MOBILITY OF CONTAMINANTS IN RELATION TO DREDGING OPERATIONS IN A MESOTIDAL ESTUARY (TAGUS ESTUARY, PORTUGAL)

During the construction of a New Bridge over the Tagus estuary 2.5 mil lion tons of sediments were dredged, part of this quantity being conta minated material. The extension and intensity of the water turbidity a ssociated with dredging operating varied with the tidal conditions but the resuspended material collected near the bucket dredger did not pr esent a concentration increment in metals and PCB, when compared to th e estuarine suspended sediments. The calculated distribution coefficie nts suggest that some contaminants in solids neat the dredger were not in equilibrium with the water. A 24-hour laboratory experiment demons trated the complexity and quickness of anoxic sediments oxidation. In such a short period of time metals in the solids change their fraction ation. A second laboratory simulation showed that mussels accumulate m etals and PCB congeners when placed in turbid aerated water. (C) 1998 IAWQ. Published by Elsevier Science Ltd.