Benthic fluxes of trace metals in the lagoon of Venice.

To assess the exchange and the mobility of trace metals between sediments and water and their geochemical behaviour, experiments were carried out within the sphere of the CORILA project for the safeguarding of the Venice lagoon. Trace element exchanges were examined for approximately 60 h at two sites in the central part of the Venice lagoon (Italy): the first one is located in front of the industrial area of Porto Marghera (Tresse) and the second one in front of Campalto, near the causeway (Campalto). The experiments were carried out using a benthic chamber monitored for pH, dissolved oxygen, salinity, and temperature. The temporal trend of metals inside the benthic chamber was examined in relation to changes of pH and dissolved oxygen. Diffusive metal fluxes were also assessed by determination of the vertical distribution of metals in pore water. Al, As, Cd, Cu, Fe, Mn, Mo, Sb, U, V and Zn were determined by ICP-SFMS. The metal concentrations for the lagoon samples were in agreement with expected values; the concentration ranges (min–max in ng/ml) were: Al 0.24–0.61, As 1.42–2.27, Cd 0.050–0.182, Cu 0.81– 2.46, Fe 0.25–1.66, Mn 11.59–31.66, Mo 6.50–10.62, Sb 0.139–0.516, U 1.7–3.3, V 0.69–3.21, Zn 5.20–21.51. Positive fluxes for the Tresse and Campalto experiments were determined for Cd (0.21 and 0.18 pmol/cm2/h), Zn (62 and 67 pmol/cm2/h), Cu (0.29 and 0.50 pmol/cm2/h) and Mn (19 and 12 pmol/cm2/h), while negative fluxes were determined for iron (-3.5 and -6.3 pmol/cm2/ h). Other elements showed differences in behaviour for the two experiments; the fluxes, for the Tresse and Campalto experiments, respectively, were 5.1 and -6.9 pmol/cm2/h for molybdenum, 0.25 and -0.18 pmol/cm2/h for arsenic and 1.3 and -8.4 pmol/cm2/h vanadium. Therefore, the different characteristics of the two areas affect the mobility of trace elements, which can derive from differences in the environmental characteristics of the two areas or seasonal difference in which the experiments were carried out.