The vertical activity distributions of fallout 238Pu, 239+240Pu, 241Am, 134Cs and 137Cs in a forest soil (Hapludult) were determined at several locations in a spruce stand separately according to their origin (global fallout or Chernobyl fallout). To determine the rate of migration of these radionuclides in each soil horizon, the observed depth profiles of the radionuclides were evaluated with a compartment model. In the top organic horizons (LOf1 and Of2), the migration rates for all radionuclides from both sources were above 0.5 cm per year. In the Oh horizon the migration rates observed for global fallout Pu, Am and Cs were similar (0.2-0.4 cm per year). Compared with Pu, however, the mobility of Am is slightly, but statistically significantly, enhanced. The highest rate in this layer was found for Chernobyl-derived radiocaesium (2 cm per year). In the layers of the mineral horizon (depth 0-2, 2-5 and 5-10 cm) the observed migration rates were very similar for global fallout Pu (0.08-0.7 cm per year) and Am (0.1-2 cm per year). In comparison, the migration rate of global fallout radiocaesium was about half in each layer. The highest rate was observed again for Chernobyl-derived radiocaesium (0.5-3 cm per year).
The association of 239+240Pu and241 Am from the global fallout with various soil components was investigated in six successive layers of an undisturbed grassland soil (Alfisol) from 0 to 30 cm by a sequential extraction procedure. In this way, the fractions readily exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter, and residual (mineral) were determined. The results revealed that these radionuclides are in most soil layers primarily attached to the soil organic matter (in general >57%) but to a considerable extent also attached to oxides and minerals. As a result, the readily exchangeable fraction of these actinides 30-40 years after their deposition is rather low (for Pu, <1%). Significant differences between the partitioning of Pu and Am are observed for all fractions, even though they are rather small for the bound to organic matter fraction [with the exception of the 5-10-cm layer in the Ah horizon, where much more Pu (67%) than Am (18%) is associated with organic matter]. In the fraction bound to oxides, more Am is found than Pu; in the residual fraction (minerals) of most soil layers, more Pu than Am is present. In all soil layers, the readily exchangeable fraction of Am is significantly higher than that of Pu (on average by a factor of 7). The results are also discussed in relation to the long-term vertical migration rates as determined recently for both radionuclides in the same soil and with respect to possible soil reclamation procedures.
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