Preliminary Results: Release of Metals from Acid-Mine Drainage Contaminated Streambed Sediments under Anaerobic Conditions

Abstract

Many miles of streams in the western U.S are contaminated with acidmine drainage (AMD) from abandoned metal mines. Treatment of these streams may include removal of the existing sediments, with subsequent burial (e.g. in a repository). Burial of previously aerobic sediments may result in release of metals through multiple processes, including reductive-dissolution of metal oxyhydroxides, with concurrent release of previously sorbed metals. This paper discusses preliminary results from a laboratory study examining the release of Cd, Cu, Fe, Mn, and Zn from aerobic streambed sediments collected from an AMDimpacted stream (North Fork Clear Creek, CO). The pH, Eh, and metal release were monitored over twenty-eight days in an anaerobic chamber from sediment slurries prepared using filtered stream water. Additionally, the effect of sediment size was examined by using two size fractions: 2 mm to 63 μm and less than 63 μm. Dissolved Cd, Cu, Fe, and Mn were released and re-sequestered over time from both sediment size fractions. Dissolved Zn, however, demonstrated a continual decrease in concentration over time, relative to the concentration present in the stream water used to prepare the slurries. Releases of Cd, Cu, Fe, and Mn varied for each of the size fractions over time. In the case of Cd and Cu, no further release was observed in either size fraction after one week. Releases of Fe and Mn declined after the first week in the smaller size fraction and after the second week in the larger size fraction. Because of the presence of SO4 in the stream water, it is hypothesized that the metals were re-sequestered as sulfide precipitates. Future studies will include replication, measurement of sulfur species, and the use of surface chemical techniques to examine the solids for the presence of metal oxides and sulfides. Additional