Changes in the total content and speciation patterns of metals in the dredged sediments after ocean dumping: Taiwan continental slope

The aim of this study was to understand the changes in heavy metals and speciation patterns in the dredged sediments after ocean dumping by a five-step sequential extraction procedure. The potential mobility and bioavailability of heavy metals in the sediments were also assessed. The results showed that the average contents of heavy metals, Cu, Zn, Ni, Cd, Cr, and Pb, in the dredged sediment of Kaohsiung Harbor are 120, 403, 33, 0.46, 112, and 40 mg/kg dw, respectively. The Cd and Zn basically bound in the carbonate fractions of the dredged sediments, which accounts for 36% and 45% of their total content on average. The highest level of Cu was present in the organic-matter fraction (92%), Cr and Pb were present in the Fe Mn oxides fraction (59% and 52%) of the dredged sediments, and the most of Ni (52%) was found in the residual fraction of sediments. Except for Ni, the total amount of all heavy metals in the dredged sediments of Kaohsiung Harbor was about 1.5-4 times higher than the surface sediment of the ocean disposal site. The speciation patterns of heavy metals in the surface sediments of disposal site were similar to those of the reference site, whereas were distinctly different with the dredged sediments. The observation implies that the dredged sediments after dumping in the ocean disposal site, their heavy metals could be diluted by the physical and chemical effects in the marine environment and eventually to the stable state.