Uranium in acidic mine drainage at the former Ogoya Mine in Ishikawa Prefecture of Japan

Uranium in acidic mine drainage from the former Ogoya Mine in Ishikawa Prefecture, Japan, and in neutral surface waters from its surrounding rivers was investigated from the viewpoint of radioactive disequilibrium in the uranium decay series. Water samples were periodically collected from the mine pithead and its surrounding rivers and their U isotopes (238U and 234U) were measured together with chemical components. The 238U concentrations in the water samples varied widely from 0.0036 to 0.78 mBq/L with a factor of about 200. High 238U concentrations were observed in the strongly acidic drainage (pH: around 3.5) from the pithead and the 234U/238U activity ratios showed significant values of as high as 10–15. By taking into account of the measurement of Th isotopes, it appeared that probable processes controlling the high 234U/238U activity ratios in acidic mine drainage were due to that the acidic water flowing from the mine pithead was formed only in the upper water layer of the pits and 234U was preferentially leached in the deeper underground water under the neutral and reducing conditions.