Depositional influences on porewater arsenic in sediments of a mining-contaminated freshwater lake.

Arsenic-containing minerals mobilized during mining activities and deposited to Lake Coeur d'Alene (CDA), Idaho sediments represent a potential source of soluble As to the overlying water. Our objective was to delineate the processes controlling porewater As concentrations within Lake CDA sediments. Sediment and porewater As concentrations were determined, and solid-phase As associations were probed using X-ray absorption near-edge structure (XANES) spectroscopy. Although maximum As in the sediment porewaters varied from 8.4 to 16.2 microM, As sorption on iron oxyhydroxides at the oxic sediment-water interface prevented flux to overlying water. Floods deposit sediment containing variable amounts of arsenopyrite (FeAsS), with majorfloods depositing large amounts of sediment that bury and preserve reduced minerals. Periods of lower deposition increase sediment residence times in the oxic zone, promoting oxidation of reduced minerals, SO4(2-) efflux, and formation of oxide precipitates. Depositional events bury oxides containing sorbed As, transitioning them into anoxic environments where they undergo dissolution, releasing As to the porewater. High Fe:S ratios limit the formation of arsenic sulfides in the anoxic zone. As a result of As sequestration at the sediment-water interface and its release upon burial, decreased concentrations of porewater As will not occur unless As-bearing erosional inputs are eliminated.

Metal(loid) diagenesis in mine-impacted sediments of Lake Coeur d'Alene, Idaho.

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.