Quantitative Analysis Of Nitrate Sources In Urban Rivers Based On Stable Isotopes

Urban rivers are an important part of urban natural ecosystems and are susceptible to significant impacts from high-intensity human activities.Factors such as fossil fuel combustion,excessive use of fertilizers in suburban areas and large industrial and domestic wastewater discharges have led to a rapid increase in nitrogen in urban rivers,which has led to a series of ecological and environmental problems.Nitrate（NO₃^-）is the main form of dissolved inorganic nitrogen（DIN）present in water bodies.Excess NO₃^-in water bodies not only leads to eutrophication and loss of biodiversity,but also poses a threat to human health and increases the prevalence of related diseases.The multiplicity of nitrogen sources and complexity of input pathways in urban rivers make the precise identification of nitrogen pollution sources in urban water bodies and the quantification of their loads an important challenge for urban water pollution management at present.As a world-class mega-city,the total nitrogen input to rivers has increased tenfold in the past decades,and the further improvement and enhancement of river water quality has encountered the bottleneck of nitrogen pollution constraints.Therefore,clarifying the sources and transformations of nitrate in urban rivers not only plays an important role in mitigating nitrogen enrichment in river networks,but also provides theoretical references for the prevention and control of nitrogen pollution in river water bodies.In this study,the main composition and concentration characteristics of DIN in water bodies were obtained by seasonal sampling and experimental analysis,taking the Suzhou River in Shanghai as the main study area,revealing the factors affecting the spatial and temporal distribution of NO₃^-concentrations in river water bodies,and further combining stable isotope(δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-)data and MixSIAR hybrid model to investigate the migration and transformation mechanisms of NO₃^-in the watershed and to quantify the contribution of different sources to NO₃^-in river water bodies.The main conclusions obtained from this study are as follows.（1）NO₃^--N is the main form of DIN in the water bodies of Suzhou River,accounting for 66%of the total year round,followed by NH₄⁺-N with 31%and NO₂^--N with only 3%.The annual average concentrations of NO₃^-and NH₄⁺in river waters were 1.86±0.85 mg/L and 0.88±0.71 mg/L,respectively,with significant seasonal differences,both showing the highest concentration in winter and the lowest concentration in summer,which were mainly influenced by the dilution of rainfall;The concentration of NO₂^-was low,with an annual average concentration of 0.10±0.08mg/L,which was mainly related to the migration transformation of NO₃^-and NH₄⁺in the water column.Spatially,the annual average concentrations of NO₃^-and NH₄⁺showed that the main stream>tributary and main stream<tributary,respectively.The annual average concentration of NO₃^-in the mainstem water body is influenced by the coastal drainage and the regulation of estuary gates,which gradually increases from upstream to downstream,with a more obvious trend in winter and a gradual decrease in summer.The spatial variation of DIN concentration is large because of the poor connectivity and strong independence among different tributaries.（2）The annual mean values ofδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-in Suzhou River water bodies were 8.38±1.54‰and 0.58±1.76‰,respectively,both of which showed significant seasonal differences,with the highestδ¹⁵N-NO₃^-value in spring and the lowest in summer,and the dry and tributary The seasonal variations were not consistent;the value ofδ¹⁸O-NO₃^-was the highest in spring,and the differences in other seasons were not significant.The NO₃^-concentration and isotopic characteristics of the river suggest that nitrification within the mainstem is weak,and NO₃^-in the water column mainly comes from direct input from external sources,and nitrification occurs before entering the river water;NO₃^-in the tributary water column may be produced by nitrification of NH₄⁺in the river,in addition to input from external sources.The results of fitting water column isotopes to NO₃^-concentrations showed that the denitrification signal was weak in the Suzhou River,with the mainstem subject to conservative mixing of both sewage and soil nitrogen sources in winter,while assimilative removal of NO₃^-occurred mainly in summer,and conservative mixing of sewage and soil nitrogen was found in tributaries only in summer.The absence of natural banks in urban rivers and the direct discharge of sewage and most rainfall runoff into the river have reduced the time and area of contact between water bodies and soil,vegetation,and sediment,reducing the potential for NO₃^-and soil interactions and making the denitrification mechanism in rivers weaker.The direct input of exogenous NO₃^-,nitrification of effluent NH₄⁺,and weaker NO₃^-removal combined to result in the accumulation of NO₃^-in the river water column.（3）The quantitative estimation of the sources of NO₃^-in the water column of Suzhou River was carried out by MixSIAR model,and the output results showed that urban sewage,soil nitrogen and nitrogen fertilizer were the main sources of NO₃^-in the river water column,with the annual contributions of 68.3±9.7%,15.7±4.8%and15.5±4.9%,respectively.Among them,the seasonal fluctuation of urban sewage contribution to NO₃^-in the main stream is large and mainly influenced by river flow,with a higher percentage of contribution in winter and spring and a lower percentage in summer and autumn,but the seasonal variation of urban sewage contribution to NO₃^-in the tributaries is small because of the existence of sewage overflow in summer.Soil nitrogen and nitrogen fertilizer are easily transported to the river by leaching in the summer and autumn seasons when there is more rainfall,so the contribution of surface source pollution to the NO₃^-of the main stream increases in that period.The contribution of atmospheric deposition to river NO₃^-was the lowest,accounting for only 0.5%throughout the year,and the seasonal differences in the contribution to rivers were not significant,which was consistent with the results of the isotope qualitative analysis.（4）Sensitivity analysis of the output results of the MixSIAR model showed that changes in the mean values ofδ¹⁵N-NO₃^-of the municipal wastewater end elements lead to large fluctuations in the proportion of contributions from different nitrogen sources in the output results,while changes in the isotopic values of the end elements of soil nitrogen,nitrogen fertilizer and atmospheric deposition do not have a significant effect on the final results,and the model results are not sensitive to these end elements The sensitivity of model results to these end elements is not high.In addition,by analyzing the uncertainty index(UI₉₀)of the model results,it was found that the uncertainty of the contribution of soil nitrogen and nitrogen fertilizer to river NO₃^-was larger,the contribution proportion of atmospheric deposition was the most stable,and municipal wastewater showed a moderate degree of uncertainty,and the model results considering the fractionation factor were more accurate,reducing the uncertainty of the contribution of different sources...