Characteristics Of Water Chemistry And Dissolved Organic Matter In Hanzhong Section Of Hanjiang River Response To Watershed Point And Non-point Source Pollution

The water quality safety of river water is currently challenged due to the expansion of agricultural and the acceleration of urban modernisation.Water chemical ions are the most mobile and abundant components in rivers,and they play an essential role in determining river water quality.The most active component of the aquatic carbon pool is dissolved organic matter（DOM）.The understanding and characterization of water chemical ions and DOM can effectively analyze the current situation of water pollution in the basin.Therefore,it is required to evaluate and explore the water chemical ions and DOM in the study area in order to clarify the pollution source,conduct quantitative analysis,and propose recommendations for river water pollution prevention.In this study,Hanjiang River（Hanzhong section）was selected as the research object,and the river water samples,soil extracts samples of different land use types and samples of wastewater treatment plants around the basin were collected to determine the water quality parameters,DOM spectral characteristics and molecular characteristics,as well as nitrogen and oxygen isotopes,so as to explore the point and non-point sources pollution of the water body in the study area.Three-dimensional fluorescence-parallel factor analysis（3D EEM-PARAFAC）was used to determine and analyze the fluorescence components and spectral characteristics of DOM,and combined with nitrogen and oxygen isotope characteristic values,the influence of land use and wastewater discharge on the DOM spectrum of the watershed was quantitatively described.Fourier ion cyclotron resonance mass spectrometry（FT-ICR MS）was also used to analyze the effect of land use on DOM molecular characteristics.The following are the study’s main conclusions:（1）According to the findings of this study,the water body of the watershed in the study region is mildly alkaline.The water chemistry types fluctuate depending on the sample season.In January,the river water chemistry type is Ca²⁺-SO₄^2-,and in July,the river water chemistry type is Ca²⁺-HCO₃^-.In general,the hydrochemical characteristics of rivers are affected by rock weathering.In addition,the main ion concentration changes in the watershed showed higher Na⁺and SO₄^2-concentrations in January and higher K⁺and Ca²⁺concentrations in July.（2）In the study area,five fluorescent components were identified,including two humic-like substances and three protein-like substances.The DOM components of soil extracts from various land use types showed no significant differences.There is also no significant difference in DOM component between wastewater and soil extracts.But relatively speaking,the FI value is quite different between the two,with the mean values of2.1 and 1.6 respectively,while the FI value of river water is in between.Therefore,FI could be used as an indicator to distinguish point and non-point source pollution of river water.Moreover,the nonpoint source pollution contribution calculated by isotopeδ¹⁵N is positively correlated with the nonpoint source pollution contribution calculated by FI.Therefore,FI can be used to quantitatively describe the point and non-point source pollution in the basin.（3）In this study,lignin compounds（81.65%）accounted for a higher proportion of the DOM in river water,while protein compounds（26.67%）and lipid compounds（13.33%）account for a higher proportion in soil extracts samples.The number of DOM specific molecules in soil extracts from different land use types varied greatly,with grassland having the most（796 species）,forestland having 29 species,orchard having 30 species and cropland having 3 species.In grassland,the specific molecules were mainly lignin,and protein in orchard and cropland,and lipid and protein in forestland.（4）The number of specific molecules in river water samples increased from upstream,downstream to main stream,and the diversity of specific molecules increased,indicating that human activities increased DOM complexity.Lignin was the main substance in DOM of river water in January and July.In contrast,the proportion of Tannins and Carbohydrates in DOM composition increased in July.Only the extracts from forestland contained Tannins in DOM specific molecules.It shows that forestland is the main influencing factors of non-point source pollution in July.In this study,3D EEM-PARAFAC and FT-ICR MS were used to analyze the spectral and molecular characteristics of DOM in the watershed,and the quantitative relationship between DOM spectral characteristics and nitrogen and oxygen isotopes to characterize the point source pollution in the watershed was established.The influence of different land use types on the molecular characteristics of DOM in the watershed was also analyzed.It provides ideas for pollution control and management in watershed.