The vertical distribution of chernobyl-derived radionuclides in a Baltic Sea sediment

The vertical distribution of radionuclides following the Chernobyl fall-out was investigated in the bottom sediment of Gälve Bay, southern Bothnian Sea. Sediment cores collected 2 years after the initial fall-out contained the highest radionuclide concentrations found in the Baltic Sea area, a reflection of the heavy fall-out over this region. The nuclide concentrations were almost constant down to about 8-cm sediment 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 community was scarce so that vertical mixing of pore water could not be explained by bioturbation. A combination of bioturbation, diffusive transport 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 bottom sediment.