137Cs distributions in soil and trees in forest ecosystems after the radioactive fallout - Comparison study between southern Finland and Fukushima, Japan

The nuclear accidents at Chernobyl and Fukushima released large amounts of Cs-137 radionuclides into the atmosphere which spread over large forest areas. We compared the Cs-137 concentration distribution in different parts of two coniferous forest ecosystems (needle litter, stems and at different depths in the soil) over short and long term periods in Finland and Japan. We also estimated the change in Cs-137 activity concentrations in needle and soil between 1995 and 2013 in Southern Finland based on the back calculated Cs-137 activity concentrations. We hypothesized that if the Cs-137 activity concentrations measured in 1995 and 2013 showed a similar decline in concentration, the Cs-137 activity concentration in the ecosystem was already stable in 1995. But if not, the Cs-137 activity concentrations were still changing in 2013. Our results showed that the vertical distribution of the Cs-137 fallout in the soil was similar in Hyytiala and Fukushima. The highest Cs-137 concentrations were observed in the uppermost surface layers of the soil, and they decreased exponentially deeper in the soil. We also observed that Cs-137 activity concentrations estimated from the samples in 1995 and 2013 in Finland showed different behavior in the surface soil layers compared to the deep soil layer. These results suggested that the Cs-137 nuclei were still mobile in the surface soil layers 27 years after the accident. Our results further indicated that, in the aboveground parts of the trees, the Cs-137 concentrations were much closer to steady-state when compared to those of the surface soil layers based on the estimated declining rates of Cs-137 concentration activity in needles which were similar in 1995 and 2013. Despite its mobility and active role in the metabolism of trees, the Cs-137 remains in the structure of the trees for decades, and there is not much exchange of Cs-137 between the heartwood and surface layers of the stem. (C) 2016 Elsevier Ltd. All rights reserved.