Study On The Response Mechanisms Of Groundwater Level And Temperature Dynamics To The Freeze-thaw Process In Seasonal Frozen Soil Areas

Freeze-thaw action in seasonal frozen soil areas controls soil hydrothermal transport processes,which in turn trigger groundwater level and temperature dynamics.This has a significant impact on the effective utilization of underground water and soil resources as well as the choice of water resources management measures.However,the mechanisms of interaction between groundwater level dynamics and soil moisture redistribution under freeze-thaw action and the response of groundwater temperature dynamics to soil temperature changes are not yet fully understood and difficult to quantify.To determine the dynamic causes of groundwater level and temperature during the freezing-thawing period,it is necessary to conduct long-term,high-frequency dynamic monitoring and quantitative analysis in the field to better comprehend the dynamic and equilibrium of water and heat in freeze-thaw process.In this study,two typical sites in Changchun and Songyuan City of Jilin Province were monitored for three years(2018-2021)and one year(2021)during the freezingthawing period.Data collected included air temperature,precipitation,snow depth,soil temperature and humidity at various depths,groundwater level and temperature.Through a thorough comparison of various meteorological,vadose zone,and hydrogeological conditions,the response mechanisms of groundwater level and groundwater temperature dynamics to the freezing-thawing process were determined.The following research findings were attained:(1)The dynamic changes of soil water and heat and their response to meteorological environment during freeze-thaw period were identified.According to the freeze-thaw process,the soil temperature in the frozen layer exhibited an air temperature-driven temporal dynamic of falling during the freezing period and rising during the thawing period,whereas the soil temperature in the unfrozen layer kept falling.The characteristics of the heat transfer between the atmosphere and soil in space were layer by layer diminishing and lagging.It was worth noting that snow cover acted as a heat barrier and insulator between the atmosphere and soil,affecting the surface 0-10 cm temperature gradient amplitude dramatically.Due to soil water phase transition and migration caused by the tight connection of soil water and heat,soil liquid water content changed spatially and temporally in response to variations in soil temperature.(2)The characteristics of groundwater level’s reaction to freeze-thaw action and its mechanism were identified.The groundwater level at the Changchun site exhibited a V-shaped dynamic pattern of decreasing during the freezing period and increasing during the thawing period.Correspondingly,the groundwater level of Songyuan site,where the air temperature was lower and the frozen soil depth was larger,was relatively stable.The dynamic difference in groundwater level suggested that the ultimate depth of groundwater under the influence of freeze-thaw can be calculated as the sum of the maximum depth at which freeze-thaw influence can occur and the maximum capillary height at which groundwater can occur.Negative accumulated surface temperature,snow cover,the lithology of the vadose zone,the distribution of soil water,and the initial depth of the water level during the freeze-thaw period all had an impact on these variables.The dynamics of groundwater level during the thawing stage revealed that the main source of recharge was the meltwater in the frozen layer,while the whole recharge from snowmelt water was constrained,accounting for only 12%-21%.(3)The dynamic process and cause of groundwater temperature during freezingthawing period were determined.Throughout the freeze-thaw process,the temperature of the groundwater continued to drop,and this dynamic change was related to the soil’s heat absorption and release states.The unfrozen layer’s heat storage demonstrated that the groundwater temperature decreased continuously because the daily heat storage of unfrozen layer was less than 0 in freezing period and the accumulated heat storage continued to decrease in thawing period.For the whole year,when the soil layer continued to absorb heat and the accumulated heat storage began to rise significantly,the groundwater temperature decreased slowly or reached a stable state from late April to mid-May.From then on,the groundwater temperature rose obviously from late May to late June until the accumulated heat storage recovered to the initial level of the freeze-thaw period and the heat absorbed by the soil in the thawing period could fully compensate for the heat lost in the freezing period.(4)Quantitative descriptions were given of the effects of groundwater level fluctuations on soil water redistribution and the groundwater temperature response to soil temperature during the freeze-thaw period.The controlling influence of lower boundary groundwater on soil water was quantitatively demonstrated using the model of water balance and water migration of meteorology-soil-groundwater as a unified system.The main performance was that during the freezing period,groundwater coordinated with the water in the unfrozen layer to recharge to the frozen layer to form ice.The water recharged to the frozen layer by upward groundwater infiltration will be redistributed in the form of recharge to groundwater and storage in the vadose zone,thereby affecting the comparison between the groundwater level at the end of the thawing period and the initial water level before freezing,as well as the changes in water volume in the vadose zone before and after freezing and thawing.From the perspective of heat equilibrium,the ratio of soil heat capacity to groundwater heat capacity can be used to roughly describe how groundwater temperature responded to changes in soil temperature over the time span when the groundwater temperature started to drop to the end of the stable trend.Specifically,the change value of groundwater temperature was roughly equal to the product of soil temperature change value at the maximum monitoring depth and the ratio of soil heat capacity to groundwater heat capacity.