Avaliação do histórico de impactos antrópicos na bacia de drenagem da lagoa de Parangaba (Fortaleza – Ce) com base nos estoques de metais pesados e compostos orgânicos em sedimento lacustre

Abstract

Lagoons are ecosystems of great importance in the urban complex and with the accelerated and disorderly process of urban expansion and industrial growth, these areas are more susceptible to the use and predatory occupation. The Parangaba lagoon is the largest lagoon in volume of water inserted in the urban mesh of Fortaleza and has suffered during the last decades several environmental impacts. In this context, this work aims to evaluate if the historical distribution of heavy metals and geochemical markers in the Parangaba lagoon in Fortaleza-CE is a function of the pattern of use and occupation of the soil of its watershed. To this purpose, a sedimentary record of 60 cm was taken from the central and deepest point of the Parangaba lagoon and fractionated every 2 cm. Each of these fractions was divided into aliquots for analysis of TOCs, radionuclides (210Pb e 226Ra) for dating, trace elements analysis, geochemical markers of PAHs, aliphatic hydrocarbons and sterols. The geochronology was applied only to the first seven fractions of sedimentary record and they presented dates ranging from 1875 to 1995. For the 10 trace elements considered in this study only Ni and Cu presented values higher than the TEL established by the USEPA, and the FE shows that only to the metals Hg, Ni and Zn the sediments can be considered slightly enriched to the samples referring to the last 100 years. Σ16PAHs, prioritized by USEPA, did not exceed the TEL and PEL effect levels at any time. Twelve diagnostic reasons were used and the relationship between them showed that there are mixed sources for the first five fractions, but the pyrogenic sources are preponderant. The diagnostic reasons used to indicate n-alkane sources suggest the predominance of biogenic contribution (higher plants) in all fractions evaluated. Regarding the fecal marker coprostanol, the diagnostic reasons and relations between them propose a strong tendency of anthropic sources for the samples of the first five layers of the profile. All groups of analyzed compounds are present in low concentrations when compared to other lakes and rivers in Brazil and in the world, allowing to classify the sediments of the Parangaba lagoon as an environment with a low level of pollution. However, they show an increase in the concentrations of the more superficial layers, this contribution was associated to the anthropic sources, due to the way of use and occupation of its watershed and surroundings in the sessions related to the last 100 years. The obtained results provide a better understanding of the lagoon's environmental quality and can be used as a reference for decision-making in future cases of increases of anthropic activities.