Forms of fallout 137Cs and 239+240Pu in successive horizons of a forest soil

In order to understand better the mobility of Pu and Cs in a spruce forest soil, the forms of these radionuclides were examined by sequential extraction analysis (modified Tessier procedure). In this way the fractions ‘readily exchangeable’ (fraction I), ‘bound to sesquioxides’ (fraction II), ‘bound to organic matter’ (fraction III), ‘persistently bound’ (fraction IV), and ‘residual’ (fraction V) of fallout 239 + 240Pu and 137Cs were assessed at 5 plots in the top organic horizons (Of1, Of2, Oh) and in the subsequent 4 layers of the mineral soil (0–2, 2–5, 5–10, and 10–20 cm). The results show that 137Cs in the organic layers is present throughout all fractions although mainly in the clay mineral fractions IV and V. Within these layers, a significant dependence on depth is observed for this radionuclide in fraction I (increase with depth) and infraction V (decrease with depth). 239 + 240Pu is associated in the organic horizons essentially with fraction III. In the mineral horizons, 137Cs is found mainly in fractions IV and V, 239 + 240Pu in fractions III and IV. A significant dependence on depth is present for 137Cs in fraction I (continuous increase with depth) and fraction V (decrease with depth). This indicates that radiocesium is presently more mobile in the deeper soil layers. 239 + 240Pu decreases with depth in fraction III, but increases simultaneously with depth in fraction IV. This could be due to the decreasing amount of organic matter present in the deeper layers of the mineral horizons and to changes in the organic matter structure (e.g. loss of functional groups). 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 reclamation procedures.