No.6 - Geochemical characterisation of sediments from marginal environments of Lima Estuary (NW Portugal)

Raquel Cardoso, Maria de Fátima Araújo, Maria da Conceição Freitas, Francisco Fatela

Abstract


Salt marshes are transitional zones between terrestrial and aquatic environments which occur along the estuarine upper intertidal zone, usually over a mud substrate and covered by halophyte vegetation. Characterized by low hydrodynamic conditions, salt marshes show a dynamic balance between erosion, transport and deposition of sediments, along with vegetation. Here, heavy metals play a special role, as their presence may affect this sensitive equilibrium. Lima estuary has several salt marsh zones which are showing reasonable environmental degradation, since the estuary surrounding area is quite populated and industrialised. To evaluate the importance of Lima estuary salt marsh environments as traps for trace metals and to establish its environmental assessment, a total of 28 surface sediment samples were collected in April of 2006 along 3 transects located in salt marshes of Lima lower estuary. Sedimentological and geochemical analysis were performed in order to determine grain size distribution, organic matter content and chemical composition for 21 elements (Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Y, Zr, Nb and Pb) by energy-dispersive X-ray fluorescence spectrometry (EDXRF). The trend of metal contamination was evaluated, taking into account the location of samples along salt marsh transects, as well as the location of these transects in the lower estuary. The results were then compared to reference values for non-polluted soils, such as Soil (Ure & Berrow, 1982), enrichment factors were calculated and inter-parameter correlations were defined. From this, it can be said that the salt marshes of Lima estuary show increasing metal concentrations from low marsh to high marsh areas, and also downstream heavy metal enrichment.


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Keywords


Tidal flat, Salt marsh, Estuary, Geochemistry, EDXRF, Environmental assessment

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