Atmospheric Direct Uptake and Long-term Fate of Radiocesium in Trees after the Fukushima Nuclear Accident

OPEN SUBJECT AREAS: POLLUTION REMEDIATION GEOCHEMISTRY Received 18 August 2014 Accepted 8 October 2014 Published 20 November 2014 Correspondence and requests for materials should be addressed to Y.M. (mahara@mta. biglobe.ne.jp) * Current address: Agriculture and Forestry Office in the Fukushima Southern District, Shirakawa, 961-0971, Fukushima, Japan. Atmospheric Direct Uptake and Long-term Fate of Radiocesium in Trees after the Fukushima Nuclear Accident Yasunori Mahara1,2, Tomoko Ohta2, Hideki Ogawa3,4 & Atsushi Kumata3* 1 Kyoto University, Kyoto, Kyoto 606-8501, Japan, 2Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan, 3Fukushima Forestry Research Centre, Koriyama, Fukushima 963-0112, Japan, 4Graduate School of Urban Environmental Science, Tokyo Metropolitan University, Hachioji, Tokyo, 192-0397, Japan. Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum 137Cs concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem 137Cs concentrations will not be affected by root uptake if active root systems occur 10 cm below the soil. T he Fukushima nuclear disaster, caused by the great earthquake and tsunami of 11 March 2011, contaminated vast forest areas with radiocesium: 137Cs at 1.5 3 1016 Bq with 134Cs/137Cs approximately equal to 1.01. Forest decontamination is an urgent issue. Although the internal contamination of trees has been roughly estimated to be small using simple compartment models and a set of parameters2 that takes into account the properties of Japanese forest soil, accurate understanding of how trees absorb fallout 137Cs is required for quick and effective decontamination. To obtain a better understanding of the uptake mechanisms of fallout radionuclides, heavy metals and volatile organic matter in the xylem of trees, we reviewed past studies on radionuclides from the Chernobyl nuclear accident3–6, nuclear weapons tests7–9 and the Nagasaki A-bomb10–12, in addition to studies on heavy metals13 and organic material14. A few studies10–14 have suggested the possibility of direct uptake (i.e. not via soil) of atmospheric radionuclides, heavy metals and organic material into the xylem of trees from bark and leaves; however, most studies3–9 on the Chernobyl accident and nuclear weapons testing have argued that fallout radionuclides are absorbed through the roots from the soil (i.e. root uptake). If atmospheric direct uptake is the main route for absorption of 137Cs into the xylem of trees in Japan, the radiocesium concentration within a tree will decrease with time after the Fukushima nuclear accident. Conversely, if the roots are the main route for radiocesium uptake, 137Cs concentration might gradually increase with a large time lag after the accident, because 137Cs reaches the active zone of roots in the forest soil after a long delay. Herein, we discuss estimates of the possibility and magnitude of direct uptake of atmospheric 137Cs into trees, and long-term prediction of changes in 137 Cs concentration in tree xylem by root uptake after atmospheric direct uptake. Results We measured the vertical profiles of stable and radioactive Cs and K in soil of two tree-harvesting yards at Koriyama, Fukushima, in 2013 (Supplementary Fig. S1). Both 134Cs and 137Cs in the soil are strongly fixed by clay minerals in soil, as shown in Figs. 1a and 1b. More than 99% of the 134Cs and 137Cs deposited from the nuclear accident were trapped in the litter layer and the top 2.5 cm of soil 2.5 years after the accident; their concentrations decrease rapidly with increasing depth. In contrast, 40K and the stable isotopes of Cs and K are almost evenly distributed through the soil profile, except stable K at two depths in the sugi yard. We harvested two Japanese common trees (konara: Quercus serrata Murray, a deciduous broadleaf tree and sugi: Cryptomeria japonica D. Don, an evergreen coniferous tree) to investigate the distribution of 137C, 134Cs and SCIENTIFIC REPORTS | 4 : 7121 | DOI: 10.1038/srep07121 1 www.nature.com/scientificreports Figure 1 | Vertical concentration profiles of 137Cs, 134Cs, 40K, stable isotopes of Cs and K in soil, 2.5 years after the Fukushima nuclear accident. (a) the konara yard and (b) the sugi yard. Open blue triangles, red circles, black squares and solid squares and triangles indicate the concentration of 40K, 134Cs, 137 Cs, and stable isotopes of Cs and K in soil, respectively. 40 K in tree rings in 2012 (Supplementary Table S1). The distributions of 137Cs and 134Cs are almost the same in the rings of both trees. We also measured the concentrations of stable isotopes of Cs and K in tree rings (Supplementary Table S2). We show the distribution of 137 Cs and 40K in the xylem in trees divided into four cardinal compass directions in Figs. 2a and 2b. We also demonstrate a directional distribution of 137Cs concentration compared with 40K concentration for konara (Fig. 3a) and a non-directional distribution for sugi (Fig. 3b). In addition, we summarize the directional distribution of radiocesium concentration in the sapwood region of trunk discs in four different directions, verified using the x2-test, in Table 1. Using the model proposed in this study, we predict changes in the concentration of 137Cs in tree rings of konara and sugi up to 120 years SCIENTIFIC REPORTS | 4 : 7121 | DOI: 10.1038/srep07121 after the nuclear accident. We show changes in the 137Cs concentration in the xylem with time from the accident (Figs. 4a and 4b), varying the magnitude of the retardation factor15 controlling the migration of 137Cs in forest soil. Discussion To confirm the occurrence of atmospheric direct uptake into trees, we hypothesized that radionuclides could be absorbed via the following two pathways. First, radionuclides absorbed through the bark could be directly transported into the sapwood, possibly along the ray tissue13–14; if this hypothesis is correct, the radionuclide concentration in tree rings would depend on differences (...truncated)