Vertical zoning characteristics and genesis mechanism of groundwater hydrochemistry: a case study of Xinjie Mining Area

Mine water, as the main potential water supply source for coal mine sites, has a good potential for comprehensive reuse. However, the complex and diverse physical and chemical reactions between the coal seam roof-filling aquifers and the surrounding rocks jointly affect the quality of mine water in coal mines. Therefore, it is of vital significance to determine the status of groundwater quality and its genesis mechanism in different water-filled aquifers for the protection and comprehensive utilization of mine water. Based on the groundwater samples and rock mineral composition data from a well field in Xinjie Taigemiao Mine, this study uses multivariate statistical analysis and hydrogeochemical techniques, and simulates the water chemical equilibrium by PHREEQC software, to explore in depth the chemical characteristics of the groundwater in different aquifers and its vertical zoning laws of mineral dissolution/precipitation, with the aim of revealing the causative mechanism. It is found that ① the chemical characteristics of the groundwater in the mining area show obvious vertical heterogeneity, in which the Zhidan Group is dominated by HCO₃-Na-Ca and HCO₃-Mg·Ca types, while the Anding Formation to Yan’an Formation is dominated by Cl-SO₄-Na and SO₄-Na types, TDS, Na⁺, and$\text{SO}_{\text{4}}^{{2-}}$may be key drivers dominating vertical water quality differentiation. ② The concentrations of key water quality indicators, such as TDS, Na⁺, and$\text{SO}_{\text{4}}^{{2-}}$, show a linear relationship with the depth of the aquifer (from the Zhidan Group to the Yan'an Formation), and the analysis of the ionic ratios reveals that the K⁺ and Na⁺ mainly originate from the dissolution of silicate minerals, and that the concentrations of Ca²⁺ and Mg²⁺ are affected by the dissolution of carbonate, sulfate and silicate minerals. ③ The depth variation of the mineral saturation index shows that dolomite, plagioclase feldspar and rock salt alternate between unsaturated and oversaturated states in different aquifers. In the groundwater of the Zhidan Group and the Anding Formation, many minerals show a tendency of further dissolution, while in the Zhiluo Formation and the Yan’an Formation, the majority of the minerals have already reached the oversaturated state. ④ Water-rock interactions control the hydrochemical composition of mine water at different depths. The hydrochemical components of the uppermost Shidan Group are mainly affected by the dissolution of minerals such as sodium feldspar, potassium feldspar, calcite, chlorite, and so on, and the deeper aquifers show a stronger reaction to the dissolution of silicate minerals, gypsum, rock salts, and pyrite accompanied by alternating adsorption of cations, resulting in the formation of highly mineralized Cl- SO₄-Na, SO₄-Na type water. This study deeply explores the vertical zoning characteristics of groundwater chemistry in different aquifers in the coal mine area and its mineral dissolution mechanism, which provides a solid scientific foundation for the precise management of mine water resources, water quality regulation and environmental protection, and has important theoretical guidance value and practical application prospects.