Vertical migration of plutonium-239 +-240, americium-241 and caesium-137 fallout in a forest soil under spruce

Abstract

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 (2cm 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).