Deposition Patterns and Rates of Mining-Contaminated Sediment within a Sedimentation Basin System, Big River, S.E. Missouri
Flooding events exert a dominant control over the deposition and formation of floodplains. The rate at which floodplains form depends on flood magnitude, frequency, and duration, and associated sediment transport capacity and supply. While it is known that sediment in the Big River is contaminated from historical mining, little is known about the patterns and rates of deposition on floodplains, especially those that have been modified for remediation purposes. The goal of this study was to evaluate the influence of flood characteristics and topography on contemporary deposition patterns and rates in a sedimentation basin system located within a floodplain along the Big River. The basin system was constructed in April 2015 with the purpose of trapping contaminated sediment and reducing downstream lead (Pb) loads. The duration the basin was inundated and cumulative flood peak had the strongest influence on the amount of sediment deposited. A majority of the sediment was deposited close to channel margins near the inlet and chute where flow velocities are reduced in the upper basin. Deposition rates in the upper basin averaged 10.3 cm/yr, which is 10 times greater than average pre-construction floodplain deposition rates. Sediment deposited in the basin system is highly contaminated with average concentrations of 1,142 ppm Pb and 1,223 ppm Zn, with both coarse (2-16 mm) and fine (<2>mm) sediment containing high (>1,000 ppm) concentrations of Pb and Zn.