Hydrogeochemical Evolution Characteristics And Simulation Of Karst Groundwater In The Jinci Spring Area

Karst groundwater has stable water quantity and good water quality in Jinci spring area.It is an important water source for industrial and agricultural water supply in Taiyuan City and its surrounding counties.In recent decades,with dense population growth,steady progress of urbanization,and rapid economic and social development in the spring area,water shortage and contradiction between supply and demand is prominent.Human activities(coal mining drainage,karst groundwater exploitation,Fenhe reservoir construction)have significantly influence on the karst groundwater system,which leads to the decrease of spring flow and the cut-off of Jinci spring.In addition,the cutoff of spring not only changes the hydrodynamic field of karst groundwater system,but also has a adverse effect on the process of hydrochemical evolution.In order to scientifically understand the evolution processes of karst groundwater quality,and rationally develop and utilize karst groundwater resources to meet the requirements of ecological civilization construction,it is necessary to analyze the hydrochemical evolution characteristics of karst groundwater in spring area.Therefore,based on the analysis of the occurrence conditions,recharge and discharge characteristics of karst groundwater,the hydrochemical characteristics and evolution law of karst groundwater were investigated,and the main sources of cation and anion were discussed,and a hydrochemical reverse simulation model was established to study the evolution processes of karst groundwater hydrochemical field.The main research results were as follows:(1)Based on borehole data,geological map and hydrogeological map,the 3D geological model was constructed by GMS software in the Jinci spring area.According to the results of geological model stratification and section display,the terrain of Jinci spring area changes greatly,and the elevation shows a decreasing trend from northwest to Southeast.The Quaternary strata are widely distributed in eastern Taiyuan basin,and sporadically distributed in valleys and intermountain basins.The Permian and Triassic strata are thick in the middle of the spring area,and the Carboniferous strata are thin and distributed in the Fenhe river valley.Cambrian and Ordovician strata are the main water bearing formations,which are exposed in the north and northwest of the spring area.The folds such as Malan syncline,Lingdi syncline and Beishicao anticline are visualized in the model,but the faults cannot be well represented in the model because GMS software assumes that the strata are continuous.(2)Based on box diagram,schukarev classification,Piper diagram,SOM neural network model,correlation analysis and scatter diagram,the hydrochemical characteristics and genesis of karst groundwater were discussed.The results showed that the dominant hydrochemical types of karst groundwater were HCO₃-Ca,HCO₃-Ca·Mg in the recharge area,HCO₃·SO₄-Ca·Mg,SO₄·HCO₃-Ca·Mg in the runoff area,and SO₄-Ca,SO₄-Ca·Mg in the discharge area.The values of T,TDS and EC increased along the seepage path.By comparing the changes of the hydrochemical field in 2001 and 2016,it is found that the spatial distribution characteristics of the TDS and the main components in 2016 are more complicated than 2001,which may be affected by the leakage of acidic drainage water and Fenhe river water.HCO₃^-,Ca²⁺and Mg²⁺originated mainly from the dissolution of carbonates;SO₄^2-came mostly from gypsum dissolution and pyrite oxidation;and the source of Cl^-and Na⁺could either be major from halite dissolution or cation exchange.(3)The Fenhe river water and karst groundwater are mixed,and the hydrochemical compositions of the mixed water are closer to the water sample with a larger proportion.Based on the analysis of the mineral saturation index,considering the possible mineral phases and constraints,the PHREEQC software was used to establish a hydrogeochemical reverse simulation model to investigate the hydrochemical evolution process that occurs along the main seepage path of karst groundwater.The results showed that different dissolution and precipitation processes and cation alternating adsorption took place in the north,middle and south of the spring area.The water-rock interactions are closely related to the type of water bearing medium,geological structure and hydrodynamic conditions.From the recharge area to the discharge area in the northern part of the spring area,calcite,dolomite and gypsum were dissolved.Because the exposed limestone area is an open system,CO₂dissolves into the groundwater,accompanied by Ca-Mg exchange.From the north to south in the discharge area,the amount of gypsum dissolution is increases,halite,dolomite and CO₂may be supersaturated,and Ca-Mg and Ca-Na cation exchange occurs.After the surface water of the Fenhe river leaked into the karst groundwater system,the dissolution of gypsum and CO₂,the precipitation of calcite and dolomite,and the exchange of Ca-Mg cations occurred.From runoff area to discharge area in the middle spring area,the dissolution of gypsum was significantly higher than calcite,halite precipitated,and Ca-Mg and Ca-Na cation exchange occurred.From the deep buried area to the discharge area,the gypsum dissolved 5.54 mmol/L,which was significantly higher than other minerals,and Ca-Mg and Ca-Na cation exchange occurred.