Mesoscale variability of vertical profiles of environmental radionuclides (40K, 226Ra, 210Pb and 137Cs) in temperate forest soils in Germany

Vertical profiles of environmental radionuclides (K-40, Ra-226, Pb-210 and Cs-137) were investigated in several temperate forest soils in Germany to estimate heterogeneity of the soil horizon of interest. Absolute values of the activity concentrations of these nuclides varied to a large extent depending on the properties of individual forests as well as local geology. Several trends were generally observed independent of the location: (1) Activity concentrations of K-40 increased with increasing soil density reflecting that most potassium is contained in mineral components of the soil. The variations in the K-40 activity with depth may relate to biological activities in subsoil, such as root uptake of the nutrients. (2) Profiles of Ra-226 with depth could be an indicator for evaluating soil heterogeneity within a horizon of interest. They are also useful to estimate anthropogenic Pb-210 (Pb-210(exc)) derived from the atmosphere via dry fallout or wet deposition. In several forests, there appeared surface enrichment of Pb-210 down to a depth of approximately 10 cm, in which the Pb-210 would have come from the atmosphere by combustion of fossil fuels. (3) Depth profiles of Cs-137 were roughly divided into three types in which (a) the activity concentration decreased exponentially with soil depth, (b) small amounts of Cs-137 existed only in the upper-most layer of the soil (0-5 cm) and (c)Cs-137 disappeared at certain depths and appeared again at deeper portions of the soil. Consequences of bioturbation could be deduced from variability in vertical profiles of the environmental radionuclides. It is probable that a site showing an exponential decrease of the Cs-137 activity with depth and also having a surface enrichment of Pb-210 is not significantly influenced by bioturbation. (C) 2003 Elsevier B.V. All rights reserved.