Research On The Source And Estimation Of Nitrate In Karst Reservoir Water Based On Nitrogen And Oxygen Isotopes ——Taking Pingzhai Reservoir As An Example

As the main form of nitrogen pollutants,the source and concentration of nitrate have an extremely important impact on the quality of water bodies.Pingzhai Reservoir is the only source reservoir of the Central Guizhou Water Control Project.Its water quality safety plays an important role in promoting sustainable economic and social development.It is of great practical significance to find out the migration and transformation law of nitrogen in the water,the source of nitrate and the contribution rate of each source.In this study,the Pingzhai Reservoir in the water source area of the Central Guizhou Water Control Project was used as the research area.Collect water sample data in November2020（autumn）,January 2021（winter）,April 2021（spring）and July 2021（summer）,and determine the main physical and chemical factors of the water body,the concentration of various nitrogen forms,nitrate nitrogen and oxygen isotopes and other indicators.The main research purpose is to use spatial interpolation to explore the temporal and spatial variation characteristics of different nitrogen forms in Pingzhai reservoir water body,and use correlation analysis to explore the interaction and restriction relationship between nitrogen forms and environmental factors,Using nitrogen and oxygen double isotope traceability technology,the potential main sources of nitrate in Pingzhai Reservoir water were identified,and then combined with stable isotope mixture model（SIAR）,the contribution rate of endmembers of each nitrate source in Pingzhai Reservoir water was calculated.The main conclusions are as follows:（1）The physical and chemical factors of Pingzhai Reservoir have obvious differentiation characteristics.WT is high in summer and low in winter;DO concentration is high,and it is generally in an aerobic environment;The p H value is higher in spring and summer,lower in winter and spring,and the water body is generally weakly alkaline;The concentration of EC in water is the highest in spring and the lowest in summer.In terms of nitrogen concentration,the concentration of TN is high;the concentration of DTN is not much different from that of TN,which is the highest in summer,followed by spring and winter,and the lowest in autumn.The DON concentration was generally in the order of summer>winter>spring>autumn.NO₃^--N is the main form of dissolved inorganic nitrogen in Pingzhai Reservoir,and the concentrations of NH₃-N and NO₂^--N are both low.（2）The correlation analysis results of nitrogen form and environmental factors showed that p H was significantly negatively correlated with NO₃^--N in winter,and a certain degree of p H reduction was beneficial to nitrification.At this time,the continuous decomposition of organic matter and the autolysis of algal cells would release DON into the water in the form of NH₃-N.And the endogenous release form had a certain impact on the nitrogen forms and concentrations in the water during winter.During summer,the conditions of WT and DO are suitable for the growth of nitrifying bacteria and ammoniating bacteria,and the nitrification process needs to consume DO.Therefore,the increase of TN concentration will reduce the concentration of DO in the water body.However,there is a positive correlation between DO in Pingzhai reservoir and the concentrations of TN and DTN,indicating that the surface runoff generated by precipitation during summer carries a large amount of nutrients into the water body,Make the foreign input occupy the main position.（3）Theδ¹⁵N-NO₃^-value of the inflow rivers and the reservoir area is spring>winter>summer>autumn,and theδ¹⁸O-NO₃^-value is su mmer>autumn>spring>winter.The large changes in the range of nitr ate concentration and nitrogen and oxygen isotope range indicated t hat nitrate may have various sources.In autumn,soil organic nitrog en and ammonium nitrogen fertilizer were the main sources of nitra te in water body.In winter and spring,the main sources of nitrate in water are domestic sewage and livestock manure.In summer,nit rate nitrogen and oxygen isotopes are mostly distributed in the end members of nitrate nitrogen fertilizer,indicating the possibility of th is source.（4）The physical and chemical characteristics of water indicate that the migration and transformation process of nitrogen is mainly affected by nitrification.SIAR calculation results show that in autumn,chemical fertilizers account for 53%,soil organic nitrogen and domestic sewage and livestock manure account for 31%and 16%,respectively.In winter and spring,domestic sewage and livestock manure were 48%and 46%,respectively,soil organic nitrogen was 29%and 33%,and chemical fertilizers were 23%and 21%,respectively.In summer,chemical fertilizer is 35%,domestic sewage and livestock manure is 33%,and soil organic nitrogen is 32%.