HA1 Contaminated Harbour and River Sediment
256 Portland Ballroom
8:00 AM - 12:00 PM, Thursday
() Mechanisms of Metal Release from Contaminated Sediments in Coastal Massachusetts.
Kalnejais, L1, Martin, W2, Francois, R2, Bothner, M3, 1 Massachusetts Institute of Technolgy-Woods Hole Oceanographic Institution Joint Program, Woods Hole, MA, USA2 Woods Hole Oceanographic Institution, Woods Hole, MA, USA3 United States Geological Survey, Woods Hole, MA, USA
ABSTRACT- The sediments of coastal Massachusetts contain elevated levels of potentially toxic metals. This study examines the geochemical and physical processes that account for metal release from sediments to the overlying water column. High resolution porewater and sediment analysis was conducted in sediment cores collected seasonally at two locations. The first is a heavily contaminated site in Boston Harbor, which received high inputs of contaminants from a nearby sewage outfall until 2000. The second site, 15 km seaward in Massachusetts Bay has lower organic carbon and solid phase trace metal concentrations. Porewater profiles of silver, lead and copper indicate these three metals behave differently in the upper 2 cm of the sediment profile but have coincident behavior at depth. Lead is strongly scavenged by iron oxide phases so becomes diagenetically focused in the oxic zone. Due to the strong scavenging there is no release of lead to the overlying water as long as there is an oxic layer in the sediments. In all seasons there is a strong gradient of dissolved copper at the sediment water interface indicating a persistent release of copper to the overlying water. Silver porewater profiles indicate a flux to the overlying water only in winter. Preliminary results indicate that sediment resuspension can enhance metal release. An erosion chamber was used to incrementally increase the shear stress at the sediment surface and induce sediment resuspension. At erosion depths of a few mm, still within the oxic zone, a significant release of dissolved copper was measured. Lead was actually removed from the dissolved phase during the erosion experiments due to adsorption onto resuspended particles. The results from the erosion chamber will be compared with the porewater fluxes to determine the relative importance of each remobilization mechanism. Integration of the results from the diagenetic studies and the resuspension experiments gives an assessment of the long term fate of the metals in sediments of Boston Harbor and Massachusetts Bay.
Key words: silver, sediment geochemistry, Boston Harbor, resuspension