Chemistry of uranium, thorium, and radium isotopes in the Ganga-Brahmaputra river system: weathering processes and fluxes to the Bay of Bengal

Abstract

The most comprehensive data set on uranium, thorium, and radium isotopes in the Ganga-Brahmaputra, one of the major river systems of the world, is reported here. The dissolved ²³⁸U concentration in these river waters ranges between 0.44 and 8.32 μ/1, and it exhibits a positive correlation with major cations (Na + K + Mg + Ca). The ²³⁸U∑Cations ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional variations observed in the [²³⁴U/²³⁸U] activity ratio are consistent with the lithology of the drainage basins. The lowland tributaries (Chambal, Betwa, Ken, and Son), draining through the igneous and metamorphic rocks of the Deccan Traps and the Vindhyan-Bundelkhand Plateau, have [²³⁴U/²³⁸U] ratio in the range 1.16 to 1.84. This range is significantly higher than the near equilibrium ratio (~1.05) observed in the highland rivers which drain through sedimentary terrains. The dissolved ²²⁶Ra concentration ranges between 0.03 and 0.22 dpm/1. The striking feature of the radium isotopes data is the distinct difference in the ²²⁸Ra and ²²⁶Ra abundances between the highland and lowland rivers. The lowland waters are enriched in ²²⁸Ra while the highland waters contain more ²²⁶Ra. This difference mainly results from the differences in their weathering regimes. The discharge-weighted mean concentration of dissolved ²³⁸U in the Ganga (at Patna) and in the Brahmaputra (at Goalpara) are 1.81 and 0.63 μ/1, respectively. The Ganga-Brahmaputra river system constitutes the major source of dissolved uranium to the Bay of Bengal. These rivers transport annually about 1000 tons of uranium to their estuaries, about 10% of the estimated global supply of dissolved uranium to the oceans via rivers. The transport of uranium by these rivers far exceeds that of the Amazon, although their water discharge is only about 20% of that of the Amazon. The high intensity of weathering of uranium in the Ganga-Brahmapura River system can also be deduced from the [²³²Th/²³⁸U] and [²³⁰Th/²³⁸U] activity ratios measured in the suspended sediments. ²³⁰Th is enriched by about 19% in the suspended sediments relative to its parent ²³⁸U. The flux of excess ²³⁰Th supplied to the Bay of Bengal via these river sediments is 980 × 10¹² dpm/a, about six times more than its in situ production from seawater in the entire Bay of Bengal.