Risk Analysis Of Water Quality Of Multi-source Water Conveyance In Flood Diversion Channel Section Of The Xiaoqing River

The Xiaoqing River diversion canal section is located in Shandong Province,and it is the intersection of the South-to-North Water Diversion Project（Ludong section）and the Yellow River diversion to the Qingdao area project,while also undertaking the task of flood control and water transport.The complexity of water quality at this location has increased due to the significant differences in hydrology and water environment between the South-to-North Water Diversion Project and the Yellow River diversion to the Qingdao area project,as well as the high number of risk sources along the line.Therefore,it has important practical significance to identify risk sources and establish a risk response mechanism.This article focused on the Xiaoqing River diversion canal section,which is borrowed for water transport by the South-to-North Water Diversion Project and the Yellow River diversion to the Qingdao area project to identify the potential risk sources,evaluate the current water quality and master the main influencing factors affecting the water quality.Then established a hydrodynamic-water quality coupling model to explore the transport and transformation laws of various pollutants indicators in the research area.Then Simulated the pollution situation for sudden water pollution and analyzed result,and researched the countermeasures of various risks.The main conclusions are as follows:（1）Industrial enterprise risk sources present the characteristics of regional concentration distribution,involving heavy pollution industries such as petroleum processing,papermaking,leather,printing and dyeing,and livestock and poultry breeding.Most industrial enterprises rely on sewage centralized treatment facilities in industrial parks or urban sewage treatment facilities in their respective areas for industrial sewage treatment.After reaching the standard,the sewage is discharged into the local surface water environment.There are more than 70 cross-canal bridges within the transport section,with an average density of 1.03 bridges/km.Some road bridges are equipped with sewage collection and emergency treatment facilities,but the lack of production bridges is severe,and there is no traffic control for vehicles carrying hazardous chemicals on road bridges across the water transport trunk.（2）During the measurement cycle,the index ofdissolved oxygen and perman-ganate of all monitoring points met the standard of Class II water.The chemical oxygen demand and the five-day biochemical oxygen demand mostly met the standard of Class III water,of which the proportion of dissolved oxygen met the standard of Class III water in all sections was 78%,and the proportion of the five-day biochemical oxygen demand was 80%.Ammonia nitrogen and total phosphorus all met the standard of Class III water and most met the standard of Class II water,with the frequency of ammonia nitrogen met the standard of Class II water at 73%,total phosphorus at 95%,and fluoride content mostly met the standard of the Class I water.Due to the high total nitrogen content in the water source,some samples in the study area exceeded the total nitrogen standard,with the proportion of samples exceeded the standard of Class V water at 69%.（3）In this article,the MIKE11 hydrodynamic-water quality model in the study area was established using collected data and monitoring data.In the term of the hydrodynamic model,the study area was divided into three parts for calibration of riverbed roughness.The final simulation value and the measured value fit well overall,with the efficiency coefficient Ens of the south city control gate flow rate was 0.86 and the determination coefficient R² was0.88.In terms of the water quality model,ten water quality indicators are selected for model building,and three verification sections were selected for comparative calibration.Finally,the relative error of the pollutant content of all water quality models was ensured to be within30%,and the simulation effect of the water quality model was good.（4）Based on the established hydrodynamic and water quality model,the transport law of water quality in the study area was analyzed,and the results are as follows:The dissolved oxygen in river diversion water,COD_Mn,BOD₅,fluoride,and chloride content in the Yangtze River water are relatively high,while the NH₃-N,TN,TP,and nitrate content in the Yellow River water are relatively high.During water transport,the dissolved oxygen,COD_Mn,and chloride content showed an increasing trend,while BOD₅ and TN showed a decreasing trend.The COD,fluoride,and nitrate content remained stable.The dissolved oxygen,NH₃-N,TN,and TP content decreased along the way,while the fluoride content increased.The COD,BOD₅,and nitrate content remained stable.Under the influence of water catchment,the dissolved oxygen,COD_Mn,fluoride,and chloride content decreased briefly,while the COD,NH₃-N,TN,and TP increased briefly.The changes of other pollutants in the water catchment area were relatively small.Overall,the changes in dissolved oxygen,COD_Mn,NH₃-N,and TP content were relatively large,while the changes in COD,BOD₅,TN,and nitrate content were relatively small and remained stable during the water transport period.（5）The pollution of sudden traffic accident was analyzed,based on the established hydrodynamic and water quality model.In the simulation of scenario one,the impact time of dissolved oxygen,COD,BOD₅,NH₃-N,TP,sulfate,solidified substances,and chloride is148h,151h,147h,149h,146h,144h,145h,and 145h.In scenario two,the impact time is67h,65h,69h,66h,66h,68h,65h,and 64h.The pollution level at the point of the accident increased by 5%-20%.The concentration of pollutants in the check section was mainly influenced by the duration of pollutant attenuation,and the flow rate and flow volume of the water control the dominant role.（6）Based on the established hydrodynamic and water quality model,the optimal water allocation scheme was studied using the WPI of the check section as the evaluation criterion.As the chemical oxygen demand and five-day biochemical oxygen demand were relatively poor indicators among the simulated indicators,the WPI of the check section was mainly affected by these two indicators.Overall,as the contribution rate of the Yangtze River increase constantly,the WPI showed a decreasing trend.When the contribution rate was 49%（the flow rate of the Yangtze River water source was 17.64m³/s）,the WPI reached its lowest value of 50.798,then showed an upward trend and finally reached 51.31.From water quality perspective,the optimal water allocation scheme for transporting water from multiple sources from the Yellow River and Yangtze River was to have a flow ratio of 0.49:0.51,with the two water sources each accounting for about half of the water,which can achieve the best water quality during transport.