STUDYING THE EFFECTS OF SEDIMENT RESUSPENSION EVENTS ON METAL-SULFIDE STABILITY IN A CONSTRUCTED WETLAND

Contamination in aquatic systems can be remediated naturally through microbially-mediated redox reactions that take place under reducing conditions such as those prevalent in wetlands. Constructed wetlands can be designed to enhance this process for removal of heavy metals by providing an abundance of organic matter, and more specifically a carbon source, in the form of aquatic plants and by supplying additional sulfate through gypsum amendments to the sediment at the time of construction. Growth of sulfate-reducing bacterial populations will be supported in this environment because they can use sulfate as a terminal electron acceptor in the process of oxidizing carbon for energy. When sulfate is microbially reduced to sulfide, sulfide is then available to precipitate with divalent metal cations, such as Cu²⁺ and Zn²⁺.

Metal-sulfide precipitates are generally stable under reducing conditions and serve as a means to sequester metals in aquatic systems. A constructed wetland on the Savannah River Site (SRS) in Aiken, SC, facilitates the removal of Cu and Zn from processing facility effluent in this manner. Metal-sulfides present in the sediments are currently being quantified using acid volatile sulfide (AVS) analysis. Also underway are sediment resuspension studies aimed at determining the risk for storm events to produce conditions under which metal-sulfide stability might be compromised, in which case harmful release of metals into the receiving waterway could potentially occur.