Variations in concentrations and bioavailability of heavy metals in rivers during sediment suspension-deposition event induced by dams: insights from sediment regulation of the Xiaolangdi Reservoir in the Yellow River

Abstract

The variations of concentrations, speciation, and bioavailability of heavy metals during large-scale sediment suspension-deposition events caused by dams in rivers are not well understood. In this study, the sediment regulation of the Xiaolangdi Reservoir of the Yellow River was chosen as a case to study the effects of large-scale sediment suspension-deposition events on the concentrations, speciation, and bioavailability of As, Cr, Cu, Ni, Pb, and Zn in river water, and the bioaccumulation and biotoxicity of heavy metals in river water samples to Daphnia magna were examined. The water in the reservoir was also collected to determine the suspended sediment (SPS) concentration and conduct bioaccumulation and biotoxicity experiments. During sediment regulation, the SPS concentration in river water increased from 0.02 g L−1 in the reservoir to 54.8 g L−1 at the dam outlet, and then it decreased along the downstream river due to SPS deposition. The heavy metal concentrations in SPS at the dam outlet were higher than those in the reservoir sediment and remained stable at downstream sites. No spatial trends were found in their exchangeable fraction. Correspondingly, the dissolved concentrations at the dam outlet were lower than those in the reservoir. The body burdens in Daphnia magna for the reservoir water samples were higher than those for the dam outlet. However, higher lethality of Daphnia magna at the dam outlet was found due to other contaminant release caused by sediment resuspension. The results revealed that the resuspended sediment which has smaller particle sizes and more sorption sites in the reservoir was a “sink” for heavy metals; the downstream SPS deposition process has no considerable influence on the heavy metal distribution. This study suggests that sediment regulation is conducive to delivering the reservoir sediment to the sea and reducing the eco-environmental risks of heavy metals in the downstream of the Xiaolangdi Reservoir. However, it might increase the total eco-environmental risks of pollutants.