Event-based diagnosis of flow connectivity and hydrogeochemical resilience in a mining-impacted karst aquifer

Karst aquifers provide essential freshwater worldwide, yet their hydrogeochemical functioning under perturbations remains difficult to resolve, especially using steady-state or time-averaged approaches. Here, we investigated hydrogeochemical dynamics in a karst aquifer affected by persistent acid mine drainage, using rainstorms as natural event-based diagnostics. Uneven rainfall over the four-month monitoring period reorganized the system into four hydrological stages. High-frequency monitoring of discharge and water temperature revealed heterogeneous hydrological responses across functionally distinct drainage outlets, attributable to contrasting connectivity to fast- and slow-flow domains. Despite a common meteoric origin, discharged waters exhibited outlet-specific isotopic variability, reflecting dynamic mixing among event water, epikarst storage, and goaf-affected deep reservoir. Hydrochemical time series showed redox-pH oscillations and episodic mobilization of reactive solutes including toxic metals. Stage-dependent concentration-discharge relationships captured shifting dominance among event-water dilution, reaction driven solute generation, and hydrologically induced colloidal transport. Event-integrated mass-volume analysis further showed first-flush metal pulses from epikarst, diluted transmission through conduits, and hydraulically forced, chemically selective release from goafs. Moreover, peak-envelop analysis revealed tight coupling between discharge and alkalinity flux at epikarst and conduit-dominated outlets but pronounced decoupling at goaf-affected outlets, demonstrating that carbonate buffering is a dynamic emergent response dictated by flow-path organization. These findings underscore the strong flow-storage-reaction interplay in disturbed karst aquifers and provide critical insights for evaluating and strengthening hydrogeochemical resilience.