Characteristics and genetic mechanism of low sulfate in high-salt groundwater of Permian sandstone in Fudong mining area, Huainan

Hydrogeochemistry research of groundwater in coal mining areas is of great significance to the prevention and control of mine water and the comprehensive utilization of water resources. In terms of the low surfate of Permian sandstone water in Fudong mining area, Anhui Huainan Coalfield, the hydrochemical characteristics, water-rock interaction and low sulfate genesis of sandstone water were studied through water sampling and test by multivariate statistical method, in combination with the hydrogeological condition analysis and the sulfur and oxygen isotope technology. The results show that the Permian sandstone water in the study area is weakly alkaline, with an average TDS of 1 842.35 mg/L, which belongs to brackish water. The main hydrochemical types are Cl-Na type, HCO₃-Na type and Cl·HCO₃-Na type. The average concentration of SO₄ ²⁻ is 37.48 mg/L, which is significantly lower than that Permian sandstone water in the eastern Panxie mining area of Huainan Coalfield and Huaibei Coalfield. The main water-rock interaction is evaporite (chloride and sulphate rock) and silicate minerals dissolution, with the alternating adsorption of cations. The Permian sandstone aquifer is a closed to semi-closed hydrogeological unit formed by the surrounding relative water-blocking boundary, which is cut by the boundary water-blocking fault. The aquifer is deeply buried, with weak water yield, poor hydrodynamic conditions and strong sulfate reduction environment, which is conducive to sulfate bacteria reduction. This results in obvious sulfate reduction, high δ³⁴S and low sulfate content in the sandstone water. The research results could provide a basis for identifying the water inrush source in mining areas with similar conditions.