Anomalously high 234U/238U activity ratios of Tatsunokuchi hot spring waters, Ishikawa Prefecture, Japan

Uranium concentration and the 234U/238U activity ratio have been measured for the Tatsunokuchi hot spring waters of Ishikawa Prefecture in Japan, collected periodically over a long period (1977-2000). The concentration of 238U varied drastically between 0.045 and 1.02 mBq/l (a factor of about 20), while the 234U concentration was almost unchanged, ranging from 2.30 to 3.07 mBq/l. Resultant 234U/238U activity ratios showed a wide range from 2.7 to 51. Equilibrium calculation by using the geochemical code showed that U for one end-member representing low uranium contents and very high 234U/238U ratios was expected to exist as UO2(CO3)22-. By using the U isotopic and 14C dating methods, the age of this water was roughly estimated to be in the range of 104-105 years.