Evolution Of Groundwater Environment In The Closed Mines Of Yushenfu Mining Area

The cycle of coal mining to pit closure cause pollution and damage to the mining environment.In particular,after the pit closure,the temperature,pressure and redox environment in the pit are changed,and the complex water gas rock interaction occurs between water and coal rock,water and the leftover materials of mining engineering,which changes the pollution components in the pit water.With the increase of water level in the pit,under the hydraulic drive,the pit water intrudes into the aquifer along the shaft,water diversion fissure,fault fissure,poorly sealed borehole and other channels,causing serious pollution to the regional groundwater environment.In recent years,hundreds of coal mines have been closed in Yushenfu mining area,and the impact of mine closure on groundwater environment has become prominent.It is of great social significance and theoretical value to carry out the study on the evolution of underground water environment in closed mines in Yushenfu mining area.Through field investigation,theoretical analysis and indoor simulation experiments,this paper finds out the basic characteristics of the groundwater environment in Yushenfu mining area,identifies the main factors affecting the groundwater environment caused by the mine closure,reveals the migration and water quality evolution of the pit water after the mine closure,and puts forward the prevention and control measures for the water environment pollution after the mine closure.The major findings are as follows:（1）It is found that the water quality of Quaternary loose layer in Yushenfu mining area is generally good,and the regional difference of bedrock fissure water is large,showing the regular characteristics of water quality deterioration with the increase of TDS with the buried depth.Before coal mining,controlled by the terrain and landform,the Quaternary loose layer water was percolated from high to low.Bedrock fissure water migrates along the rock layer from high to low pressure.The two aquifers only alternate runoff in the bedrock outcrop area,and the vertical hydraulic connection is weak.During coal mining,the Quaternary loose layer water and bedrock fissure water discharge toward the mine,.The water accumulates from the surrounding to the mine,generating mine water.After the mine closure,the pit water level rises,and the mine water evolves into pit water.The high-level pit water invades the low-level aquifer along the shaft roadway,mining fissures and other channels,causing pollution to the water environment in the low-level area.（2）It is found that after the mine is closed,underground water flow through the closed mine in two ways.A part of water flows downstream along the original aquifer,and the other part of the water flows through the mine pit via the roof water conducting fissure–tunnel—roof water conducting fissure channel.The water flowing through the pit is the pollution source of the Quaternary loose aquifer.The pit?water has a great impact on the Quaternary loose aquifer in the shallow buried area,and a small impact on the medium and deep buried areas.With the same water head,the water flow through the pit and the water flow along the original aquifer have a mutual relationship.The roadway is a strong channel for water diversion.Blocking the roadway can reduce about 40%of the total water flowing through the closed mining area.The isolation of the production system can effectively reduce the water flow through the pit and reduce the pollution of the pit water to the Quaternary loose layer.（3）Mining activities pollute the mine water by organic matter,petroleum,trace metals and nonmetals.After the closure of the mine,the man-made pollution sources disappeares,and the Hg,Pb and Cd contents in the pit water gradually decrease.The mine water still partly comes from the bedrock fissure aquifer.The chemical reaction between the mine water and coal continues.The salt,Mn and As concentrations in the mine water gradually increase with the increase of closing time.The concentration of salt content,Mn and As increase in the Quaternary aquifer.（4）It is found in the laboratory test that the conductivity,salinity,hardness,SO₄^2-and Cl-concentrations are linearly correlated with the mixing ratio of water in different aquifers.The background concentration of water environment does not affect the dissolution and precipitation of SO₄^2-and Na⁺in coal,but it will affect the dissolution of carbonate minerals in coal.The background water quality makes the concentration of Na⁺and SO₄^2-in mine water increase linearly,and the concentration of Ca²⁺,Mg²⁺and HCO₃^-increase.The change of redox conditions does not change the main type of reaction between water and coal（rock）,but affects the formation rate and equilibrium concentration of solute.It is found that the mineralization,hardness and SO₄^2-release of Yan’an group 2 coal（after soaking）conform to the second-order reaction kinetic model,and the reaction rate constant is-10^-7L/（mg·d）;Cl^-release conforms to the first-order reaction kinetic model,and the reaction rate constant is-10^-3L/（mg d）.（5）It is found that the permeability coefficient of the water conducting fracture zone is the key factor affecting the water flow through the pit after the blocking of large passages.A formula is established for calculating the water flow along the mining water conducting fracture flowing through the pit and injected into the downstream aquifer.The injection model of Quaternary aquifer polluted by mine water is determined as one-dimensional continuous injection flow field.The differential equation of pollutant migration,initial conditions and boundary conditions are given,and the quantitative evaluation method of mine water polluted aquifer is proposed.