Seasonal formation and stability of dissolved metal particles in mining-impacted, lacustrine sediments

Formation of dissolved metal particles (<450 nm) in mining-impacted environments is a concern because of their potential for greater mobility and ecotoxicity compared to free ion and(or) sediment-bound metals. Metal-contaminated environments may produce soluble metal(loid) particles whose stability and transportability are determined by environmental conditions and particle composition. The Coeur d'Alene River Basin of northern Idaho, USA, is impacted by legacy mine waste-estimated 56 million tonnes of waste rock containing 900,000 t of Pb and 700,000 t of Zn were discharged into the Coeur d'Alene River and its tributaries during mining of argentiferous galena-sphalerite deposits. These legacy disposal practices resulted in substantial metal contamination-including As, Cd, Fe, Pb, Mn, and Zn-of floodplain sediments. Monthly lakewater samples and sediment cores were collected along the shoreline of a metal-contaminated lateral lake of the Coeur d'Alene River. Porewater was extracted from upper and lower sediments to evaluate the formation and stability of dissolved metal particles during seasonal changes. Substantial concentrations of Fe, Pb, Mn, and Zn were present in 450-nm filtered porewater during each month, with variable increases and decreases of metal concentrations in filtered lakewater according to seasonal changes. Dissolved metal particles with an average diameter of 180 ± 115 nm were present in the porewater of the upper and lower sediments with size increases in early spring and decreases in fall. Particles in the lower sediment porewater were typically more stable, as indicated by more negative ζ potential values, and the greatest particle stability occurred during summer. Differences between upper and lower porewater metal particles correspond to changes in sediment S speciation and bond relocation resulting from an input of oxygenated groundwater. Transport of the dissolved metal particles in and from the sediments likely occurs with a change in the hydraulic gradient during a spring-to-summer transition that induces redox changes and increases particle stability. The presence of mining-related minerals and seasonal changes in environmental conditions allow for formation of dissolved metal particles, but the limited stability of the particles and/or low permeability of the sediments appear to limit, but not fully restrict, possible transport of metal particles to the overlying lakewater.