Analysis And Evaluation On Groundwater Recharge Of Snowmelt Infiltration In A Seasonal Frozen Soil Area

China's seasonal permafrost is mainly located in the arid and semi-arid regions north of 30°N latitude,accounting for about 53.5% of the country's land area,and the freezing and thawing period is several months long.As an important part of the natural water cycle,water migration in the seasonal permafrost system plays an extremely important role in ecological environment,industrial and agricultural production and environmental engineering.Whether seasonal permafrost can effectively recharge groundwater when the snow melts in the spring is still a subject for further research.A deeper study of this issue will help to deepen the understanding of the formation mechanism of groundwater resources in seasonal permafrost areas,and thus make a reasonable evaluation of the amount of groundwater resources.Based on in-situ monitoring data from the test site,this paper systematically analyses the changes in soil temperature,water content and groundwater level during the non-freezing,freezing and thawing periods and their response relationships;explores the role of snowmelt infiltration on groundwater recharge based on hydrochemical and isotopic methods;and quantitatively evaluates the amount of snowmelt infiltration using water balance methods and numerical simulation of groundwater flow,and obtains the following main conclusions:(1)The analysis of the atmospheric temperature and the characteristics of the permafrost(ground temperature)at the test site shows that the air temperature determines the starting point of the freezing process,while the ground temperature determines the end point of the thawing process.The entire freeze-thaw period is divided into five periods,namely the unstable freezing period,the stable freezing period,the unstable thawing period,the stable thawing period and the non-freezing period,using a combination of these two indicators.(2)During the freeze-thaw period,there were significant differences in the characteristics of soil moisture content between the 0.5m and 1.5m depths below the test site surface:temperature had a strong influence on soil moisture content at the0.5m depth,and precipitation and evaporation also caused fluctuations in soil moisture content;soil moisture content at the 1.5m depth remained more or less constant at this depth,as the site was located in the capillary water-filled zone and in the non-freezing zone.(3)The factors influencing the fluctuations in groundwater levels at various times are systematically analysed:groundwater level rise is mainly influenced by infiltration recharge from precipitation and snowmelt as well as by melting permafrost;the stable freezing period leads to a slow decline in groundwater levels due to the presence of icing potential;when the permafrost layer melts through for a period of time,the temperature continues to rise and there is a degree of weakening in the response of groundwater levels to atmospheric precipitation under the influence of evaporation and lateral groundwater runoff.(4)Water chemistry and isotopic analysis based on two water bodies(groundwater and snowmelt)collected at the test site showed that snowmelt differs from groundwater in terms of water chemistry type and isotopic composition,and that the water chemistry and isotopic characteristics are not sufficiently indicative of snowmelt infiltration due to the low precipitation of the year.(5)Based on the results of the groundwater water balance method and numerical simulation of groundwater flow,the amount of groundwater recharge from precipitation(snowmelt)infiltration at different periods in the study area was calculated and evaluated,combined with the analysis of groundwater level dynamics and the response of environmental factors.