Potential ecological risk by metals in Sepetiba bay (SE Brazil): Exporting metals to the oceanic region

Abstract

Sepetiba Bay (SB) is one of the Brazilian coastal regions most impacted by anthropogenic activities due to increased population density, the installation of tourist and industrial mega-developments, and essential port, agricultural and fishing activities. Given the great concern about its environmental quality, several studies have focused on it, especially since the 1980s. This study aims to analyze the current ecological risk by potentially toxic elements (PTEs) in the bottom sediments of Sepetiba Bay (SB). The work integrates numerous variables (physicochemical, textural, and geochemical) analyzed at 73 stations in SB and the transition area to Ilha Grande Bay. The results show that the sediments in the eastern-inner region of Sepetiba Bay, along the Itacuruçá coast, and in some areas near the Marambaia Barrier Island (MBI) are severely polluted by Cd (4.47 mg kg−1) > Sn (7.31 mg kg−1) > Zn (824 mg kg−1) > Co (12.50 mg kg−1) > Pb (36.6 mg kg−1) > Cr (65.0 mg kg−1) > Fe (4.98 %) > Ni (21.4 mg kg−1) > V (66.0 mg kg−1), (maximum values), and highly polluted by U (2.8 mg kg−1) > Mn (857 mg kg−1) > Sb (0.32 mg kg−1) > Tl (0.63 ppm) > Hg (170 μg kg−1) > As (11.2 mg kg−1), maximum values. The pollution load index (PLI) and contamination degree (Cdeg) indexes also indicate considerable to very high contamination, indicating severe pollution in that zone. The Potential Ecological Risk Index (PERI) suggests high (160≤RI < 320) and very high (RI ≥ 320) ecological risk in the inner and middle areas of Sepetiba Bay by Cd > Hg > Cu > Co > Pb > Sn > Tl > Ni. Relatively low pH and Eh values were identified in the inner region of Sepetiba Bay, indicating eutrophication processes. A comparison of the concentration factors in the surface sediments of SB in 2009–2011 and 2022 suggests that there has been a general increase in contamination by metals, namely Fe, Mn, Pb, Co, Cr, Ni, U, V, and Hg. Although metals accumulate more pronounced in the inner and the most impacted marginal areas of Sepetiba Bay, their enrichment factors (EF) indicate an increase in contamination in the outer sector. Metals are being imported from the nearby oceanic region, where there is a dredging site, to the outer sector of Sepetiba Bay. Thus, the choice of dredged spoil disposal sites and how contaminated dredged material is deposited needs to be better addressed and is still a challenging task. This aspect often results in problems being transferred from one region to another. The results suggest that mitigation measures must be adopted to prevent the situation from worsening.