Hydrogeochemical Process Of Typical Thermokarst Lake In Qinghai-tibet Plateau

Under the situation of increasing global warming and human engineering activities,temperature rising rate in Qinghai-Tibet Plateau known as the global climate "amplifier" and "driver",is obviously higher than the global average level.Due to temperature changes,glaciers and frozen soil in Qinghai-Tibet Plateau are degraded to a great extent.The formation and development of thermokarst lakes in permafrost area is a typical thermal thaw disaster caused by permafrost degradation.This process directly or indirectly affects the area in many aspects such as hydrologic cycle,ecological cycle,carbon cycle in land and atmosphere and to some extent destroyed the local ecosystem.Therefore,on the basis of consulting a large number of literatures at home and abroad,this paper takes the typical small watershed of thermokarst lakes distributed in the Beilu River basin as the research object and 11 underground water samples,16 lake water samples and 11 drilled soil samples are collected.The conventional ion detection data,stable isotope data and mineral composition data collected from field investigation are analyzed and studied in detail by means of combination of hydrogeochemistry and statistics.A preliminary understanding of the hydrogeochemical processes of lakes during the melting of frozen soil in summer has been made to lay a foundation for the later study of the hydrological effects of lakes.The research results obtained in this paper are as follows:(1)The major machanisms that controls the chemical composition of the lakes and grounawater are the same,and contains the evaporation-frctional crystallization process and rock weathering.There are three main hydrogeochemical processes that occur during the formation of lakes and groundwater hydrochemistry: dissolution/precipitation,cation exchange and evaporation-frctional crystallization.The direction of cation exchange between lakes and groundwater is basically the same,but the exchange amount of lakes is less than that of groundwater.(2)The main dissolved minerals in lakes and groundwater are halite and carbonate,and there is a small amount of silicate dissolved.The mainly carbonate is dolomite,and a small amount of calcite is dissolved in some areas.The difference between lakes and groundwater is that the solubility of sulphate in groundwater is slightly higher than that in lakes.(3)The calcite,dolomite and aragonite in lakes and groundwater have reached saturation,and the halite is not saturated.However,the saturation state of gypsum and anhydrite in the two water bodies is different,and most of the two minerals in groundwater have reached saturation state.None of the lakes reached saturation.(4)According to the study of isotope relationship map,it can be determined that the source of recharge of thermokarst lakes includes atmospheric precipitation,groundwater and permafrost meltwater.The E/I ratio of all thermokarst lakes in the study area waries from 0.17 to 0.68,showing that all the lakes are in the stage of continuous epansion.It is indrectly proved by isotope endmember analysis that the characteristics of water chemical composition are not only affected by the chemical characteristics of recharge source water,but also by evaporation.In summary,during the melting of the permafrost in summer,the groundwater in the study area mainly receive the recharge of atmospheric precipitation and permafrost meltwater,while discharge through evaporation and flowing to lakes.The thermokarst lakes in the study area mainly receive the recharge of atmospheric precipitation,groundwater and permafrost meltwater,while discharge through evaporation.In the formation process of groundwater and lakes chemical components,evaporation and concentration,cation exchange as well as weathering dissolution and precipitation of some minerals mainly occur.At the same time,groundwater is a source of recharge for lakes,and its characteristics of hydrochemistry also have a certain impact on lakes water quality.