Histórico das fontes da matéria orgânica através da determinação de hidrocarbonetos e esteróis em testemunho sedimentar do manguezal da lagoa de Itaipu, Niterói – RJ

Abstract

On the coast of the state of Rio de Janeiro, mangrove forests are found associated with major coastal systems, which have been influenced by diverse human activities. Mangroves are considered deposition environments where there is naturally high organic matter production, and conditions favor its preservation regardless of its allochthonous and/or autochthonous origin. The pressures of urban occupation combined with rapid economic development are responsible for potential alterations in the flows of natural or non-natural organic material in this environment. Therefore, monitoring the quality of this environment is necessary given its significant ecological and socioeconomic importance. One way to determine the presence of pollution in these environments is through geochemical organic markers, such as hydrocarbons and fecal sterols. This study aims to evaluate the origin of organic matter through the distribution of molecular markers, polycyclic aromatic hydrocarbons (PAHs), and fecal sterols (EFs), as well as the elemental composition of carbon (Corg) and total nitrogen (NT) in a sediment core from the mangrove adjacent to Itaipu lagoon. One sediment core was collected from the mangrove of Itaipu lagoon (Niterói, RJ) for the determination of elemental markers (carbon and nitrogen) and molecular markers (PAHs and sterols). Another core was used for determining the fines content and dating using 210Pb gamma spectrometry. Identification and quantification were performed using gas chromatography coupled with mass spectrometry (EPA 8270D). The concentrations of Corg and NT were determined using dry combustion in an elemental analyzer. The use of the CRS model resulted in an average sedimentation rate of 0.55 cm per year, indicating an increase in sedimentation rate in the 1980s and late 2000s. The Corg concentration ranged from 1.4% to 14.5%, and NT from 0.15% to 1.05% along the sediment core, with a median fines content of 77%. The C/N ratio showed variability along the core, with values oscillating between 6 and 20. The results indicate that the sediment in the Itaipu mangrove is composed of organic matter of terrestrial and marine origin, suggesting that its distribution may result from the hydrodynamics of the Itaipu-Piratininga lagoon system, river and stream runoff in the region, and mangrove forest production. Analyzing the diagnostic ratios of PAHs, a predominance of combustion sources and/or mixed sources is observed for this system. Fecal sterols, on the other hand, showed concentrations ranging from 0.22 to 6.30 μg.g-1 (median of 1.51 μg.g-1). There is evidence of low to moderate fecal contamination, probably due to the significant influx of domestic effluents from the expansion of urban density in the coastal region, where sewage treatment infrastructure does not reach informal settlements. Additionally, the profile demonstrated an increase in anthropogenic organic matter sources from the 1970s, the year of the construction of the Rio-Niterói bridge. Thus, a more significant influx of these compounds was observed in recent decades, reflecting the increase in PAH concentrations and the presence of compounds from domestic sewage.