| As a typical fragile area of ecological environment in the world,under the influence of increasing human activities and greenhouse effect,the permafrost layer in Qinghai-Tibet Plateau is deteriorating year by year,causing lots of irreversible effects on the permafrost region.In the interaction between lake and the surrounding groundwater,the permeability of the sediment in the lake bed of thermokarst lake directly affects the transformation intensity of the lake and groundwater and the strength of the hydraulic connection.Therefore,it is of great significance to carry out the research on permafrost region of Qinghai-Tibet Plateau,rationally exploit and utilize land as well as groundwater resources and maintain the ecological health of alpine environment in permafrost region by studying the spatial variability of sediment hydraulic conductivity of lake bed,discussing the correlation between hydraulic conductivity and various influencing factors,analyzing transformation relationship and intensity between lake and groundwater.In this paper,two thermokarst lake of the Qinghai-Tibet Plateau are selected as the research objects.Based on in-situ vertical tube method and laboratory particle analysis experiment,the hydraulic conductivity Kv of sediment in two typical lake beds of Gehurst Lake were measured and calculated.The method of ground temperature monitoring is used to monitor the ground temperature for a long time.The conclusions are as follows:（1）Based on the traditional statistical method,the vertical hydraulic conductivity Kv in lake bed sediment of thermokarst lake was analyzed.Kv of lake A was found to be 2×10-5～5×10-4 cm/s and lake B was about 5×10-6～4.5×10-4 cm/s.The spatial variation degree of hydraulic conductivity of lake A bed sediment is of medium variability,and that of lake B is of strong variability.（2）The normal distribution test is carried out on hydraulic conductivity Kv,and the result shows that the Kv does not obey the normal distribution,and LnKv obeys the normal distribution.After systematic analysis of spatial variability of sediment permeability coefficient by using semi-variance variation function in geostatistics software,it is concluded that the sediment permeability coefficient LnKv of two typical lake beds in thermokarst is well fitted with the gaussian model,and the spatial variability of hydraulic conductivity is very strong,which is approximately a spatial independent variable.（3）Through the long-term monitoring of the lake water and ground temperature of two typical thermokarst lakes,it is concluded that the frozen soil layer at the bottom of the B lake has been melted through for many years.At the lowest temperature,there are still exchange activities between the lake and groundwater,and there are other sources of heat near the lake,which continue to provide heat for the lake,so that the lake does not freeze in a year.（4）The current correlation analysis on thermokarst lake bed shows that there is a strong correlation between sediment hydraulic conductivity,sediment size gradation and lake deposition.The characteristic particle size d50 is significantly related to the hydraulic conductivity Kv of lake bed sediment at 0.01 level,and the correlation coefficient is 0.783.Therefore it can be used as the characteristic particle size to evaluate the hydraulic conductivity Kv,and it fits best with the quadratic function model,the fitting degree is 0.642,the empirical formula is:Kv=1.21×10-6d502+1.68×10-8d50+0.000046.（5）The groundwater is continuously recharged to the lake in the form of permafrost from upstream and both sides,some of the lake water is converted into the downstream upper frozen soil layer water,some flow through the bottom of the frozen soil layer to the under frozen soil layer,and some remain in the lake.As a result,the water level in the lake keeps in a high level,the lake water is constantly draining to the east coast low-lying areas,causing land subsidence,gullies and other environmental geological problems.The conversion intensity of lake water to frozen soil water was 0.01667～0.05 m/d from October to May,average conversion strength was0.0177 m/d.From June to September,the conversion intensity of the water from the lake to the frozen soil layer was higher than that of the former,which was restricted by the measurement conditions,no quantitative analysis here.Above all,combined with the value and variation law of sediment hydraulic conductivity,size gradation of sediment at the lake bed,the indication significance of ground temperature,and comprehensive analysis of the transformation relationship between the two lakes and groundwater as well as the spatial variability of sediment hydraulic conductivity in lake bed,which provides a good theoretical basis for further study of plateau freeze-thaw phenomenon. |
