THE VERTICAL-DISTRIBUTION OF CHERNOBYL-DERIVED RADIONUCLIDES IN A BALTIC SEA SEDIMENT

The vertical distribution of radionuclides following the Chernobyl fal l-out was investigated in the bottom sediment of Galve Bay, southern B othnian Sea. Sediment cores collected 2 years after the initial fall-o ut contained the highest radionuclide concentrations found in the Balt ic Sea al ea, a reflection of the heavy fall-out over this region. The nuclide concentrations were almost constant clown to about 8-cm sedim ent depth. The calculated diffusivity through the porous sediment was compared with the measured radionuclide concentrations. The calculated values were much below those observed, and diffusion alone could not account for the distribution pattern in the sediment. A macrobenthic c ommunity was scarce so that vertical mixing of pore water could not be explained by bioturbation. A combination of bioturbation, diffusive t ransport and vertical displacement of pore water, caused by changes in density of the overlying water column, is a possible mechanism which could explain the rapid penetration of fall-out nuclides in the soft b ottom sediment. Copyright (C) 1996 Published by Elsevier Science Limit ed