ORIGIN AND FATE OF ARTIFICIAL RADIONUCLIDES IN THE SCHELDT ESTUARY

The distribution of artificial radionuclides (Co-60, Ru-106, Sb-125, Cs-134, Cs-137, Pu-238, Pu-239+240) in the suspended matter and sediments of the Scheldt estuary was investigated after the Chernobyl accident from 1986 to 1991 and was compared to the distribution of the same elements determined during a previous study in 1979-1984. The aim is to gain a better understanding of the geochemical behaviour of solids in the estuary. The longitudinal profiles indicate that Ru-106, Sb-125 and Pu-239+240 in the particles are of marine origin and that they are actively transported by tidal action upwards at least at 100 km from the mouth. On the other hand, Co-60 and Pu-238 are mainly discharged in the estuarine zone itself and a large fraction of these radionuclides are trapped in the upper zone of the estuary, in an area of intensive shoaling. The hypothesis that most of the continental suspended matter transported by the river is deposited in the area of Antwerp and does not reach the sea except during high floods is confirmed by the distribution of Cs-134 and Cs-137 resulting from the fall-out after the Chernobyl accident. Longitudinal distribution of dissolved Pu and Co, as well as scavenging experiments of Co and Cs, using radiotracers, have been performed over the entire salinity range of the estuary. This study shows scavenging of Pu in the anaerobic zone of the estuary. In contrast, Co is strongly and rapidly removed from solution when aerobic conditions are restored and Cs is described when the salinity increases.