Distribution and fractionation of heavy metals in the surface sediments ofthe Ganges-Brahmaputra-Meghna river system in the Bengal basin

The lower Ganges-Brahmaputra-Meghna (G-B-M) drainage basin occupies the tot al Bengal Basin, which is one of the unique basins of the world because of its location and size, density of population, and catastrophic deposition o f sediments. The increased heavy metal concentration in the 63 m fraction o f surface sediments shows similarity among major segments of the G-B-M syst em in the basin, which reflects the homogenization of lithologic and chemic al diversity of the greater denudation regime by the river processes. The d ifferences in heavy metal concentation in the lower GB-IM system with that of its upper and middle counterpart is mainly related to the contrast betwe en Himalayan rivers and the other major South Asian rivers, and may be due to the geological differences of their denudation regime. Heavy metals in t he Lower G-B-M system have an affinity towards the clay fraction of the sed iments. The correlation matix of heavy metals in the lower Brahmaputra and Meghna suggests the importance of FeMn oxyhydroxides in their accumulations . Iron, Ti and Mn are higher in the Meghna main channel, Zn is higher in th e Meghna tributaries, and Cr is higher in both the Brahmaputra and Meghna c ompared to the value for standard shale. The enrichment factor is less than or equal to 1 for most of the metals except Mn which is relatively higher in the Meghna and lower Ganges main channels. The geoaccumulation index (I- geo) for most of the heavy metals lies below grade zero, suggesting unpollu ted sediment quality. The lower Ganges system shows relatively higher conce ntration in the nondetrital fraction of heavy metals, probably due to the p resence of petroleum refinery, industrial and mining effluents, and agricul tural runoff in the drainage basin. The relative uniformity in concentratio n of heavy metals in vertical profiles may be due to the uniformity in sedi ment grain size and catastrophic deposition of sediments, where the time pe riod represented by the vertical sediment column is not enough to reflect t he cultural accumulation of heavy metals. The Bengal basin thus represents a relatively-unperturbed alluvial basin with regards to heavy metal polluti on.