This paper describes the geochemistry of acid mine drainage (AMD) and river water around the Date Mine in southwest Hokkaido, Japan. The mine is located in a sulfide deposit that contains pyrite and generates AMD, which is currently being neutralized; however, the introduction of a passive treatment system (PTS) is being considered. In preparation for the introduction of the PTS, this study aims to evaluate the environmental impact of untreated AMD flowing into rivers. The flow rate of the river depends on precipitation, and the ratio of AMD to that of the river was in the range 0.0014–0.018. In mixing experiments of AMD and river water based on measured results, the formation of iron and copper precipitates with increasing pH was observed, which suggests the possibility of self-purification phenomena of heavy metals in the river. The calculated oxidation rate of Fe(II) increased with pH. The surface complexation model in PHREEQC reproduced Cu concentration decrease due to coprecipitation with iron hydroxide. Assuming that untreated AMD is mixed with the river water, the Fe2+ concentration decreased from 0.19 mg L–1 to 0.003 mg L–1 in the river.
