ABSTRACT

Uranium concentrations in the rivers of Okinawa Island, Japan were determined in this study. Most rivers in the southwestern area of the island showed high U concentrations, the highest of which was 87 and 11 times the average U levels in Japanese rivers and worldwide, respectively. The U concentration in two rivers exceeded the Japanese guideline for public waters. Thermodynamic calculations revealed that the predominant U species in the rivers was Ca₂UO₂(CO₃)₃⁰ (aq) because of high concentrations of calcium and carbonate ions. However, the U concentrations were not explained by either the congruent dissolution of limestone or the input of seawater and/or sea salt aerosols. Therefore, selective dissolution of U from limestone as well as from other rocks and soils via the formation of calcium-uranyl-carbonate complexes may be a possible mechanism for the high riverine U concentrations.