Study On The Evolution Mechanism Of Lake-groundwater System In Desert Coal Mining Area

Coal mining activities in arid and semi-arid areas often lead to a series of ecological and environmental problems,such as aquifer drainage,lake shrinkage and disappearance,vegetation decline,surface desertification,etc.,which has become a major limiting factor for the sustainable development of regional economy and ecological and environmental security.Bojianghaizi Mine Field and Youfanghao Mine Field are located in the transition zone between Mu Us Sandy Land and Kubuqi Desert in the coal mining area of Bojianghaizi Basin.During the development process,it is easy to cause serious influence on the lakes and groundwater in the basin.Taking Bojianghaizi Basin as an example,this paper studies the evolution and genesis of lake-groundwater system in desert coal mining area.Based on the physical geography and hydrogeology data of the study area and field investigation and sampling,remote sensing interpretation,mathematical statistics,Piper trigram,Gibbs map,correlation analysis,ion proportion analysis,saturation index and contour map were used to analyze the interannual variation of the main lakes and its driving factors.The characteristics of lake-groundwater system hydrochemistry,the sources of main components and the temporal and spatial evolution of groundwater chemical field in the basin were studied.The main conclusions are as follows:(1)The area of the core lake in the study area showed a shrinking trend in the past 40 years,and the area decreased by 82%.The process of lake area change can be divided into three stages:the normal fluctuation stage from1979 to 1999;From 2000 to 2015,the period of continuous contraction;From2015 to 2017,the period of recovery growth.Climatic warming and drying is the climatic factors that affect the lake area change,while the construction of water conservancy projects,ecological restoration projects and coal mining are the human factors that lead to the shrinking of lake area.The decrease of surface and underground runoff caused by human activities and the lack of effective water supply are the main reasons for the shrinking area of Tao-Ahaizi Lake.(2)Both the regional unconfined water and the confined water are weakly alkaline,and the content of TDS in unconfined water ranges from 554.75 to1192.48mg/L.The confined water was 554.41?1298.07mg/L.Sodium ion is the dominant cation,followed by calcium and magnesium ion.In the anions,carbonate ion and sulfate ion are the most,followed by chloride and nitrate ion.The average TDS value of the lake is 4385.21mg/L,and the average p H value is 9.90,which belongs to the strongly alkaline and slightly saline lake.The contents of Na⁺and Cl^-in the lake water were the highest,and the content of F^-fluoride in the lake water exceeded the standard.The main hydrochemical types of unconfined and confined water are HCO₃-Na type,and the lake water is Cl-Na type.(3)The lake is significantly affected by evaporation and concentration,and the groundwater is jointly controlled by evaporation and concentration and water-rock interaction.The main sources of Na⁺and Cl^-in regional groundwater are the dissolution and evaporation and concentration of rock salt,and Na⁺also comes from other sodium-containing minerals and cation exchange.Ca²⁺and Mg²⁺are mainly derived from the weathering and dissolution of dolomite and gypsum.Carbonate rocks,such as rock salt,gypsum,dolomite and calcite,are the main dissolution of groundwater in the study area.Silicate rock is not the main component of groundwater,and F^-in groundwater mainly comes from the dissolution of fluorite.(4)The p H value,TDS,EC,COD and other water quality indexes in the northern mining area of the study area are obviously lower than those in the southern non-affected area,and the cation concentrations such as Na⁺,Ca²⁺,Mg²⁺and anion concentrations such as HCO₃^-,Cl^-,SO₄^2-in the coal mining affected area are lower than those in the non-affected area.This is due to the large amount of drainage of groundwater in the process of coal mining,which makes the groundwater in the lake develop towards the direction of desalination.At the same time,the evaporation and concentration of the lake area is strong,and the surface pollution is more serious.(5)In the process of coal mining,compared with 2012 and 2019,the temporal evolution characteristics of groundwater in mining areas are as follows:p H keeps decreasing.The concentrations of TDS,Th,COD and Cl^-in groundwater have a decreasing trend,and the groundwater is developing towards desalination.A large amount of SO₄^2-is produced by the oxidation reaction of sulfur-bearing coal and pyrite in mining activities,and the content of SO₄^2-remains stable under the condition of desalination of the whole groundwater environment.With the development of agriculture,NO₃^--N in almost all areas of the study area is in the state of exceeding the standard in2019.