Source Analysis Of Nitrate Pollution In Typical Rivers In The Coastal Zone Based On Stable Isotopes

With increasing population and economy development in coastal areas,intense human activities have posed great pressure to ecosystems of rivers,estuaries and offshore areas.According to the“Qingdao Marine Environment Bulletin”,the water environmental quality of Jiaozhou Bay has been severe for recent years,and hence the water eutrophication has become the most serious environmental problem for Jiaozhou Bay.At the Dagu river estuary,the concentration of dissolved inorganic nitrogen?DIN?has been higher than the environmental standard.Reducing the nitrogen flux from the Dagu River is critical to control the land-sourced nitrogen pollutant of Jiaozhou Bay.Identifying the NO₃^-sources and their contributions of the Dagu River can provide important information for the control of land-sourced nitrogen pollutant input and water eutrophication for Jiaozhou Bay.This study,taking the Dagu River as an example and foucusing on NO₃^-which is the main form of DIN of river water,has explored the spatiotemporal characteristics,main sources,controlling factors,and transformation processes of nitrate,using hydrochemical analysis,nitrate nitrogen and oxygen isotopes(?¹⁵N-NO₃^-and?¹⁸O-NO₃^-),the output coefficient model and the MixSIAR model.The primary conclusions of this study were shown as follows:?1?The DIN and NO₃^-concentrations exhibited a significant seasonal difference?p<0.05?,which were higher in the dry season than those in the wet season,mainly due to the effects of seasonally varied precipitation,temperature and dissolved oxygen?DO?.Spatially,the DIN and NO₃^-concentration was higher in the upper reaches than those in the middle and lower reaches,which were primarily attributed to sewage discharge,river damming and land use type distribution.?2??¹⁵N-NO₃^-and?¹⁸O-NO₃^-showed a significant variation,which were largest in the normal season and lowest in the dry season.Spatially,they were higher in river reaches of the main stream with dams.Sewage,animal manure and chemical fertilizers were the main NO₃^-sources of the Dagu River.At river reachs with high NO₃^-concentrations(>3.2mg·L^-1),sewage and manure were the main NO₃^-sources,while the NO₃^-had complicated sources at river reachs with low NO₃^-concentrations(?3.2 mg·L^-1).The nitrification and denitrification were two important biogeochemical transformation processes of NO₃^-in the Dagu River.The more positive?¹⁵N-NO₃^-and?¹⁸O-NO₃^-in river reaches with dams mostly located in the middle and lower reaches were mainly owing to the strong evaporation effects as well as the coupling mechanism of nitrification and denitrification caused by river damming.?3?The quantitative estimation by the MixSIAR stable isotope model showed that sewage,manure and fertilizer were the main NO₃^-sources of the Dagu River.Their contribution ratios were 48.7±13.1%?57.9±11.3%and 49.1±10.9%,18.5±14.6%?14.3±11.9%and 18.4±14.0%,as well as 14.0±10.8%?10.6±8.6%and 13.4±10.3%in the wet season,normal season and dry season,respectively.The contribution of soil organic nitrogen was relatively small,which was 11.0±8.7%,9.0±17.3%and 12.6±9.8%in these three periods respectively.Atmospheric precipitation contributed the least with 7.8±3.7%,8.2±4.6%and 6.5±4.1%,respectively.The calculated results by the output coefficient model indicated that domestic sewage,farmland fertilization and livestock manure were the main sources of total nitrogen load in the Dagu River Basin,consistent with the results of the MixSIAR model.This phemominon confirmed the accuracy of model estimation.