Hydrochemical Characteristics And Source Identification Of Nitrogen And Oxygen Isotopes Of Wetland In Sanmenxia Reservoir Area

The special Yellow River water and sediment conditions and the operation of Sanmenxia Reservoir form the wetland in Sanmenxia Reservoir area,which is a highly dynamic and complex ecosystem.This paper takes the wetland in Sanmenxia Reservoir area as a typical study area.By using five sampling data of the reservoir's water storage and discharge periods in three years from 2018 to 2019.Based on ion concentration analysis and stable isotope technique,the characteristics of chemical distribution of wetland water during the storage and discharge of Sanmenxia Reservoir were analyzed systematical.And the distribution characteristics of NH₄⁺,NO₃^-and NO₂^-as well as the rules of nitrogen migration and transformation.The correlation between the change of water level and nitrogen sources is preliminarily proved.The research results can provide scientific basis for the protection and management of wetland in Sanmenxia Reservoir.The main research results of this paper are as follows:Based on five sampling data during the period of storage and discharge of Sanmenxia Reservoir from 2018 to 2020,the basic physical and chemical parameters,ion composition characteristics and influencing factors of the surface water of the wetland in Sanmenxia Reservoir area were systematically analyzed.The results show that the surface water of the wetland in Sanmenxia Reservoir area is weakly alkaline,and the p H value varies from 7.03 to 8.64 during the storage period and from 7.21 to11.25 during the drainage period.The hydrochemical type of surface water is mainly Na·Ca-HCO₃·SO₄.The hydrochemical types of wetland in Sanmenxia Reservoir area are influenced by natural factors and human activities.The natural factors are rock weathering,including the weathering of silicate salt and the dissolution of evaporative salt.The human activities are mainly caused by the discharge of waste water during the storage period.(2)Systematic analysis of the NH₄⁺,NO₃^-and the NO₂^-of the concentration distribution in the surface water and groundwater in the Sanmenxia Reservoir area wetland.The results of sample analysis showed that NO₃^-and NO₂^-are super class III water index in surface water storage period,NO₃^-and NH₄⁺are super class III water index during the drainage of surface water.The contents of NO₃^-in groundwater are high,but NO₂^-and NH₄⁺could be ignored.(3)The composition characteristics of nitrogen and oxygen isotopes of wetland during storage and drainage in Sanmenxia Reservoir area were systematically analyzed,and the source of nitrogen in the wetland was discussed.During the storage period,the?¹⁵N-NO₃^-and?¹⁸O-NO₃^-range from 5.84‰to 13.46‰and 1.39‰to33.28‰,during the drainage period,the?¹⁵N-NO₃^-and?¹⁸O-NO₃^-range from-10.62‰to 15.46‰and 12.90‰to 8.92‰.During the storage period,the values of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-in the groundwater of the wetland are between-3.38‰?7.99‰and-3.46‰?0.01‰,and between 6.02‰?15.59‰and-6.97‰?1.72‰during the drainage period.During the storage period,the values of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-in the soil water of the wetland are between-6.99‰?13.54‰and 5.79‰?50.96‰,and between-6.42‰?7.33‰and-3.4‰?51.6‰during the drainage period.According to the analysis of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-,it is preliminarily determined that nitrogen migrates from groundwater and soil water to surface water during the storage period,accompanied by the occurrence of nitrification and denitrification.Both nitrification and denitrification occur in surface water during the drainage period,and nitrification occurs in both groundwater and soil water,but the denitrification is weak.According to the abundance analysis of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-of different nitrogen sources,the nitrogen sources of surface water and groundwater in Sanmenxia Reservoir area wetland are mainly atmospheric precipitation,chemical fertilizer,soil,waste sewage and manure,and the soil water and nitrogen sources are mainly atmospheric precipitation,chemical fertilizer and soil.(4)Nitrogen isotopic sources of surface water,groundwater and soil water in the wetland storage and drainage period in Sanmenxia Reservoir were identified based on the Iso Source model with atmospheric precipitation,chemical fertilizer,soil,waste water and manure as the control end elements.The results showed that soil,waste water and manure were the main nitrogen sources in the wetland surface water of Sanmenxia Reservoir.The contribution rates of atmospheric precipitation,chemical fertilizer,soil,waste water and manure were 7.69%,12.95%,40.14%and 39.23%,respectively.The contribution rates of drainage period were 20.30%,17.08%,27.32%and 35.31%,respectively.Waste water and manure are the main sources of nitrogen in groundwater.In the storage period,the contribution rates of atmospheric precipitation,chemical fertilizer,soil,waste water and manure are 12.15%,25.9%,31.42%and30.52%,respectively,and in the discharge period,they are 15.6%,11.14%,23.00%and 50.26%.Soil nitrogen was the main source of nitrate in soil water.The contribution rates of precipitation,chemical fertilizer and soil nitrogen were 21.63%,24.15%and 54.23%in the storage period,and 34.47%,16.62%and 48.92%in the drainage period,respectively.