Characteristics Of Agricultural Non-point Source Pollution In Nanyang City And Evaluation Of Control Effect In Typical Small Watershed

Agricultural non-point source pollution（ANSP）has become the main source of water pollution,which seriously restricts the improvement of water environment quality and the sustainable development of agriculture.Nanyang is not only a big agricultural city in Henan Province,but also the canal head and the core water source area of the Middle Route Project of South-to-north water diversion.It is of great significance to strengthen the research and treatment and reduce the load and risk of ANSP,so as to ensure the long-term stability of water quality in Danjiangkou Reservoir and the sustainable agriculture development in Nanyang city.In this study,statistical data,literature and laboratory experimental data were used to estimate the emission load of chemical oxygen demand（CODcr）,total nitrogen（TN）and total phosphorus（TP）of ANSP caused by chemical fertilizer application,crop straw,livestock and poultry breeding,rural life and aquaculture in Nanyang City and its counties（cities and districts）from 1995 to 2018.The main pollutants and main pollution sources of ANSP were determined by using the equal standard pollution load method,and their temporal and spatial distribution characteristics were analyzed.At the same time,combined with the“Pilot Project of Comprehensive Treatment of ANSP in Xichuan County,Danjiangkou Reservoir Area”,the treatment effect of ANSP in typical small watershed was evaluated.The main results were as follows:（1）The loads of COD_Cr,TN and TP of ANSP in Nanyang city from 1995 to 2018were 124.6～233.0,68.1～120.9 and 92.0～154.0 kilo tons·a^-1,respectively,and the corresponding equivalent standard pollution loads were 0.62×10¹⁰～1.16×10¹⁰,6.81×10¹⁰～12.09×10¹⁰ and 4.59×10¹⁰～7.71×10¹⁰ m³·a^-1,and the total equivalent pollution load was 12.06×10¹⁰～20.97×10¹⁰ m³·a^-1.ANSP was an important source of water pollution in Nanyang city.TN and TP were the main pollutants of ANSP in Nanyang City,and their contributions were 55.22%～58.97%and 33.93%～39.36%respectively.TN and TP emissions from livestock breeding and chemical fertilizer application were the key to the prevention and control of ANSP in Nanyang city.（2）The main pollution sources of ANSP in Nanyang city from 1995 to 2018 were livestock breeding and chemical fertilizer,accounting for 41.51%and 30.84%of the total pollution load,respectively.The pollution caused by rural life,crop straw and aquaculture was relatively small,accounting for 14.91%,10.12%and 2.62%of the total pollution load,respectively;In livestock and poultry breeding,rural life,crop straw and aquaculture,big livestock,domestic sewage,wheat and fish had the largest pollution load,which should be focused on in the prevention and control of ANSP in the future.（3）In 1995-2006,the pollution load of the majority of counties（cities and districts）in Nanyang city increased with time,regardless of the total unit area equivalent pollution load or COD_Cr,TN and TP equivalent standard pollution load,while most counties（city and district）decreased after 2006;From 1995 to 2018,the equivalent standard pollution load of fertilizer,straw and livestock and poultry breeding in each county（city and district）of Nanyang city was generally increased first and then decreased.However,there were differences in the year when the peak value appeared in different counties（cities and districts）.The equivalent standard pollution load of rural living unit area in all counties（cities and districts）decreased with time,and the aquaculture was increasing.TN is the most important pollutant in Nanyang city,but its proportion decreased slowly with time,while TP proportion was rising slowly;The proportion of equivalent standard pollution load per unit area of livestock and poultry breeding and rural living in each county（city,district）decreased with time.The proportion of equivalent standard pollution load of fertilizer,straw and aquaculture per unit area increased with time.Especially fertilizer,it had become the most important pollution source in most counties（cities and districts）.（4）The overall spatial distribution pattern of ANSP load per unit area in Nanyang city was increasing from northwest to southeast（except Tongbai county）.Xinye county,Tanghe county,Sheqi county,Dengzhou city,Wancheng district and Wolong district were areas with relatively serious equivalent standard pollution load per unit area,especially Xinye county,its total equivalent standard pollution load per unit area in each stage from1995 to 2018 was 105.30 to 161.80 kilo m³·hm^-2;The areas with relatively low equivalent standard pollution load per unit area included Xixia county,Neixiang county and Nanzhao county,especially Xixia county,and its total equivalent standard pollution load per unit area was 10.00～11.70 kilo m³·hm^-2 from 1995 to 2018,and the difference between regions were obvious.（5）From the perspective of global spatial autocorrelation,there was obvious spatial aggregation of ANSP in Nanyang city,however,from 1995-2000 to 2011-2015,the spatial aggregation of equivalent standard pollution loads of total,COD_Cr,TN,TP,chemical fertilizer,straw,livestock and poultry breeding and rural life per unit area showed a decreasing trend with time,while from 2016-2018,the spatial aggregation increased.From the perspective of local spatial autocorrelation,the significant（P<0.05）hot spots of the equivalent standard pollution loads of COD_Cr,TN and TP per unit area in 1995-2000 and2001-2005 were Xinye county and Wancheng district,and the significant（P<0.05）cold spots were Xixia county,and there were no significant（P>0.05）cold and hot spots in2006-2010,2011-2015 and 2016-2018;The equivalent standard pollution loads of chemical fertilizer,straw,livestock and poultry breeding and rural life per unit area were significantly different in cold and hot areas,however,Xinye county and Wancheng district were significant hot spots most of the time,while Xixia county was significant cold spots in most of the time.（6）Total nitrogen was the main pollutant of ANSP in Jinzhu river small watershed,followed by TP,and the NH₃-N and NO₃-N contents were low during the whole monitoring period,which reached the class II of surface water quality standard.The dissolved oxygen（DO）(4.35～5.02 mg·L^-1),TN(4.91～5.10 mg·L^-1)and TP contents(0.12～0.19 mg·L^-1)of ecological ditch intake（XX）,outlet（i.e.wetland intake,XM）and wetland outlet（XR）were class IV,V and III surface water quality standards,respectively,before the ICP.The water quality of each section was significantly deteriorated during the period of the ICP.The DO(8.25～9.30 mg·L^-1)and TN(0.97～0.98 mg·L^-1)TP(0.03～0.04 mg·L^-1)contents in the XR section were up to class I,III and II surface water quality standards,respectively,after the ICP.Before,during and after the ICP,the total increase rate of DO by ecological ditch and wetland was 15.40%,–1.25%～–17.37%and 13.48%～24.00%,respectively.The total removal rates of TN and TP were 0.04%,–40.32%～–28.53%,30.50%～42.28%,and–25.86%,–20.00%～–16.67%and 49.06%～67.44%,respectively.The integrated index of water quality showed that the water quality of each section before and during the ICP was XX>XR>XM,and it was XX<XM<XR after the ICP.These findings demonstrated that the ICP presents promising control effect on non-point source pollution in Jinzhu river small watershed,but it is necessary to pay attention to the problem of pollution aggravation during the construction of the project.The treatment mode is suitable for widespread application in agricultural non-point source pollution control in small watershed of Danjiangkou Reservoir area.