Geochemistry of phosphorus release along transect of sediments from a tributary backwater zone in the Three Gorges Reservoir

摘要

As an important phosphorus (P) source in water, sediment P release can influence water quality in the tributary backwater zone of "river-type" reservoir. Chemical sequential extraction and diffusive gradients in thin films technique (DGT) were applied to study sediment P mobility in the Daning River backwater zone (DNB) in the Three Gorges Reservoir (TGR). Influenced by the TGR backwater intrusion, both of water P forms and sediment P forms in the DNB zone exhibited increased spatial distributions from upper reach to lower reach. Sediment particles in the upper, middle, lower reaches were mainly derived from the settled particles (with high Ca contents) carried by the Daning River outflow, the former and sedimentary dead algal, the settled particles carried by the TGR backwater, respectively. According to P adsorption/desorption experiment results and concentration gradients of DGT-labile P at the sediment-overlying interface (SWI), sediments in the DNB zone acted as P source for overlying water during the sampling period. In the lower reach of the DNB zone and the adjacent TGR mainstream, iron oxides reduction in the anoxic or anaerobic sediments drove the synchronous release of labile P and Fe(Ⅱ). Whereas, algal decomposition directly produced labile P and S(-Ⅱ) in the top 0 to -10 mm sediment layer in the middle reach, and then the downward increases of labile P and Fe(Ⅱ) in the depth of-30 to -80 mm were caused by iron oxides reduction. In the upper reach, the released labile P in sediments might be re-trapped by precipitating with Ca(Ⅱ) or being adsorbed onto the newly formed iron oxides near the SWI. Sediment P release in DNB zone had a limited influence on water quality, but the influence would be significant in the future time after the external P source is reduced efficiently.