Nitrate contamination in a karst aquifer affected by acid mine drainage during a storm event: Insights from stable isotopes and hydrochemical tracers.

Hydroclimatic extremes increasingly threaten groundwater quality, but storm-driven nitrate (NO3-) dynamics within highly permeable karst networks remain poorly constrained. Integrating high-frequency monitoring, artificial tracer tests and multi-isotope tracking (δD/δ18O-H2O, δ15N/δ18O-NO3- and δ13C-DIC), this study deciphered the coupled evolution of hydrological pathways, water quality and nitrogen transformations during an extreme storm event (116 mm within 2.9 h) in an acid mine drainage (AMD)-affected karst watershed in China. During flood recession, a rapid shift in NO3- source dominance was observed. The contribution of manure and sewage declined from 49 ± 14% to 16 ± 11%, whereas the AMD contribution increased from 44 ± 15% to 80 ± 12%, collectively accounting for > 88% of the total NO3- load. This dynamic provenance shift induced severe nitrogen pollution. NO3- concentrations surged from 19.67 mg L-1 to 44.37 mg L-1, and ammonium (NH4+) increased from below the detection limit to 1.92 mg L-1, approaching or exceeding WHO and Chinese drinking water thresholds. Mechanistically, whilst elevated NH4+ and persistently oxic conduit conditions favoured localised nitrification, directly driving rapid NO3- accumulation, the apparent denitrification signal captured at the conduit outlet was attributed to the flushing of residual NO3- that had undergone prior denitrification within the epikarst hydrologic network. Furthermore, the storm mobilised 1.69 × 104 kg of NO3- from the watershed, underscoring the disproportionate effect of extreme precipitation on mass nutrient export. These findings elucidate how hydraulically connected anthropogenic point sources can rapidly amplify groundwater nitrogen hazards and highlight the critical need for event-based monitoring and targeted AMD mitigation in vulnerable karst terrains.