Temporal And Spatial Distribution Characteristics And Source Analysis Of DOM And Nitrate Nitrogen In Qingshui River, A Typical Tributary Of The Upper Yellow Rive

Ningxia Hui Autonomous Region is located in the upper reach of the Yellow River.The ecological protection and high-quality development of the Yellow River Basin according to local conditions meet the national requirements.The Qingshui River,as a tributary of the Yellow River in Ningxia,is the study area.The land use types along the Qingshui River are mainly urban,industry and agriculture,which are mainly affected by the tail water of sewage plants,urban and rural non-point source pollution.The pollution of dissolved organic matter（DOM）and nitrate nitrogen in the Qingshui River is becoming increasingly prominent,and the pollution prevention and control is difficult,threatening the water environmental quality in Ningxia.Through water quality evaluation,DOM three-dimensional fluorescence spectrum analysis and nitrate nitrogen and oxygen isotope analysis,the water quality status of the Qingshui River and the spatial and temporal distribution characteristics,sources of main pollutants are clarified.And it is explored that the relationship between nitrogen and oxygen isotopes of nitrate and DOM under fluoride concentrations.The main conclusions are:（1）The overall water quality of the Qingshui River is medium and below.The water quality is the best in spring and the worst in winter.During the irrigation season,the water quality is poor.The Qingshui River is mainly polluted by organic matter and nitrate nitrogen.The spatial and temporal changes of physical and chemical parameters of water quality are obvious,and irrigation has a great influence on the concentration of nitrogenous substances in water.The main cation ions in the Qingshui River are sodium ions and magnesium ions.Irrigation has little effect on the spatial and temporal distribution of cations.（2）DOM is mainly protein-like substances.Tryptophan protein-like components C1,C2and humus-like components C3 are analyzed.The origin of components C1 and C2 are the same.The fluorescence intensity of DOM is larger in summer.The parameter values of FI,BIX and HIX show that the self-generation characteristics of DOM in the Qingshui River are strong,and the seasonal variation characteristics are obvious.UV-visible absorption parameters show that the humus in the Qingshui River is mainly fulvic acid with small molecular weight.The humification degree,hydrophobic component concentration and aromaticity intensity of DOM in summer and autumn are higher,while the molecular condensation degree is opposite.Irrigation increases the concentration of DOM in the Qingshui River,but has little effect on the source of DOM.The main components of DOM were tryptophan-like protein component C1,humus-like components C2 and C3.The three DOM components have similar sources,and the components C2 and C3 may be terrestrial inputs.Irrigation activities have influence on the composition of humus components and molecular condensation degree of DOM.（3）The spatial and temporal differences ofδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-are obvious.In terms of nitrate isotope composition,irrigation increase the contribution ratio of chemical nitrogen fertilizer and soil organic nitrogen to nitrate.In spring,the main source is domestic sewage and manure,with a contribution ratio of 32%.In summer,nitrate nitrogen in water mainly comes from soil organic nitrogen and chemical nitrogen fertilizer（contribution ratio of 29%）.In autumn and winter,domestic sewage and manure contribute the most,57%and 40%,respectively.In the irrigation season,the main source of nitrate nitrogen is chemical nitrogen fertilizer（contribution ratio of 31%）,but the main source is domestic sewage and manure in the non-irrigation season（contribution ratio of 39%）.（4）In the presence of fluoride,the correlation between DOM and nitrate nitrogen and oxygen isotopes is weak.The correlation results show that farmland irrigations increase the proportion of chemical nitrogen fertilizer sources in nitrate nitrogen sources.There is a negative correlation between F^-andδ¹⁸O-NO₃^-（p<0.05）,and a positive correlation between HIX andδ¹⁸O-NO₃^-（p<0.05）.In the presence of irrigated and non-irrigated seasons,δ¹⁸O-NO₃^-is positively correlated with BIX（p<0.05）,but is negatively correlated with HIX（p<0.01）.