FLOODPLAIN STORAGE OF MINING SEDIMENT IN THE BIG RIVER, OLD LEAD BELT, SE MISSOURI

The Old Lead Belt is a historic Pb-Zn mining district within St. Francois County in Southeast Missouri which was a leading producer of lead worldwide from 1869 to 1972. Previous studies by government agencies focused on understanding the dispersal of tailings and lead in channel sediments in order to evaluate toxic effects on fish and mussel communities. However, the role that floodplain storage of Pb might play on long-term contamination trends has not been evaluated. The purpose of this study is to quantify the spatial trends and volumes of mining sediment and Pb contamination within historical overbank deposits in the Big River. Geochemical profiles were examined in over 50 cores collected at cut-bank exposures and across the valley floor with a push-tube coring machine. Peak lead concentrations of >5,000 ppm extend from the town of Leadwood (river km 171) to its confluence with the Meramec River near Eureka (river km 0). Contaminated (>400 ppm Pb) soil depths on floodplains range from 1 to 6 m and average about 2 m. During peak mining periods, heavily contaminated sediments were apparently flushed down the river with little dilution in some cases. However, Pb concentrations in recent floodplain sediments are decreasing both vertically and downstream. The floodplain storage of Pb in the river system is extensive. Bank erosion is now a significant source of Pb contamination along the lower segment of the river.