Six-year monitoring study of radiocesium transfer in forest environments following the Fukushima nuclear power plant accident.

The study investigated temporal changes in the Cs concentrations in vegetal and hydrological samples collected from various forests in Yamakiya District, Kawamata Town of Fukushima prefecture over six years following the Fukushima Dai-ichi nuclear power plant accident. Cesium-137 was detected in all forest environmental samples. However, the concentration in most samples decreased exponentially with time. The Cs concentrations in throughfall samples exhibited a double-exponential decreasing trend with time. Temporal changes in the Cs concentration in vegetal samples and stemflow were approximated by using a single-exponential equation. A comparison of the decline coefficient for the latter observation period (>2 y since the accident) revealed that the declining trend of Cs concentrations varied between foliage and the outer barks of the Japanese cedar and Japanese konara oak trees. The Cs concentration in cedar needles decreased exponentially while that in konara oak leaves was constant over the last six years. Conversely, the declining trend of Cs concentration in the outer bark of konara oak exceeded that of cedar. The results suggested that self-decontamination processes and internal recycling of Cs varied among tree species and different tree parts. The results indicated that the leaching of Cs in the throughfall in Japanese cedar was dependent on the Cs concentration in needles. However, a comparison of Cs concentrations in vegetal and hydrological samples from each sampling year showed that the leaching rate decreased with time. Conversely, the Cs concentrations in the stemflow were independent of the concentrations in the outer bark. The declining trend of Cs concentrations in litterfall (λ: 0.31-0.33 y) was similar to that of the mean of new/old needles (λ: 0.26-0.33 y) for cedar stands. With respect to the hydrological components, the Cs concentration in the stemflow (λ: 0.32-0.33 y) decreased at a slightly slower rate than that in the throughfall (λ: 0.36-0.54 y) for the cedar forest. The decline coefficients of Cs concentration in the aforementioned types of hydrological components slightly exceeded that for the vegetal samples. The results suggest that monitoring of Cs concentrations in hydrological components and vegetal samples can aid in further understanding the leaching mechanisms of Cs from trees to rainwater.