43.-An environmental outlook of the hydro-geochemistry of the Brahmaputra river system

Abstract

The Brahmaputra is perhaps the most poorly studied large river of Asia. This is despite the fact that global estimate of river solute transport is incomplete without taking into account such a high sediment yielding river. Water, suspended and bed sediment samples collected from different locations over a stretch of about 750 km during monsoon and dry seasons were analysed for their elemental chemistry. The seasonal and spatial distribution of major elements over the basin was evaluated. The size distribution of the suspended sediments and the mineralogy of the suspended sediments were derived to find the percentages of different size fractions and that of detritals, carbonates and clay minerals. Average ion chemistry and sediment composition of the Brahmaputra river were compared with those of other major rivers of India. The ion chemistry of the Brahmaputra is characterised by high bi-carbonate content and source rock influence. Sediment chemistry reveals abundance of Si and Al, and amongst the heavy metals, dominance of Fe, Mn, Zn and Cu. Suspended sediments vary in size from fine sand to clay. Quartz, Feldspar and clay minerals constitute 90% of the suspended sediment mineralogy. Sediment chemistry reveals marked difference between the Brahmaputra and the Ganges, with respect to HCO3, SO, and Ca as well as Al, Fe and Zn.