Study On Formation And Evolution Of Groundwater In Changbai Mountain Area Under The Influence Of Seasonal Freeze-thaw

Seasonal frozen soil has a large distribution range.Around 513.7×10⁴km²or 53.5%of China’s total land area is covered by seasonally frozen ground.Seasonal freeze-thawing environments and complicated topographic and geological conditions in mountainous areas are the significant elements affecting groundwater formation,and make it more challenging to study the formation and evolution of groundwater and quantify.Previous relevant studies are relatively weak and lack systematization.Research on the formation and precise quantification of groundwater resources in freeze-thaw mountainous areas is not only of great significance for the sustainable development and utilization of regional groundwater resources but also of strategic significance for regional water resources management and the construction of hydrological ecological civilization.Changbai Mountain area is the source of three large rivers,namely the Songhua River,Yalu River,and Tumen River.The source area of"Three Rivers"in Changbai Mountain is taken as the study area.Through data collection and analysis,field investigation,in-situ monitoring of groundwater and soil,collection and detection of water samples,variable temperature seepage experiments,multi-field coupling numerical simulation,in addition to the control effect of mountainous terrain geological conditions on groundwater flow system and the influence of the freeze-thaw process on groundwater time allocation.The mechanism of the key links of groundwater formation and evolution in seasonal freeze-thaw mountainous areas was revealed,the spatiotemporal variation characteristics of groundwater resource quantity were described in detail,and the rational development and utilization pattern of groundwater was proposed.The main conclusions are as follows:（1）Characteristics of soil freeze-thaw process and complex groundwater flow system in the Changbai Mountain area.The soil freeze-thaw process in Changbai Mountain was separated into four periods and five time nodes.The differences in characteristic time nodes and time lengths in the freeze-thaw process at different spatial locations were clarified.The initial freezing time in the study area was from October 5 to 23,the stable freezing time in the study area was from November 19 to 20,the initial thawing time in the study area was from March 11 to 26,the stable thawing time in the study area was from April 14 to 18,and the complete melting time was from April 25 to June 7.The freeze-thawing process lasted 185 to 246 days.The groundwater flow system in the study area was controlled by topography,different levels of surface watersheds,aquifer system structure,and groundwater recharge conditions.The complicated groundwater flow system was separated into three first-level systems and eight second-level subsystems based on flow system theory.The study on the freeze-thaw process of soil and the characteristics of the groundwater flow system laid a foundation for the formation and evolution of groundwater and water quality and quantitative evaluation in the study area.（2）Mechanism of groundwater formation and circulation in freeze-thaw environment.The groundwater in middle mountain area is charged by atmospheric precipitation and meltwater infiltration,which accounts for about 80%in freezing-thawing period.The middle mountain area was a water resource formation area,which was supplied to the platform and intermountain basin by lateral runoff,with an elevation ranges from 517 m to 1477 m.The proportion of groundwater recharge by meltwater in the platform and intermountain basin during the freezing-thawing period was around 22%to 60%.During the runoff process,the groundwater was directly discharged into the valley or overflows to the valley in the form of spring,which is controlled by the topography.The simulation results of groundwater-surface water conversion water-thermal coupling model show that temperature changes significantly affect the permeability of the streambed undercurrent zone,which was the fundamental reason for the change of groundwater to surface water transformation intensity.The temperature of the streambed undercurrent zone declined by 12℃from the no freezing period to the stable freezing period,the intensity of groundwater discharge to river water decreased by 28.85%.In terms of spatial distribution pattern,the hydrochemistry evolves from HCO₃-Na to HCO₃-Na·Ca,HCO₃-Mg·Ca/Ca·Mg,and HCO₃·SO₄-Ca,and the content of TDS increases gradually from the volcanic middle mountain,basalt platform to the intermountain basin.The hydrochemical evolution of the high altitude subsystem was mainly affected by atmospheric precipitation recharge（ice and snow meltwater）and weathering of silicate rocks.The excess Ca²⁺,Mg²⁺,and Na⁺in the high altitude subsystem was controlled by albite dissolution in the basalt.The low-altitude subsystem was affected by weathering of silicate rocks and carbonate rocks.In terms of time,from the no freezing period to the freeze-thaw period,anions evolve from SO₄·HCO₃to HCO₃·SO₄and from SO₄·HCO₃to HCO₃,and the cations evolve from Mg·Ca to Ca·Mg.（3）Coupled evolution patterns of groundwater flow and hydrochemistry in seasonal freeze-thaw mountainous areas.Three two-dimensional numerical models of groundwater flow for three representative sections of the first-level systems were established using the COMSOL software.The results show that there were three vertical groundwater flow modes:local,intermediate,and regional.The flow field of the local flow pattern was strictly controlled by topographic and geomorphic conditions.The cycle depth was less than 100 m,the groundwater-surface water transformation was strong,and the local flow pattern disappears during the freezing period;The intermediate flow pattern has a wide range of influence,and the cycle depth was less than 500 m;The runoff path of the regional flow pattern was long and the cycle depth was greater than 500 m.The coupling model of groundwater flow and hydrochemistry under the influence of freeze-thaw was constructed from the perspectives of landform,geological structure,the composition of recharge sources,flow system structure（local,intermediate,regional）,and drainage（river,spring）,which laid a theoretical foundation for the calculation of seasonal freeze-thaw groundwater resources in mountainous areas.（4）Spatiotemporal variation of groundwater resource quantity in seasonal freeze-thaw mountainous areasBased on the flow system concept and considering the influence of the freeze-thaw environment,the unit and period division method of groundwater resource quantity calculation was proposed.The spatiotemporal variation characteristics of groundwater resources in freeze-thaw mountainous areas were described more precisely from a new perspective.The calculation results show that the soil freeze-thaw process and complicated geological environment in seasonal freeze-thaw mountainous areas have significant impact on the spatiotemporal distribution pattern of groundwater.The groundwater resource quantity was 14.38×10⁸m³/a in the study area.The groundwater resource quantity in the three first-level systems（I,II,and III）was 2.71×10⁸m³/a,6.75×10⁸m³/a,and 4.92×10⁸m³/a,respectively.The groundwater distribution of system I was controlled by hydrogeological units.The groundwater distribution of system II was controlled by topography.The groundwater distribution of system III was controlled by hydrogeological units and acreage.The unstable freezing period lasted 27–46 days and the stable freezing period lasted 111–127days,and the groundwater resource quantity accounted for 10.11%and 12.62%of the total,respectively;The unstable melting period lasted 22–34 days and the stable melting period lasted 12–51 days,and the groundwater resource quantity accounted for 6.11%and 8.46%of the total,respectively;The no freezing period lasted 120–172 days,and the groundwater resource quantity accounted for 62.70%of the total.Based on the quantitative calculation of spatial and temporal distribution of groundwater resources,13 groundwater resource utilization zones in the study area were divided,and the rational development and utilization mode of groundwater resources was put forward.The research results provide a scientific basis for the rational development,utilization,and scientific management of groundwater in the seasonal freeze-thaw mountainous areas.