Coupled hydrological pulse and geochemical buffering controls on acid mine drainage evolution: Insights from typical abandoned pyrite mines in Southern Shaanxi, China.

Acid mine drainage (AMD) has emerged as a major global water-environment threat because legacy sulfide mining continuously releases acidity and toxic metals into surrounding water systems, causing long-term ecological degradation and threatening water-resource security. This study combined hydrochemical analysis, hydrogeochemical modeling, and a novel Carbonate-adjusted Modified Acid Mine Drainage Index (MAMDI_C) to investigate AMD evolution and iron mineral transformation processes across four representative mining areas (DBS, TLG, LHC, and JCG) in Southern Shaanxi, China. Results showed that mine adit drainage and waste-rock leachate were the primary pollution sources, characterized by extremely low pH and elevated concentrations of sulfate and dissolved metals (e.g., Fe, Mn, Zn). Hydrochemical facies progressively evolved from SO4-Ca-type waters in mining-affected zones to HCO3-rich waters downstream due to dilution and carbonate buffering. Precipitation significantly enhanced infiltration, discharge, and hydrological connectivity, promoting the flushing and downstream transport of stored oxidation products, particularly in the TLG mining area. The MAMDI_C index effectively differentiated high-risk adit drainage, transitional reaches, and relatively stable upstream waters by integrating pollution intensity with carbonate-buffering capacity. Hydrogeochemical modeling revealed a pH-dependent sequence of Fe transformation from dissolved Fe2+ and sulfate-complexed species under acidic conditions to secondary minerals such as jarosite-K, goethite, and hematite during downstream neutralization. These mineral transformations played a dominant role in controlling trace metal mobility and long-term immobilization. This study provides a process-sensitive framework for understanding AMD evolution, assessing AMD risks, and developing targeted remediation strategies in carbonate-buffered mining watersheds.