The Dynamic Change Of Nitrogen And Eutrophication Assessment In The Water Source Area Of The Middle Line Of South-to-North Water Diversion Project

Danjiangkou reservoir is the water source of middle Line of South-to-North Water Diversion,covering Danjiangkou city of Hubei province and whole Xichuan county of Henan province.Danjiangkou reservoir is national-level water source conservation area as well as the largest artificial lake in Asian.At present,the water quality of Danjiangkou reservoir is overall in good condition,it meets the category ? water quality requirements of GB 3838-2002 Surface Water Environmental Quality Standards?total nitrogen was not involved in the evaluation?.However,the total nitrogen content of the water is higher,and the water in the reservoir area exceeds class ?,and some of the river water flowing into the reservoir reaches ? to inferior ?.They are making a potential threat to the water quality protection and eutrophication in the reservoir area.The concentration and structure of nitrogen nutrients in Danjiangkou reservoir and its upstream tributaries will not only affect the water environment and the succession of water ecosystem in the reservoir area,but also affect the water security in the downstream water receiving area.In order to fully understand the nitrogen pollution and grasp the dynamic change characteristics of nitrogen in the water source area of the middle line of the south-to-north water diversion project,here their influence and possible sources are explored so as to provide scientific basis for the water quality protection and prevention of eutrophication in the water source area of the middle line of the south-to-north water diversion project.In this study,from May 2016to May 2018,the water quality routine monitoring and measurement was carried out on 8monitoring stations in the reservoir area and 12 monitoring stations in the upstream tributaries Danjiang river and Laoguan river,different forms of nitrogen,total phosphorus,permanganate index,phosphate,chlorophyll A content and fecal coliform,then analyze the spatial and temporal distribution and source of nitrogen,and explore the correlation of water environmental factors and the response to water level changes.And use the comprehensive nutrition index method and potential nutrition index method to evaluate the nutritional status of water.The main research results are as follows:1.From May 2016 to May 2018,the variation range of TN,NH₄⁺-N,NO₃^--N,NO₂^--N,DIN and ON in the water body of Danjiangkou Reservoir was 0.630?2.733,0.037?0.180,0.350?1.971,0.001?0.050,0.429?2.066 and 0.008?0.868 mg/L.DIN was the main stable form of nitrogen in Danjiangkou Reservoir,NO₃^--N was the main form of DIN;The concentration of TN,NO₃^--N and DIN in Danjiangkou Reservoir increased in autumn,summer,spring and winter by turns,while the seasonal differences between NH₄⁺-N and NO₂^--N were small.The concentration of TN,NO₃^--N and DIN in flood season was lower than that in dry season and in average season.Danjiangkou reservoir accept the Danjiang River and Laoguan River at S8 point station,TN and nitrogen content are the highest,followed by S2 point station in Hanku;The S1 point station in the middle line of South-to-North Water Transfer,TN and NO₃^--N content are the lowest.2.From May 2016 to May 2018,the variation range of TN,NH₄⁺-N,NO₃^--N,NO₂^--N,DIN and ON in the water body of Danjiang River,the upstream tributary of Danjiangkou Reservoir,was 0.340?10.500,0.035?0.287,0.159?7.831,0.002?0.144,0.253?8.027,0.001?2.831 and 0.001?2.473 mg/L,NO₃^--N is the main form of nitrogen in Danjiang River;TN concentration in Danjiang River is low in summer and autumn,high in winter and spring;TN concentration in Danjiang River rises in turn in autumn,summer,spring and winter;seasonal trends of TN,NO₃^--N and DIN are the same;NH₄⁺-N and NO₂^--N concentration were lower and seasonal differences are small;The contents of TN,NO₃^--N,DIN and ON in dry season were higher than those in flood season and average season.The TN and nitrogen contents in Danjiang River are higher at source than at outlet.The TN content in Danjiangkou Reservoir is lower at reservoir entrance,but still higher than the reservoir area.The water quality of Qihe River is better than that of Danjiang River.3.From May 2016 to May 2018,the variation range of TN,NH₄⁺-N,NO₃^--N,NO₂^--N,DIN and ON in the water body Laoguan River,the upstream tributary of Danjiangkou Reservoir,was 0.503?10.782,0.032?0.341,0.207?9.751,0.001?0.158,0.303?10.011 and0.023?3.273 mg/L;TN concentration in Laoguan River was low in summer and autumn,high in winter and spring,and increased in turn in summer,autumn,spring and winter.The low TN content in the flood season and the high TN content in the dry season indicate that the nitrogen nutrients in the Laoguan River are mainly affected by the upstream inflow and point source pollution.The maximum values of TN and nitrogen forms appeared at S16 in Laoguan River,and the contents of TN,NO₃^--N and DIN in Laoguan River showed the characteristics that the source was higher than the outlet.4.When the water level is above the high water level of 160m,The concentrations of TN,NH₄⁺-N,NO₃^--N,DIN and ON are higher than the low water level,the concentration of NO₂^--N is opposite;TN,NO₃^--N and DIN are positively correlated with water level,pH,TP,and Chla are positively correlated with water level,while COD_Mnn and NO₂^--N are negatively correlated with water level,and T,SD and ON are negatively correlated with water level.5.From May 2016 to May 2018,the average Chla content of Danjiangkou Reservoir was 2.27 mg/m³.The Chla content gradually increased from winter,spring,summer to autumn.The Chla content of Danjiangkou Reservoir was 1.94 mg/m³ in winter,2.05mg/m³ in spring,2.24 mg/m³ in summer,and the highest Chla content was 2.87 mg/m³ in autumn.Carlson's nutritional status index?TSI?ranged from 45.12 to 54.23.The average TSI index at Danku 8 was 54.23.The water quality was eutrophic.The average TSI at other sites was low,ranging from 37<TSI<53.The water quality of the reservoir was in the medium nutritional state.6.The comprehensive nutritional status index method showed that the Danjiangkou Reservoir was in the medium nutritional state.The evaluation of the potential eutrophication status showed that the Danjiangkou Reservoir had higher DIN content,and the Danjiangkou Reservoir water body showed phosphorus limitation potential eutrophication.