Abstract
Metal pollution is a global environmental problem. In order to understand the effects of the cascade hydropower development on Cd in reservoir sediments, this study investigated the migration and deposition mechanisms of Cd in reservoir sediments by using the film diffusion gradient technique (DGT) for the cascade large deep reservoirs of the Lancang River (LCR), Nuozhadu (NZD) and Xiaowan (XW). The results show that: (1) Strong correlations amongst the deposition processes of Cd, Mn and S were found in the sediments of the NZD reservoir; weak correlations between the deposition processes of Cd and Mn were found in the sediments of the XW reservoir. (2) Part of labile Cd in the pore water of the NZD reservoir was supplemented by silt and sand, and another part was inherent in the pore water; most of labile Cd in the pore water of the XW reservoir was inherent in the pore water, and minimal replenishment of sediment was noted. (3) Mn is the main factor controlling the migration of Cd in the NZD reservoir. This paper provides an effective resource for understanding the migration and deposition of Cd in the sediments of large deep reservoirs along the LCR.
Highlights
Cd is a toxic, bioaccumulating trace metal
The oxidation conditions of the overlying water at all sampling points in the NZD and XW reservoirs are similar, and these points do not appear to be in the hypoxic zone
The average pH of the overlying water of the two cascade large deep reservoirs is weakly alkaline, and trace metals in this water may precipitate in the sediments [11]
Summary
Cd is a toxic, bioaccumulating trace metal. Xu et al [3] compared sediment quality criteria and potential ecological risks and found high ecological risks due to Cd. The cascade construction of large deep reservoirs affects the continuity of rivers and changes the scope of the inflow of regional sediments. The intercepted water of upstream reservoirs increases sediment deposition rates and reduces the transportation of sediments in downstream reservoirs [4,5]. Retention of sediments in upstream reservoirs and the influx of sediments in downstream areas simultaneously affect the cycle of biological matter in the sediments of downstream reservoirs and pose a severe threat to the water quality, freshwater resources and aquatic ecosystem of these reservoirs [6]
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