Metal remobilisation during resuspension of anoxic contaminated sediment: short-term laboratory study

The sediments of the Tagus estuary North Channel are characterised by high concentrations of trace metals and monosulphides. During dredging operations Cd, Cu and Pb were analysed in water and suspended sediments collected 50 to 100 m around the dredging point. Concentrations in both fractions fluctuated randomly: 2-3 fold for Cd and Cu and 10 for Pb. Since sampling in the dredging point reflects integration of rapid chemical reactions, a short-term laboratory experiment was conducted to follow the geochemical alterations occurring in the highest turbidity sites. The experiment was monitored as a function of time over a period of 4 hr in short time intervals. Dissolved oxygen, pH, EH, AVS, SO42-, Cl- and metals were monitored in the slurry samples. Iron, Mn, Cd, Pb and Cu were determined in the dissolved fraction (<0.45 mu m), in the reactive solid phase and in the total fraction. Resuspension resulted in a significant release of Fe, Mn, Cd, Cu and Pb from the solids. Following the release Pb and Cu were almost totally scavenged in the 4 hr by the newly precipitated Fe oxyhydroxides, while more than 50% of the mobilised Cd remained in the dissolved fraction. The less efficient removal of Cd from solution implies a prolonged availability of this metal in the environment.