This paper addresses the mechanism of acid mine drainage generation in tailings from an abandoned mine site and predicts the evolution of zinc (Zn), copper (Cu), and iron (Fe) concentrations. Batch leaching experiments and sequential extractions were conducted to investigate the leaching behavior of these contaminants from the tailings and to understand their solid-phase partitioning. Acid-base accounting and principal component analysis (PCA) were used to confirm factors affecting Zn, Cu, and Fe leaching and acid formation based on the leaching experiments. There were strong positive correlations between Zn, Fe, or EC and SO4 2-, indicating that pyrite and sphalerite are the major minerals releasing Zn and Fe. This aligns with the PCA results. In the upper part of the tailings, the water-soluble and sulfide fractions of Zn, Cu, and Fe were almost flushed out, whereas they remained high in the deeper tailings. This implies that the tailings will likely continue to release these contaminants (Zn > Cu > Fe) for a long time unless remedial measures are taken.

Acid mine drainage (AMD), the very acidic and highly contaminated leachate generated in closed/abandoned mines, is commonly managed by neutralization to raise the pH and precipitate most of the heavy metals. Although effective, this approach does not generate any product of economic value, so it is very costly and unsustainable in the long-term. Unfortunately, there are currently no effective alternatives to neutralization, and one way to improve the sustainability of this process is to reduce the volume of AMD generated and/or the concentration of heavy metals. The tailings dam investigated in this study is located in northern Hokkaido, Japan. Detailed characterization of borehole core samples showed that even after almost 40 years of exposure to the environment, the tailings still contain pyrite (FeS2) and substantial amounts of copper (Cu) and zinc (Zn). Reactive-transport modeling using Visual MODFLOW predicted that AMD quality would likely continue to deteriorate with time and that treatment should be continued for at least 1,000 years. The model also predicted that a barrier with low permeability installed downstream of the tailings dam or ground sealing techniques for recharge reduction could lower the volume of AMD and concentration of Zn from the site.

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