Partitioning of arsenic and metals in reducing sulfidic sediments

Sediment in a reservoir of Clark Fork River, MT, is laden with heavy metals. This sediment is the source of groundwater contamination in the adjacent alluvial aquifer. The processes transferring arsenic to the groundwater by the formation of diagenetic sulfides in the sediment were investigated. Vertical trends in a core through oxidized surface sediment into reducing sediment show that concentration and partitioning of metals are controlled by the redox interface. Solid phases of As, copper, and zinc change from dominantly oxyhydroxide and organic phase to sulfide phases across the interface. The change in redox conditions upon burial results in a system where the growth of diagenetic Cu, Zn, and As sulfides controls the distribution and partitioning of metals in the sediment and the release of As to the groundwater system.