Geochemical Characteristics And Traceability Of Dissolved Nitrate In Rivers In Industrial Cluster Areas

Nitrate（NO₃^-）is an important component of nitrogen in water bodies and one of the limiting factors for eutrophication.The sources of nitrate in natural surface water bodies are complex and diverse,including atmospheric nitrogen deposition,soil organic nitrogen,rock weathering,agricultural activities,domestic sewage,industrial wastewater and effluent from sewage treatment plant,etc.The processes of transformation for it often include the Nitrification,denitrification,assimilation and mixing processes.Nitrate pollution has become a major challenge for global water pollution,so it is urgent to investigate its migration and transformation process in water bodies and to identify its major sources.The Maozhou River in Guangdong is a tributary of the Pearl River system and flows through the Chang’an industrial cluster area（abbreviated as the Chang’an section）in Dongguan City.In recent years,the water quality of Maozhou River has been improved,but the migration and transformation process of nitrate and its major sources are not clear.There is still a lack of scientific basis for nitrate pollution under the influence of human activities.In this study,we used elemental and stable isotope geochemistry as well as mathematical modeling to study the migration and transformation processes and reveal the major sources of dissolved nitrate in the river water in wet and dry seasons.This study aims to further reveal the influence of industrial clusters on the geochemical cycling processes of nitrate in the river,and to provide a theoretical basis for the prevention and control of nitrate pollution for the Maozhou River in Guangdong.The major conclusions are as follows.（1）The water quality type of the Chang’an section of the Maozhou River during wet and dry seasons is Na⁺-Cl^--SO₄^2-type,showing the characteristics of a typical industrial cluster area water system.The proportion of Na⁺among the total cations is the highest in both wet and dry seasons,accounting for 71%and 69%respectively.The proportion of Cl^-among the total anions is the highest in both wet and dry seasons,accounting for 41%and 31%respectively.The major ion composition,Pearson correlation analysis and Gibbs distribution diagram show that the major ions of the river water in Chang’an section of the Maozhou River are little affected by rock weathering and atmospheric precipitation but more influenced by human activities.（2）The major chemical form of inorganic nitrogen in the Chang’an section of the Maozhou River is NO₃^--N.The proportion of NO₃^--N in different water periods was higher than 60%.The NO₃^--N concentration at each sampling point along the flow is gradually increasing in wet season but decreasing in dry season,respectively.The reason for these trends may attributed to more influences from human activities in wet season than that from the dry season.The NO₃^--N concentration at each sampling site in dry season is higher than that in wet season,which is likely due to the stronger dilution effect of rainwater in wet season than in dry season.The redundancy analysis between inorganic nitrogen and conventional water quality indexes found that the correlation between inorganic nitrogen and conventional water quality indexes differed in the two seasons.NO₂^--N is significantly positively correlated with Na⁺and Cl^-in wet season and Cl^-and SO₄^2-in dry season,demonstrating great impacts from human activities in the industrial cluster area.（3）Bothδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-in the Chang’an section show a spatial trend of increasing and then decreasing along the flow direction,showing the confluent mixing effect of tributaries.These isotopic composition indicate that there was nitrification but little denitrification occurring during the process of nitrate forming.Theδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-values in wet and dry seasons varied widely,with the ranges ofδ¹⁵N-NO₃^-values from 9.08‰to 14.83‰and 7.87‰to 16.24‰,respectively,and the ranges ofδ¹⁸O-NO₃^-values from 0.1‰to 5.06‰and 2.96‰to6.21‰,respectively.The seasonal variation is likely due to the difference in the intensity of fractionation in each river section.The nitrogen and oxygen stable isotopes of the river water samples are trends to the endmember of domestic sewage,revealing the dominant source of the nitrate.According to the mass balance mixing model and SIAR model,the contribution of domestic sewage to the nitrate pollution in the Chang’an section of the Maozhou River is the largest,and are all higher than80%in wet and dry periods.The results show significant difference from the non-industrial river water.（4）To address the needs of tracing sources for dissolved nitrate pollution in rivers in industrial cluster areas,this paper takes the Chang’an section of the Maozhou River as an example to construct a preliminary early warning feedback model of tracing sources based on the water quality parameters of the river section,actual measurement data and the QUAL2K water quality model.Through the model,the migration and transformation rules of ammonia and nitrate nitrogen were simulated and tested.Based on the analysis of the model,the dissolved nitrate in Chang’an section of Maoczhou River majorly originated from the service coverage of upstream sewage treatment plant.The results are identical to those showed by above stable isotopes,showing that the model has great potential of exploration.In conclusion,the prevention and control measures of water pollution,especially industrial wastewater,in the industrial cluster area of Chang’an section of Maozhou River are effective and appropriate.The tracing results of nitrate by nitrate nitrogen and oxygen stable isotope technology and river traceability mathematical model can confirm each other.How to organically combine the two to develop a new traceability model will be the focus of the next step.