ACCUMULATION OF ZN, PB, CU, CR AND NI IN SEDIMENTS BETWEEN ROOTS OF THE TAGUS ESTUARY SALT MARSHES, PORTUGAL

Sediment cores of 60 cm length were collected from two Tagus estuary s alt marshes. At each salt marsh, samples were taken from a non-vegetat ed zone and one from each of areas dominated by Halimione portulacoide s, Spartina maritima and Arthrocnemum fruticosum. Cores were sliced in situ and, at each sediment layer, redox potential and pH were measure d, and the organic matter content (LOI), grain size, and concentration s of Zn, Cu, Pb, Ni and Cr were determined. Sediment between roots and non-vegetated sediments of the same depth (5-15 cm) were extracted wi th several acid solutions, and the metal concentrations were compared. Metal residues were determined in roots of vascular plants. Sediment between roots was more oxidative, more acidic and richer in organic ma tter than non-vegetated sediment. Profiles of Zn, Pb and Cu concentrat ions in vegetated sediments differed from those recorded in non-vegeta ted areas: at subsurface layers (where root density is higher), Zn, Pb and Cu were enriched. The percentages of Zn, Pb and Cu removed by ace tic acid, nitric acid and DTPA extractions from sediment between roots were much lower than those from non-vegetated sediments, being prefer entially linked to the residual fraction. Chromium and Ni behave diffe rently no subsurface enrichment being found and their associations bei ng similar in the two types of sediment. Furthermore, Ni concentration s in roots were much lower than in bulk sediments, while levels of Zn and Fb were similar and Cu values higher. These results point out that plants are an important feature for metal accumulation in salt marshe s. (C) 1996 Academic Press Limited