The migration of fallout 134Cs, 137Cs and 106Ru from Chernobyl and of 137Cs from weapons testing in a forest soil

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Abstract
To estimate the accumulation and vertical migration of radiocesium and radioruthenium, the activity concentrations of Chernobyl‐derived 134Cs, 137Cs and 106Ru as well as of 137Cs from the global fallout of weapons testing observed in the upper horizons of a forest soil (Hapludult, spruce stand) were evaluated with a compartment model. The resulting residence half‐times were used to estimate the mean rates of transport of these radionuclides. For Chernobyl‐derived 134Cs and 137Cs within the time period of 200–600 days after the beginning of the fallout the rates were between 4 cm/yr (Of1‐horizon) and 2 cm/yr (Oh‐horizon), and for 106Ru between 4 cm/yr (Of1‐horizon) and 7 cm/yr (Oh‐horizon). These rates, though considerably slower than observed in the same soil during the initial infiltration of these radionuclides with a rain shower, are (depending on the soil horizon) still higher by a factor of 3–6, when compared to the rates of transport of 137Cs from the global fallout of weapons testing in the same soil. Because global fallout 137Cs is in the soil since about 20 years, these results suggest that the fixation of radiocesium in the forest soil is a rather slow process.