Studies Of Exchange Behavior Of Nutrients On The Sediment-water Interface In The Tidal Reach

Estuaries are the transition zone linking rivers ecosystems to ocean ecosystems, and bio-geochemical behaviors of nutrients in estuary have significant effect on its output to the ocean. Sediment-water interface is the boundary where physical chemistry and biological features of estuary water are the most significant differences and material transport and exchange are the most frequent. The intersection of freshwater and saltwater inevitably leads nutrition elements redistribute between solid and liquid and leads form of nutrition elements change, which affects nutrition elements contant and components characteristics input to the ocean from the land. As a typical urban pollutant carrying rivers, Licun River estuary polluted seriously. The migration and transformation of nutrient elements in tidal flat sediments have important implications on biological production process in the estuary and jiaozhou bay. This article studies nutrition adsorption and release kinetics process and adsorption thermodynamics process to discusses nutrient's adsorption/release feature on the surface of suspended particle, then further analyze the environmental factors on the influence of adsorption/release behavior; Through the simulation of nutrient exchange process on sediments - water interface to obtain the nutrient exchange flux and on the basis of which study the influence of environmental factor on nutrient exchange process; Through the field observation of current velocity on the bottom and wind speed, analyzing the hydrodynamic mechanism of sediment resuspension,then simulated and calibrated resuspended conditions, and on the basis of which studies the changing of nutrient content in sediment resuspension and settlement process. Based on the field observation and the laboratory simulations, known as follows:1. The overlying water temperature, Eh and COD change over time show rose at first, then decreased; And the change of depth, salinity and dissolved oxygen content and pH present the trend of first decreases then increases. Nutrient index levels in water (ammonia nitrogen, nitrate, TN and phosphate) present increased at first then reduce trends. Sediment particle size is reletate to sediment position. Ammonia and nitrate content of the sediments in 3 sampling point substantially all increases with depth. Along with the increase of depth, the slope of the TP reduce increased gradually.2. The adsorption kinetics of NH₄⁺-N and NO₃^--N are complex , NO₃^--N adsorption kinetics can fitting well with Parabolic diffusion model and Elovich model, NH₄⁺-N adsorption kinetics can highly fitting with Elovich model. Level 1 dynamic model fitting poor with the two adsorption kinetics, showing NO₃^--N, NH₄⁺-N adsorption effect is affected by many kinds of environmental factor.3. Salinity is a major factor affecting the adsorption and release of estuary nutrient. Raising of salinity can promote release and inhibit PO₄^3--P adsorption. Raising temperature have stimulative effect on PO₄^3--P adsorption and release, which reaction in mixed system is primary, and behaved for promoting which one. Raising of oscillation frequency promote the release of PO₄^3--P, the sediment particles whether under suspended state is the key factor of PO₄^3--P release. Particle concentration is also a important factor which influence PO₄^3--P release, along with the increase of overlying water, PO43 - P release quantity are increased.4. Laboratory incubation experiments were carried out to study the exchange fluxes of nutrients between sediments and water at the tidal estuary .It was shown that the diffusion was the key process that influenced the exchange of nutrients. The exchange rate at the sediment-water interface was mainly controlled by the difference of the concentrations between the pore water and the overlying water. The NO₂^--N, NH₄⁺-N, PO₄^3--P showed a net exchange flux from sediment to water.5. The relationships between the nutrients exchange and the potential environmental controls were also investigated. It was observed that temperature and bioturbation were the key factors during the exchange process. When DO levels were changed, the fluxes of NO₃^--N, NH₄⁺-N, PO₄^3--P were also changed. However the effects of DO levels on NO₂^--N were unclear. The exchange flux of NO₂^--N and NH₄⁺-N were higher when salinity was increased, but the exchange flux of PO₄^3--P were influenced few with salinity.6. According to wind speed and underflow field observation data, wave factors, wave shear stress and underflow stress are calculated. And the results show that in small wind speed condition, wave shear stress and underflow stress are roughly equal, and both of them are lower than their critical shear stress. In medium and large wind speed conditions, wave shear stress is much larger than underflow shear stress, also exceeds the critical wave shear stress. The wave shear stress which is correlated significantly with wind speed is the key factor causing sediment starting and resuspension. Therefore, the wind speed can be taken as the major index to measure resuspension degree.7. The changes of suspended substance concentrations under different disturbance frequencies are measured, and the quantitative relation of the suspended substance concentration(C50) in the distance of 50cm from the sediments - water interface and disturbance frequency (n) is determined as C50=0.00044exp(n/0.486)+24.875. Furthermore, the disturbance frequency corresponding with the measured C50 under different wind speeds can be obtained, so the quantitative corresponding relation between the disturbance frequency (n) and site wind speed (v) can be settled as: v=4.3091n-17.001. In normal conditions of Small, medium and large winds, the corresponding C50 are 37.80, 91.92 and 307.96 mg/L, respectively, which show wave disturbance strength and sediment resuspension degree are in positive correlation.8. Sediments resuspension and settlement process are simulated, nutrient response in the process is discussed. And the results show that: the nutrient content in the overlying water changes with the change of disturbance intensity, the initial 0.5h in the settlement process is the main stage of the suspended substances concentration decreasing. In small and medium wind conditions, with the gradual increase of suspended substance concentration, the ammonia nitrogen content decreases. In large wind condition, although suspended substance concentration is higher, the ammonia nitrogen content increases compared to that in medium wind condition. The phosphate content and suspended substance content in the water body are in negative correlation. As the disturbance enhances, the phosphate content reduces gradually. Along with the increase of overlying water salinity, the adsorption capacity of sediment adsorbing N, P decreases, which causes the increase of the ammonia nitrogen and phosphate contents in the water body. Biological effect causes the contents of the ammonia nitrogen and phosphate decreasing.