Natural and artificial radionuclides in forest and bog soils: Tracers for migration processes and soil development

Radionuclide distributions in undisturbed forest and bog soils, mostly situ ated in Saxony, Germany (Erzgebirge), were studied. Low concentrations of n aturally occurring U and Th decay series nuclides, including Pb-210, and ar tificial radioisotopes (Sb-125, (CS)-C-134, Cs-137, Am-241) were determined using low-level gamma -spectrometry. In addition, the activities of Pu-238 and Pu-239,Pu-240 were determined by radiochemical separation and alpha -s pectrometry, C-14 and excess Pb-210 dating methods were used to date the sa mpled bog profiles. The different radionuclides show characteristic depth d istributions in the forest and bog soil horizons, which were sub-sampled as thin slices. Sb-125, Am-241, Pu-238 and Pu-239,Pu-210 are strongly fixed i n soil organic matter. In spruce forest soils, the influence of soil horizo ns with distinct properties dominates the vertical time-dependent distribut ion. In ombrotrophic bogs, the peak positions correlate with the year of ma ximum input of each nuclide. The Sb, Am and Pu "time markers" and the C-14 and Pb-210 dating results correspond very well. Although Cs seems to be rel atively mobile in organic as well as mineral forest soil horizons, it is en riched in the organic material. In ombrotrophic bogs, Cs is very mobile in the peat deposit. In Sphagnum peat, Cs is translocated continuously towards the growing apices of the Sphagnum mosses, where it is accumulated.