Study On Risk Assessment Of Endogenous Pollution In Daye Reservoir

With the rapid development of the social economy and the improvement of people’s quality of life,water quality safety has become a common concern of the Party and the country.Daye Reservoir is one of the important water sources in Jinan City,After long-term optimization and control,external pollution from the reservoir has been effectively controlled.However,the reservoir was built in the 1950s and there were few times of dredging;At the same time,the seasonal characteristics of precipitation and upstream inflow are obvious,and the hydraulic retention time in the reservoir area is relatively long throughout the year,posing a risk of endogenous pollution.Therefore,this article takes the Daye Reservoir as the research object,and based on the analysis of the spatiotemporal changes in the physicochemical indicators of water-suspended solids-sedimentation and the analysis of the transport and transformation characteristics of endogenous pollutants under different environmental conditions,carries out research on the risk assessment of endogenous pollution.Based on reservoir engineering,hydrology,environment,and other conditions,set up monitoring sections,and dynamically layer by layer set sampling points according to changes in water depth;using the test data,analyze the spatial-temporal change(3D dynamic for short)characteristics of physical and chemical indicators in water,suspended solids and sediment,and identify the main risk factor affecting water quality;based on the identification of risk factor,indoor experiments were designed to study and analyze the characteristics of endogenous pollutant transport and transformation(referred to as endogenous transport)under different environmental conditions;based on the analysis of three-dimensional dynamics and endogenous transport characteristics,the main risk factor were selected to establish a multi-dimensional endogenous pollution risk assessment index system of water,suspended solids and sediment;in view of the fuzzy uncertainty of risk factor,a variable fuzzy evaluation model of endogenous pollution risk is constructed to evaluate the endogenous pollution risk of Daye Reservoir.The main research content and conclusions are as follows:(1)Analysis of spatiotemporal variation characteristics of various physicochemical indicators in water,suspended solids and sedimentsTo analyze the spatiotemporal variation characteristics of water quality in Daye Reservoir,monitoring sections were set up at important locations such as river inlet,reservoir center,water intake,and dam front,and sampling points were dynamically layered according to changes in water depth;from June to October 2022,monthly on-site measurement of water quality indicators such as pH,DO,water temperature,and conductivity in different sections and layers of water,and sampling was conducted to experimentally detect BOD5,CODMn,total nitrogen(TNw),and total phosphorus(TPw)in different layers of water,as well as total nitrogen(TNss,TNs,respectively)and total phosphorus(TPss,TPs,respectively)in suspended solids and sediments;using the test data,analyze the temporal and spatial variation characteristics of physical and chemical indicators,and identify the main risk factor affecting water quality.The results show that:1)From June to August,water temperature,pH and DO all form vertical stratification.The average bottom water temperature is 20℃,the maximum temperature difference between the surface and bottom is 15.6℃,and the lowest pH value and DO concentration in the bottom water are 6.7 and 0.84mg/L,respectively;the highest concentrations of TNw and TPw were 6.34mg/L and 0.15mg/L,respectively,higher than the surface layer;the concentrations of TNs and TPs decreased compared to other months,with the lowest being 0.97g/kg and 0.494g/kg,respectively.2)The stratification phenomenon subsided from September to October,with DO concentrations in each layer around 9mg/L and pH values around 8.5;the concentrations of TNw and TPw in the bottom layer are close to those in the surface layer,with average values of 5.12mg/L and 0.035mg/L,respectively;the concentrations of TNs and TPs increased from June to August,with the highest being 2.02g/kg and 0.889g/kg,respectively.3)The hydraulic disturbance during the flood season has a significant impact on the upstream section,with an increase in TNss and TPss concentrations,with the highest being 2.17g/kg and 1.08g/kg,respectively.The above results show that water temperature,DO,pH and disturbance factors have obvious effects on the concentration changes of total nitrogen and total phosphorus in water,suspended solids and sediment,which are the main risk factor affecting water quality.