Using ?¹⁵N-NO₃^- And ?¹⁸O-NO₃^- To Trace The Sources Of Nitrate In The Nanchang Section Of The Ganjiang River

With the increase of population and the rapid development of industry,agriculture and economy,wastewater from the domestic sewage,industrial and agricultural contain high concentrations of nitrogen species are discharged into river ecosystems,resulting in a series of ecological and environmental problems.Such as eutrophication of water body,deterioration of water quality,sharp decline of biodiversity,acidification of surface water and soil,etc.Nitrogen excess has aroused a widely concern of water pollution in China and even over the world.Therefore,to identify the sources of reactive nitrogen in water can help the government to govern the water pollution.Ganjiang River,is the largest river in Jiangxi Province,and also the largest tributary of Poyang Lake.The concentration of nitrate in Ganjiang River was obviously higher than that in other tributaries of Poyang Lake,and the Ganjiang River was the dominantly source of nitrate for the Poyang Lake.It is an important factor to affect the water quality of Poyang Lake.At the same time,Ganjiang River is also the source of domestic water and agricultural irrigation water in many cities over the Ganjiang River Basin,it is inevitable that high concentration of nitrate in Ganjiang River may affect the water security for the whole Ganjiang River Basin.To explore this question of nitrogen pollution in Ganjiang River,the Nanchang section of Ganjiang River was selected as a study case.Water samples were collected from the upstream,midstream and downstream of Nanchang section of Ganjiang River from January to December 2017,and also sampled from the Fuhe River which located in the Nanchang City.The spatial and temporal characteristics of water-soluble ions in Nanchang section of Ganjiang River were analyzed and compared.Nitrate nitrogen and oxygen isotopic technology coupled with water-soluble ions were used to investigate the potential sources of nitrate in Nanchang section of Ganjiang River.Our study provides a scientific suggestions for the Ganjiang River Basin water resources management and environmental protection.The mainly conclusions are as follows:?1?Ca²⁺is the main cation in the water of Nanchang section of Ganjiang River during dry,flat and abundant periods,accounting for average 54%of the total cations?calculated in units of equivalent?.The dominant anions are SO₄^2-,Cl^-,and both account for about 42%of the total anions respectively.The orders of the main cations are Ca²⁺>Na⁺>Mg²⁺>K⁺>NH₄⁺,for the anions,the orders are SO₄^2->Cl^->NO₃^-.?2?There are significant correlations among Ca²⁺,Mg²⁺,Na⁺,K⁺,Cl^-,SO₄^2-,NH₄⁺in the Nanchang section of Ganjiang River,which indicated those ions similar sources or similar chemical reactions.Domestic sewage,industrial sewage,agricultural sewage and rock weathering are the possible sources of water-soluble ions in Nanchang section of Ganjiang River.Especially,rock weathering is the mainly chemical weathering of silicate rocks and carbonate rocks.?3?There were an obviously spatial and temporal characteristics for the water-soluble ions in Nanchang section of Ganjiang River.Over all,the periods of the highest concentrations of water-soluble ions was the dry season,and the lowest periods was the flood season,which may attribute to difference of precipitation between dry and flood season.Moreover,concentrations of water-soluble ions in the midstream of Nanchang section of Ganjiang River were obviously higher than those in upstream or downstream,this patterns may explain by the emitting of wastewater from Nanchang.Furthermore,the nitrification may be another possible sources of nitrate.?4?The average values of?¹⁵N-NO₃^-in Nanchang sections of Ganjiang River during dry,plain and flood periods are+7.78‰,+6.26‰,and+6.07‰.Based on the nitrogen and oxygen isotopic compositions,NO₃^-in Nanchang section of Ganjiang River may mainly originate from the following sources:nitrogen-contained fertilizer,soil organic nitrogen,domestic sewage.?5?Based on the mass balance model,the source contributions of nitrate during dry periods in the Nanchang section of Ganjiang River were 50%,36%,and 14%from the domestic sewage,nitrogen-contained fertilizer and soil organic nitrogen during the dry periods,respectively.But during the plain periods,nitrogen-contained fertilizer was the biggest contributor,which account for 42%of nitrate to the total nitrate,and the contribution fractions were similar for domestic sewage?28%?and soil organic nitrogen?30%?.In the flood periods,the main source of nitrate was nitrogen-contained fertilizer,accounting for 46%of the total nitrate,followed by domestic sewage?accounting for 31%?and soil organic nitrogen?accounting for 23%?.