Sources Of Nitrogen Pollution Under The Background Of Cascade Reservoir Construction In Lancang River Basin

Since the development of the Lancang River Basin,the environment in the basin has been increasingly affected by human activities.The environmental problems in the Lancang River Basin have attracted more and more attention from scholars at home and abroad.In this study,the Yunnan section of Lancang River Basin was taken as the research object.In summer of 2017 and winter of 2018,water samples from different sections of Yunnan section of Lancang River Basin and the main pollution sources around were sampled and analyzed,including domestic sewage,chemical fertilizer,farm manure,coastal plants,coastal soil and other samples.The distribution regularity of various forms of nitrogen in the watershed was studied and analyzed.The contribution of different sources of nitrogen pollution in the Lancang River Basin was quantitatively calculated by using isotope tracer technology combined with the measured source values of surrounding pollutants.The source of nitrogen pollution was traced to effectively control the input of nitrogen in the environment.At the same time,the mechanism of isotope fractionation was used to explain the large amount of nitrogen pollution.Effects of dam construction on nitrogen transport and transformation in river basin.This paper is of great significance to alleviate the environmental pollution in the river basin and reduce the input of pollution sources from the source.The specific work and results of this paper are as follows:（1）Through traditional hydrochemical experiments,it was found that the total nitrogen in Lancang River watershed varied from 0.33 to 1.76 mg·L-1 in January.The concentration of nitrate was between 0.11 and 1.12 mg·L^-1,and the concentration of nitrogen in six watersheds was lower than that in January.The total nitrogen was between0.22 and 0.95 mg·L^-1,and the nitrate concentration was between 0.11 and 0.50mg·L^-1.Nitrate concentration in the basin increased along the river in both quarters,which indicated that nitrogen pollution in the basin increased gradually along the river,and the reason for the change could not be explained by the concentration alone.In order to further analyze the reasons for the increase of nitrogen concentration in water,the isotopes in suspended particulate matter and nitrate in Lancang River were analyzed and traced,and the following results were obtained:（2）The carbon and nitrogen isotope values of suspended particulate matter in water during winter and summer were measured by isotope gas chromatography and mass spectrometry.It was found that the nitrogen isotope values of suspended particulate matter in summer ranged from 6.7‰to 15.6‰and in winter ranged from 2.3‰to 11.8‰respectively.The isotope abundance in summer was significantly heavier than that in winter,while the isotope values in both seasons increased.The carbon isotope values of suspended particulate matter in summer ranged from-27.41‰to-23.21‰,while in winter the carbon isotope values were slightly lower,ranging from-28.61‰to 23.42‰.The carbon isotope values of suspended particulate matter in the two quarters showed a trend of lightening.The source of suspended particulate matter in different sections of the river basin was calculated by using the multivariate mixing model.The results showed that the input of suspended particulate matter in the upstream section of the natural river and the organic matter in the coastal soil were the main sources of suspended particulate matter in the water body,accounting for 33.7%and 34.3%of the total sources,respectively.The main factors affecting suspended particulate matter in the downstream reservoir section were domestic sewage and upstream section.The input of suspended particulate matter accounts for 28.0%and 25.2%of the total source,respectively.Through the analysis of suspended particulate matter,it can be found that there is an increasing trend of domestic sewage discharge along the way,but suspended particulate matter can not directly reflect the situation of dissolved nitrogen in water.（3）It is found that the nitrogen isotope value of nitrate is between 2.1‰～6.2‰,and the oxygen isotope value of nitrate fluctuates between 4.0‰and 8.4‰,and tends to be lighter along the course.Isosource software was used to analyze the sources of various forms of nitrogen along the Lancang River Basin.The results showed that nitrate in surface water was mainly composed of nitrate from upstream section,accounting for 30.5%of the total contribution rate.There were differences in nitrate composition between upstream and downstream sections.Nitrate and soil from upstream section of upstream natural river were the main factors affecting nitrate.The average contribution rates of the two are 35.5%and30.4%respectively.The main sources affecting the nitrate composition in the downstream reservoir section are domestic sewage and nitrate input from the upstream section,which account for 25.5%and 26.9%of the total contribution rates in the downstream section.（4）Gases released from surface and bottom water in reservoirs are collected by inverted funnel device,and nitrogen concentration and isotope values are measured.The results show that:The concentration of bubbles nitrogen in different sections of water is between 0.41 and 3.21 mmol·L^-1.The concentration of bubbles nitrogen in surface water is higher than that in bottom water bubbles;Nitrogen isotope values in water bubbles ranged from 10.64‰to 20.28‰and nitrogen isotope values in surface bubbles were lower than those in bottom bubbles.（5）The fractionation coefficients of nitrogen in water and the isotope values of nitrogen generated by denitrification at different sites were calculated by steady state model,and the isotope values of nitrogen generated by denitrification were brought into the mass conservation equation.The results show that:Among the sites of Miaowei,Heihui River and Dachaoshan,denitrification was found in the middle and surface water bodies.The contribution of denitrification to surface nitrogen was 41.7%,41.7%and35.5%respectively.The contribution of denitrification in Gongguoqiao Reservoir is 9.8%.The main source of surface nitrogen at this point is sediment denitrification,and its contribution rate is 78.0%.The denitrification of Xiaowan is weak.The nitrogen deposited in the air is the main source of surface nitrogen,accounting for 48.3%of the total source.（6）By calculating the value of delta△δ¹⁵N∶△δ¹⁸O of nitrogen and oxygen isotopes in nitrates,it is found that the value of delta△δ¹⁵N∶△δ¹⁸O of nitrates in the upstream natural river reaches varies slightly from-0.244‰to 0.394‰,indicating that the upper reaches are less affected by assimilation and nitrification and denitrification,while the value of△δ¹⁵N∶△δ¹⁸O in the downstream reservoirs ranges from-1.98‰to 3.33‰,and the difference between the values and the values of 0 indicates that the construction of The process of nitrogen cycle in water is affected,resulting in the difference of nitrogen cycle between natural river section and reservoir section.At the same time,different reservoir sections in the lower reaches have different effects on nitrogen cycle due to different climate,environment and retention time of water conservancy.Among them,Miaowei,Nuozhadu,Dachaoshan and Jinghong reservoirs have obvious denitrification effect,which plays a role of nitrogen sink.The three reservoirs,Gongguoqiao,Xiaowan and Manwan,have the process of meditation nitrification and nitrogen fixation,showing the role of"source"of nitrogen.（7）The denitrification of Miaowei reservoir,Nuozhadu reservoir,Dachaoshan reservoir and Jinghong reservoir was analyzed by Rayleigh equation.The results show that the denitrification effect of these reservoirs is from 1.91‰～3.33‰,and the denitrification effect of these reservoirs is from-4.5‰～-1.49‰.Under the condition of removing the effect of fractionation,the multi-distance mixing model was used to analyze the sources of water in front of the dam.The results showed that the effects of denitrification in different reservoirs were different.Nitrate in four reservoirs was mainly affected by denitrification and domestic sewage.The average ratios of denitrification in the four reservoirs were36.9%and 21.9%,respectively.The effects of denitrification in different reservoirs were quite different.The value ranges from 19.1%to 65.4%.This phenomenon is caused by environmental and hydraulic factors of different reservoirs.