Iron (Fe) isotopes decipher Fe fate in acid mine drainage within a karst watershed

Abstract

The fate of Fe in acid mine drainage (AMD) profoundly affects water quality and pollutant migration. In karst watersheds, although it is acknowledged that carbonate buffering strongly controls the fate of Fe in AMD, the impacts of hydrochemical variations induced by karst hydrology and anthropogenic remediation measures remain poorly understood. This study employs Fe isotopes to elucidate the Fe sources, transformation pathways, and controlling factors in a karst watershed impacted by AMD. Results reveal that dissolved Fe(II) is primarily derived from the oxidative dissolution of coal-hosted pyrite. As AMD migrates underground, the enrichment of heavy Fe isotopes in shaft sediments (Δ⁵⁶Fe_{sediment-water} = +0.86‰) indicates additional isotope fractionation during subsequent Fe(II) oxidation-precipitation, as supported by mass-balance calculations. Furthermore, following anthropogenic source reduction measures (e.g., grout sealing), the δ⁵⁶Fe values of outfall waters exhibit a transient shift (<1 year) toward lighter signatures. Partial least squares structural equation modeling (PLS-SEM) confirms that this isotopic lightening is driven by temporary enhancement of carbonate buffering that subsequently elevates the Fe(II) oxidation ratio. At the surface, Δ⁵⁶Fe_{sediment–water} values are notably high at acidic outfalls (+0.85‰ to +1.49‰), yet decrease (+0.18‰ to +0.94‰) upon mixing with alkaline karst river water. These different isotope fractionations reflect the sensitivity of Fe(II) oxidation to pH, as alkalinity promotes a higher oxidation ratio shifting the fractionation mechanism from an oxidation-controlled to a precipitation-dominated regime. Collectively, these results establish a process-dependent framework for Fe isotope fractionation in AMD-impacted watershed and highlight the limitation of carbonate attenuation under both natural and engineered hydrological perturbations.