Abstract: The Yellow River, as an important river flowing into the Bohai Sea, is the main source of trace metal elements in the Bohai Sea. The transformation behavior of cadmium (Cd) directly affects the ecological security of the basin and the environmental quality of the Bohai Sea. Since the implementation of the water and sediment regulation project in 2002, the regulation effect of the project on sediment flux has been well studied, but there is a lack of systematic research on the flux, speciation distribution and ecological risk of Cd. We conducted systematic samplings at seven sites along the main stream of the Yellow River in 2023 and then analyzed the migration and transformation characteristics of Cd and their driving mechanisms during the water and sediment regulation period. The results showed that Cd was mainly transported in the particulate form. The particulate flux at the Huayuankou site was 23463 kg·a^-1, and the dissolved flux was 2591 kg·a^-1. At the Lijin site, the particulate flux was 17776 kg·a^-1, and the dissolved flux was 1169 kg·a^-1. The Cd flux during the sediment regulation period was significantly higher than that in other periods, mainly regulated by water flow velocity, suspended sediment concentration and the intensity of bottom sediment disturbance. The concentrations of available Cd in suspended particulate matter and sediments were relatively high, indicating a high potential ecological risk. The pollution index E_ri value indicated that particulate matter had a high risk of environmental pollution. The resuspension of Cd during the sediment regulation period led to an increase in short-term risk, while the accumulation of Cd in sediments during the dry season might cause long-term ecological threats. Further attention should be paid to its bioavailability after entering the sea. By revealing the significant impact of the water and sediment regulation project on the environmental behavior of Cd, this study provides a scientific basis for improving water environmental quality and ensuring ecological security in the Yellow River Basin.

Key words: water-sediment regulation, cadmium speciation, flux to the sea, suspended particulate matter, ecological risk