FATE OF CHEMICALLY PERSISTENT METALS IN RIPARIAN WETLAND SYSTEMS

Short term observations indicate that heavy metals are effectively scavenged by riparian wetlands, minimizing transport into adjacent open water bodies. However, studies of sediment cores from wetland and open water environments in north Mississippi indicate that wetland metal sequestration in the past has been a temporary phenomenon. Open water sediments often contain higher concentrations of metals relative to contemporaneous wetland sediments, even after normalization with respect to clay and organic content. One hypothesis is that inflowing metals are initially scavenged by leaf litter, but decomposition over multiple cycles of flooding and aeration result in the remobilization of the metals, with a significant fraction transported out of the wetland.

This hypothesis is being tested in a physical model constructed to simulate transport in a riparian wetland, subject to periodic inundation and aeration, with an adjacent open water body that is continuously flooded. At the start of the study, the wetland cell was layered with naturally senesced cypress needles before inundating both cells with turbid water. Sediment at the upper end of the wetland cell was spiked with metal salts (Co, Cu, Ni, Pb, Zn) before beginning simulation of repeated cycles of low-energy flooding of both sides by sediment-laden water, and subsequent aeration of the wetland cell. Every forth cycle includes a fresh layer of senesced cypress litter in the wetland cell. Sediment and litter samples are being collected after every three inundation-aeration cycles and analyzed for metal concentrations to track migration of the spiked metals. Redox measurements document dramatic changes in redox potential in the wetland cell during transitions between inundated and aerated conditions in the wetland cell, creating variable decomposition conditions in the system; the open water cell remains permanently reducing. Preliminary results indicate that metals are being redistributed within the model as hypothesized. Though not part of the planned study, areas of the model free of algal growth appear to be tracking the lateral migration of the spiked metals.