Environmental Characteristics Analysis And Pollution Load Estimation Of Qiaobian River

The small and medium rivers is an important source of material exchange in the Yangtze River basin,and the water environment has received extensive attention.Under the background of the protection of the Yangtze River,the characteristics of water pollution and its temporal and spatial distribution in small and medium rivers is the basis and prerequisite for achieving precise water pollution control.Therefore,the research on water environment quality assessment and total pollution load control in the river basin has important theoretical and practical value for the improvement of water environment quality of small and medium-sized rivers and the treatment of water pollution.In this paper,the Qiaobian River Basin,a first-level tributary of the Yangtze River,is taken as the research object,combined with the actual conditions of the Qiaobian River Basin,in 2019 and 2020 respectively in January（dry season）and August（high water season）the Qiaobian River Basin.In the water environment and aquatic ecological survey,the basic information database of the bridge river basin was established through data collection,field survey and sampling,and indoor experimental analysis,and the temporal and spatial distribution characteristics of each index were analyzed.The single-factor evaluation method and the comprehensive index evaluation method were used to evaluate water environment quality of Qiaobian River;On this basis,with the help of one-dimensional mathematical model and MIKE11 model to calculate the pollution holding capacity of the study area,and conduct an in-depth analysis for it according to the calculation results of the model,and then construct the flow-holding capacity Relationship curve;By establishing the Qiaobian River Basin SWAT model,analyzing the causes of pollution through the temporal and spatial evolution characteristics of the pollution load in the basin,and proposing corresponding management measures,then they could provide a theoretical basis for pollution prevention,governance in the Qiaobian River Basin.The main research contents and conclusions are as follows:（1）Based on the field survey data of four high and low water periods,The eight environmental factors including total nitrogen（TN）,soluble total nitrogen（DTN）,ammonia nitrogen（NH₃-N）,nitrate nitrogen（NO₃-N）,total phosphorus（TP）,dissolved total phosphorus（DTP）,orthophosphate(PO₄^3-)and permanganate index(COD_Mn),were the research objects,and their temporal and spatial distribution patterns were analyzed separately.The results showed that most of the indicators were going up from upstream to downstream.Increasingly,the concentration of TN,NO₃-N,TP,PO₄^3-,DTP,and COD_Mn during dry season was slightly higher than that during wet season,while the concentration of NH₃-N and DTN was just opposite.The results of the single-factor evaluation method showed that most of the water quality of the Qiaobian River is between Grade III and Grade V,and the pollution was relatively serious.The overall water quality of the Qiaobian River was poor.The main reason was that TN indicator exceeds the standard.The results of the comprehensive index evaluation method show that the overall water quality of the Qiaobian River was in a good-light pollution state,and the water quality in the dry season was slightly better than that in the wet season.（2）According to the division of the water function area of the Qiaobian river basin and the value of related parameters.From traditional one-dimensional mathematical model and the hydrodynamic-water quality model of the Qiaobian river basin are constructed on the base of the MIKE11 model,and it could be calculated at 90 present and 50 present guarantee during high and low water periods under design hydrological conditions.Comparative analysis shows that the overall pollution holding capacity results calculated according to MIKE11 are lower than that by the traditional one-dimensional mathematical model.The COD holding capacity range is 143.93-522.44t/a,the NH₃-N holding capacity range is7.65-27.47t/a,and the TP holding capacity range is 9.74-64.68t/a.And it could be further calculated the pollution holding capacity under different design flow conditions,and then constructed the flow-pollution holding capacity relationship curve.The results showed that the pollution holding capacity and the flow rate of different pollutants were in a significant linear relationship,and the linear regression coefficients are all Greater than 0.9.（3）The SWAT model was used to simulate the water environment of the Qiaobian River Basin,the study area was divided into 33 sub-basins and 237 hydrological response units,the daily calibration method was selected,and the SWAT-CUP software was used to calibrate and verify the parameters in the model.For sensitivity analysis,the SUFI-2 algorithm could be celected,and the spatial analysis function of Arc GIS were used to obtain the temporal and spatial distribution of nitrogen and phosphorus load in the river basin by the bridge.The results showed that the nitrogen and phosphorus loads were mainly concentrated on both sides of the entire river bank of the Qiaobian River Basin,and the main pollution sources were concentrated in the middle and lower reaches.The pollution load during the wet season is higher than that in the dry season,but there are some differences.Finally,according to the analysis results,different region could adopt corresponding management measures.