SULFUR SPECIES, BONDING ENVIRONMENT, AND METAL RELEASE IN MINING-IMPACTED LAKE SEDIMENTS UNDER EUXINIC CONDITIONS

The oxidation state of sulfur [S] is a primary control on mobility of metals in metal- and S-rich sediments impacted by legacy mining practices. An estimated 75 Mt of trace element-rich sediments, including 470,000 t of Pb, along with substantial masses of As, Cd, Fe, Mn, Zn, and associated S, were discharged into Coeur d’Alene Lake of northern Idaho over the past 100+ years. The presence of mining waste and the alteration of environmental conditions with seasonal changes in lake sediments allow for remobilization and release of S and metals into the lake water column. Future conditions of the lake are expected to include deposition of algal detritus to the sediment-water interface that may alter S and metal remobilization during the seasonal euxinic environment. Cores of the lake sediments were exposed to anoxic and anoxic + algal detritus conditions for eight weeks at 4 °C. Over the eight-week period and at a location about 2.5 cm below the sediment-water interface, anoxic conditions promoted a shift in S species distribution to continually larger concentrations of reduced species and an associated shift in the bonding environment reflective of increased sulfur-metal bonds. Over the eight-week period and at a location about 12.5 cm deep in the sediments, anoxic condition produced no changes in S species distribution in this continuously reducing environment. These results likely parallel the current lake conditions during seasonal stratification. The addition of algal detritus to the anoxic conditions suppressed the increasing trend of reduced S species at the 2.5 cm location and had no effect on S species at the deeper sediment location. The addition of algal detritus at the sediment-water interface induced greater release of Mn in the upper sediments compared to anoxic only conditions over the first six weeks of the experiment but did not appear to influence the release of As, Cd, or Fe. The addition of complex organic matter as algal detritus to the sediment-water interface of these metal- and S-rich sediments mobilized Mn likely because of dissimilatory metal reduction where algae oxidation stimulated Mn reduction but did not substantially change As or Fe reduction or Cd release from the upper sediments. Results of the study indicate that future metal release from the lake sediments will be altered with the likely deposition of algal detritus, but the effect probably will not influence the release of acutely toxic metals such as As or Cd or substantially influence the redox cycle of Fe in the sediments.