Partitioning of arsenic and metals in reducing sulfidic sediments

The sediment in a reservoir on the Clark Fork River in western Montana is contaminated with arsenic, copper, zinc, and other elements. This sediment is the source of groundwater contamination in the adjacent alluvial aquifer, and this study elucidates the processes transferring arsenic to the groundwater by the formation of diagenetic sulfides in the sediment. Vertical trends in a core through oxidized surface sediment into reducing sediment below show that concentration and partitioning of metals and arsenic are controlled by the redox interface. Solid phases of arsenic, copper, and zinc change from dominantly oxyhydroxide and organic phases to sulfide phases across the interface. Arsenic, copper, zinc, manganese, and iron in the pore water are controlled by the solubility of iron and manganese oxyhydroxides in the oxidized zone and metal sulfides in the reduced zone. The change in redox conditions upon burial results in a system where the growth of diagenetic copper, zinc, and arsenic sulfides controls the distribution and partitioning of metals and arsenic in the sediment and the speciation and release of arsenic to the adjacent groundwater system.