RESTORATION AND LONG-TERM RECOVERY OF CONTAMINATED SEDIMENT IN BIG RIVER, OLD LEAD BELT, SOUTHEAST MISSOURI

The Old Lead Belt in the southeast Missouri Ozarks was a global producer of lead (Pb) from 1864 to 1972. Off-site releases of mill wastes resulted in Pb contamination of channel and floodplain deposits along 170 km of the Big River from Leadwood, Missouri to its confluence with the Meramec River near Eureka, Missouri. Present-day transport and remobilization of contaminated sediment poses water quality concerns and toxic risks to aquatic life. This study reports on the effectiveness of two mitigation structures designed to trap and remove contaminated river sediment for land disposal. In spring 2015, the US Army Corp of Engineers installed a large riffle ramp structure in the channel of Big River above Bonne Terre, Missouri.  In addition, an off-line sedimentation basin was constructed within the adjacent floodplain to trap suspended sediment. These structures interrupt the transport of sediment downstream and provide enhanced channel storage capacity in excess of normal river conditions. Monitoring efforts indicate that channel banks are responding by geomorphic adjustment due to channel modifications, and frequent floods have deposited relatively large volumes of sediment in the floodplain basin. The riffle structure has caused increased deposition in upstream bars and chutes to some degree. However, given the amount of available contaminated sediment in the river system, it may take more than a century and/or many more mitigation sites to reduce Pb contamination risk in Big River.