Research On Self-purification Characteristics And Water Environment Capacity In Longbao River Backwater

Wanzhou District of Chongqing City is located in the backwater of the Three Gorges Reservoir area.The Longbao River is a tributary of the right bank of the Yangtze River.It is an important body of water in Wanzhou District and an important part of the Yangtze River.Due to the rapid development of the urbanized economy,the Longbao River has been in disrepair for a long time,and the river water body has been seriously polluted.After the black and odorous water body has been rectified,it has improved slightly.Long-term water pollution prevention and control are needed to improve the water quality and ensure the water quality of the Yangtze River.Objectively evaluate the current status of the water environment of the Longbao River and conduct a comprehensive survey of pollution sources and pollution loads in the basin.Objective evaluation of the present situation of Longbao river water environment,comprehensive survey of pollution sources and pollution load.In view of the unique hydrological and water quality changes in the backwater area,the analysis of self-purification characteristics of water bodies and the study of water environmental capacity are of great significance to the protection of water sources in the Three Gorges Reservoir area and the prevention and control of urban water ecology.This article takes the Longbao River as the research object,according to the existing data in the basin and long-term monitoring of rainfall,hydrology,water quality and other data,the hydrological and water quality spatial and temporal distribution characteristics of the Longbao River backwater area and river channel pollutant factors are analyzed and obtained.The self-purification characteristics of water bodies in three modes:natural statics,disturbance（strength 50～500 r/min）,aeration（dissolved oxygen 2～8 mg/L）were studied by indoor simulation test device.The self-purification characteristics and degradation coefficients of major pollutant factors under different mode conditions were analyzed.The method of combining in-situ measurement and theory was used to calculate the pollutant load in the river,and the one-dimensional water environment capacity model was optimized with parameters to calculate the water environment capacity and propose corresponding pollution reduction countermeasures.The main conclusions are as follows:（1）There are obvious spatial and temporal changes of hydrology and water quality in the Longbao River backwater area.The hydrological characteristics are mainly reflected in the periodic changes of water level;the water quality is mainly in the spatial position with the direction of the water flow,which firstly fluctuates and then rises to the peak and then fluctuates.The trend is that the concentration of pollutants in the non-backwater area is at a low level and stable small fluctuations.It rises to a high level and large fluctuations in the backwater area.Most of them reach two peaks in the gem bridge and shantytowns.The concentration of NH₃-N showed a"U"shape change from January to December,and the concentrations of TP and COD showed a relatively gentle jagged fluctuation.（2）Indoor simulation experiments found that both plants and sediments have a greater strengthening effect on the pollution and degradation of water bodies.Plants have a stronger ability to degrade nitrogen in water than sediment,sediments have a stronger ability to degrade phosphorus and organic matter in water,and sediments have stronger impact load resistance to water.The best self-purification ability of the water body in the standing group is the plant with bottom mud,and the degradation of the device mainly occurs in the early stage under the condition of good plant growth activity.The resolution rates of TN,TP and COD in the first 6 days reached 52.9%,81.1%and 52.94%respectively.（3）Both disturbance and aeration can enhance the self-purification ability of water to a certain extent.The disturbance has a small effect on the dissolved oxygen in the water body,which basically increases with the intensity,but the increase is not large;Therefore,the disturbance effect is mainly reflected in the uniform mixing of the water body,which is conducive to the physical diffusion and dilution of the pollutants,and also strengthens the contact between the microorganisms and the water pollutants to a certain extent.Aeration also has certain disturbance to the water body due to the influence of aeration position,time,intensity,etc.But the main impact is reflected in the change of dissolved oxygen in the water body.The degradation of most pollutants depends on the biochemical reaction process of microorganisms,and the dissolved oxygen environment directly limits the biochemical conditions of microorganisms,which in turn affects the degradation of pollutants.（4）The pollutant load of Longbao River is mainly non-point source,while the non-point source load is mainly rainwater runoff pollution load,and the load of each pollution factor is mainly COD and TN;It is calculated that the pollution load is COD1180.01t/a,TN58.36t/a,TP8.79t/a,NH₃-N45.35t/a.By improving the parameters of water environmental capacity model for non-uniformity coefficient and water quality compliance rate,the calculation method of water environmental capacity suitable for Longbao River basin is obtained.The COD、TN、TP、NH3-N water environmental capacity calculated after improvement is 496.61 t/a、9.93t/a、2.64t/a、19.67t/a respectively.The pollution load（COD,TN,TP,NH₃-N）of the Longbao River Basin needs to be reduced by 59.99%,83.84%,71.41%,and 58.79%,respectively,to meet the relevant environmental capacity requirements.