Evaluation of urban pollution in a tropical lacustrine ecosystem by using n-alkanes and sterols as biomarkers.

The Jacarepaguá Lagoon System (JLS) receives industrial and domestic waste in an urban area with high population density and intense economic activity. The hydrography of the lagoons favours the sedimentation of particulate material transferred from the drainage basin. Water engineering, such as channel dredging and subsea outfall, did not satisfactorily mitigate pollution effects. Therefore, the environment is highly eutrophic, presents frequent blooms of algae and generates high emissions of greenhouse gases. There is no record in the literature on the analysis of organic compounds in the water compartment. The present work applies sterols as biomarkers to quantify the degree of pollution caused by biogenic compounds in riverine and lacustrine water of the JLS. n-Alkanes were applied to estimate the fractions of petrogenic contaminants. The sums of n-alkanes and sterols analysed had average concentrations of 21 ± 20 μg L and 10 ± 8 μg L respectively, in the river samples and 235 ± 156 μg L and 30 ± 28 μg L, respectively, in the lagoon samples. The work also showed that the organic compounds inside the lagoons are evenly distributed, and approximately 7% of them are transferred to the marine ecosystem. Biogenic biomarkers and the absolute concentrations of sterols showed that sewage contaminants transferred by the rivers are partially decomposed in the lagoons. The correlations between indices and physicochemical parameters indicated that the degradation of organic compounds in the lagoons occurs mainly in the sediment compartment under anoxic conditions. The indices for sewage indicate that the ecosystem has exceeded its carrying capacity. The indices based on n-alkanes reported strong contamination at all sampling stations and inferred that 75-100% of these compounds were derived from petrogenic sources. These indices did not show any difference between rivers and the lagoon, which demonstrates the resilience of these compounds in the ecosystem.