Nitrate Source Identification In Surface Water Of A Typical Peri-urban Watershed In The Loess Tableland Gully Region

With the rapid development of industry and agriculture and urbanization,surface water pollution has become one of the main water environment problems in many areas of our country.Since the beginning of the new century,point source pollution and non-point source pollution have led to the continuous rise of nitrate in surface water,and the sources of nitrate pollution in water are complex and susceptible to the mixture of multiple sources of pollution.Identifying the sources of nitrate pollution and taking effective prevention and control measures can improve the current situation of surface water pollution,which is the basis for ensuring the effective utilization of water resources and realizing regional water security.Qualitative and quantitative analysis of surface water nitrate provides data support for surface water pollution prevention and control,and provides theoretical basis for the prevention and control of surface water nitrate sources and pollution control in the Loess Plateau.In this study,Yanwachuan Basin,a typical suburban basin in the Loess Plateau gully region,was selected as the study area.Based on field investigation and physical and chemical indexes of various samples,the water-chemical characteristics analysis method,nitrogen and oxygen bistable isotope technology and SIAR model were used to analyze the spatio-temporal variation characteristics of surface water chemistry,and to explore the control mechanism of main ion sources in surface water.The temporal and spatial variation of nitrate in surface water and its potential pollution sources in the basin were analyzed,the possible transformation process of nitrate was discussed,the contribution rates of different pollution sources of nitrate in surface water in the dry season and rainy season in the study area were quantitatively identified,and the main reasons for seasonal differences of different pollution sources were clarified,so as to provide reference for accelerating the restoration and reconstruction of water ecological environment and the management of water resources in the basin.The main conclusions of this study are as follows:（1）Surface water in different seasons of Yanwachuan watershed is in aerobic environment and weak alkaline water.TDS of surface water in Kongtonggou watershed,Qingshuigou watershed and main stream of Yanwachuan watershed are significantly higher than those of other tributaries.The concentration of major ions decreased gradually in the process of the tributaries gathering to the main stream,and the change trend of K⁺,Na⁺,Ca²⁺,Mg²⁺,HCO₃^-,Cl^-and NO₃^-concentrations was basically the same.The NO₃^-and Cl^-plasmas,which were closely related to human activities,were relatively volatile,which might be related to the discharge of urban domestic sewage.Surface water in different seasons belongs to SO₄-Cl-Na type and HCO₃-Na type,and its hydrochemical characteristics are mainly affected by rock weathering,evaporation and concentration,and to some extent by human activities.（2）The main forms of inorganic nitrogen in the surface water of Yanwachuan watershed are NO₃^--N and NO₂^--N,showing significant spatio-temporal differences.The average concentration of NO₃^--N and NO₂^--N in surface water in rainy season was higher than that in dry season,while the average concentration of NH₄⁺-N showed opposite characteristics.In different seasons,the concentrations of NO₃^--N and NO₂^--N in surface water of Qingshuigou sub-watershed and Qijiachuan sub-watershed affected by domestic sewage discharge in Xifeng urban area were significantly higher than those in other sub-basins,and the concentration of NH₄⁺-N at urban flood discharge outlet was significantly higher than that in other sampling points,reaching 79.97 mg·L^-1 in dry season.（3）The main source of nitrate in surface water of Yanwachuan watershed in rainy season is manure sewage,while the main source of nitrate in dry season is manure sewage and soil nitrogen,followed by ammonium fertilizer.Nitrification was dominant in the whole nitrogen cycle of the basin,and no obvious denitrification occurred.In both the dry season and the rainy season,the contribution rate of sewage was the highest（31.40%and 65.66%）,and the influence of sewage on nitrate in the rainy season was much higher than that in the dry season.The main reasons for this phenomenon include two aspects:after summer,the water consumption and displacement of urban areas increase rapidly,and a large amount of domestic sewage is discharged into the river;In addition,the vegetation coverage in rainy season is higher than that in dry season,and rainfall can effectively trap and infiltrate in situ,which reduces the contribution rate of pollution from non-point sources such as soil organic nitrogen,fertilizer and manure to a certain extent.