Modeling 137Cs migration processes in lake sediments.

Lake sediments can be considered as the "diary" of the lake: new layers are continuously deposited on top of the sediments. The migration behavior of radionuclides, particularly of (137)Cs, can be analyzed by modeling the input into and the vertical distribution within the sediment. For this purpose, a model consisting of sedimentation-diffusion equations was developed. Solved with a finite element method, this model can cover the time period from the nuclear weapons testing to the present. It takes into account fixation and redissolution, compaction of sediments, and the influence of competing ions on the retarded diffusion within the sediments. The outcome of the model is compared to water and sediment measurements from Lago Maggiore. The results of an optimization process can produce the sedimentation rate and (137)Cs distribution coefficients which determine the uptake of activity into the sediment and also the retarded diffusion within the sediment.