TEXTURE AND MINERALOGY OF SEDIMENTS FROM THE GANGES-BRAHMAPUTRA-MEGHNA RIVER SYSTEM IN THE BENGAL BASIN, BANGLADESH AND THEIR ENVIRONMENTALIMPLICATIONS

The Bengal basin, Bangladesh, represents one of the most densely popul ated recent floodplains of the world. The sediment flux through the ba sin is one of the highest on a global scale. A significant portion of this sediment load find its sink in the basin itself because of its lo wer elevation and frequent flooding. The textural, mineralogical and c hemical nature of the sediments thus have an important bearing on the environmental quality of the basin as well as for the Bay of Bengal. T he sediment load of the Ganges-Brahmaputra-Meghna (GBM) river system c onsists exclusively of fine sand, silt and clay at their lower reaches within the Bengal basin, Bangladesh, and is deposited under uniformly fluctuating, unidirectional energy conditions. The sediments have a c lose similarity in grain size with the sediments of the surrounding fl oodplain. The mineral assemblage is dominated by quartz and feldspars. Illite and kaolinite are the major clay minerals, and occur in almost equal proportion in bed sediments. The heavy mineral assemblage is do minated by unstable minerals which are mostly derived from high-rank m etamorphic rocks. The characteristic smaller grain-size, i.e. having l arge surface-to-mass ratios, and the mineralogy of sediments suggests that they are susceptible to large chemical adsorptive reactions and t hus could serve as a potential trap for contaminants. However, the sed iments of the GEM river system in the Bengal basin, Bangladesh, shows lower concentration of Pb, Hg and As, and a marginally higher value fo r Cd as compared to that of standard shale. Considering population den sity and extensive agricultural practice in the basin, the sediments c an in the long run become contaminated.