Study On Spatial And Temporal Characteristics Of Nitrogen Pollution Sources In Wuliangsuhai Watershed

With the rapid development of urbanization in China,a large amount of nitrogen has been released into the lake ecosystem,and excessive nitrogen input has posed a potential threat to the water environment health in China.Aiming at the problem of nitrogen pollution in typical agricultural recession lakes in northern China,the Wuliangsuhai basin,where the large-scale irrigation area in the middle reaches of the Yellow River is located,is taken as the research object.Based on the analysis of the temporal and spatial distribution characteristics of total nitrogen,ammonia nitrogen and nitrate in surface water and pollution endmember of the basin,nitrogen and oxygen dual isotope tracer technology and Iso Source model are used to analyze the distribution of nitrogen and oxygen isotope composition in drainage and Lake area The source and contribution rate of nitrate were analyzed,and the SWAT model of nitrogen migration in Wuliangsuhai watershed was constructed to estimate the total nitrogen load of each watershed and identify the main contribution areas of nitrogen load,so as to provide theoretical basis for regional water environment nitrogen pollution control.The main conclusions are as follows.（1）By analyzing the temporal and spatial distribution characteristics of TN,NH₃-N and NO₃^-concentrations in the outlets,drains and lakes of Wuliangsuhai River Basin,it is found that TN and NO₃^-concentrations in the outlets of enterprises in autumn are significantly higher than those in spring and summer,and TN input in the fifth and seventh drains is higher than that in other drains.The seasonal differences of TN,NH₃-N and NO₃^-concentrations were significant.The maximum concentrations of TN,NH₃-N and NO₃^-were 3.01 mg/L,0.94 mg/L and 1.16 mg/L respectively in winter,which have exceeded the class IV standard of surface water environmental quality.（2）Based on nitrogen and oxygen isotope tracing of surface water nitrate and source contribution analysis of IsoSource model,the main sources of drained nitrate are manure and sewage,with a contribution rate of 36.6%～39.5%,and the contribution rate is relatively high in spring.Among them,the contribution rate of manure and sewage of fifth drain and seventh drain that are located in urban area is the highest,reaching 38.8%～50.4%;The agricultural contribution rate from chemical fertilizer and soil organic nitrogen of sixth drain and eighth drain that are located in agricultural area was higher,up to 37.5%～53.6%.The contribution rate of nitrate in lake water mainly came from manure and wastewater,accounting for 28.05%～39.43%.The contribution rate of wastewater in spring and summer was higher than that in autumn,which was consistent with the seasonal production characteristics of the basin.（3）According to the applicability evaluation of the model,the SWAT model of nitrogen pollution was established（R²>0.80,NSE>0.78,PBIAS<2%）with good applicability.Combined with the tracing results of nitrogen and oxygen isotope pollution,it is concluded that the urban area of Hetao Plain is the main contribution area of nitrogen load,and the load is 6.9～9.5 kg/ha,in which the total nitrogen load loss of the area where the fifth drain and seventh drain undertake urban sewage is larger.