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Mercury Concentrations in Sediment Profiles of a Degraded Tropical Coastal Environment

The present and past degrees of contamination by mercury were evaluated in the Guanabara Bay by the study of five sediment cores. The cores were sampled from the deltaic mouth of the São João de Merití River (cores SJ1, SJ2, SJ3, and SJ4), adjacent to a large mangrove stand and in a mangrove stand in Guapimirim (opposite side of the bay). The study area receives most of the domestic and industrial wastes from the metropolitan region of Rio de Janeiro city and, in consequence it becomes increasingly degraded. Among the sources of pollutants, mercury-spilling industries are inventoried in the São João de Merití River basin. Total mercury concentrations were measured in the sediments by cold vapour atomic absorption spectrometry. The fraction smaller than 63 µm, organic carbon and total sulphur concentrations were also measured. The results show that concentrations of mercury can reach high values (up to 37 µg g−1 at a depth of 34 cm in the core SJ4). The base of core SJ1 shows mercury concentrations that probably represent background values, as established elsewhere for the Guanabara Bay (51 ng g−1 at the depth of 32 cm). Among the studied cores, only core SJ1 seems to reach background levels.