Changes of effective half-lives of 137Cs in three herbaceous plants and bioavailable 137Cs fraction in soil after the Fukushima nuclear accident

- 137Cs concentrations in three wild edible herbaceous plants were measured in June 2011-May 2016.
- The 137Cs changes with time fitted well with two-component exponential decline curves.
- The bioavailable fraction in the soil would not change much for a long-time period from 2014.
- Tag of Cs from soil to wild edible herbaceous plants were reported for the first time.

We measured cesium-137 concentrations from June 2011 to May 2016 in three wild edible herbaceous species, i.e. giant butterbur (Petasites japonicus), knotweed (Trifolium pratense), and mugwort (Artemisia indica var. maximowiczii), that were grown on the same sampling field in Chiba Prefecture, Japan. The pattern of plant 137Cs concentrations versus time could be described well with the two-component exponential decline (short-term and long-term components), and the half-lives of these three plant species were between 103 and 230 d for the short component and 970-3830 d for the long component. During April 2014 to May 2016, due to the slow decline in the 137Cs concentrations in these three plant species, we assumed that the bioavailable 137Cs fraction in the observation ground soil was almost the same in this period. Then, values of the aggregated transfer factor from soil to plant (Tag) were calculated for these plant species, and the values ranged from 5.3 × 10−5 to 1.6 × 10−4 m2 kg−1-fresh. We also calculated Tag values for wild edible herbaceous plants in Fukushima Prefecture using open data sources that were reported from January 2014 to June 2016. The five plant species included were giant butterbur (P. japonicus), western bracken fern (Pteridium aquilinum), Japanese royal fern (Osmunda japonica), ostrich fern (Matteuccia struthiopteris) and momijigasa (Parasenecio delphiniifolius). The Tag values ranged from 1.4 × 10−4 to 7.0 × 10−4 m2 kg−1-fresh and were similar to those measured in our Chiba field study.