Dynamics of radiocaesium within forests in Fukushima-results and analysis of a model inter-comparison

Radiocesium mobility in different parts of the two major tree species in Fukushima

Radiocesium (¹³⁷Cs) released in the Fukushima Dai-ichi Nuclear Power Plant accident is still cycling in the forest ecosystem. We examined the mobility of ¹³⁷Cs in the external parts-leaves/needles, branches, and bark-of the two major tree species in Fukushima, Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata). This variable mobility will likely lead to spatial heterogeneity of ¹³⁷Cs and difficulty in predicting its dynamics for decades. We conducted leaching experiments on these samples by using ultrapure water and ammonium acetate. In Japanese cedar, the ¹³⁷Cs percentage leached from current-year needles was 26-45% (ultrapure water) and 27-60% (ammonium acetate)-similar to those from old needles and branches. In konara oak, the ¹³⁷Cs percentage leached from leaves was 47-72% (ultrapure water) and 70-100% (ammonium acetate)-comparable to those from current-year and old branches. Relatively poor ¹³⁷Cs mobility was observed in the outer bark of Japanese cedar and in organic layer samples from both species. Comparison of the results from corresponding parts revealed greater ¹³⁷Cs mobility in konara oak than in Japanese cedar. We suggest that more active cycling of ¹³⁷Cs occurs in konara oak.

Decadal trends in 137Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident

Understanding the actual situation of radiocesium (¹³⁷Cs) contamination of trees caused by the Fukushima nuclear accident is essential for predicting the future contamination of wood. Particularly important is determining whether the ¹³⁷Cs dynamics within forests and trees have reached apparent steady state. We conducted a monitoring survey of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites. Using a dynamic linear model, we analyzed the temporal trends in ¹³⁷Cs activity concentrations in the bark (whole), outer bark, inner bark, wood (whole), sapwood, and heartwood during the 2011–2020 period. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay. The ¹³⁷Cs concentrations in the whole and outer bark samples showed an exponential decrease in most plots but a flat trend in one plot, where ¹³⁷Cs root uptake is considered to be high. The ¹³⁷Cs concentration ratio (CR) of inner bark/sapwood showed a flat trend but the CR of heartwood/sapwood increased in many plots, indicating that the ¹³⁷Cs dynamics reached apparent steady state within one year in the biologically active parts (inner bark and sapwood) and after several to more than 10 years in the inactive part (heartwood). The ¹³⁷Cs concentration in the whole wood showed an increasing trend in six plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend. Overall, the results show that the ¹³⁷Cs dynamics within forests and trees have reached apparent steady state in many plots, although the amount of ¹³⁷Cs root uptake in some plots is possibly still increasing 10 years after the accident. Clarifying the mechanisms and key factors determining the amount of ¹³⁷Cs root uptake will be crucial for predicting wood contamination.