Seasonal and basinal influences on the formation and transport of dissolved trace metal forms in a mining-impacted riverine environment

Abstract

The release of nanophase metal particles from sulfide mineral decomposition in mining-impacted environments is a growing concern because of the potential for the transport of nanoscale particles that could increase the distribution of the metals and their environmental impact. An analysis of total (unfiltered) and dissolved (450-nm filtered) metal concentrations in the mining-impacted Coeur d'Alene River indicates the leaching of dissolved metal forms from sediments and transport to and within the river. The distribution of metals between total and dissolved forms is driven by seasonal temperatures, hydraulic gradients, and ligand availability. Cd and Zn were the least influenced by changes in gradient and biological productivity between the upper and lower basins. Cd and Zn primarily travel as dissolved forms, with the lowest ratio of dissolved-to-total concentrations in spring and the highest in summer. Fe and Pb primarily travel as suspended particles, but their dissolved forms were greater during all seasons in the lower basin. A principal components analysis of upper basin data indicates that temperature and conductivity were correlated with dissolved Cd and Zn, and total Fe and Pb were correlated with streamflow. In the lower basin, dissolved Cd and Zn, conductivity, and temperature were correlated, and suspended sediment, total metals, and dissolved Pb, but not streamflow, were correlated. The correlation of metals and sediment in the lower basin is not from erosion but the availability of organic matter and Fe that form a range of dissolved to suspended metal particles. The summer decrease in surface water levels releases sediment porewater containing nanoscale-to-microscale metal particles that are transported to open water, where they may impact human and wildlife health. Such releases are unmitigated with current remediation strategies of sediment stabilization.