Uptake and translocation of radiocesium in cedar leaves following the Fukushima nuclear accident

Cryptomeria japonica trees in the area surrounding Fukushima, Japan, intercepted 137Cs present in atmospheric deposits soon after the Fukushima nuclear accident in March 2011. To study the uptake and translocation of 137Cs in C. japonica leaves, we analyzed activity concentrations of 137Cs and the concentration ratios of 137Cs to 133Cs (137Cs/133Cs) in old and new leaves of C. japonica collected from a forest on Mount Tsukuba between 9 and 15months after the accident. Both isotopes were also analyzed in throughfall, bulk precipitation and soil extracts. Water of atmospheric and soil origin were used as proxies for deciphering the absorption from leaf surfaces and root systems, respectively. Results indicate that 20–40% of foliar 137Cs existed inside the leaf, while 60–80% adhered to the leaf surface. The 137Cs/133Cs ratios inside leaves that had sprouted before the accident were considerably higher than that of the soil extract and lower than that of throughfall and bulk precipitation. Additionally, more than 80% of 137Cs in throughfall and bulk precipitation was present in the dissolved form, which is available for foliar uptake, indicating that a portion of the 137Cs inside old leaves was presumably absorbed from the leaf surface. New leaves that sprouted after the accident had similar 137Cs/133Cs ratios to that of the old leaves, suggesting that internal 137Cs was translocated from old to new leaves. For 17 species of woody plants other than C. japonica, new leaves that sprouted after the accident also contained 137Cs, and their 137Cs/133Cs ratios were equal to or higher than that of the soil extract. These results suggested that foliar uptake and further translocation of 137Cs is an important vector of contamination in various tree species during or just after radioactive fallout.