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Heavy metal assessment for surface sediments from three areas of the Portuguese continental shelf

Journal article
Authors Mario Mil-homens
Rodney Stevens
F. Abrantes
Ingemar Cato
Published in Continental Shelf Research
Volume 26
Issue 10
Pages 1184-1205
ISSN 0278-4343
Publication year 2006
Published at Department of Earth Sciences
Pages 1184-1205
Language en
Keywords portugal, continental shelves, sediment chemistry, geochemistry, environmental assessment, river-basin portugal, estuarine sediments, macrobenthic community, marine-sediments, douro estuary, trace-metals, suspended sediment, particulate metals, reference element, united-states
Subject categories Earth and Related Environmental Sciences


Twenty-nine surface samples from the Portuguese shelf, recovered offshore from the mouths of the Ave, Douro, Lis and Mira rivers, were analysed using ICP-OES for selected major and trace elements, after total dissolution. Organic carbon, carbonate content and grain size were also determined. Five evaluation tools have been applied in order to compare the three study areas and to evaluate sediment geochemistry and other sediment compositional variability in the acquired samples: (1) empirical methods based on comparison with standard reference criteria, e.g. the NOAA sediment quality guidelines, (2) normalisation ratios using a grain-size proxy element, (3) "Gradient Method", plotting contaminant vs. organic matter or Al, (4) definition of a regional geochemical baseline from a compiled database, and (5) enrichment factors. The evaluation of element and component associations indicates differences related both to the onshore drainage areas and to the environmental shelf setting. Despite the considerable variability in total metal contents indicated by our results, the sediment metal composition is largely of natural origin. Metal enrichments observed in the Mira area are associated with the drainage of mineralised areas rich in Cu, Pb, Zn, Fe and Mn. The near absence of human impact on shelf sediments, despite the vicinity to urban areas with high industrialisation levels, such as the Ave-Douro and Lis areas, is attributed to effective trapping in the estuaries and coastal zones, as well dilution with less contaminated sediments shelf sediments and removal with fine fractions due to grain-size sorting. The character of the contaminated sediments transported to these shelf areas is further influenced by grain-size sorting as well as by dilution with less contaminated marine sediments. The results obtained individually by the different methods complement each other and allow more specific interpretations. (c) 2006 Elsevier Ltd. All rights reserved.

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