Phosphorus Cycling Characteristics Under The Influence Of Suspended Particulate Matter In Slow Moving River

Slow-moving rivers are common in water resource scarce areas in China,often accompanied by slow velocity and poor water quality.Suspended particulate matter(SPM)as an important carrier of phosphorus migration in river,is under strong chemical weathering and the weaker physical weathering,leading to the possible changes of its characteristics,this change will pose great impact on the migration process of phosphorus in river system,resulting in the possible changes of phosphorus cycling.However,the characteristics of phosphorus cycling in slow moving rivers under the influence of SPM are still unclear.This thesis choose Niuwei River(slow-moving river)and Luanhe River(fast-moving river)in Haihe River Basin as the research object,based on the differences of water environment and hydrodynamic conditions between the two rivers,their influences on SPM were investigated.And the phosphorus cycling characteristics was identified by the method of phosphorus species and oxygen isotope ratios of phosphate(?¹⁸O_p)technique from qualitative and relative quantitative point of view.It is revealed that SPM is more involved in phosphorus exchange with overlying water than sediment.Based on the change of phosphorus cycling characteristics in slow-moving river,their influences on water environment management were analyzed,and the countermeasures for water environment management were proposed.The main research contents and results are as follows:(1)Water environment and hydrodynamic characteristics of slow-moving river and its influence on SPMThe differences in water environment and hydrodynamic conditions between Niuwei River and Luanhe River were clarified.In terms of water environment conditions,the water quality of Niuwei River was worse than that of Luanhe River,and COD and conductivity(COND)were higher than that of Luanhe River.The eutrophication index also showed that the eutrophication of Niuwei River was more serious.In hydrodynamic condition,the velocity of Niuwei River is lower than that of Luanhe River.The average velocity of luanhe River is 1.14m/s in dry season and 1.45m/s in rainy season,while the average velocity of Niuwei River is 0.12m/s in dry season and 0.18m/s in rainy season.The results of scanning electron microscopy(SEM)observation and simulated weathering experiments show that the poor water environment and weak hydrodynamic conditions in Niuwei River lead to more organic coating on the surface of SPM.(2)Characteristics of phosphorus cycling in slow-moving river based on phosphorus speciesThe characteristics of phosphorus species in sediments and SPM of Niuwei River and Luanhe River were studied by SMT method,the results showed that SPM are more involved in the process of phosphorus adsorption and desorption than sediments.An improved formula of Freundlich was applied to investigate the role of SPM and sediment in the phosphorus cycling.The results showed that the SPM was more likely to act as the sink of phosphorus,SPM can adsorb and desorb phosphorus in overlying water preferentially than sediment and act as the potential phosphorus source.The phosphorus cycling pattern of slow-moving rivers may be changing from the traditional one-way deposition cycle to the dynamic balance of phosphorus adsorption and desorption between overlying water and SPM.(3)Characteristics of phosphorus cycling in slow-moving river based on oxygen isotope ratios of phosphate(?¹⁸O_p)techniqueThe ?¹⁸O_ppretreatment method suitable for overlying water,SPM and sediment of the slow-moving river was optimized and constructed.Based on these methods,the?¹⁸O_pof overlying water,SPM and sediment of the Niuwei River were collected and interpreted.The results showed that sediment had the highest?¹⁸O_pratio,followed by SPM,and finally overlying water.The variation of?¹⁸O_pratio of overlying water was smaller than that of SPM and sediment.Based on the role of SPM and sediment as phosphorus sinks,it was believed that the exchange of phosphorus between overlying water and SPM was more frequent and close,resulting in closer?¹⁸O_pbetween the two.From the relative qualitative point of view,it was clear that the phosphorus cycling characteristics of the slow-moving river had changed from one-way deposition cycle to dynamic balance of phosphorus adsorption and desorption between SPM and overlying water.(4)Effects of phosphorus cycling changes on water environment management in slow moving riversThis paper summarizes the management measures of phosphorus involved mainly from point source,non-point source and endogenous source in China,and points out the influence of phosphorus cycling characteristics changes on these measures.In point source,the current environmental standard of phosphorus involved has lost the support of basic theory by influencing the environmental standard of phosphorus involved,which leads to the unreasonable phosphorus discharge standard for slow-moving river.The inflow of phosphorus from non-point source will affect the migration and transformation path of phosphorus.The SPM in the slow-moving river is more involved in the phosphorus cycling process than the sediment.The sediment in the slow-moving river is no longer the primary risk source of endogenous release,and the effect of endogenous phosphorus control will be challenged.In view of the changes of phosphorus cycling characteristics in slow-moving rivers,reasonable standard of point source phosphorus control,strengthening control of non-point source phosphorus input and careful selection of endogenous source phosphorus control measures were proposed.