Abstract
Sequential extraction was used to study the operationally determined chemical forms of Co, Cr, Cu, Ni, Pb and Zn and their spatial distribution in the sediments of the Tapi estuary. It was found that oxidizable binding fraction was the most important phase for binding of all metals followed by Fe-Mn oxide and carbonate fractions. Zn bound to the Fe-Mn oxide fraction had a significant relationship with reducible Mn and reducible Fe concentrations suggesting that Fe-Mn oxides may be the main carriers of Zn from the fluvial environment to the estuarine body. The Zn bound to the Fe-Mn oxides and Cu bound to the oxidizable fraction showed a general distinctive decrease from the middle of the estuary (ES-3) to sea mouth (ES-5) and from upstream ES-2 to the mouth of the estuary (ES-2). Zn, Pb, and Cr were not correlated with bottom layers and had a high enrichment factor (up to 60 times) representing the influence of anthropogenic activities and thus are the main precursors of metal pollution. The average Igeo of six metals in the sediments from the middle of the estuary is 3.67 and is larger than the estuary mouth (1.24) and seaside sampling locations (2.33). This suggests that the middle of the estuary is strongly contaminated with heavy metals. Interestingly higher enrichment in the sediments is not linearly related to toxicity response factor which may be due to metal immobilization.
