn, and Al (a reference element). Dating of a core with¹³⁷Cs indicated that inputs of these metals increased markedly in the late 1950s, peaked in the early 1970s, and decreased in the late 1970s to 1981. Sediment enrichment factors decreased in the order Pb>Cd>Cu⩾Cr>Zn>Ni. The depth of the most highly contaminated strata in the core profiles decreased longitudinally from upstream to downstream—paralleling the upstreamto-downstream decrease in sedimentation rates. Changes in metal inputs to the lake would be most readily detected by analysis of sediments from the uppermost reaches, but the potential for release of sediment-associated metals to the water column is probably greatest in downstream reaches.