Gecchimica ef Ccmmxhimifa Acta Vol. 54, pp. 1387-1396 Copyrisht @I990 Fqamon ResspIc. Printed in U.S.A. ~16.7037/~/$3.~ t.00 Chemistry of uranium, thorium, and radium isotopes in the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal M. M. SARIN, ’ S. KRISHNASWAMI,’ 3. L. K. SOMAYAJULU,’ and W. S. MOORER ‘Physical Research Laboratory, Ahmedabad 38~9, India ?Jniversity of South Carolina, Columbia, SC 29208, USA {Received September 26, 1989; arcepted in revised~orm February 16, 1990) 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 pg/l, and it exhibits a positive correlation with major cations(Na + K + Mg + Ca). The 238U/ ZCations ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional vari- ations 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 ( - I .05 ) observed in the highland rivers which dram through sedimentary terrains. The dissolved 226Ra concentmtion ranges between 0.03 and 0.22 dpm/l. The striking feature of the radium isotopes data is the distinct difference in the ‘**Ra 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 1.8 1 and 0.63 pg/l, 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 [2xTh/238U] activity ratios measured in the suspended sediments. ‘@Th is enriched by about 19% in the suspended sediments relative to its parent 238U.The flux of excess 2qh supplied to the Bay of Bengal via these river sediments is 980 X 10 I2 dpm /a, about six times more than its in situ production from seawater in the entire Bay of Bengal. INTRODUCTION RIVERS ARE THE MAJOR pathways for the transport of weath- ered materials from the land to the oceans. The geochemical studies on river waters provide an insight into the weathering processes that control the distribution of elements in dissolved and particulate phases and their fluxes to the estuaries. Con- sequently, a detailed and systematic study of the major ion composition and radionuclide abundances in the dissolved and suspended phases of the Ganga-Br~maputm rivers was undertaken during 1982-83 (SARIN, 1983; SARIN and KRISHNASWAMI, 1984; SARIN et al., 1989). We present here data on the dissolved uranium and radium isotopes in these river waters and on some of the U-Th decay series nuclides in the suspended sediments. Prior to this work, BHAT and KRI~HNASWAMI ( 1969) have reported the downstream and seasonal variations in the concentrations of dissolved uranium and radium isotopes in the Ganga main stream. The primary objective of our study is to identify the parameters controlling the supply of uranium and radium isotopes to these river waters during weathering, and to determine their fluxes to the estuaries. The results presented in this paper represent the most ~mp~hensive data set on uranium and radium isotopes yet published for a major river system. EXPERIMENTAL TECHNIQUES Water samples for this study were collected from several locations along the Ganga and Brahmaputra main streams and horn their major tributaries (Fig. 1). The Ganga and the Brahmaputra are the two largest rivers of India. Gn a global scale, this river system ranks fourth in terms of water discharge and first in terms of sediment transport to the oceans (HOLEMAN, 1968; MILUMAN and MEADE, 1983 f . Together these rivers transport, annually, about 10 I’ liters of water and about 2 X 10” g of suspended sediments to the Bay of Bengal. The geologic and hvdrokxic regimes of the Ganaa and Brah- map&a river basins have-been &sc&ed in detail bySARIN et at. ( 1989). The Ganga and its tributaries were sampled during March, September, and December 1982 and November 1983, to determine temporal variations in radionuclide abundances. Samples from the Brahmaputra and Manas were collected during April and December 1982. Large volume samples for the U-Th series nuclides were collected in plastic containers from the mid-channel of the rivers. The sus- pended sediments were allowed to settle for lo- 12 h and water was then filtered through 3 pm Gelman cartridge filters for the analysis of dissolved uranium and radium isotopes. Uranium isotopes were extracted kom the water (-20 liters) by coptipitation with Fe(OH), in presence of 23?J yield tracer. Radium isotopes from the filtered water were absorbed onto MnOr impregnated acrylic fibers ( MICHEL et al., i981; MOORE, 1984). The Fe(OHb precipitates and MnQ fibers were processed in the laboratory following the procedures of SARIN ( 1983) and MOORE ( 1984) for the destination of uranium and radium isotopes. 1387