Evaluation of metals availability in sediments of the Bertioga Channel (Santos Estuarine Complex - SP - Brazil): A tool for chemical pollution monitoring

Abstract

The Bertioga Channel (SE Brazilian coast) is part of the Santos Estuarine Complex, situated at the Baixada Santista. The Baixada Santista is a very important economic region of Brazil which, due to its intensive economic development, has been experiencing several interventions on its coastal environment. Both the largest harbor, located on the city of Santos, and the Cubatão industrial complex, which presents large chemical, petrochemical and steel industries, are found near the Bertioga Channel. As a consequence, this socially and environmentally relevant channel is exposed to various sources of pollution that can increase the content of contaminating metals in sediments. For these reasons, the main objective of this study was to analyze the availability of potentially toxic elements (Cd, Cr, Cu, Ni, Pb and Zn) in sediment cores sampled along the Bertioga Channel. In this study, the term availability refers to the possibility of metal remobilization in sediments. By applying the chemical procedure of sequential extraction of metals, it was determined the contents of the elements of interest associated with the sediments' main components, enabling the assessment of the mobile fraction and the behavior of the elements regarding their remobilization. Therefore, this study can serve as a management tool for monitoring chemical pollution in the region. In order to evaluate the chemical contamination in sediments, the elements' available levels (excepting Ni) were compared to Canadian quality guidelines (ISQG and PEL). Due to the absence of comparative values of Ni for this guide, this element was compared to values described on the CONAMA 454/2012 resolution for dredging material from the Brazilian Ministry of Environment. For the environment risk assessment, the Risk Assessment Code - RAC - was employed toward a better understanding of the risk concerning the elements' remobilization. The maximum available levels, estimated from the sum of all mobile fraction, were 22.06 mg kg-1 for Cr, 5.57 mg kg-1 for Cu, 11.53 mg kg-1 for Ni, 36.43 mg kg-1 for Pb and 57.53 mg kg-1 for Zn. Cd presented levels below its quantification limit for all samples. It was observed that, excepting Cd, there is potential for the remobilization of all those elements, even though they exist in very low concentration and are not associated, mainly, to the acid-soluble fraction (considered the most environmentally relevant one due to its mobility level). Changes on local conditions can induce the remobilization of those metals and, thus, allow them to become available to the environment. However, even when released possibly they should not be hazardous to the biota as a result of their low contents in the sediments. Despite the low concentrations of Cr, Cu, Ni, Pb and Zn and considering that the observations made in this study were based on their available contents, the results highlight an anthropic contribution of those metals for the region, the potential for their accumulation and showing the importance of such monitoring for the future protection of the local communities.