Nitrate-Nitrogen Pollution Sources Of An Underground River In Karst Agricultural Area Uing ¹⁵N And ¹⁸O Isotope Technique And Isosource Model

With the development of modern agriculture,excessive and unreasonable use of pesticides and compound fertilizers,and the discharge of livestock and poultry excrement and domestic sewage from large-scale breeding,water nitrate pollution has gradually become an environmental problem concerned by the world.Groundwater resources in karst areas in southwest China account for one quarter of the country's water resources and play an important role in promoting national economic construction and sustainable development.The karst groundwater system develops a double-layer space structure on the surface and underground,with rapid transformation between rain water,surface water and groundwater,poor self-purification ability,easy entry of pollutants,resulting in serious nitrate pollution.Given the vulnerability of karst groundwater system and the importance of groundwater resources,studying the southwest karst agricultural geochemical features of the underground river system and the nitrate pollution situation,parsing the nitrate pollution source,comparing the interannual scale change nitrate pollution degree and the source,the contributions to the quantitative evaluation of each pollution source side,for karst environmental protection,the rational use of karst water resources and provide a scientific basis for regional sustainable development,lay theoretical basis for the control of groundwater nitrate pollution.This study takes Qingmuguan underground river basin in Chongqing city as the research object.There are 7 sample points in the basin,including atmospheric precipitation,surface water,soil water and underground water and so on.In May 2017 to April 2018 by field hydrogeological investigation,periodic sampling,indoor test and model analysis method,to study the dynamic change law of hydrogeochemical month process in underground water and the water source of chemical composition,spatial and temporal variation characteristics nitrate nitrogen,nitrate pollution source identification.The levels and sources of nitrate pollution from 2008 to 2017were compared and put forward the corresponding pollution control measures.The results are analyzed as follows.Atmospheric precipitation,surface water and soil water temperature in the study area are all affected by air temperature.The temperature in summer and autumn is high,while that in winter and spring is low.However,the water temperature of Jiangjia Spring has little change throughout the year,which is basically consistent with the local average temperature.The pH values of Jiangjia Spring are 7.15±0.15,and the pH values of Fishpond,Yankou sinkhole,the soil water of dry land and woodland are similar,and both of them show the characteristics of high dry season and low rainy season,while the pH values of rain water and Daluchi are low.Ca²⁺and Mg²⁺are the main cations in each water body,while HCO₃^–,SO₄^2–and Cl^–are the main anions.The main ions have their own characteristics of spatial and temporal changes,but they are all affected by geological background,karstification,human activities and other factors.Principal component analysis was carried out on 12 kinds of hydrochemical indexes of underground river water,and three principal components were extracted.PC1 reflects the water-rock interaction and nitrate pollution between the underground river and carbonate rocks during the migration process.The plasma mass concentration of HCO₃^–,Ca²⁺,Mg²⁺and NO₃^–-N increases,along with the electrical conductivity value.PC2 is highly positively correlated with Cl^–,K⁺,Na⁺and electrical conductivity,reflecting the influence of human activities such as agricultural fertilization and sewage discharge on underground rivers.PC3 reflects the higher SO₄^2–background value in the study area.There was a great risk of nitrate pollution in the underground river system,because most NO₃^–-N concentrations of the sampling points exceeded the threshold.Spatially,NO₃^–-N concentrations in the underground river increased from upstream to downstream due to the continuous intake of rainwater,surface water,soil water and other water bodies along the way.Temporally,NO₃^–-N concentration of rainwater and Daluchi was always low and did not change much,while NO₃^–-N concentration of soil water was always high and decreased after the fallow period.NO₃^–-N concentration of the upstream Yankou sinkhole and Fishpond and the downstream Jiangjia Spring were impacted by agricultural fertilization from May to June and fluctuated from June to September due to precipitation and after September with the decline of agricultural activities fluctuating decline.With the use of ¹⁵N-NO₃^–and ¹⁸O-NO₃^–isotopes and IsoSource model,the source of nitrate pollution in each water body in the study area was analyzed qualitatively and quantitatively.The Daluchi is located in the residential area with a relatively dense population,where assimilation takes place.The nitrate pollution mainly comes from soil organic nitrogen,NH₄⁺in fertilizer and rain,with an average contribution rate of 48.3%and 26.1%,respectively.The characteristic values of nitrogen and oxygen isotopes in Fishpond varied greatly,indicating that the mixing manure and sewage played a dominant role in the pollution sources,with an average contribution rate of 59.6%,followed by soil organic nitrogen and NO₃^–in precipitation.Under the influence of Fishpond water inflow,the nitrate pollution sources in the Yankou sinkhole are similar to each other.The average contribution rate of manure and sewage is as high as 65.1%,followed by soil organic nitrogen.Soil water in layers A and B in upland was affected by background values and agricultural activities,and nitrate mainly came from soil organic nitrogen and NH₄⁺in fertilizer and rain.The nitrate of soil water in layer A and B of forest land mainly comes from soil organic nitrogen,and the average contribution rate of NO₃^–in precipitation is higher than other sample points.The source of nitrate in Jiangjia Spring is the most complex,which is the mixing manure and sewage,soil organic nitrogen,NH₄⁺in fertilizer and rain and NO₃^–in precipitation.Under the influence of changes in land use types,the average NO₃^–-N concentration of Yankou sinkhole and Jiangjia Spring increased by 8 times and nearly 5 times respectively in the past decade,indicating that the nitrate pollution in the underground river system is increasingly serious.The average value of ¹⁵N-NO₃^–was increased from-0.25±2.09‰,4.19±0.92‰in 2008to 10.68±4.93‰,8.53±3.12‰,indicating that the nitrate pollution source also changed.Due to a large number of new agritainment near Yankou sinkhole,the pollution sources are mainly affected by the mixing manure and sewage.However,the source of pollution in Jiangjia Spring has become more complex.On the original basis,the mixing manure and sewage,NO₃^–in precipitation have been added as two pollution sources.