Study On The Evolution Mechanism Of Regional Groundwater Circulation Under The Condition Of Plateau Permafrost Degradation

The mechanism of groundwater circulation change caused by permafrost thawing in the alpine region is one of the key scientific problems in the basic theory research of hydrogeology and environmental geology.Understanding the dynamic response of such groundwater to permafrost changes is of great value to water resources protection,ecological environment and engineering construction in cold regions.This paper takes Datong river source area of Qinghai province as the research area to study the regional groundwater circulation evolution mechanism under the condition of plateau permafrost degradation.By means of monitoring,statistical analysis,water chemical isotopes,numerical simulation and other methods,the process of permafrost degradation were systematically analyzed,and the changes of permafrost microstructure and seepage characteristics and the evolution mechanism of regional groundwater circulation were revealed.The main results are as follows:1)By using remote sensing interpretation,multivariate statistics,ground temperature monitoring,and scanning electron microscopy,the types and degradation stages of permafrost were divided,and the variation rules of area,upper and lower limits,boundary,ground temperature and microstructure of permafrost at different stages of degradation in Datong river source region were revealed.Through multivariate statistical analysis and water balance calculation,the response law of groundwater recharge diameter and discharge is expounded.On the basis of the insignificant increase in rainfall,the spring flow,surface water runoff and groundwater natural resources in the region showed an increasing trend,which was caused by the degradation of permafrost and the increase of groundwater recharge channels and the melting water from underground ice.According to the water equilibrium budget,the proportion of underground ice melt water is about 17%2)Through temperature-controlled CT scan,mercury injection experiment and temperature-controlled permeability test,the law of microstructure and permeability of permafrost change under degradation condition were revealed.During the warming process,the permafrost particles and pores are redistributed,and the large pores and connectivity of permafrost are increased.At the beginning of the process,the permeability coefficient of permafrost increases rapidly.When temperature reaches-0.5?,the permeability coefficient doubles,indicating that the "high-temperature permafrost" has a certain permeability and must not melt completely before it changes from a "water-proof layer" to an "aquifer".The quantitative relationship equations of temperature,microstructure and permeability were established.3)Different groundwater sources and circulation pathways were identified by sulfur,boron,strontium and uranium isotopes.The multi-water transformation relationships were calculated.The results show that under the condition of permafrost degradation,the relationship between superpermafrost water and infrapermafrost water becomes close,and the groundwater system changes from closed to open.Underground ice melt water is involved in groundwater circulation,and the proportion of it recharges infrapermafrost water in continuous permafrost area,discontinuous permafrost area and island permafrost area is 9%,17% and 11% respectively.The proportion of replenishment in superpermafrost water is 18%,24% and 20% respectively.Finally,the groundwater circulation model of mountain basin in alpine river source area was established under the condition of permafrost degradation.4)By using the secondary development function of COMSOL multi-physics software system,the multi-field coupling model is improved,and the quantitative simulation and prediction of regional groundwater cycle evolution under the condition of permafrost degradation with medium and long period time scale is realized.5)The evolution mechanism of regional groundwater circulation under the condition of permafrost degradation is revealed: temperature ? permafrost type ? microstructure ? permeability coefficient ? hydrogeological structure ? evolution of groundwater circulation model ? release of permafrost water ? change of water quality.Finally,based on the theory of earth science system,the countermeasures and suggestions for the protection of permafrost environment and groundwater resources are put forward in view of the possible resource and environmental effects of permafrost degradation from the perspective of multi-layer interaction zone.