Groundwater System Evolution Of The Opencast Coalmine Area In Hulun Buir Prairie,Inner Mongolia,North China

A typical and representative Northern prairie,the Hulunbuir prairie is one of the natural grazing lands maintaining complete primary appearances in China.As mining economy develops rapidly here,mine exploitation has caused tremendous influence on ecological environment around.The open-pit coal mining activities have profoundly changed the natural occurrence environment and regional water cycle conditions of groundwater,leading a series of correlated environmental problems more and more serious.As the important impact factor of geological environment changes,groundwater has a significant indication of the size and strength of human activities in the mining area,so the analysis of the cause of these environmental problems in the steppe mining area all depends on the study of groundwater flow,occurrence and water quality evolution.Therefore,the study of groundwater environment evolution in Hulunbuir Grassland open-pit coal mine area is of great concern.The groundwater system is mainly introduced in association with the historical data of hydrogeology and groundwater evolution of YiMin coal mining area,using groundwater simulation to simulate the evolution of grourndwater flow field.A comprehensive study on the temporal and spatial evolution law of water chemical characteristics in the coal mining area by means of mathematical statistics,contour map comparison,Piper three line chart,Kurllov's formula method,Gibbs method,ion proportion coefficient method and PHREEQC has been done.The main results of this study are as follows:?1?The opencast mining activities lead to regional groundwater depression cone.In the mine pit,the groundwater level downs 124 meters,the area of groundwater level decreased obviously by 100km²,among which,the area of groundwater level dropped more than 20m was 84.1km²,taking the mine as the center.The groundwater level dropped 5-10m around the ChaiDamian Lake.Moreover,the groundwater level decreased less than 2m northern of it.The groundwater table lowers greatly lead to the drainage of strata,mainly distributed in the south of the study area,which has a depressive trend as drill down.The drainage area of the Quaternary strata is about 69.2km²;the drainage area of the second,the third and the fourth aquifers is 44.6km²,29.9km² and 22.7km²,moreover,the fifth aquifer and the sixth layer of water-resisting layer are drained in the mine pit area.?2?Opencast mining activities change the groundwater recharge and drainage conditions,the amount of precipitation infiltration decreased from 11,532,000 to 8,213,000 m³/a.Evaporation decreased from 37,937,000 to 29,574,000m³/a.Yimin River lateral recharge increased from 4,567,000 t.o 18,365,000 m³/a,four times of the amount of natural recharge.And the length of Yimin River recharging of groundwater showed a pronounced increasing tendency by increasing froml.7km to 9.8km,with the flow velocity increasing substantially about 10 times than before.The regional lateral supply of groundwater,with the speed of 35,102,000m³/a,which is more than the natural conditions by 16,737,000 m³/a.The mine drainage becomes the main way of groundwater discharge in the study area,at the speed of 25,418,000m³/a.?3?Open-pit mining activities greatly accelerated the circulation speed of groundwater,the area of groundwater flow velocity obviously increased is by 103.2km² in the south of the study area,with the mean flow rate is 1.06 m/d,8 times that of natural conditions.In the north of the study area,the area of groundwater flow velocity without the slightest influenced is about 82km²,with the mean flow rate is 0.17m/d.?4?The evolution characteristics of water chemical composition before and after coal mining is that the pH value decreased significantly,the total hardness-TH,Ca²⁺,Mg²⁺ and SO₄^2-concentrations increased,while TDS,COD,EC,Na++K+ and HCO₃^-concentrations showed a decreasing trend.The hydrochemical type of groundwater changes from carbonate type to carbonate-sulfate type,which is the HCO₃^-Na-Ca-Mg type to the HCO3 · SO4-Na · Ca-Mg type,with an evolutional tendency from simplicity to complexity,and singleness to pluralism.?5?Under the influence of the groundwater drainage of the opencast coal mining,groundwater evolved in the direction of desalination,the chemical indicators such as TDS,COD,EC,Na⁺?K⁺ and HCO₃^-showed a decreasing trend in the center area of the mining.Comparing the data before and after coal mining,except for TH,Ca²⁺,Mg²⁺ and SO₄^2-ions,the location of the mining pit is evolved from a high ion concentration region to a low ion concentration region.The concentration of each index in the affected area was significantly lower than that in the non affected area,this is closely related to the groundwater evolution of desalination;Meanwhile,it is also related to the longterm acceptation and accommodation of surface pollutants and the strong effect of evaporation of the ChaiDamin Lake,which leading to the concentration of each index in the northern non affected area is high.?6?Under the natural condition of the study area,the groundwater ions mainly come from the weathering and dissolution of feldspar and gypsum,precipitation of calcite and dolomite and so on.The chemical composition of groundwater is mainly controlled by water-rock interaction and evaporation concentration;The strong drainage of groundwater accelerated the water-rock interaction,also made calcite,dolomite and gypsum in the state of saturation and continuously dissolving,the water-rock interaction played the main role in controlling the hydrochemical composition in mining area;Comparing to the data before coal mining,the content of SO₄^2-increased obviously and the content of HCO₃^-decreased.This is mainly due to the oxidation of sulfide and pyrite in the coal seam and the decarbonation caused by the decrease of hydrostatic pressure;the cation exchange and adsorption between Ca²⁺,Mg²⁺ and Na⁺ has occurred in the groundwater discharge area.