Distribuição dos metais pesados em sedimentos da Lagoa Rodrigo de Freitas

Abstract

This work discusses the variation of heavy metals concentrations in the sediments of a hypertrophic coastal lagoon located in the south zone of Rio de Janeiro City. The Rodrigo de Freitas Lagoon watershed includes a highly urbanized area without industrial activities. The only connection with the sea is a channel constantly dredged due to siltation. Six sediment cores were collected in the lagoon between May and July of 2003. The concentration of the heavy metals (Fe, Al, Mn, Hg, Zn, Pb, Cu, Cd, Cr e Ni) in the sediment was analysed. With the objective of better understanding the behaviour of the metals along the sedimentary column, othercomplementary parameters were analysed, such as: organic matter and fine fraction contents, concentration of carbon, nitrogen, phosphorous and sulphur in the sediment; and determination of the sedimentation rate through the 210Pb method. The sedimentation rate of a sediment core from the lagoon central area was estimated as 0.75 cm year-1, indicating that a 60 cm layer was deposited since approximately 1923. The alterations in the environmental quality of the Lagoon caused by anthropogenic activities were clearly evidenced. The results showed significant changes in the perceptuals of fine fraction and organic matter contents before and after the period of the channel opening. The results showed a gradual increment in the concentration of carbon, phosphorus and sulphur above 60 cm depth, indicating an increase in the sewage input. The analysis of the cores showed a strong increase in the concentrations of Hg, Cu, Pb, Zn and Cd, from the layer corresponding to period of channel opening, while a decrease of Mn and Ni concentrations was observed. These results may indicate a shift in the oxygenation conditions of the lagoon waters in that time. In the more recent sedimentary layers the influence of the urbanization process was strongly evidenced.