Chemical composition of sediments and suspended matter from the Cauvery and Brahmaputra rivers (India)

Abstract

Suspended matter, surface and core sediments from the Cauvery River and core sediments from the Brahmaputra River, India were analysed (15 elements) by the energy-dispersive X-ray fluorescence technique. The Cauvery River load indicates a relative predominance of solute vs. suspended transport. Cauvery River suspended matter is depleted in Al and Si, and enriched in Ca, Mn, Cr, Ni, Cu, Zn and Sr relative to the world's average river suspended matter. As the sampling web covered mainly the Cauvery Delta, the enhanced heavy metal concentrations in the suspended matter are explainable in terms of chemical–physical processes in the zone of mixing. Flocculation of the dissolved metal species causes their transformation in suspended state. Consequently, deltaic suspended matter is one–two orders of magnitude richer in heavy metals than the main river course background. Cauvery surface sediments reflect the average Indian river sediment composition. They have a homogenous composition along the river course and a heavy metal content lower than that of the Cauvery suspended matter. Sedimentation rates in the Cauvery and Brahmaputra are comparable with those published for other Indian rivers. Sediment flux to the Brahmaputra bed is higher than for the Cauvery. Mass accumulation rates for heavy metals in the sediments were determined. It is supposed that the large and synchronous fluctuations in the temporal distribution of the heavy metal mass accumulation rates have been caused by cyclic climatic changes over the Indian sub-continent. An additional supply of transition elements, due to urbanisation and industrial activities, could be assumed to cause the enrichment in the uppermost core layers.