Uranium isotopes and radium in the Bhagirathi Alaknanda river system - evidence for high uranium mobilization in the Himalaya

Abstract

Extensive measurements of dissolved ²³⁸U and ²²⁶Ra concentrations and ²³⁴U/²³⁸U activity ratio have been made on samples collected from the Bhagirathi, Alaknanda and their tributaries-the source waters of the Ganga. The objectives of this study are to determine (i) the sources of U and Ra to the Ganga river; (ii) the weathering rate of uranium in the Himalaya, and (iii) the role of Himalayan-Tibetan rivers on the marine budget of uranium. The dissolved ²³⁸U and ²²⁶Ra concentrations in the Ganga source waters are typically ~ 2-μgl^-1 and ~ 0.2 dpml^-1 respectively. The low ²²⁶Ra concentrations relative to ²³⁸U in these waters indicate that Ra is far less mobile. The Bhagirathi and Alaknanda weather uranium from their drainage basins at a rate of ~2 kgkm^-2yr^-1 comparable to that of the other Himalayan rivers like the Yamuna, Gandak and Ghaghara, but orders of magnitude higher than that derived for some of the world's major rivers (Amazon and Congo). These results suggest that large-scale mobilization of uranium in the Himalaya by rivers is ubiquitous. In the global context, the rivers draining the Himalayan-Tibetan region could be a major source of uranium to the oceans and that its supply via these rivers may have considerably influenced the marine budget of uranium.