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 137Cs radionuclides into the atmosphere which spread over large forest areas. We compared the 137Cs 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 137Cs activity concentrations in needle and soil between 1995 and 2013 in Southern Finland based on the back-calculated 137Cs activity concentrations. We hypothesized that if the 137Cs activity concentrations measured in 1995 and 2013 showed a similar decline in concentration, the 137Cs activity concentration in the ecosystem was already stable in 1995. But if not, the 137Cs activity concentrations were still changing in 2013. Our results showed that the vertical distribution of the 137Cs fallout in the soil was similar in Hyytiälä and Fukushima. The highest 137Cs concentrations were observed in the uppermost surface layers of the soil, and they decreased exponentially deeper in the soil. We also observed that 137Cs 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 137Cs 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 137Cs concentrations were much closer to steady-state when compared to those of the surface soil layers based on the estimated declining rates of 137Cs concentration activity in needles which were similar in 1995 and 2013. Despite its mobility and active role in the metabolism of trees, the 137Cs remains in the structure of the trees for decades, and there is not much exchange of 137Cs between the heartwood and surface layers of the stem.