Effect of mixing layer on the vertical distribution of radionuclide in lacustrine and coastal sediments

Vertical mixing of radionuclide in lacustrine and coastal sediments is described in terms of a non-steady state advection-diffusion model. The characteristic properties of the model are dependent only on the parameter G (G=D/LS, D: Mixing coefficient, L: Thickness of mixing layer, S: Accumulation rate). With greater G a material falling on the surface of sediment is mixed immediately to depth L. If G becomes smaller, the concentration profile of the original source is preserved in the mixing layer. The model applied to analysis of the vertical distribution of Cs-137 in the sediment collected from the south basin of Lake Biwa, Japan. The concentration profile of Cs-137 in the sediment was estimated by using the advection-diffusion equation for its annual fallout from 1954 to 1990. The observed profile of Cs-137 in the sediment was not compatible with the simulated profile. This difference indicate that Cs-137 may migrate in the sediment through the interstitial water. (author)