Metal(loid) Diagenesis in Mine-Impacted Sediments of Lake Coeur d'Alene, Idaho

Abstract

Mining activity along the South Fork of the Coeur d' Alene River in northern Idaho has resulted in fluvial mine tailings enriched in Pb, As, Ag, Sb, Hg, Cd, and Zn deposited on the lakebed of Lake Coeur d'Alene, thus serving as a potential benthic source of inorganic contaminants. Our objective was to characterize the dominant solid phase materials and diagenetic processes controlling metal(loid) solubilities, and thus their potential release to the overlying water column. Aqueous and solid concentrations of metal(loid) contaminants were examined along with distinct species of Fe and S within sediments and interstitial water. A gradient from oxic conditions at the sediment-water interface to anoxic conditions below 15 cm exists at all sites, resulting in a dynamic redox environment that controls the partitioning of contaminants. Fluvial deposition from frequent seasonal flood events bury ferric oxides residing at the sediment-water interface leading to reductive dissolution as they transition to the anoxic zone, consequently releasing associated metal(loids) to the interstitial water. Insufficient sulfur limits the formation of sulfidic minerals, but high carbonate content of this mining region buffers pH and promotes formation of siderite. Diagenetic reactions create chemical gradients encouraging the diffusion of metal(loids) toward the sediment--water interface, thereby, increasing the potential for release into the overlying water.