Research On The Application Of The Sponge City Concept In Urban River Governance

As ecological civilization construction continues to advance,urban sewage treatment systems are becoming more and more perfect,and point source pollution has basically been effectively controlled,while Non-point source pollution from rainfall is gradually becoming an important factor that threatens the health of the water bodies.By studying the impact mechanism of rainfall runoff pollutants on the water environment of old urban rivers before and after sponge city renovation,with a view to providing some data support for the comprehensive management of urban rivers and their long-lasting governance.In this paper,takes the river channel from the section of Longbaohe Pengcheng Primary School in Wanzhou District of Chongqing to Xujiahuayuan Community as the research object.By monitoring the runoff water quality of the three types of underlying surfaces（residential,commercial and campus）in the watershed under different rainfall conditions,the changes of SS,COD,TP,TN and NH₃-N with the duration of rainfall and pollution load are analyzed;Combined with the nature of the watershed land and the outlet of the drainage pipe network,the monitoring points of the river section were selected to analyze the relationship between the water quality of the river and the nature of the sediment;the SWMM model was used to establish the water quantity and water quality model of the Longbao River Basin.Variation characteristics of pollutants entering the river.The main conclusions are as follows:（1）Different underlying surfaces and different rainfall events have different changes in the range of rainfall and peak formation time,but the overall trend is that with the increase of rainfall duration,the concentration of pollutants gradually decreases and stabilizes after the peak of the initial rainfall The peak value of runoff pollutants is within5-30min of rainfall runoff.The most polluted is the rainfall runoff in the commercial area,and the pollution in the campus area is lighter.（2）The EMC of the runoff pollutants of the 3 types of underlying surfaces are:commercial area>residential area>campus area.The average EMC values of SS,COD,TP,TN,and NH₃-N in the rainfall runoff of commercial areas are 498.02,107.58,0.64,2.25,and 1.3 mg/L,respectively;the corresponding values of rainwater runoff in residential areas are 264.28,81.44,0.52,2.03 and 1.19 mg/L,respectively;the corresponding values of rainwater runoff in the campus area are 248.70,68.61,0.61,1.76and 1.03mg/L,respectively.The EMC of COD and TP in the 3 types of underlays exceeded ClassⅤwater standard,and the initial runoff pollution was serious.The EMC of rain and runoff pollutants TN in commercial and residential areas exceeds ClassⅤwater standards.The EMC of campus TN meets Class IV water standards,and the EMC of NH₃-N in commercial,residential and campus areas meets Class IV water standards.The analysis of the initial erosion effect shows that the three types of underlying surfaces all show a certain degree of erosion,and most of them are weak erosion.There is a certain correlation between runoff pollutants.In commercial areas,SS has a good correlation with COD,TP,TN,and NH₃-N,and the average correlation coefficient R is 0.858,0.82,0.867,0.858;The correlation is good,the mean values of R are 0.926 and 0.898respectively;the correlation between SS and TP,TN,NH₃-N in campus area is good,the mean values of R are 0.939,0.891 and 0.847 respectively.（3）River water quality has significant seasonal differences.The pH value of each section of Longbao River was 6.96-8.09,which was neutral;DO,TN,NH₃-N,TP,COD were 6.06-8.84 mg/L,1.45-9.02 mg/L,0.64-4.25 mg/L,0.14-2.97 mg/L,15-150 mg/L.Both pH and DO meet surface water quality standards.The TP changes significantly with the seasons,the highest in spring is 1.24 mg/L,and the lowest in summer is 0.3 mg/L,and then gradually increases,exceeding the categoryⅤwater standard.TN exceeded the categoryⅤwater standard throughout the year,and the seasonal difference was obvious.high.The average concentrations of NH₃-N in the four seasons were 2.5,1.77,2.26,and 3.37 mg/L,which accounted for more than 55%of TN,indicating that the nitrogen pollution load mainly comes from NH₃-N.The average COD concentrations in the four seasons were 81.20,54.91,29.97,and 26.80 mg/L,respectively.The water quality in autumn and winter belonged to categoryⅣwater,while spring and summer belonged to inferior categoryⅤwater.The single-factor water quality evaluation and the Nemero index evaluation show that the pollution of nitrogen and phosphorus in the river is relatively heavy,and that of organic matter is relatively light.Principal component analysis shows that the main pollutants of Longbao River are TN and TP,which is the same as the water quality evaluation results.（4）The average organic content of the river channel in summer,autumn and winter seasons is 15.49,26.21 and 26.15 g/kg,respectively.The organic content in autumn and winter is higher than that in summer,which may be due to the pollutants generated by the construction of sponge cities around the river and entering the river.The average content of total nitrogen in rivers in summer,autumn and winter is 3.67,2.57 and 2.30 g/kg;the average content of total phosphorus in rivers in summer,autumn and winter is 0.50,0.33and 0.24 g/kg.The total nitrogen and phosphorus content are higher in summer and lower in winter.The nitrogen and phosphorus elements in the sediment may be cultivated with the farmland on both sides of the bank.A large amount of nitrogen and phosphorus elements will converge into the river channel with rainfall runoff and eventually settle in the sediment.Regarding the correlation between the channel water quality and the bottom sediment,there is no significant relationship between the TP in the water and the total phosphorus content in the bottom sediment,the pollutant TN and the total nitrogen content in the bottom sediment,and the COD in the channel water and the organic matter content in the bottom sediment.The water quality of Longbao River is mainly affected by external pollution,namely rainfall runoff and point source discharge.（5）After establishing the SWMM model and arranging green facilities such as green roofs,permeable pavements,and biological retention zones in the watershed,under different rain recurrence periods,the initial river water quality still exceeds the surface water class V standard.With the increase of rainfall intensity,the reduction rate of pollutants SS,COD,TP and TN gradually decreased after the sponge transformation.The reduction rate of corresponding pollutants under the 1-year recurrence period was 72%,70%,64%,and 66%,respectively;the corresponding pollutant reduction rates in the 50-year recurrence period are 47%,56%,48%and 49%,respectively,but they can achieve the runoff pollution control target of Wanzhou District in different recurrence periods（47%）.Through the source-middle-end of the governance mode,using permeable pavement,sunken green space,biological retention zone and other LID facilities,the sponge city is reconstructed at the source of the catchment area,the rain and sewage pipe network is reconstructed in the middle,and set up storage or treatment facilities at the end,including the initial rainwater storage ponds,rainwater treatment wetlands,etc.,the construction of ecological shoreline,control of pollution into the river,reduce the pollution of the water environment,reduce the pollution from surface sources into the river pollutants,and at the same time with the combination of measures to enhance the water environment of the river,improve the river’s own impact resistance to pollutants and water body self-cleaning ability,so as to achieve the effect of river governance.