Temporal And Spatial Variation Characteristics And Pollution Sources Analysis Of Nitrate In Surface Water Of Jinxiuchuan River Basin

Water is the basis on which human beings live.With the acceleration of urbanization,the pollution of rivers caused by human activities has become more and more serious,resulting in a threat to the water quality of water sources.Nitrate pollution is widespread in surface water environment.Nitrate can lead to water environment problems such as eutrophication and water quality degradation;excessive concentrations of nitrates are reduced to nitrites in the human body,increasing the risk of diseases such as methemoglobin,esophageal and stomach cancers.In addition,the transformation process of nitrate is complex and difficult to manage.Therefore,identifying the sources of nitrate pollution and its transformation process is of great practical importance for the prevention and control of pollution in the water environment and for the long-term protection of human health and safety.In this study,the spatial and temporal distribution of nitrate concentration in the surface water of the Jinxiuchuan River Basin was investigated by collecting surface water from the river basin in January（dry season）,May（normal season）and July（wet season）in 2022.The surface water hydrochemistry,redundancy analysis（RDA）,nitrogen and oxygen isotope technique,Iso Source model and SIAR model were combined to elucidate the mechanisms of nitrate transport and transformation in surface water,and to qualitatively and quantitatively determine the various sources of nitrate in surface water and their contribution,and to analyze the uncertainty and sensitivity of the SIAR model through the data of this study.The following main conclusions were drawn:（1）There are significant temporal and spatial differences in NO₃^--N concentration in Jinxiuchuan River Basin.In terms of time,the concentration of NO₃^--N in January and July was higher than that in May.Spatially,the concentration of NO₃^--N in the upstream is low in January and May,but it is higher in the downstream after the inflow of the upper tributary with higher concentration of NO₃^--N.In July,the concentration of NO₃^--N in upstream and upper tributaries was higher than that in downstream.（2）Several qualitative analyses revealed that NO₃^--N concentrations in January and May were related to sewage and manure sources,while nitrate pollution in July was mainly from agricultural pollution;land use and water quality dates were analyzed by RDA analysis and found that changes in NO₃^--N concentrations in January and May were related to grassland and transport land,while July was more related to woodland.（3）The isotopic fractionation of nitrate nitrogen and oxygen is mainly influenced by denitrification and assimilation.In this study,denitrification and assimilation were not obvious during the nitrate transformation process in different periods in the study area,all of them were dominated by nitrification,and the fractionation coefficient was negligible when the nitrification reaction occurred completely.Therefore,the effect of isotopic fractionation was not considered in this study during the quantitative analysis.（4）The quantitative estimation by SIAR and Iso Source models revealed significant temporal differences in the contribution of nitrate sources in the basin.The main sources of nitrate in January were manure and sewage with 43.0%and 24.2%contribution respectively,the main sources of nitrate in May were manure,sewage and soil with 72.3%and 22.0%contribution respectively,and the main sources of nitrate in July were soil and fertilizer with 56.36%and 34.04%contribution respectively.Spatially,there was no significant difference between January and July,with manure and sewage contributing more than soil in the upper tributaries in May.（5）The uncertainty analysis of SIAR from the data in this study concluded that the uncertainty was greater for sources with larger contributions.The sensitivity analysis of the model shows that sources with larger initial values of nitrogen isotopes are more sensitive,so accurate collection of such sources during the sampling process will make the estimation results more accurate.