Analysis Of Factors Affecting Storm Runoff Pollution Of Wanzhou,Chongqing

With the acceleration of the urbanization and the increase of impervious land on the underlying surface,storm runoff pollution has become the most serious non-point source pollution.In order to control storm runoff pollution from the begin,low-impact development?LID?through decentralized control has become an extremely effective method,but the selection of LID measures is greatly affected by runoff water quality.Therefore,in order to provide reference for the selection and improvement of LID facilities in the future construction of sponge city,it is necessary to conduct research on the characteristics of rainfall runoff water quality in the study area,understand the non-point source pollution in the basin,and analyze the influencing factors of runoff pollution.In this research,two reconstruction areas of Wanzhou,Chongqing City were used as research areas to monitor and analyze the spatial and temporal distribution characteristics and pollution characteristics of SS,COD,BOD₅,TN,NO₃^--N,NH₃-N and TP in different rainfall and underlying conditions.according to regional water body monitoring data,the river pollution situation is analyzed by principal component analysis and comprehensive water quality index method,and the influence of rainfall non-point source pollution on water body is explored in combination with rainfall pollution characteristics;.the self-organizing map?SOM?is applied to analyzes the effects of rainfall,rainfall interval,rainfall duration,slope,and permeability coefficient on the concentration of runoff pollutants.According to the monitoring data of imported and outgoing water of LID facilities,runoff pollution caused by rainwater detention facilities and permeable pavement were analyzed.The reduction effect of the material,and the pollution load value of each pollutant surface area of each typical underlying surface is calculated according to the monitoring runoff water quality data.The main conclusions are as follows:?1?The different water quality indexes of Longbao river are greatly affected by sediment dredging and sponge city reconstruction.The trends of ORP,COD,DO,TN,TP,NH₃-N and NO₃^--N are generally the same.Two peaks appeared on September 20th.and June 1st.Before the renovation of the sponge city?May 10th?,the COD in the section of the automobile scrapping factory and the section of Shenyu pharmaceutical glass factory was relatively high,reaching 91mg/L and 99mg/L respectively,,the concentrations were 0.665mg/L and 0.595 mg/L.During the process of sponge city reconstruction?June 10th?,the organic pollution of Shenyu pharmaceutical glass factory section was the most serious,and the COD content in the river section was 248mg/L.?2?The eutrophication of Longbao River is more serious in the rainy season.The water dilution in the dry season is weakened.The basic concentration values of all pollution indicators except the DO in the rainy season are basically higher than the dry season,especially COD,but some water quality parameters such as COD TN still cannot meet the V water quality requirements of enviromental quality standards for surface water?GB 3838-2002?.The main pollution section of Longbao River is the corresponding section of the sewage discharge,the front section of the automobile scrapping factory and the Shenyu pharmaceutical glass factory.The main pollutants are COD and TN.Before the renovation of the sponge city,the water quality of each monitoring section was inferior to V and partially accompanied by black odor.The water quality of the sponge city was partially improved during the renovation process.In addition to the sewage discharge port and the Shenyu pharmaceutical glass factory,the water environment function was basically achieved.?3?Different rainfall and underlying surface conditions,the variation of runoff pollutant concentration is quite different,but most of the pollutants in the rainfall runoff have peaks at the beginning,and then decrease with the extension of rainfall time.The peak occurs mainly in the runoff 10-30min,and then the trend before the decline is steep and slow,the municipal roads and residential areas of the Longbaohe area?old town?in the underlying surface are the most polluted,with the highest COD reaching1090mg/L and 642mg/L respectively.?4?In the research of the first flush effects of different rainfall intensities,the heavy rain on September 20 was the most significant for the first flush,and the effect of the municipal roads in the old city was the most obvious.The first flush of COD and TN was more significant.The COD content in the path of the municipal road in the Tianzi lake area?new urban area?is lower than that in the old city,but the concentration value during the heavy rain on September 20 is more than four times that of the surface water of the V type.The newly-built urban school roads,community roads and commercial plazas are less polluted.In addition to COD,they basically meet the reuse of urban recycling water--Water quality standard for urban miscellaneous water consumption?GB/T118920-2002?.Under the same date,the main pollutants of rainfall runoff have high consistency with the main pollutants of water bodies.Urban runoff pollution is also one of the influencing factors of black odor in Longbao river.?5?The SOM neural network was used to analyze the concentration of each pollutant in the rainwater samples collected in 2018.The correlation between SS and COD was very strong.The correlation between TP and NO₃^--N,NH₃-N and BOD₅ was good.The results were tested with Pearson coefficient.The main influencing factors of runoff pollution are rainfall,slope,rainfall duration and permeability coefficient.Rainfall,slope and permeability coefficient have strong positive correlation with pollutant concentration in runoff,and the influence of rainfall interval time is not clear.The effect of rainfall on SS,COD,NO₃^--N and TP is stronger,and the effect of rainfall duration on NO₃^--N and NH₃-N is stronger.The simulation uses different influencing factors as input,and the EMC value of the pollutant is predicted as the output.Except for BOD₅ and individual extreme values,the error between the predicted result and the expected value is small.?6?LID facilities have strong ability to indirectly control runoff pollution by controlling runoff.For example,light rain and partial heavy rain?33.5mm heavy rain on August 22,2018,long duration rainfall,low instantaneous rainfall intensity?did not produce enough runoff at the LID outlet,and runoff pollutants were all intercepted and not discharged.In terms of pollutant control,the removal effect of pollutants from rain gardens is stronger than that of permeable pavement.The instantaneous removal rate of SS and COD can reach 85.3%and 80.4%.But due to the precipitation of nutrients in vegetation and soil,the nitrogen and phosphorus salts are elevated from the effluent,causing secondary pollution.