Hydrochemical Evolution of Karst Groundwater and Mining-Induced Activation Effects in a Coal Mining Area: A Case Study from the Tengxian Coalfield, China

The prevention, control, and utilization of karst groundwater are prevalent issues in coal mining operations across North China. Karst water serves as both a potential hazard to safe coal production and an essential resource for domestic and agricultural use. However, decades of intensive, large-scale mining have significantly disrupted subsurface karst systems and intensified hydrogeochemical processes, particularly water–rock interactions. This study focuses on the Tengxian Coalfield in North China, where Ordovician and 14th limestone aquifers were sampled, and a multiwell drainage experiment was conducted in the Ordovician aquifer. Through a combination of multimethod approaches, the hydrochemical characteristics of karst groundwater under natural conditions and mining disturbances were analyzed. The evolutionary patterns and controlling factors of major ion concentrations in karst water were revealed. Results indicate that SO₄^2– and Na⁺ are the dominant anion and cation, respectively, with Cl–Ca identified as the primary water type, alongside a subset displaying SO₄–Na·Ca characteristics. Major ion sources include rock weathering and evaporative concentration, while mining-induced chemical evolution is primarily driven by cation exchange and anthropogenic activities. Under natural conditions, ion concentrations in the Ordovician limestone water remained relatively stable, indicating limited groundwater circulation and weak runoff. During water inrush events, enhanced water–rock interactions caused a transition in hydrochemical composition from low Na⁺(K⁺) and high Ca²⁺(Mg²⁺) to high Na⁺(K⁺) and low Ca²⁺(Mg²⁺). This research provides insights into the hydrochemical behavior and flow field evolution of karst groundwater in mining areas and offers a scientific basis for water hazard control and resource utilization under similar geological conditions.