Trace metal partitioning in a shallow estuary, Muthupet, India

Abstract

Dissolved trace metal concentrations of Fe, Cd, Cu and Zn were analysed in the Muthupet estuary, a well-mixed micro-tidal estuary in India during the post-monsoon and pre-monsoon in 2014. There was a significant variation in the salinity and suspended sediment concentration over the seasonal scale with peak salinity occurring at the downstream mouth during the post-monsoon and at intermediate locations during the pre-monsoon. Strong seasonal variations were observed in the concentration of trace metals. The concentration of Fe, Cd and Zn exceeded the USEPA standards, while that of Cu was within the permissible limits. Fe, Cu and Zn showed non-conservative behavior with peak concentrations at mid-estuarine reaches, irrespective of the location of salinity maxima, while the peak concentration of Cd occurred at high salinity reaches. The complexation of Cd as chlorocomplexes enhanced the mobilization of Cd, resulting in a peak value at high salinity reaches. The study indicated that the estuary is influenced by oceanic water and the anthropogenic input is secondary. Resuspension of bed sediments is identified as the major process controlling the trace metal concentration. Partition coefficient (Kd) of Fe, Cu and Zn was observed to be an increasing function of salinity, of which, the effect on Fe was much significant. The coagulation of suspended sediments at high salinity reaches enhanced the removal of Fe as Fe oxyhydroxides.