Geochemical and Isotopic Fingerprint-Based Identification of Sulfate Source Regional Characteristics and Evolution of Groundwater Impacted by Acid Mine Drainage (AMD) from a Nonferrous Metal Mining Area

Identifying hydrochemical processes in groundwater systems is a critical prerequisite for implementing pollution abatement in high-sulfur nonferrous metal mining areas. However, the sulfate sources and evolution mechanisms remain unclear due to complex hydrogeological conditions and intense anthropogenic disturbances. This study combines self-organizing maps (SOM), MixSIAR modeling, hydrochemical analysis, and multi-isotope tracers to clarify the sources and evolution of sulfate in groundwater from the Tianmashan sulfur–gold mining area. Results reveal a distinct depth-dependent pattern: shallow aquifers are dominated by sulfate derived from skarn sulfide oxidation (e.g., pyrite), enhanced by Fe 3+ transport from adjacent closed mines and mining-induced fractures. In contrast, deep aquifers, characterized by sluggish flow and limited connectivity, are governed by gypsum dissolution and cation exchange, with minor contributions from atmospheric precipitation via vertical shafts. A conceptual model highlights the roles of intermine hydraulic connectivity and redox zonation in controlling sulfate evolution. These findings provide a scientific basis for targeted acid mine drainage (AMD) management, advocating a “contain the shallow, utilize the deep” strategy for sustainable groundwater remediation.