Study On Evolution And Ecological Response Of Groundwater-lake System In Hulunberier Grassland Coal Mining Area

The Hulunbuir grassland is not only the grassland with the most biodiversity,but also an area with large coal resources.In recent years,with the continuous increase of the mining scale and mining period in the coal mining area,the original area ecosystem has been severely damaged,which has led to a series of ecological and environmental geological problems such as hydrological cycle imbalance,soil desertification,and reduced vegetation richness.In this study,the groundwater,lake wetland,and vegetation communities of the Yimin open-pit coal mine in the Hulunbuir grassland were used as research objects.Based on field surveys and explorations,remote sensing technology,model analysis and other methods were adopted to analyze the groundwater level,lake area,and even the vegetation around the mining area over a long series,and then explore the impact of coal mining on the surrounding ecological environment for many years;at the same time,Darcy's law,water balance principle and geometric knowledge were uesd to build a model of the relationship between groundwater level and lake area and the response model of groundwater-soil-vegetation.The relationship model further reveals the evolution of groundwater-lake systems and the surface vegetation response mechanism under the influence of coal mining activities.The conclusions of the study are as follows:(1)The coal mining has led to a sharp drop of groundwater level.The groundwater level in the study area before coal mining(1982)fluctuated steadily,The groundwater level is lowest in the northern part of the study area,and highest near the lake,The difference between high and low water level is only about 15m.The dredging project started after the coal mining which makes the groundwater level fluctuate violently in the research area,especially around the mining pit in the south of the study area,the groundwater level dropped to 111m.Until 2017,the groundwater level in the study area stabilized and there were no longer significant fluctuations.(2)The area and number of lakes in the study area showed a significant decreasing trend from 1982 to 2017.There are five lake groups in the research area before coal mining However,after the coal mining by2000,there are only two lake groups(Camel Neck Grass Kulun Lake and Chaidaminor Lake),the lake area has decreased from 0.55 km~2 and2.36km~2 before coal mining to 0.42 km~2 and 2.00 km~2 respectively.As of2017,the total area of lakes had reduced from 6.94 km~2 before coal mining to 1.12 km~2,with an area reduction rate of 84%.(3)Over the past 35 years,the growth of vegetation in the study area has generally shown a downward trend.Before coal mining in 1982,the NDVI was mainly concentrated in the range of 0.5-0.8.Among them,the number of NDVI pixels in the 0.6-0.7 interval was nearly 90,000,and the vegetation coverage was extensive and dense,vegetation grew well.With the continuous increase of mining years and mining scales,the NDVI concentration interval and the number of pixels gradually decrease.As of2017,NDVI was mainly concentrated between 0.1-0.4,and the vegetation coverage was reduced and sparse,vegetation grew generally poor.(4)The groundwater-lake recharge type in the mining area has generally evolved from groundwater recharge lake type to lake recharge groundwater type.In this study,groundwater level and lake area coupling model was constructed using the principle of water balance,and the goodness of fit reached over 0.80 after testing.This model can be used to predict the change of the area of round-table lakes with the groundwater level.(5)There is a correlation between the groundwater level and the content that characterizes the soil and vegetation.With the increase of the groundwater table burial depth,the soil water content and soil bulk density generally showed a downward trend;The groundwater table burial depth and TP and AK showed a significant linear negative correlation,R~2 was 0.92 and 0.86,respectively.With the increase of groundwater depth,the contents of TP and AK decreased significantly.Species richness decreases with the increase of groundwater depth,while aboveground biomass and community height show a local downward trend with the increase of groundwater depth.(6)A multiple linear regression model based on mathematical principles found that vegetation species richness was negatively correlated with groundwater table burial depth,soil pH,and soil EC,and positively correlated with soil moisture content and soil bulk density.After the significance test,the regression model is highly significant,and the model can represent the relationship of groundwater-soil-vegetation.