Abstract
Natural background concentrations of dissolved uranium in 194 Japanese rivers were measured by inductively coupled plasma mass spectrometry. The mean concentration of uranium in 194 rivers (249 samples) was 40.8 ng L-1. However, this value decreased to 30.3 ng L-1 when four rivers in Okinawa that showed very high concentrations were excluded. Both of these values were lower than the mean value of rivers worldwide. Although there was no correlation between the concentration of uranium and that of other chemical species in all samples collected, the concentration of total major cations and the alkalinity showed a good linear relationship with uranium concentration in several districts, including Kinki and Kyushu. The drainage area of the high uranium concentration rivers showed high uranium concentration in the earth’s surface based on a geochemical map of Japan (Imai et al., 2004), and was also in good accordance with the granite distribution area. These results suggested that uranium has entered the river water through weathering of granite. However, in some rivers, especially those in Okinawa, the uranium concentration in the earth’s surface was low despite the high value in river water. These areas contained widespread limestone; therefore, these findings suggest that carbonate ions selectively dissolved uranium from limestone by forming soluble complexes with uranyl ion.
