A sediment record of trace metal loadings in the Upper Mississippi River

Sediment cores from Lake Pepin, a natural lake on the Upper Mississippi River, reveal the historical trends in trace metal use and discharge in the watershed. Lead-210 dated concentration profiles of trace metals (Ag, Cd, Cr, Cu, Hg, Pb, V, Zn) in sediment cores from throughout the lake generally showed low and stable concentrations prior to settlement (circa 1830), peak concentrations between 1940 and 1975, and substantial decreases thereafter. Whole-lake sediment accumulation rates increased greatly over the period of record, from 79,000 metric tons year−1 prior to 1830, to 876,000 metric tons year−1 during the 1990s. Whole-lake accumulation rates of most trace metals peaked in the 1960s but decreased sharply after that. Sediment and trace metal accumulation rates decreased in the downstream direction, and approximately two-thirds of the sediment and trace metal mass accumulated in Lake Pepin since 1800 was deposited in the upper 30% (by area) of the lake. The dramatic declines in trace metal concentrations and accumulation rates in Lake Pepin sediments since 1970 coincide with increased pollution control and prevention efforts throughout the watershed, including the implementation of secondary treatment at a large municipal wastewater treatment plant upstream.