New perspectives on fluxes of metal release from contaminated sediments

摘要

Contaminated sediment from a marine harbour was maintained for 16 months in two flumes that continuously re-circulated the overlying water and sustained a smooth flow at the sediment surface. The sediment was placed in one flume intact, while for the other it was pre-homogenised. Concentrations of trace metals in the pore water were measured at a vertical resolution of 2 mm using the technique of diffusive equilibration in thin-films (DET). Local mobilisation of metals was measured in 2-dimensional arrays by deploying DGT (diffusive gradients in thin- films) probes. A clearly defined vertical distribution of metals quickly established and was maintained even when fresh fauna and reactive organic material were added. The locaised mobilisation of metals was associated with recent diagenetic processes, rather than the depositional or disturbance history (e. g. , dredging). There were sharply-defined maxima of Cu and Cd within 2 mm of the sediment-water interface and substantial fluxes to the overlying water, consistent with their release from organic material as it is oxidised. There was a maximum concentration of Co about 5-8 mm lower than the Cu and Cd maxima, coincidental with Mn mobilisation. While there were clear Ni maxima, their location appeared to vary from being coincident with Co to a few mm above the Co maxima. Even though there were steep gradients of Fe, Mn, Ni and Co within 1 cm of the sediment-water interface, there was no clear evidence for any substantial metal fluxes across the interface. These data show that conventional measurements of metals in pore waters may markedly under- or overestimate release fluxes from sediments.