Seasonal and depth variability of reduced sulphur species and metal ions in mud-flat pore-waters of the Venice Lagoon

An in-situ sampler was employed to monitor seasonal changes in the composition of pore-waters at different depths in a mud-flat located in the central part of the Lagoon of Venice, in proximity of the industrial area of Porto Marghera. The use of voltammetric methods of analysis allowed the simultaneous determination of concentrations of reduced S(-II), thiosulphate, Fe(II) and Mn(LI) as well as zinc, lead, cadmium and copper ions. Other parameters such as the concentration of sulphate, Eh, pH and conductivity were also measured. Seasonal increases of S(-II) and Fe(II) concentrations in summer and autumn samples were observed, with the highest concentrations located at about 25 and 55 cm depth, respectively. An opposite seasonal dependence was observed for Mn(II) and S2O32- concentrations, which displayed higher values in winter. the highest concentrations were observed at a depth of 45 cm for Mn(II) and of 65 cm for thiosulphate. The concentrations of Cu, Cd, Pb and Zn ions showed a less evident seasonal variability. Top and bottom samples revealed high concentration values for zinc and copper ions, with the highest values in the 1,000-1,600 nM and 100-300 nM ranges, respectively. Cadmium and lead ions were present at lower concentrations, always less than 70 nM and 30 nM, respectively. (C) 1997 Elsevier Science B.V.