Impact of effluents from a closed mine on the surrounding water resources: Groundwater contamination of the historical abandoned mine site in Japan

The extraction of mineral deposits is often associated with the occurrence of acid mine drainage (AMD), which can persist even after mine closure due to remaining sulfide minerals. This study investigates a 200-year-old abandoned mine and its impacts on nearby water resources. The study area is well known for Kuroko ore deposits located upstream of spring and river water resources. To elucidate the impacts of the abandoned mine site, mine water and spring and river water samples were collected, and their geochemical properties were monitored between 2021 and 2022. Groundwater, seepage, and surface water at the mine site showed AMD characteristics with Ca²⁺–SO₄²⁻/Mg²⁺–SO₄²⁻ type. AMD-affected mine water showed a low pH range of 3.40–4.84, with elevated SO₄²⁻ of up to 326 mg/L. At the downstream of the mine site, one of the groundwater samples showed pH of 3.55 and average concentrations of 5.03 mg/L of Al, 2.06 mg/L of Cu, 2.06 mg/L of Fe, 0.42 mg/L of Pb, and 8.04 mg/L of Zn, inferring the contaminant transport. Saturation indices of the mine water also indicated that the solubility controlling phases, anglesite, gibbsite, ferrihydrite, and jarosite influence the concentrations of Al, Fe, and Pb at the mine site. Meanwhile, spring and river water samples showed Ca²⁺–HCO₃⁻, Ca²⁺–SO₄²⁻, and Na⁺–K⁺–HCO₃⁻ type with a circumneutral pH range of 5.59–8.02 and they were unaffected by AMD. The principal component analysis (PCA) of the spring and river water samples also showed higher loadings for Ca, Mg, NO₃⁻, and Cl⁻ reflecting the abundance of carbonate and evaporite minerals while the mine water and groundwater downstream showed higher loadings of Cl⁻, Fe, SO₄²⁻, and Zn. The results suggest that the past mining activities only influenced the mine site and groundwater downstream. Consequently, the fate and migration of contaminants in the downstream of the mine site should be evaluated in the near future.