Sediments deposited in sewer pipes have been concerned as one of the main pollution loads from combined sewer overflow (CSO). Since the toxicity, mobility and bioavailability of heavy metals depend on their speciation, it is necessary to consider not only their amounts but also their speciation to evaluate the adverse effects on aquatic organisms in receiving waters. Sequential extraction method was applied to the size-fractionated sediments to evaluate heavy metal speciation. Although the speciation of Cr, Ni, Cu, Zn and Pb differed among sampling points as well as their contents, the content of easily exchangeable fraction was strongly correlated with total content for all the metals except for Cu. The result of heavy metals speciation revealed that Ni, Zn and Pb in combined sewer sediments could have high mobility potential. The result of size-fractionated sediments indicated that the contents and mobility potential of the heavy metals were larger as particle size was finer. Thus, it is important to control fine sediments in combined sewer to reduce the impact of CSO pollution.

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