(2)Study on the transport and transformation characteristics of endogenous pollutants under different environmental conditionsBased on the analysis and identification of factors such as water temperature,DO,pH,and disturbance based on three-dimensional dynamic features,internal transport characteristics were studied through indoor simulation experiments to further reveal the mechanism of endogenous pollution.Based on on-site testing and actual situation analysis,the temperatures were set at 5℃,15℃,and 25℃,with pH values of 5,7,and 9,and DO concentrations of 10mg/L(aerobic)and Omg/L(anaerobic),respectively.Considering the sediment resuspension caused by runoff inflow during the flood season,refer to relevant literature and set the flow rate as 5cm/s,15cm/s,25cm/s,35cm/s,45cm/s,55cm/s,and 65cm/s.Conduct nitrogen and phosphorus nutrient transport and transformation experiments using collected water samples and sediments under different conditions such as temperature,DO,pH value,and flow rate;using detection data,study and analyze the concentration changes of nitrogen and phosphorus nutrients in overlying water,the release rate of sediment,and the transformation of various forms of nitrogen and phosphorus.The results show that:1)Under temperature conditions such as 5℃,15℃,and 25℃,as the temperature increased,the release rates of TNs and TPs gradually increased.The average release rates of TNs were 13.60,27.64,and 56.43mg/(m2·d),respectively,while the average release rates of TPs were 0.47,1.42,and 1.85mg/(m2·d),respectively.2)Under aerobic and anaerobic conditions,the average release rates of TNs were 34.90 and 52.23 mg/(m2·d),respectively,and the average release rates of TPs were 0.76 and 1.16 mg/(m2·d),respectively.Under anaerobic conditions,the release rates of TNs and TPs increased compared to aerobic conditions.3)At pH values of 5,7,and 9,the average release rates of TNs were 43.39,34.81,and 54.10mg/(m2·d),and the average release rates of TPs were 1.88,1.49,and 2.08 mg/(m2·d),respectively.The release rates of TNs and TPs showed a U-shaped trend.4)At a flow rate of 5～65 cm/s,the concentrations of TNw and TPw in the overlying water increase exponentially or more with the increase of flow rate.At a flow rate of 5～45cm/s,the concentrations of TNss and TPss continued to increase,but slightly decreased at 55～65cm/s.The suspended solids underwent a transition from "converge" to "source".5)Under an environment of 25℃ anaerobic,pH=9,and flow rate of 65cm/s,the concentration of NH4+N in the water increased by 0.87,0.60,0.75,and 0.47mg/L compared to the initial value,and the concentration of NaOH-P decreased by 25.04,34.31,28.48,and 7.79mg/kg,respectively.The conversion of nitrate nitrogen to NH4+-N,NaOH-P to soluble phosphorus,and the increase of NH4+-N and soluble phosphorus concentration can easily lead to eutrophication of the water body,further indicating that temperature,DO,pH and hydraulic retention time are the main risk factor for endogenous pollution(3)Multidimensional comprehensive evaluation of endogenous pollution risksBased on the research and analysis of three-dimensional dynamics and endogenous transport characteristics,the main risk factor such as total nitrogen and total phosphorus in water,suspended solids and sediments,and water temperature,DO,pH and hydraulic retention time in environmental factors were selected to establish a multi-dimensional endogenous pollution risk assessment indicator system for water,suspended solids and sediments;using Analytic Hierarchy Process and Entropy Weight Method to comprehensively determine the weights;in view of the fuzzy uncertainty of risk factor,this paper uses the variable fuzzy set theory to build an endogenous pollution risk assessment model and carry out endogenous pollution risk assessment for Daye Reservoir.The results show that:1)The constructed multi-dimensional evaluation index system based on water,suspended solids and sediment can comprehensively evaluate the endogenous pollution risk of reservoirs compared to only considering single risk factors such as water body,suspended solids or sediment,and the evaluation results are more reasonable.2)The average characteristic level of endogenous pollution risk in Daye Reservoir from June to October ranges from 3.20 to 3.46,which is basically at level Ⅲ risk.Among them,the risk levels of water bodies,suspended solids,and sediments are Ⅳ,Ⅲ,and Ⅳ,respectively,with the average risk level characteristics ranging from 3.20 to 3.75,2.81 to 3.10,and 3.51 to 3.63,respectively.The evaluation results are consistent with the actual situation that triggers endogenous pollution risks,further verifying the reliability of the evaluation model.