Activity concentrations of radionuclides in lichens following the Fukushima nuclear accident

The activity concentration of ¹³¹I, ¹³⁴Cs and ¹³⁷Cs radionuclides in lichens was traced one and a half months after the Fukushima nuclear accident. The samples were collected in Tsukuba City, which is located c. 170 km south of the Fukushima Daiichi nuclear power plant (NPP). The activity concentrations differed depending on species and habitat. For example, the maximum activity concentration of ¹³⁷Cs was 22596 Bq kg^-1 dry weight in Physcia orientalis (collected from the trunk of Zelkova serrata on 30 June 2011), and 1928 Bq kg^-1 in Hyperphyscia crocata (from the trunk of Quercus myrsinaefolia collected on 8 March 2012). The activity concentration of ¹³⁷Cs in Dirinaria applanata and Phaeophyscia spinellosa growing on vertical habitats decreased by c. 50% within a year, indicating radionuclides might have been washed off by rain. The radionuclides were apparently derived from the Fukushima NPP accident because: 1) one specimen collected at the same place one year before the accident did not contain radionuclides, 2) high activity concentrations of radionuclides were detected after the accident, 3) ¹³¹I, which has a short half-life of 8 days, was detected one and a half months after the accident, and 4) the ratio of ¹³⁴Cs/¹³⁷Cs in lichens was 0·90-0·98 on 26 April 2011, which is consistent with the values reported for radiocesium from the Fukushima NPP accident.