Study On The Key Processes Of Nitrogen And Phosphorus Migration At Sediment-water Interface In Xiangjiaba Reservoir Area

The water conservancy project of water blocking and damming on the river plays a huge comprehensive effect,and also changes the original hydrological characteristics and material transfer and transformation characteristics of the river.Xiangjiaba Reservoir is regarded as the end of the cascade reservoir group in the lower reaches of Jinsha River,and it is very important for the migration and transformation of nutrients in the reservoir area to have an influence on the water environment of the whole river basin.Based on this,this thesis studies the vertical distribution characteristics of the nutrient occurrence form and the migration process of sediment-water interface at the Xiangjiaba reservoir area,so as to provide a scientific basis for understanding the impact of the Xiangjiaba reservoir on the nutrient salt cycle of the water ecosystem and the ecological effect on the water environment.The research results are as follows:（1）The vertical distribution characteristics of nutrient salts in Xiangjiaba reservoir area show that:The vertical distribution of NH₄⁺-N in the water in the reservoir area is not significant,The main nitrogen occurrence form of the three tributaries is granular,and PTN accounts for more than 52.31%of TN,while the form of the main stream nitrogen is mainly dissolved,accounting for more than 69.31%of TN.The vertical distribution of phosphorotrophin in the water of Xiangjiaba reservoir area fluctuates greatly,and DTP generally compares the surface layer as larger than the bottom layer,and the vertical distribution characteristics of PP and TP have good similarity.SS increases with water depth and shows bottom enrichment;the vertical distribution characteristics of solubility Si O₂and KMn O₄index fluctuate with water depth,but the overall change range is small;the distribution of COD in the whole study area shows obvious spatial heterogeneity.The water quality in this study area meets Class II of surface water.PTN accounts for a large proportion of particulate matter,while SS and PTN responses are more obvious.The granular nitrogen and phosphorotrophin salts show a bottom cumulative effect,And good settlement.（2）Indoor control test was used to study the nitrogen and phosphorus adsorption and desorption process of sediment-water interface in Xiangjiaba reservoir area,and the results showed that:within a certain range,the increase of temperature,the enhancement of hydraulic disturbance,the decrease of the ratio between water and water and the increase of organic matter content all accelerate the exchange of nitrogen and phosphorus at the sediment-water interface,and affect the migration process of nitrogen and phosphorus at the sediment-water interface.The migration of nitrogen and phosphorus at the sediment-water interface in the area of this study shows the release of ammonia nitrogen and phosphate to the water.The adsorption-desorption process of NH₄⁺-N and H₃PO₄in the simulation test conforms to the Henry model overall,The fitting effect was better,R²≥0.9348;When the simulation test mainly shows the adsorption state,The Langmuir model does not conform to the adsorption of NH₄⁺-N in Xiangjiaba reservoir area,The Freundlich model describes the adsorption of NH₄⁺-N in Xiangjiaba reservoir area,The R²is 0.9700-0.9967;However,the Langmuir model is compared with the Freundlich model to describe the adsorption characteristics of H₃PO₄in Xiangjiaba reservoir area,The R²is 0.9951-0.9998.The migration behavior of nitrogen and phosphorus at the sediment-water interface between the monitoring points in Xiangjiaba reservoir area is greatly affected by temperature,water disturbance,water-water ratio,soil-water ratio and organic matter content,which shows that the completion and operation of Xiangjiaba reservoir has an obvious impact on the transfer of nutrient salts.（3）The difference between the equilibrium concentration and the overlay water nitrogen and phosphorus concentration obtained by fitting the thermodynamic data with the Henry isothermal adsorption model,the estimated release flux of ammonia nitrogen and phosphate in the sediment of Xiangjiaba reservoir area is 0.56～3.76mg/L,-0.01～1.51mg/L,respectively.The sediment ammonia nitrogen release flux is greatly affected by the hydraulic disturbance,while the release flux of the sediment phosphate is overall less affected by the various factors.The risk change range of the sediment ammonia nitrogen and phosphate release in Xiangjiaba reservoir area was-79.63%～87.37%and-93.33%～2.5,respectively.The release risk level of the sediment ammonia nitrogen was lower than that of the sediment phosphate release level,and the risk of the sediment ammonia nitrogen release decreased on the whole,while the release risk of the sediment phosphate was increased.