Geochemical Journal, Vol. 49 (No. 4), pp. e9-e14, 2015
Akihito Mochizuki*, Ko Hosoda and Masahito Sugiyama
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo, Kyoto 606-8501, Japan
(Received March 27, 2015; Accepted June 18, 2015; Online published July 13, 2015)
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.
Key words: uranium, river, Okinawa Island, guideline value, supply mechanism