Hydrochemical differentiation and heavy metal migration along karst fracture-conduit runoff paths in an acid mine drainage-affected watershed.

Abstract

Acid mine drainage (AMD) from karst sulfide mines threatens regional groundwater security, yet water-rock-pollutant coupling across heterogeneous runoff media remains poorly understood. Using hydrogeological, hydrochemical and isotopic data, we examined four water types-tailings-pond water (M-type), fracture-spring and conduit water (S-type, G-type) and surface river water (R-type)-along fracture-conduit runoff zones in an AMD-affected karst watershed during the August 2024 wet season. All waters were circumneutral to weakly alkaline, carbonate-neutralized AMD-affected waters, not acidic leachate. Isotopes (δ²H-H₂O, δ¹⁸O-H₂O, δ³⁴S-SO₄ and δ¹⁸O-SO₄) and hydrochemistry showed that M water was dominated by sulfuric-acid dissolution, S-type water by mixed carbonic-sulfuric-acid dissolution, R-type water by coexisting multi-acid dissolution and G-type water by carbonic-acid dissolution. S-type water showed stronger apparent bidirectional AMD buffering than R/G-type water, although dilution and immobilization could not be distinguished. Isotope patterns were consistent with preferential bacterial sulfate reduction (BSR) in fractures, one of several interpretations pending microbiological verification. Sulfide oxidation elevated dissolved metals in M-type water; dilution reduced them downstream in R-type water. S-type water likely attenuated metals through carbonate buffering and weakly reducing conditions, with a possible unverified BSR contribution; G-type water buffered mainly through carbonate dissolution. These findings support karst AMD prevention and control.