Particle size variation and retention characteristics of phosphorus in suspended sediments in Three Gorges Reservoir Area

Abstract

[Objective] Phosphorus is the main pollutant and biogenic element in the Yangtze River, and the completion of the Three Gorges Dam has led to the interception of a large number of suspended sediment(particulate phosphorus) carrying phosphorus, which has affected the ecological environment of the Yangtze River Basin.Particle size(D) is an important physical property of suspended sediment, but there is a lack of information on particle size(D) in the study of particulate phosphorus in the Yangtze River.[Methods] Suspended sediment particle size gradation analysis and suspended sediment phosphorus content(PP′) monitoring were carried out monthly in eight hydrological sections of the Yangtze River mainstream and its tributaries Jialing River and Wujiang River in the reservoir area in 2021, and the PP′ values of suspended sediment at different time and different particle size sections were obtained.Sediment transport rate, flow velocity and other hydrological parameters, as well as total phosphorus(TP), dissolved phosphorus(DP) and particulate phosphorus(PP) concentrations, the temporal and spatial variation of suspended sediment particle size characteristics and phosphorus content in the reservoir area were summarized, the main suspended sediment components dominating the transport of particulate phosphorus in the reservoir area were analyzed, and the interception effect of the reservoir area on different suspended sediment components was estimated.[Results] The results show that:(1) The suspended sediment in the reservoir area was dominated by fine particles with D≤0.062 mm, and there was a trend of refinement from upstream to downstream. Silt is the main carrier of particulate phosphorus in the reservoir area, accounting for 77% of the annual suspended sediment flux(D≤0.062 mm) and 82% of the annual suspended sediment flux(D≤0.062 mm).(2) The average phosphorus content of suspended sediment in each section of the Yangtze River mainstream in the non-flood season is greater than that in the flood season, and the smaller the particle size is, the greater the PP′value is, but in the flood season, the clay with 0.002<D <0. 004 mm contains the most phosphorus, and the sediment transport rate determines the amount of suspended sediment flux, which is the main factor affecting the transport of particulate phosphorus. (3) The phosphorus in the reservoir water is mainly DP in the non-flood season and PP in the flood season, and the suspended sediment and PP are retained throughout the year, especially in the flood season, and the suspended sediment and particulate phosphorus retention in the flood period account for about 94.5% and 97.1% of the annual retention, respectively. The retention rate of silt particulate phosphorus in the reservoir area is more than 80%, and that of clay particulate phosphorus is more than 60%. [Conclusion] The results show that the combined monitoring method of particle size distribution of suspended sediment and soil phosphorus measurement could determine the phosphorus content of suspended sediment with different particle sizes in the reservoir area, which provided a new technical reference for phosphorus monitoring in the reservoir area. The ratio of suspended sediment composition has greater influence on the transport of particulate phosphorus than the particle size of suspended sediment in the reservoir area, and the greater the ratio of suspended sediment composition is, the greater the influence on the transport of particulate phosphorus is. Therefore, phosphorus control in the upper reaches of the Yangtze River should focus more on the ratio of suspension sediment composition.

Keywords

Three Gorges Reservoir Area / suspended sediment / particle size / phosphorus / interception rate / spatiotemporal variation / water quality