Geochemical Characterization of Suspended Sediments in the Nethravati Estuary, Southwest Coast of India: Insights to Redox Processes, Metal Sorption, and Pollution Aspect

Estuaries are the major conduits of material transfer from the continental river basins to the marine environments. In this study, suspended particulate matter (SPM) was collected from the estuarine region of Nethravati River, southwest India, over a one-year period on a seasonal basis to assess the metal fluxes to the eastern Arabian Sea. The focus was to understand the effect of secondary fluvial geochemical processes on metal chemistry as well as the role of Fe and Mn particles in heavy metal transport along a tropical micro-tidal estuary. The SPM content and its metal concentrations in the Nethravati estuary showed strong seasonal and spatial variability. The Fe-Mn oxyhydroxides formed the important carriers of metals in the estuarine region; however, it depended on the seasonality. The heavy metals displayed higher concentrations at a low Fe/Mn ratio in the estuary, which suggests a dominant role of Mn-oxyhydroxides as the metal carrier phase. Thus, the geochemi. cal assessment of SPM in the Nethravati estuary demonstrates redox cycling of metals coupled with adsorption-desorption of heavy metals onto the metal oxyhydroxides. Higher concentrations of heavy metals were observed in the estuarine bed sediments than in the suspended sediments. However, from the pollution point of view, the heavy metal concentrations were not enriched and were found to be within the limits of the National Oceanic and Atmospheric Administration (NOAA) guidelines.