Spatial And Temporal Variations And Source Identification Of Nitrogen Pollution In The Upper Han River

With the development of economy and increased population,industrial wastewater and domestic sewage N poured into rivers and lead to increased nitrogen?N?concentration in river water.Increased N have resulted in eutrophication in inland waters worldwide,causing blooms of toxic algae,loss of aquatic life,and loss of biodiversity.Excessive N has been the main problem of water pollution at a global scale,particularly in China.Thus,it's an urgent task to control N pollution for the management of water pollution and improvement of water quality.As the largest tributary of Yangtze River and also the water source area of the Middle Route of China's south to North Water Transfer Project,the upper Han River's water quality is of great importance.In this study,we selected two distinctly different rivers,namely,upper Han River which is greatly influenced by human activities and Jinshui River?a tributary of upper Han River?where the water quality is generally good to address the question of N pollution in upper Han Rivern On this basis,spatial and temporal variations of N pollution were analyzed,and then N-oxygen isotopic technique?815N and ?18O?,principle component analysis?PCA?or cluster analysis?CA?and information of land use were constructed to identify the sources of NO₃^-and SPM and reveal the biogeochemical significance.The results in this research can provide theoretical guidance for water quality management and N pollution controlling in watersheds.The main results are as follows:?1?Nitrate-N is the main form of N in water,and presented similar spatial and temporal variations with total N?TN?concentration.The water quality in some areas is good,and the concentrations of N dramatically increased in frequent human activity areas.The concentrations ofNO₃^-and TN in 2014-2015 were significantly higher than those in other two years.Water quality was the worst and N pollution was the most serious in 2014-2015 among three years.The water quality of upper Han River in 2015-2016 improved a lot and N pollution also reduced.Based on the relationships of N species and its sources,the upper Han River was greatly influenced by agricultural activities in 2014-2015,which is mainly polluted by urban or industrial sewage in 2015-2016.As a whole,the N concentrations in water showed obviously seasonal variations.But the annual changing trend of N pollution was not obvious as a whole.?2?The sources of nitrate differed significantly among seasons in upper Han River,and were influenced by complicated external inputs.As November 2013?autumn?and January 2014?winter?were fallow seasons,nitrate in water mainly came from industrial waste and domestic sewage.In April 2014,nitrate in the upper Han River mainly derived from soil organic N.atmospheric deposition and chemical fertilizer,which resulted from the increase in pollutants pouring into river caused by human activities.Industrial sewage also polluted the water of a few sampling sites located in cities.Most sampling sites in July 2014 were mainly polluted by soil organic N or industrial sewage,while others were polluted by domestic sewage or animal waste.There were many sources of nitrate in April 2015.With a limitation of rainfall,the sources were mainly chemical fertilizer,domestic sewage,soil organic N and industrial waste.With a high level of ?15N-NO₃^-in July 2015,NO₃^-in water mainly derived from rainfall,domestic sewage animal waste and chemical fertilizer.In November 2014?autumn?and January 2015?winter?,nitrate in water mainly came from atmospheric deposition,industrial waste and soil organic N,while atmospheric deposition could be neglected in winter.Large scales of ?15N-NO₃^-distribution from November 2015 to July 2016 revealed the complicated sources in these seasons.And turbidity was greatly correlated with ?15N-NO₃^-by PCA of water chemical parameters and N isotopic values.Combined with the characteristics of N pollution in different sampling sites,eight representative sampling sites were chosen to analyze?180-NO₃^-in water.The sources of nitrate in these sites were identified accurately depending on the results.Meanwhile,our results indicated that variations of nitrate concentrations were possibly driven by nitrification and denitrification.?3?Similar with study of the upper Han River,our results showed that nitrate was the dominant form of dissolved inorganic N in Jinshui River.There were higher concentrations of N pollution in wet seasons than those in dry seasons,and the minimum occurred in November 2013.According to the spatial and temporal distributions of N,water in the middle stream was less polluted and showed much better quality,whereas the upstream and downstream were disturbed by human activities to some extent.?4?Based on the isotopic techniques and different physicochemical parameters in water,nitrate in Jinshui River mainly derived from atmospheric deposition,chemical fertilizer,soil organic N and domestic sewage.N in SPM mainly came from atmospheric deposition and chemical fertilizer,and greatly differed among four seasons.Meanwhile,SPM in Jinshui River was mainly influenced by human disturbance during winter season.The 815N-SPM significantly differed among seasons with peaked value in July followed by January and November,and bottomed in April.In addition to the dominant factors of nitrification in January,assimilation and decomposition occurred in each season,likely because the strong decomposition could lead to the much high ?15N-SPM.In summary,this study demonstrated the temporal and spatial variation of N pollution and identifies the N sources in the above two study areas based on the monitoring data and isotopic data analysis.The results in this research would provide scientific basis for water quality improvement in these two rivers,and also could supply methods to trace pollutants in other watersheds.