Geochemistry characteristics of sediment phosphorus in a typical area prone to deposition in the Three Gorges Reservoir

The mainstream reach from Zhongxian to Yunyang in the Three Gorges Reservoir. In this study, the modified sequential extraction method (SEDEX) and diffusive gradients in thin films technique (DGT) were utilized to reveal geochemistry characteristics of sediment phosphorus (P) in a typical mainstream reach from Zhongxian (ZX) to Yunyang (YY) in the Three Gorges Reservoir (TGR). A hydrodynamic model was established for the study reach to clarify hydrodynamic characteristics in the pro-TGR and post-TGR periods and its effects on sediment deposition and P release. Results illustrated that total phosphorus (TP) in the surface sediments was about 693.31–805.92 mg/kg, and inorganic phosphorus (IP) and organic phosphorus (OP) were 648–781 mg/kg and 23–45 mg/kg. Ca-bound P (Ca-P) made a major contribution (39.18–54.08%) to TP. Vertical distribution of labile P has a positive correlation with Fe(II) and S(-II), respectively, and labile P exhibited a synchronous change with Fe(II). Diffusion flux (Fd) of labile P was about 0.32–0.56 mg/(m2·d), and the sediment paid a role as P source during the sampling period. Affected by the operation of the TGR, velocity and sediment carrying capacity (S*) reduced significantly in the study reach. The velocity was about 1.0–3.0 m/s and 0.0–0.7 m/s, and S* ranged from 0–10 kg/m3 and 0–0.1 kg/m3 in the pro-TGR and post-TGR periods, respectively. A highlight of this work is that impacts of hydrodynamics on P mobility in a typical mainstream reach prone to deposition are systematically investigated by recreating the hydrodynamic processes in the pro-TGR and post-TGR periods.