Study On Mechanism Of Hydrochemical Stratification Classification And Mixing Of Groundwater In Typical Coal Mines

Coal mining destroys the natural storage environment of groundwater,resulting in close hydraulic connection of groundwater,complex and diverse hydrochemical characteristics of mixed groundwater from aquifers of different sources,resulting in a series of severe problems such as water quality deterioration,water inrush and water inrush.To study the hydrogeological and geochemical stratification,classification and mixing mechanism of groundwater in coal mine area,so as to provide reference for water disaster prevention and comprehensive utilization of water resources in coal mine.Therefore,this paper selects Jiaozuo coal mine area,a typical North China coal mine,as the research area.Based on the measured data of groundwater hydrochemistry and hydrogen and oxygen isotopes of the main aquifers in the coal mine area,the piper PCA BP water quality layered classification model and PCA Bayes four terminal element hybrid model are constructed by means of mathematical statistics,BP neural network evaluation,hydrogen and oxygen isotope tracing and hydrogeochemical simulation,This paper expounds the hydrochemical evolution process and main controlling factors of groundwater,quantitatively analyzes the mixing process of deep groundwater,and reveals the hydrochemical stratification classification and mixing mechanism of coal mine groundwater.The results show that:（1）According to the methods of mathematical statistics,correlation analysis,Kriging interpolation and Piper three line diagram,the hydrochemical characteristics of groundwater show that the Quaternary pore water is mainly Ca-Mg-Na-HCO₃type,the main hydrochemical type of Permian sandstone water is Na-HCO₃type,and the main hydrochemical types of deep limestone water are Ca-Mg-HCO₃type and Ca-Mg-HCO₃-SO₄ type;This result expounds the spatial distribution law of typical ions such as Ca²⁺,Mg²⁺,SO₄^2-.（2）The Piper-PCA-BP water quality hierarchical classification model of groundwater in coal mine is constructed（the accuracy is 98.6%）,and the water quality evaluation results indicate that the Quaternary pore water has been polluted to a certain extent.The groundwater quality of Permian,Carboniferous and Ordovician aquifers is good,which can be used for drinking,industry and agriculture around the mine.（3）δD andδ¹⁸O analysis shows that the precipitation is the main supply source for deep limestone water,there is a certain hydraulic connection between deep limestone water,Quaternary pore water and Permian sandstone water,and the mixing of groundwater in coal mining area is significant.（4）By comprehensively applying Gibbs diagram,chlor-alkali index diagram,typical ion combination ratio and multivariate statistical analysis methods,it is clarified that the groundwater hydrochemistry of the main aquifers in coal mines is controlled by coal mining,mixing and water-rock interaction.（5）Based on principal component analysis,the endmember of groundwater recharge was determined,and the PCA-Bayes four-endmember mixed model was constructed.The calculation results show that the proportion of Carboniferous limestone water receiving precipitation recharge is 23.70%～41.10%,and the highest proportion of Ordovician limestone water mixing is 36.1%.The accuracy of the mixed model is verified byδ¹⁸O,and the overall error is less than 10%.（6）Reverse hydrogeochemical simulation was used to quantitatively study the hydrochemical evolution process of mixed groundwater runoff.The results show that the water-rock interaction in the mixing process is mainly the dissolution of minerals such as dolomite,gypsum and rock salt and the precipitation of calcite,cation alternate adsorption continues to occur,and Carboniferous limestone water accepts A small amount of Ordovician limestone water is mixed and replenished,accounting for about 20-28%.（7）Forward hydrogeochemical modeling was used to predict the relationship between inter-aquifer mixing and changes in groundwater hydrochemical composition.In the initial stage,with the intensification of deep limestone-water mixing,the groundwater flow rate becomes faster,the water-rock interaction weakens,and the content of Ca²⁺,Mg²⁺,and SO₄^2-decreases.However,in the later stage,the water chemical components in groundwater are close to saturation,the increase of Ca²⁺and Mg²⁺is small,and the SO₄^2-is decreasing continuously.In general,the change of TDS in groundwater showed a decreasing trend.