Source Identification And Quantitative Estimation Of Nitrate In Karst Caves

The continuous increase of atmospheric CO₂ concentration has led to global warming.The resulting temperature rise,water cycle intensification and extreme weather events have become more frequent.Researches such as clarifying the global carbon cycle path and estimating the karst carbon flux have attracted domestic and foreign scholars.s concern.As an important part of karst underground space,karst caves are where carbon accumulates,and their contribution to the global carbon sink effect cannot be ignored.In the process of quantitatively estimating the effect of karst carbon sinks,it is found that nitric acid participates in the chemical weathering of carbonate rocks,which makes the estimation of carbon sinks in karst areas very uncertain.The unique dual structure of karst areas makes the difference between surface water and groundwater.The hydraulic connection between them is relatively active,and surface nitrate materials can easily enter the ground hydrological system through karst pipelines,which affects the erosion and weathering intensity of carbonate rocks by groundwater,which in turn leads to deviations in the estimation results of karst carbon sinks.In the process of calculating karst carbon sinks It is necessary to remove the influence of nitric acid on the karst carbon sink effect.Therefore,exploring the source of nitrate in karst cave water and quantitatively estimating the contribution ratio of different sources is of great significance for the accurate estimation of karst carbon sink flux in the later stage,and provides a scientific reference for further research on the carbon cycle of karst cave systems.There are few reports on the research of cave nitrate source identification in the early stage.This research breaks through the research status of nitrate source identification staying on the surface and shallow groundwater,and provides a new direction for subsequent cave research.This article takes Mahuang Cave,a branch cave of the Shuanghe Cave system in Suiyang,Guizhou,as the research object.Six water-gas monitoring points are set up in the cave,namely Mahuang No.1 water point to Mahuang No.6 water point（in the text,MH1-According to MH6）,the water-air environment of Mahuang Cave will be monitored monthly and water samples will be collected from January to December2020,and water samples for testing isotopes will be collected in July and December 2020.Through data analysis to understand the air environment and cave hydrogeochemical characteristics of Mahuang Cave,use nitrogen and oxygen isotope tracer technology to identify the source of nitrate,and use the SIAR mixed model to quantitatively estimate the contribution ratio of different sources of nitrate.The main conclusions of this paper are as follows:（1）Affected by local climatic conditions,the temperature,relative humidity and CO₂ concentration in Mahuang Cave show seasonal changes of high in summer and low in winter.The lower the temperature and the higher the relative humidity and CO₂ concentration in the cave,the smaller the changes in the concentration of the three cave air indicators,and reach a relatively stable state at a certain position,and the cave air environment gradually stabilizes.（2）The water chemistry type of Mahuang Cave is HCO₃^--Ca·Mg type.In the water chemistry characteristics,the p H value shows the seasonal change characteristics of high in winter and low in summer,and the higher the p H value is in the cave in space.The size of the EC value is shown in space as dripping water>perennial fast fissure water>seasonal wall water>underground river water.In terms of time,the EC value of point MH6 did not change significantly.The EC value of points MH1-MH5 showed a high level in the rainy season and low in the dry season in terms of time.Affected by the extreme dry weather in August,the EC value had two peaks.The change of DO concentration in the water of Mahuang Cave is low in summer and high in winter,with small spatial differences.The water temperature in the cave shows a trend of first decreasing and then increasing in space from the entrance of the cave to the inside of the cave.The more the cave is,the smaller the change in water temperature,and the water temperature is higher in summer and lower in winter in time.（3）The concentration of Ca²⁺in the water of Mahuang Cave is the highest,and the average concentration of Ca²⁺in point MH5 is the lowest in space.The difference in Ca²⁺concentration in the water of the other five monitoring points is small;in terms of time,affected by the extreme weather in August,the Ca²⁺concentration appears to be two.Sub-peak,the overall time change was high in summer and low in winter.The Mg²⁺concentration is spatially expressed as dripping water>underground river water>perennial fast fissure water>seasonal wall water.The concentration of Sr²⁺gradually increased from the mouth of the cave to the inside of the cave,and the concentrations of Mg²⁺and Sr²⁺had no obvious seasonal changes.The concentration of HCO₃^-,Cl^-and NO₃^-in the water of Mahuang cave changes with time.The characteristics of HCO₃^-,Cl^-and NO₃^-at the slow dripping points MH2 and MH6 are not obvious.MH1,MH3,MH4,MH5 The concentration of HCO₃^-ions at the number point is high in the rainy season and low in the dry season.The concentration of Cl^-and NO₃^-is low in the rainy season and high in the dry season.The concentration of HCO₃^-,Cl^-and NO₃^-responds to extreme drought weather.More significant.The value of SO₄^2-ion concentration in space is shown as dripping water>perennial fast fissure water>seasonal wall water>underground river water,with no time-changing characteristics.（4）Theδ¹⁵N-NO₃^-,δ¹⁸O-NO₃^-,andδ¹⁸O-H₂O values of Mahuang cave water are positive in the rainy season and negative in the dry season.The nitrate nitrogen and oxygen isotope tracer technology was used to qualitatively identify the source of nitrate,and it was determined that the nitrate in the water of Mahuang Cave was mainly derived from ammonium nitrogen fertilizer,soil nitrogen and manure sewage.The water body of Mahuang Cave is neutral and slightly alkaline,with sufficient DO and isotope composition,which indicate that the nitrate conversion process in the water body in the cave is mainly affected by nitrification.（5）Based on the quantitative estimation of the SIAR mixed model,it is concluded that the contribution rates of different sources of nitrate in the water of Mahuang Cave in the rainy season are ammonium nitrogen fertilizer（41.2%）,soil nitrogen（35.3%）,manure and domestic sewage（23.5%）;in the dry season,the contribution rate is ammonium nitrogen fertilizer（41.2%）,soil nitrogen（35.3%）,and sewage（23.5%）.Ammonium nitrogen fertilizer（62.1%）,soil nitrogen（30.4%）,manure and domestic sewage（7.5%）.The contribution rate of the three sources to nitrate in the water body is:ammonium nitrogen fertilizer>soil nitrogen>manure and domestic sewage.