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Abstract This study investigated the transport of 137Cs within a forest ecosystem by examining temporal changes in the inventory and determining the major pathways of transfer following significant atmospheric deposition. A forested area of eastern Japan was monitored for four years immediately after the Fukushima nuclear power plant accident in March 2011 that released a large amount of radionuclides. The long physical half-life of 137Cs means that contamination can persist for decades, so it is vital to understand the mechanisms underlying the 137Cs dynamics in ecosystems. We sampled litterfall, throughfall, and soil, mainly from a cedar stand, over a four-year period, and analyzed the 137Cs concentrations of each sample to determine the transfer rate and total inventory. After validating our methodology through a comparison with results from an earlier study, we determined the temporal changes in the 137Cs distribution and in the major transfer pathway. Results showed that most 137Cs intercepted by canopies was transferred rapidly over the first nine months, and that the major pathway was not litterfall but throughfall. The ecological half-life of the 137Cs stocked in the canopy was calculated for both the early and later stages of contamination. Although the former is consistent with previous results, the latter ecological half-life is somewhat longer, probably because of dependence on the meteorological and tree physiological conditions at the site. This study presents valuable new data on the post-Fukushima 137Cs contamination, enhancing our understanding of the associated dynamics in forest ecosystems. Graphical abstract Unlabelled Image Highlights • The Fukushima accident provided an opportunity to understand the fate of radionuclides • An area in Japan was monitored for 137Cs transfer from canopies to the forest floor • Throughfall transferred much more 137Cs than litterfall, regardless of tree species. • At a cedar stand, most 137Cs was transferred within nine months of the accident. • General trends obtained by comparisons with previous studies are also discussed. [ABSTRACT FROM AUTHOR]

Japanese cedar (Cryptomeria japonica) is the main timber species in Japan. The prediction of the temporal changes in the 137Cs concentration in the stemwood of Japanese cedar after the Fukushima nuclear accident is essential for optimizing forest management in contaminated areas. However, it is difficult to estimate the respective contributions of root and foliar uptake to 137Cs accumulation in stemwood from simple field measurements, especially in trees that contain the residue of initially-deposited 137Cs. In this study, we devised a method for estimating the rate of 137Cs root uptake into stemwood using the 133Cs content in stemwood and the 137Cs/133Cs ratio in the exchangeable fraction of soil. As a trial, the method was applied to a cedar stand in Fukushima Prefecture, using available monitoring data from prior studies over 5 years from August 2011 to August 2016. The mean annual rate of 137Cs root uptake into stemwood over this period was estimated as 53 ± 20 Bq m−2 yr−1. We note that our method likely provided a maximum estimate, because it is based on the assumptions that 133Cs in wood is exclusively supplied by root uptake, and that Cs isotopes are taken up by roots in the top 5 cm of mineral soil. Moreover, the mean annual increase of the 137Cs inventory in stemwood during the study period was measured as 108 Bq m−2 yr−1, although this value was associated with considerable uncertainty (95% confidence interval from −109 to 324 Bq m−2 yr−1). As a result, the maximum estimated rate of 137Cs root uptake into stemwood accounted for around half of the measured rate of 137Cs accumulation in stemwood. Our results show that the Cs isotopic approach has potential to distinguish the main pathway of stemwood contamination (i.e., root vs. foliar uptake) following radioactive fallout. Unlabelled Image • A method was devised for estimating the rate of 137Cs root uptake into stemwood. • We tested the method at a cedar stand in Fukushima using data from 2011 to 2016. • Maximum estimate accounted for the half of the total 137Cs accumulation in stemwood. • This isotopic approach can contribute to revealing the origin of 137Cs in stemwood. [ABSTRACT FROM AUTHOR]

The temperature and oxygen environment play important roles in the desorption of 137Cs from freshwater lake sediment to lake water. In this study, 12 quarterly surveys were performed to measure the dissolved 137Cs concentration in surface and bottom lake water, the vertical distribution of water temperature, and the dissolved oxygen (DO) concentration at the upstream, midstream, and downstream sites of the Yokokawa Dam Lake in Fukushima Prefecture, Japan. Higher concentrations of dissolved 137Cs were detected in the bottom water than in the surface water, especially in the summer and midstream lake regions at depths of 8–21 m owing to higher temperatures, which activated the bacterial decomposition of organic matter, and anaerobization, which enhanced the NH 4 + in the pore water and 137Cs desorption from mineral particles. To compare the effects of anaerobization and increasing temperature on 137Cs desorption from sediment particles, intact sediment core samples were collected from the lake midstream and incubated for 1–14 days in a chamber under three controlled temperature and oxygen environment conditions: aerobic +10 °C; anaerobic +10 °C; and anaerobic +20 °C. The vertical distribution of 137Cs in the sediment pore water showed a similar profile as NH 4 + and K+, and both the increased temperature and reduced DO concentration enhanced the 137Cs desorption. A comparison of the standard partial regression coefficients of temperature and DO concentration in the multiple regression equation for the 137Cs concentration in pore water shows that the reduction of DO from saturation to zero at the water–sediment interface accelerated the desorption of 137Cs more strongly than did the temperature rise from 10 to 20 °C. The experimental results show a nearly inverse proportional relationship between NH 4 + and the distribution coefficients of 137Cs after thermodynamic correction, except in the surface layer. These findings indicate that the 137Cs concentration in pore water can be explained by the temperature-dependent chemical distribution between the solid–aqueous phase and its equilibrium with NH 4 +. [Display omitted] • Dissolved 137Cs in the midstream area of bottom lakewater is high in summer. • Reduced dissolved oxygen promotes Cs desorption more than temperature rise. • 137Cs desorption in sediment appears to be determined by NH 4 + dynamics. [ABSTRACT FROM AUTHOR]