Examination of sediment in the Derwent River by selective extraction

The distribution and bioavailability of zinc, lead, copper and cadmium in sediment samples from the Derwent River has been examined by means of sequential selective extraction. Two types of sediments; core sediment and surface sediments, were used in the examination. They were taken from two sampling sites, one near the Derwent Sailing Squadron and the other in Prince of Wales Bay. The majority of the metals in the core sediment were in residual form and appeared to be accumulated in the top part. By contrast, the metals in the surface sediment were found mainly in the bioavailable (non-residual) forms. The core and surface sediments had a different physical appearance and a different pH which may account for in part the apparently anomalous results. The other explanation is that sediments are parts of a dynamic system, and their ecotoxicological impact will be different in different pH environments. Compared to the total metals concentration measured in the previous studies from similar sites, the results in this study indicated a decrease in the level of the metals concentrations, except for mercury which showed a slight increase in its level.