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

Uranium concentration and the U-234/U-238 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 U-238 varied drastically between 0.045 and 1.02 mBq/l (a factor of about 20), while the U-234 concentration was almost unchanged, ranging from 2.30 to 3.07 mBq/l. Resultant U-234/U-238 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 U-234/U-238 ratios was expected to exist as UO2(CO3)(2)(2-). By using the U isotopic and C-14 dating methods, the age of this water was roughly estimated to be in the range of 10(4)-10(5) years.