Hydrogeochemical Process Of The Typical Thermal Karst Lake On The Qinghai-Tibet Plateau Under The Conditions Of Active Layer Freeze-Thaw

With the gradual increase of the average temperature on the Tibetan Plateau and the intensification of human activities,the permafrost in the Qinghai-Tibet Plateau has shown a trend of continuous degradation.The specific manifestations are that the thickness of the permafrost layer becomes smaller or even disappears,and the upper limit of the permafrost layer moves downward.During the continuous degradation of permafrost,accompanied by a series of geological disasters,the most typical is the development of the thermal karst lake.Thermal karst lake has an important impact on the ecosystem of the Qinghai-Tibet Plateau,the carbon cycle in land and the atmosphere.Therefore,on the basis of consulting relevant literatures,the lake and groundwater were sampled four times during the process of melting and refreezing the active layer in the study area in 2019.Each sampling included 15 lake water samples and 9 groundwater samples,and the water chemical composition and isotope content in the water sample were determined.The method of combining hydrogeochemistry and mathematical statistics was used to analyze the causes of changes in water chemical composition and stable hydrogen and oxygen isotopes in the lake and groundwater in the study area during the freezing and thawing of the active layer,and quantitatively calculated the ratio of lake evaporation to groundwater inflow at different stages.The hydrogeochemical process of the thermal karst lake in the process of freezing and thawing of the active layer was initially grasped,which laid a certain foundation for the future research on the thermal karst lake.The research results obtained in this paper are as follows:(1)The hydrochemical types of lakes in the north are mainly HCO₃·Cl-Na·Ca·Mg or Cl·HCO₃-Na·Mg,and the hydrochemical types of lakes in the central and southern parts are mainly Cl-Na·Mg.The chemical types of groundwater are mainly HCO₃·Cl-Na·Ca·Mg,which are converted to Cl·HCO₃-Na·Ca·Mg during the freezing period of the active layer.(2)At different stages of the active layer melting to freezing,the water chemical components of the lake and groundwater in the study area were affected by both evaporation and concentration and rock weathering.(3)The main ion ratio coefficients of lake water and groundwater indicate that Na⁺and Cl^-are mainly from the dissolution of rock salt,and Ca²⁺and Mg²⁺may be from the dissolution of dolomite.The saturation indices of related minerals in lake water and groundwater also indicate that during the different stages of the active layer melting to freezing,dolomite and calcite control the relevant components of water chemistry in the study area.The salt rock did not reach saturation state,indicating that there is no rock salt mineral in the soil rock layer in the study area.(4)The photosynthesis of aquatic plants at the bottom of the lake is the main factor causing the changes in the concentration of p H,HCO₃^-,CO₃^2-,Ca²⁺,and Mg²⁺in the lake water.Atmospheric precipitation,evaporation and freezing and desalination of the active layer are the main factors that cause changes in Na⁺and Cl^-concentrations.From the early stage to the late stage of the melting of the active layer,the CO₂ produced by the respiration of microorganisms in the soil and the melting layer released by the active layer increase the solubility of calcite and dolomite in the groundwater.From the late stage of the active layer to the freezing period,the respiration of microorganisms is almost stagnant.HCO₃^-in the groundwater can react with Ca²⁺and Mg²⁺to produce Ca CO₃ and Mg Ca(CO₃)₂ precipitation,which is the main reason for the decrease of the HCO₃^-concentration in groundwater during the freezing period The reason,and the increase of other ions during the freezing period of the active layer is the effect of the freezing and desalination of the active layer.(5)Continuous precipitation factors may be the main reason for the?D and?¹⁸O in atmospheric precipitation in August being lower than in June;?D and?¹⁸O fluctuations in lake water and groundwater are mainly affected by groundwater runoff,temperature and evaporation.(6)There are significant differences of E/I(Ratio of Lake evaporation to groundwater inflow of lakes)in the different stages from the melting of the active layer to freezing.In the middle and late stages of melting of the active layer,the E/I reaches the maximum,and the recharge of groundwater to the lake reaches the maximum.In the initial stage of thawing or freezing period,the E/I becomes smaller,and the recharge of groundwater to the lake decreases,which also explains the effect of freeze-thaw action of the active layer on the water cycle.