STORAGE OF MINING-RELATED ZINC IN FLOODPLAIN SEDIMENTS, BLUE RIVER, WISCONSIN

This study investigates spatial patterns of sediment-associated zinc ( Zn) storage in floodplain deposits and the potential reintroduction of these metal-contaminated sediments to the active channel by bank eros ion. We estimate patterns of Zn mass storage by combining longitudinal trends in Zn concentrations with measurements of sediment mass storag e in overbank and point-bar deposits. Overbank deposits are the larges t contaminant sink, storing five times more Zn than the point-bar depo sits. While Zn concentrations decrease downstream because of dilution effects, the total mass of tn stored in floodplains is greatest in bot h the upstream reaches and in the wider lower valleys where low channe l gradients promote rapid sedimentation. Zn storage is low in middle r eaches where steep, narrow valleys with high stream power favor sedime nt transport over deposition. Overall, more than half of the Zn releas ed by mining remains stored in floodplain deposits within the watershe d. The remobilization of Zn from storage is more likely in the upstrea m and mid-basin reaches where high stream power increases rates of lat eral channel migration. Channels in the lower valley lack the stream p ower to migrate laterally and remobilize the large Zn mass stored in o verbank sediments.