休廃止鉱山周辺の河川水質に対する坑廃水の影響評価

抄録

This study involved a mine in Akita Prefecture, Japan, which was closed in the 1970s and from which acid mine drainage (AMD) flows directly into a nearby river. Local residents use river water downstream of the mine, at a point-of-use, for agricultural purposes. In selecting factors to be used as a contribution index, the flow rate at multiple points including tributaries, and the concentrations of dissolved ions and heavy metals, were measured periodically and their trends evaluated. The river flow rate increases with rainfall, the AMD flow increases during the snowmelt season. These two periods and the river low-flow period were selected for study. Mine drainage is acidic (pH 3.0-3.6), and comprises Mg-SO₄ type water quality throughout the year, with higher concentrations of SO₄^2-, S-Fe, Cu, and Zn than those of river water. Downstream of the mine, where the AMD merges, the concentrations of heavy metals gradually decreased with increasing distance downstream, particularly during the low-flow period. The SO₄^2- fluxes did not change from above to below the AMD merge-point, and the heavy metals fluxes decreased after merging in the order S-Fe > Cu > Zn. As for the factors that decrease the concentration and fluxes, the possibility is considered that S-Fe has an effect of oxidizing and precipitating Fe²⁺ due to the increase in pH by the mixing with river water and dissolved oxygen. And Cu, Zn has an effect of adsorption accompanying the formation of surface complex of iron precipitates. The AMD contains high concentrations (320-400 mg L^-1) of SO₄^2- due to the acidification of pyrite, and no precipitation of heavy metals was observed in the stream. Since precipitation phenomena such as heavy metals are not recognized in SO₄^2-, an index indicating the degree of impact of AMD on the water quality of the point-of-use was calculated using SO₄^2- as a contribution ratio; CR. The CR of the AMD to water quality at the point-of-use is in the range of 2%-12%.