Study On Response Mechanism Of Sediment Oxygen Consumption To Environmental Factors Based On Eddy Measurement Technology

Dissolved oxygen as an important indicator for evaluating water quality,its content plays an important role in the survival of aquatic organisms and the maintenance of self-purification of water bodies.The oxygen flux（also called sediment oxygen consumption） of sediment-water interface（SWI） is an important component of water oxygen balance.Therefore,the study of SWI oxygen flux is of great significance for water environment management and restoration.In this paper,the eddy correlation method,a non-invasive measurement technique,is used to study the response relationship between SWI oxygen flux and environmental factors.The eddy measurement system is constructed by an acoustic Doppler flow point flow meter and a dissolved oxygen sensor,and the key factors affecting the oxygen flux measurement results such as the sensor distance and the sample height are tested in lab to determine the optimal arrangement of the system.Based on the eddy measurement system,the diffusion boundary layer（DBL） thickness and SWI oxygen flux at different horizontal flow velocity were observed in lab tests to investigate the response relationship between SWI oxygen flux and hydrodynamic conditions.The eddy measurement system is used to measure the oxygen flux of SWI in the on-campus ring river,off-campus Hongni River and Daheiting Reservoir.The variation law of oxygen flux and environmental factors such as water temperature and flow velocity is analyzed,and the system is improved to make it more suitable for long-term measurements in the field.The main contents are as follows.（1）For the optimal arrangement of eddy measurement systems,when the horizontal distance between the ARO-EC tip and the sampling volume is less than 0.5cm,the flow velocity measurement will be disturbed.And considering the measurement principle of the eddy correlation method,the appropriate horizontal distance is 0.5cm～2cm.As the height of the sampling volume gradually increases,the change of oxygen flux is increased-stationary-reduced.According to the position of the peak and stable section of the oxygen flux,the appropriate sampling volume height is 4cm～5.5cm.（2）For the response relationship between oxygen flux and hydrodynamic conditions,DBL thickness decreased with increasing hydrodynamic conditions（characterized by Batchelor scale）,while SWI oxygen flux increased.By analysing of the relationship between hydrodynamic conditions,DBL thickness and SWI oxygen flux in lab tests and related studies,there is positive correlation between the DBL thickness and Batchelor scale,and the fitting relationship is δ_DBL =0.89L_B^*+0.07,which shows that the DBL thickness can be more easily obtained by the Batchelor scale.The SWI oxygen flux is in negative correlation with DBL thickness and the fitting relationship is Flux_O2=-3.41δ_DBL^-1-3.91.The DBL thickness variation has a strong influence on the SWI oxygen flux when less than 0.5mm of DBL thickness,while the DBL thickness is more than 0.5mm,the SWI oxygen flux remains basically stable.（3）For field test of on-campus ring river and off-campus Hongni River,due to the small depth of the on-campus ring river,the changes in environmental factors such as flow velocity,water temperature and light intensity are random affected by surface wind and solar radiation,and the factors are interrelated,leading to the dominant factors affecting oxygen flux are unclear and oxygen flux changes are uncertain.The water depth of the off-campus Hongni River is relatively large,and the water temperature remains stable during the measurement.The change trend of oxygen flux and flow velocity is consistent with the results of the response relationship between oxygen flux and hydrodynamic conditions in lab test,that is,the oxygen flux increases with increase of the flow velocity.（4）For field test of Daheiting Reservoir,the improved real-time transmission eddy correlation system can meet long-term measurements in the field.Comparing the oxygen flux measurements at different water temperatures,it was found that the oxygen flux increases with increase of the water temperature due to the promoted the redox reaction in the sediment.Comparing the measurement results under different aeration conditions,it was found that the aeration had a significant effect on the oxygen flux.The oxygen flux during continuous aeration was -0.32mmol/（m²?d）,and the oxygen flux after stopping the aeration was -1.77mmol/（m²?d）.