Multi-layer aquifer hydrogeological responses to deep coal mining and their controlling mechanisms: A hydrochemical and multi-isotope coupling study

Abstract

Study region The Dahaize Coal Mine lies on the southern margin of the Mu Us Sandy Land, a key part of the large coal energy base in northern Shaanxi, China. Study focus Deep coal mining severely disturbs regional groundwater stability, yet the long-term and multi-scale responses of hydrogeological functions of multi-layer aquifers in mining areas remain poorly understood. This study combines hydrochemical approaches with multiple isotopic tracers (δ 2 H, δ 18 O, δ 34 S, δ 18 O sulfate , and 222 Rn) to elucidate spatiotemporal evolution features and dominant driving mechanisms of multi-layer aquifer hydrogeological functions across the whole coal mining cycle. New hydrological insights for the region Phreatic water (PW) and Anding Formation confined water (ADCW) maintained stable hydrogeological functions. PW was mainly affected by sulfate leached from coal gangue under rainfall infiltration, while ADCW was slightly improved owing to weakened vertical leakage recharge. By contrast, Zhiluo Formation confined water (ZLCW) suffered severe functional degradation driven by expanding water-conducting fracture zones (WCFZs). WCFZs lowered hydraulic gradients, slowed groundwater flow and promoted evaporite dissolution. As preferential flow pathways and geochemical reaction interfaces, WCFZs strengthened hydraulic connectivity between ZLCW and mine water (MW), accelerated cation exchange, carbonate dissolution and radon emission, and caused simultaneous enrichment of SO 4 2 - , δ 18 O, δ 34 S, and 222 Rn in the two aquifers. This study deepens the understanding of the driving mechanisms for the hydrogeological functional evolution of aquifers in mining areas.