Comparative Analysis Of Non-point Source Pollution Load In Mountainous Small Watershed And Plain Small Watershed

The pollution of water environment has seriously restricted and threatened the survival and development of human beings,and has become one of the important environmental problems to be solved.Nansha River is the main river in the northern part of Haidian District of Beijing and the main stream of the Shahe reservoir.Its water quality and ecology are the important factors affecting the whole environment of Haidian District and the water ecological environment of Shahe Reservoir and the sub-center of the downstream city.In recent years,although the control of point source pollution has been intensified,the water quality of Nansha River is still poor,and the water is often worse than class V.The research and analysis of the non point source pollution load in the small watershed is the basic work to improve the ecological environment of the Nansha River,at the same time,it can provide a reference for the treatment of water pollution in Beijing small watershed.Two typical representative small watersheds Bei'an River(mountainous area type)and Qianzhang Village drainage ditch(plain type)were selected for field investigation of their pollution sources,and the main pollution source types were determined in combination with the collected data.The output coefficient empirical method was used to estimate the load of non-point source pollutants of COD and ammonia nitrogen in rural life,farmland runoff and scattered livestock and poultry.At the same time,through the soil erosion model of Beijing mountainous area,the runoff and soil erosion amount of Bei'an River small watershed were simulated by using SCS runoff curve number and general soil loss equation(USLE),based on the simulation results,the amount of total nitrogen and total phosphorus of non-point source pollutants produced by soil and water loss in Bei'an River watershed was predicted.The main conclusions are as follows:(1)The total amount of non-point source pollutants COD and ammonia nitrogen in Bei'an small watershed were 14.97 t/a and 2.55 t/a respectively,and the total amount of non-point source pollutants COD and ammonia nitrogen in Qianzhang drainage ditch small watershed were 17.74 t/a and 1.97 t/a respectively;(2)Through comparative analysis,runoff of farmland and livestock and poultry non-point source pollutants of COD and ammonia nitrogen enteringinto the river were as follows:Bei'an River small watershed>Qianzhang Village drainage ditch smallwatershed,rural life non-point source pollutants of COD and ammonia nitrogen entering into the river were as follows:Qianzhang village drainage ditch small watershed>Bei'an River small watershed;(3)From 2007 to 2017,the annual average rainfall of Bei'an River watershed was 566.3 mm,the average runoff depth was 42.15 mm,the average soil erosion modulus was 311.92 t/km2,the average soil erosion amount was 12651.29 t/a,and the small watershed belonged to micro-erosion in general,soil erosion was mainly distributed in shallow mountainous area of Sujiatuo Town.In addition,according to the simulation results,the annual average total nitrogen loss and total phosphorus loss were 1.15t and 0.78t respectively.