Research On Non-Point Source Pollution Load And Quantity Control In Small Watershed In Surburbs

With the intensification of human activities and the development of social economy,the water environment problems are becoming increasingly prominent.While point source pollution is gradually being controlled,the non-point source pollution control is becoming more and more urgent.How to effectively control non-point source pollution has become the key to inhibit water pollution and to improve water environment.The Yuntaishan River Basin is located in the Jiangning District of Nanjing City and is one of the important sub-basins of the Qinhuai River Basin.The water environment in Yuntaishan River is poor and the total nitrogen is seriously exceeded,which has a serious impact on the water environment of the whole basin.Therefore,studying the non-point source pollution load and water environment capacity of the Yuntaishan River Basin is of great significance to the ecological water environment security of Qinhuai River Basin.It also provides scientific basis for the non-point source pollution control in small watershed in suburb areas.This paper simulates the rainfall-runoff process in the Yuntaishan River Basin based on the semi-distributed hydrological model.The model calculates the production flow and confluence process of four divided subzones.Based on the generation module and processing module of the non-point source pollution load model,the total nitrogen and total phosphorus production and inflow into the river are estimated.At the same time,based on the actual control target of the Yuntaishan River,the estimated pollutant holding capacity and the reduction / surplus of pollutants into the river are calculated.Based on the results,corresponding countermeasures are provided for the non-point source pollution control of the river basin.The specific results are as follows:(1)Based on DEM and field investigation,we extract the boundary of Yuntaishan river basin.The total area of the river basin is 183km2.Based on remote sensing images,the underlying surface is interpreted as 5 types,including urban land,water surface,farmland,forest land and unused land.The areas are respectively 40.3km2,4.7 km2,69.4 km2,33.8 km2 and 35.0 km2.We also divided the Yuntaishan River Basin into four sub-areas,namely the Shengli River area,the Yunba River area,the main stream area,and the Yangshan River area.(2)The semi-distributed hydrological model is based on C ++ and divided into two modules: the runoff module and the confluence module.The runoff module takes raster data as input to calculate the daily yield of the four underlying surfaces,including towns,water surfaces,paddy fields,and drylands.After simulating the confluence of the slope and the confluence of the river,the flow of the river is simulated.The results show that the total output in 2019 is 34.84 million m~3,of which the Shengli River area and Yunba River area in the upper reaches have the largest output flow,accounting for 60% of the total output flow of the basin.The water volume is about 34.26 million m3,and the average annual discharge is only 1.08 m3 /s.The river flow is small and the river water is relatively stagnant.(3)The non-point source estimation model is divided into two parts,including the pollutant generation module and the treatment module.The pollutant sources include rural life and livestock breeding that are not related to rainfall,and agricultural and town pollution that are related to rainfall.According to the calculations,the total nitrogen production of the Yuntaishan River Basin is 581 t/a,of which the most from rural life and agricultural runoff are 208 t and 201 t,respectively accounting for more than 70% of the total nitrogen production.The total phosphorus production is 82 t/a,the largest source is livestock,which is 51 t,accounting for 62%of the total phosphorus production.The total nitrogen into the river is 187.8 t/a.The largest sources are rural life,farmland runoff and urban runoff,respectively 54.3 t,76.0 t and 36.3 t,accounting for 86% of the total inflow.The total phosphorus into river is 13.9 t/a,and the proportion of each pollution source is basically similar.Compared with the monthly measured water quality results,the error of the simulated results is 14%,which is acceptable for the following analysis..(4)Based on the target water quality standard of the Yuntaishan River,we estimate the pollution capacity of the Yuntaishan River under hydrological conditions of 2019.The results show that the total nitrogen capacity is 50.5 t,and the total phosphorus is 10.3t.The Shengli River has the largest capacity.The total nitrogen and total phosphorus are 17.6t and 3.5t respectively.Then we calculate the amount of pollutant reduction in the Yuntaishan River Basin.The total nitrogen reduction in the basin is 137.3 t/a,which is more than twice the capacity of the whole Basin.The total phosphorus reduction is 3.6 t/a,which exceeds 40% of the pollution-receiving capacity of the whole basin..(5)Based on the balance of pollutant reduction,the total nitrogen control targets of different pollution sources is determined.Total nitrogen reduction in rural life is39.7t,livestock and poultry breeding is 15.5t,farmland runoff is 55.5t and urban runoff is 36.6t.Through measures such as the control of pollutant production and the optimization of the route to the river,the total nitrogen of each pollution source can be reduced by 32 t,15t,65 t and 30 t,a total of 141 t,which reaches the pollution control goal.The Shengli River area and the main stream area are mainly rural life pollution control and the farmland pollution control.The Yunba River area is mainly rural life pollution control,and the farmland pollution control is supplementary.The Yangshan River Basin is mainly runoff pollution and the farmland control.