Distribution of Selected Heavy Metals in Sediments of the Águeda River (Central Portugal)

Abstract

The state of river water deterioration in the Águeda hydrographic basin, mostly in the western part, partly reflects the high rate of housing and industrial development in this area in recent years. The streams have acted as a sink for organic and inorganic loads from several origins: domestic and industrial sewage and agricultural waste. The contents of the heavy metals Cr, Cd, Ni, Cu, Pb, and Zn were studied by sequential chemical extraction of the principal geochemical phases of streambed sediments, in the <63 μm fraction, in order to assess their potential availability to the environment, investigating the metal concentrations, assemblages, and trends. The granulometric and mineralogical characteristics of this sediment fraction were also studied. This study revealed clear pollution by Cr, Cd, Ni, Cu, Zn, and Pb, as a result from both natural and anthropogenic origins. The chemical transport of metals appears to be essentially by the following geochemical phases, in decreasing order of significance: (exchangeable + carbonates) ≫ (organics) ≫ (Mn and Fe oxides and hydroxides). The (exchangeable + carbonate) phase plays an important part in the fixation of Cu, Ni, Zn, and Cd. The organic phase is important in the fixation of Cr, Pb, and also Cu and Ni. Analyzing the metal contents in the residual fraction, we conclude that Zn and Cd are the most mobile, and Cr and Pb are less mobile than Cu and Ni. The proximity of the pollutant sources and the timing of the influx of contaminated material control the distribution of the contaminant-related sediments locally and on the network scale.

Keywords:

• Hydrographic basin
• Fluvial sediments
• Sequential extraction
• Metal pollution

ACKNOWLEDGMENTS

This work was carried out with the scientific and laboratory support of the Postgraduate Research Institute for Sedimentology (University of Reading). The authors acknowledge this support with gratitude.

Notes

*This paper was presented in the third workshop organized by SedNet working group on “Monitoring Sediment Quality at River Basin Scale” held at IPIMAR, Lisbon, Portugal, January 29–31, 2004.