Pollutant Loads From Rainfall Runoff:A Case Study Of The Demonstration Area On The West Bank Of Wangyu River

With the rapid development of urbanization and industrialization in China,the impervious ground is increasing rapidly,and the problem of rainfall runoff pollution has been increasingly prominent.How to calculate the pollutant load of rainfall runoff quantitatively is an important content of water pollution control.At the same time,during the field experiment,it is found that the curbs in urban construction and field ridges in the farmland system will have significant effects on the volume of surface rainfall runoff,which is often ignored in previous studies.Therefore,taking the comprehensive demonstration area on the West Bank of Wangyu River with serious water pollution as a case,this study took the reduction effect of curbs and field ridges on rainfall runoff into consideration,and quantitatively measured the load of four major pollutants,including total phosphorus（TP）,total nitrogen（TN）,ammonia nitrogen（NH₃-N）and chemical oxygen demand（COD）.The comprehensive demonstration area on the West Bank of Wangyu River is experiencing rapid urbanization,and it is also the area where clear water corridor of Wangyu River"Yangtze-Taihu Water Diversion"Project is flowing through.Therefore,it is of great significance to study the pollutant load of rainfall runoff for the formulation of non-point source pollution reduction plan and even for the protection of the water quality of Taihu Lake.At the same time,the research on the calculation method of rainfall runoff pollutant load has reference for many urban areas in China.Based on Landsat 8 satellite image and Geo Eye-1 high-resolution commercial remote sensing image of the comprehensive demonstration area on the West Bank of Wangyu River,the land use type map of nine types of land is interpreted,including commercial land,industrial and mining storage land,urban residential land,rural residential land,transportation land,forest land,cultivated land,grassland,unused land,etc.Then,natural rainfall observation and artificial rainfall simulation experiments are carried out on each land use type to obtain parameters required for runoff depth calculation under different land use types and runoff pollutant concentration.On this basis,the height distribution of curbs and field ridges in the study area is investigated,and the impact of different heights of curbs and field ridges on rainfall runoff is studied.Finally,combined with the daily rainfall data of 10rainfall observation stations in the study area from 2017 to 2019,using SCS-CN rainfall runoff model to calculate the rainfall runoff pollutant load in the study area from 2017 to 2019,the results of which are refined to the day on the time scale and the high resolution on the spacial scale is 5 m*5 m.Based on the above work,this study completed the calculation of the pollutant load of rainfall runoff in the comprehensive demonstration area on the West Bank of Wangyu River,realized the daily analysis on the time scale and the high-resolution of5 m*5 m grid analysis on the spatial scale,and finally constructed the spatial database of high-resolution rainfall runoff pollution sources in the comprehensive demonstration area on the West Bank of Wangyu River.The results showed that:（1）the recommended initial loss rate of green land and farmland in the study area is 0.3,and that of impervious surface is 0.9.（2）Runoff depth and pollutant load had significant positive correlations with rainfall.The annual load of TP,TN,NH₃-N and COD in rainfall runoff of the study area decreased from125 tons,1243 tons,432 tons and 14965 tons in 2017 to 80 tons,882 tons,306 tons and 10521 tons in 2019 respectively.（3）From 2017 to 2019,the reduction rate of runoff volume generated on green land by curb is 9.2%,17.5%and 21.1%respectively,the reduction rate of runoff volume generated on farmland by field ridge is 87.4%,91.1%and 92.6%respectively,and the reduction rate of annual TP load by curb and field ridge is 34.3%,33.3%and 32.7%respectively.While the reduction rates of annual TN,NH₃-N,COD load by curb and field ridge are all less than 10%.（4）In terms of spatial distribution,Liangxi District with the highest degree of urbanization has the largest rainfall runoff depth,while Xishan District and Xinwu District have the highest loads of major pollutants.The land use type with the highest TN and COD load per unit area is rural residential land,and the land use type with the highest TP load per unit area is unused land.In addition,the load per unit area of four pollutants in grassland is the lowest,followed by that in forest land.（5）From the point of view of temporal change,the peak months of rainfall runoff volume and pollutant load are from July to September in summer,which should be paid more attention.Based on the above study results,the following countermeasures and suggestions are put forward.（1）The planning,supervision and management of land use types should be strengthened.（2）The design regulations on the height of curbs and field ridges should be strengthen.Limiting the existing convex design in the green space planning and construction,and using the concave design as much as possible to reduce the rainfall runoff.（3）The cleaning and renovation of rural residential land far away from the urban area,commercial land with frequent commercial activities and unused land should be strengthened by the sanitation department.（4）Investigation of rainwater and sewage pipe network in the study area should be strengthened to ensure rainwater and sewage diversion and reduce the situation of sewage directly discharged into Wangyu River water body due to wrong connection and leakage of pipe network.（5）Treatment and control of runoff pollution should be strengthened,focusing on the collection and treatment of initial rainwater.The results of this study provide the basic data for water pollution control and basin management,and also supply the method reference for the quantitative calculation of rainwater runoff pollutant load.