Research On The Influence Of Urban Rainstorm Runoff On River Water Quality

With the acceleration of urbanization,the problem of urban non-point source pollution is increasingly prominent.As a typical type of urban non-point source pollution,runoff pollution carries pollutants concentration several times higher than the national standard,which seriously affects the quality of the water of river and may also damage the urban water ecological environment.Runoff pollution has gradually become an important factor of water pollution of urban river,which has high research value.Taking Wuqing district of Tianjin as the research object,this paper constructed a runoff water quality model in Wuqing district,calculated the runoff and pollutants load of the river under different conditions,and analyzed the influence of the designed rainstorm on river water quality.The main research contents and results are as follows:（1）Monitoring and analysis of river and rainfall runoff quality in Wuqing district.From may to October in 2019 collected river water samples,and the determination of COD,TN and TP in water samples and NH₃-N concentration,found with dangerous levels of COD and TN concentrations for worseⅤclass water.Qiangguo road was selected as the research object to collect water samples from three times of rainfall and runoff during the flood season in 2019 and measure the concentration of COD,TN,TP and NH₃-N.Results show that the concentration of COD,TN and TP in the instantaneous and average concentration higher thanⅤclass water standard,average concentration respectivelyⅤclass water standard 2.5 times,2.6 times and 1.8 times.（2）Simulation analysis of river inflow runoff and total pollutants load.Runoff water quality models in typical areas and Wuqing district were constructed,and the runoff quality models in typical areas were calibrated and verified with the measured data.Applied the calibration parameters to Wuqing district and simulated the runoff and the total pollutants load of the designed rainstorm in Wuqing district.Under the same rainfall duration,with the increase of rainfall return period,the total runoff and pollutants load of river inflow increased,but the average pollutants concentration of event decreased.Under the same return period,with the increase of the duration of the designed rainstorm,the total runoff and pollutants load of the river increased.（3）Simulation analysis of the influence of design rainstorm on river water quality under different conditions.The cross section of Xijie canal was selected as the research object to simulate and analyze the influence of design rainstorm on river water quality under different conditions.The results showed that the COD and TN concentrations changed in the same trend under the same design rainstorm conditions in the same return period,and there were two peak values.The first peak value of the design rainstorm in 1h was the earliest and the peak value was the smallest,the design rainstorm in 2h was the second,and the design rainstorm in 3h was the latest and the peak value was the largest.The second peak occurred in the same order as the first,but in the opposite order.The concentrations of TP and NH₃-N were consistent,and decreased at the beginning of rainfall.After the rainfall,the upstream river inflow discharged the upstream pollutants to the Xijie canal,and the concentration gradually increased.As the duration of rainfall increased,the peak value appeared later and the peak value decreased.In addition,under the condition of design rainstorm with the same rainfall duration,COD and TN concentrations also showed two peak values.As the return period increased,the earlier the first peak value appeared,the higher the peak value.The second peak appears later and gradually decreases.The concentrations of TP and NH₃-N were consistent.Except for the design rainstorm once a year,the concentrations of TP and NH₃-N decreased in the early stage of rainfall.After the rainfall,the upstream inflow discharged the upstream pollutants to the Xijie canal,and the concentration of TP and NH₃-N began to increase.With the increase of the return period,the peak appeared later.（4）Analysis the effects of rainfall on water quality of different rivers in Wuqing district.The typical sections of the Qizhi canals,the Xijie canal and the Erzhi canal were selected as the analysis objects to analyze the water quality effects of the design rainstorm once every 2 years（2h）on different rivers in wuqing district.Analysis results showed that the Qizhi canal section of COD and TN concentrations gradually rise,the peak value forⅤclass 1.4～1.7 times,Then the concentration gradually decreases and finally stabilizes at a certain concentration.The Xijie canal and the Erzhi canal cross section at the beginning of the rains also presents the downward trend after rising first,but due to upstream inflow,the Xijie canal and Erzhi canal pollutants concentration will appear again and again the trend of rising and falling,peak water standard forⅤclass1.5～1.75 times,and restoreⅤclass standard need for longer periods of time,water downstream of the river pollution is more serious.