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

Abstract

Radionuclide distributions in undisturbed forest and bog soils, mostly situated in Saxony, Germany (Erzgebirge), were studied. Low concentrations of naturally-occurring U and Th decay series nuclides, including 210Pb, and artificial radioisotopes (125Sb, 134Cs, 137Cs, 241Am) were determined using low-level γ-spectrometry. In addition, the activities of 238Pu and 239,240Pu were determined by radiochemical separation and α-spectrometry. 14C and excess 210Pb dating methods were used to date the sampled bog profiles. The different radionuclides show characteristic depth distributions in the forest and bog soil horizons, which were sub-sampled as thin slices. 125Sb, 241Am, 238Pu and 239,240Pu are strongly fixed in soil organic matter. In spruce forest soils, the influence of soil horizons with distinct properties dominates the vertical time-dependent distribution. In ombrotrophic bogs, the peak positions correlate with the year of maximum input of each nuclide. The Sb, Am and Pu “time markers“ and the 14C and 210Pb dating results correspond very well. Although Cs seems to be relatively mobile in organic as well as mineral forest soil horizons, it is enriched 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.