Distribuição espacial e avaliação de indicadores de contaminação de sedimentos por metais em um estuário tropical de macromaré, Baía de São Marcos, Norte do Brasil

Abstract

The estuaries are areas of great ecological importance for having the role of nursery, feeding and reproduction area for several aquatic species, in addition to housing large metropolitan and industrial. However, they are exposed to various types of contaminants, including metals because of anthropic action. In this context, the estuary of São Marcos Bay (SMB) in northern Brazil, which has an urban area with more than one million inhabitants and the port area with the largest cargo transport in the country (182 million tons in 2021). The present study evaluated the presence of metals (Cr, Cu, Ni, Pb and Zn) in the surface sediments of São Marcos Bay adjacent to São Luís Island and the quality of the sediments, their relationship with granulometry, total organic carbon (TOC) and in the classification of geochemical indices. Surface sediments were collected with the aid of a Gibbs dredger, in two sectors of the SMB, one adjacent to the port area and the other to the São Luís metropolitan região. In the laboratory, the samples were submitted to the granulometric analysis process by laser diffraction; determination of metals by Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and the total organic carbon content by means of the catalytic combustion method at high temperature. The results demonstrate a strong correlation between metal concentrations, granulometry and TOC. Since Cr and Zn showed high concentrations observed in sector A in areas with a predominance of fine sediments and high TOC content. In sector A the median was 16 μg/g (Cr), 4.6 μg/g (Cu), 6.5 μg/g (Ni), 4.6 μg/g (Pb) and 18 μg/g (Zn). While in sector B: 9 μg/g (Cr), 2.2 μg/g (Cu), 3.7 μg/g (Ni), 3 μg/g (Pb) and 7 μg/g (Zn). The evaluation of the concentration of metals in the BSM sediments showed that, according to the criteria adopted by CONAMA 454/12 and NOAA, the analyzed metals present concentrations below those that could cause some type of adverse effect on the biota, and therefore, affect the balance of the ecosystem. The geoaccumulation index (Igeo) shows variations from moderately to heavily polluted for the analyzed metals. However, these classifications are not supported by other methods. For example, the contamination factor and the enrichment factor appreciated that the area has low contamination and a low enrichment for metals. The use of local background values, observing that the use of reference values for the continental crust or for shales (PASS) tends to overestimate the infection indicators.