Water-sediment regulation scheme of the Xiaolangdi Dam influences redistribution and accumulation of heavy metals in sediments in the middle and lower reaches of the Yellow River

Water and sediment regulation by dams influence the erosion, deposition, and transport of sediments, which should cause the redistribution and accumulation of contaminants. In this study, we investigated the impacts of the water-sediment regulation scheme (WSRS) of the Xiaolangdi Dam (XLD) on the distribution and accumulation of six heavy metals (HMs) (Cr, Ni, Cu, Zn, Cd, and Pb) in sediments in the Yellow River. Our results showed that the concentrations of HMs were higher in sediments in the study area compared to background levels, and the accumulation of HMs mainly occurred in the downstream reaches during the WSRS. However, following the WSRS, the HMs were mainly distributed in the reservoir area. While not significantly different between the reservoir area and downstream reaches, HM concentrations varied spatially and were significantly different (p < 0.05) between seasons. Accumulation of HMs occurred in sediments in the study area, particularly Zn and Cd in the downstream reaches in post-WSRS seasons, and was driven by agricultural and urban industrial activities. The accumulation of Cd induced a very high contamination level and ecological risk of sediments, while Pb induced a considerable contamination and moderate ecological risk in the downstream reaches in the post-WSRS II season. In conclusion, the WSRS led to high HM concentrations in downstream reaches during the WSRS, whereas the XLD led to high HM concentrations in the reservoir area after the WSRS. These results are critical for better understanding the effects of the WSRS on contaminants and for informing the management and restoration of regulated rivers.