Epiphytic fruticose lichens as biomonitors for retrospective evaluation of the 134Cs/137Cs ratio in Fukushima fallout

Abstract

In 2011–2013, sampling of epiphytic fruticose lichens of the genera Usnea, Bryoria and Alectoria was carried out on Sakhalin and Kuril Islands (the Sakhalin region, Russia) to investigate contamination of these organisms with the Fukushima-derived 134Cs and 137Cs. Activities of the radionuclides were determined in all 56 samples of lichens taken for the analysis. After correction for radioactive decay (on 15 March 2011), the activity concentrations ranged from 2.1 Bq kg−1 (d.w.) to 52 Bq kg−1 for 134Cs and from 2.3 Bq kg−1 to 52 Bq kg−1 for 137Cs. Cesium-134 and 137Cs activities for the whole set of lichens (n = 56) were strongly positively correlated; Spearman's rank correlation coefficient was calculated as 0.991 (P < 0.01). The activity concentrations of 134Cs and 137Cs in Usnea lichens from the Sakhalin and Kunashir islands declined with a factor of three in the period from 2011 to 2013. The average biological half-time for both cesium radionuclides in lichens of the genus Usnea is estimated as 1.3 y. The mean of 0.99 ± 0.10 and median of 0.99 were calculated for the decay corrected 134Cs/137Cs activities ratios in the lichens (n = 56). The radionuclides ratio in the lichens did not depend on location of sampling site, species and the time that had passed after the Fukushima accident. The regression analysis has shown the background pre-Fukushima level of 137Cs of 0.4 ± 0.3 Bq kg−1, whereas the ratio between the Fukushima-borne 134Cs and 137Cs in the lichens was estimated as 1.04. The 134Cs/137Cs activities ratio in lichens from the Sakhalin region is consistent with the ratios reported by others for the heavy contaminated areas on Honshu Island in Japan following the Fukushima accident. The activity concentrations of natural 7Be in lichens from the Sakhalin region varied between 100 Bq kg−1 and 600 Bq kg−1; the activity concentrations did not exhibit temporal variations during a 2y-period of observations. The applicability of epiphytic fruticose lichens as retrospective bio-monitors for the air-borne radiocesium contamination of the environment is discussed.