URANIUM IN RIVER WATER

The concentration of dissolved uranium has been determined in over 250 river waters from the Orinoco, Amazon, and Ganges basins. Uranium con centrations are largely determined by dissolution of limestones, altho ugh weathering of black shales represents an important additional sour ce in some basins. In shield terrains the level of dissolved U is tran sport limited. Data from the Amazon indicate that floodplains do not r epresent a significant source of U in river waters. In addition, we ha ve determined dissolved U levels in forty rivers from around the world and coupled these data with previous measurements to obtain an estima te for the global flux of dissolved U to the oceans. The average conce ntration of U in river waters is 1.3 nmol/kg, but this value is biased by very high levels observed in the Ganges-Brahmaputra and Yellow riv ers. When these river systems are excluded from the budget, the global average falls to 0.78 nmol/kg. The global riverine U flux lies in the range of 3-6 x 10(7) mol/yr. The major uncertainty that restricts the accuracy of this estimate (and that of all other dissolved riverine f luxes) is the difficulty in obtaining representative samples from rive rs which show large seasonal and annual variations in runoff and disso lved load.