METAL CONCENTRATIONS IN RECENTLY DEPOSITED SEDIMENTS OF COCHIN BACKWATERS, INDIA

The concentrations of a number of metals (Cd, Cu, Co, Cr, Fe, Mn, Ni, Pb and Zn) were determined in recently deposited sediments before, during and after flood conditions in the lower reaches of two large tropical rivers and in their adjoining estuarine beds of the Cochin backwater system, southwest India. Markedly elevated concentrations (ppm) of some metals, Cd (0.6-1.8), Cr (20-35), Ni (20-45) and Pb (12-18), were observed near the discharge outlets of industrial units. The results show enrichment of metals during post-flood periods in selected regions of the estuary, indicative of accumulation in sedimentary material. There were pronounced seasonal variations in the distribution of trace metals in the sediments which coincided with the flushing of bottom bed material to nearshore coastal plains. This study reveals that the transport of trace metals associated with bottom sediments occurs predominantly during the monsoonal months (flood conditions). Analyses of dissolved and particulate metals in overlying waters revealed that Cu and Cd concentrations consistently covaried, while Zn, Ni and Pb covaried on a seasonal basis with sediment-associated metals. There was no correlation between trace metal concentrations and organic carbon content of the sediments. © 1990.