| Since the Three Gorges Reservoir(TGR)generated water storage,the velocity of the water in the reservoir area slowed down,the self-purification capacity of the water body weakened,and the water environmental carrying capacity decreased,and the sub-branch returning water area frequently overflowed in recent years,attracting extensive attention from scholars at home and abroad.When the external load control was reduced,the effect of endogenous release on nutrient elements in water was more and more significant.Therefore,it is of great significance to study the flux and influence mechanism of sediment release from tributaries of the TGR in order to solve the eutrophication problem of tributaries and similar water bodies and to ensure the health of the water ecosystem of the TGR.Taking the Xiangxi Bay(XXB),a typical tributary of the TGR,as an object,this paper conducts quarterly in-situ observation of different monitoring sections of XXB from January to November 2017,combined with laboratory tests and theoretical analysis using mathematical models,etc.The temporal and spatial characteristics of sediment storage environment in XXB,the characteristics of phosphorus adsorption and desorption in sediments of different water levels were analyzed,and the diffusion of phosphorus molecules and the resuspension release flux of phosphorus molecules in sediments of XXB were estimated quantitatively.In order to lay a theoretical foundation for the calculation and control of the source phosphorus load in the tributaries of the TGR,the influence mechanism of phosphorus release from sediment in XXB is revealed.The main tasks and understandings are as follows:Temporal and spatial characteristics of sediment environment in XXB:1)In different water level period corresponding to different forms of intrusion into XXB,the upstream flow generally flows downstream from the XXB in the form of a gradient flow,average velocity of water is higher than that on the bottom of the middle reaches of the region,downstream;The overlying water temperature decreased gradually from the downstream of the estuary to the upstream,the p H of the overlying water gradually increased from the downstream of the estuary to the upstream of the river,and the electric conductivity was the same as that of the temperature.The water content and porosity of the surface sediments of the XXB gradually decreased from the lower reaches of the estuary to the upper reaches and the change trend of bulk density was opposite,and the organic matter of sediment changed at 9.58~13.82 g/kg.The content of organic matter in the surface sediments decreased gradually from the downstream to the upper reaches during the period before flood,and increased gradually in the other periods,and silt and clay dominated the sediment types in XXB.The relatively fine particles in the sediments account for the majority.2)The phosphorus content of sediment interstitial water was higher than that of overlying water on the whole,and the particle phosphorus content was higher than that of dissolved total phosphorus as a whole,the dissolved total phosphorus ?(DTP)is mainly exchanged in the form of ?(PO43--P)in the sediment-water system;?(PO43--P)accounts for more than 85% of ?(DTP),and ?(PO43--P)in the upper and middle reaches is larger than that in the lower reaches;The main form of phosphorus in sediments for inorganic phosphorus,XXB in different regions of ?(HCl-P)was significantly higher than that of ?(Na OH-P)and ?(OP),?(OP)from downstream to upstream a gradual increase trend,?(IP)on the contrary,?(HCl-P)and ?(IP)trend in space.The characteristics of phosphorus adsorption and desorption in the sediments of XXB:1)The adsorption kinetics test showed that the adsorption rate was 32.66mg/(kg·h)and the average adsorption rate was 32.66 mg/(kg·h)within 12 hours.The average adsorption rate is 7.85mg/(kg·h)and the amount of sediment adsorption is as follows: water storage period>dry water running period>flood season>flood fall period.The time of net adsorption of sediment is: water storage period>flood season>dry water operation period>fall period before flood,and the adsorption rate of phosphorus in the lower reaches of different water levels is obviously higher than that in the middle and upper reaches.The adsorption of phosphorus by sediments in the XXB is more in line with the quasi-first order kinetic equation.2)It is found that the Langmuir model can better describe and characterize the phosphorus adsorption characteristics of XXB sediment,and when the overlying water temperature and p H increase within a certain range,the phosphate adsorption capacity increases gradually.The amount of phosphorus adsorbed by sediment in the lower reaches is obviously higher than that in the middle and upper reaches,and the net adsorption capacity is as follows: flood season>pre-flood fall period>water storage period>dry water operation period.Study on flux and mechanism of XXB sediment phosphorus release:1)The sediments of XXB in the dry water running period is generally a "sink" of phosphorus,and the sediment is generally a "source" of phosphorus during the fall period;the contribution rates of phosphorus diffusion at the sediment-water interface of XXB during high and low water level to the overlying water are-0.045~-0.011%(sink)?0.128~1.872%(source).The contribution rate of phosphorus diffusion from the sediment to water interface to the overlying water increases gradually;the annual phosphate diffusivity of XXB sediment is 0.55 t under static condition,and the annual phosphate release amount is 44.34 t under resuspension condition,the main way of phosphorus contribution from XXB sediment to water is dynamic release.2)There is a significant correlation between the overlying water temperature and the sediment organic matter in Xiangxi River;temperature plays an extremely important role in the process of phosphorus transport and transformation in sediment-water system of XXB,the decomposition and accumulation of overlying water p H and organic matter have a certain effect on the phosphorus transformation of sediment;for the XXB,the lower the backflow infiltration point?the lower the operating water level and the greater the biological disturbance will accelerate the sediment phosphorus release. |
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