Temporal variations of 7Be, 40K, 134Cs and 137Cs in epiphytic lichens (genus Usnea) at the Sakhalin and Kunashir islands after the Fukushima accident

Temporal variations of radionuclide levels in the epiphytic Usnea sp. lichens from the two islands Sakhalin and Kunashir, the Sakhalin region, Russia have been evaluated using the already published (2011–2013) and new experimental data (2015). A total of 62 lichen samples were measured using high purity germanium γ-ray detectors and multichannel analyzers. In the period 2011–2015, activity concentrations of the anthropogenic radionuclides 134Cs and 137Cs and the natural radionuclides 7Be and 40K were found to be in the range of (<0.53)–41.3, 0.55–50.6, 99–603 and 35–95 Bq kg–1 on dry weight, respectively. The activity concentrations of 134Cs and 137Cs in lichens were statistically significantly higher at Kunashir than at Sakhalin. The present-day levels of radiocesium activity concentrations in lichens are low: < 6 Bq kg–1 for 137Cs and <1 Bq kg–1 for 134Cs. A decline in the annual median 137Cs activity concentrations in lichens from 2011 to 2015 corresponds to a biological half-life of 1.2 y for Kunashir and 1.1 y for Sakhalin. The activity concentrations of 137Cs and 134Cs in lichens were strongly correlated (r=0.978, P<0.01) and the 134Cs biological half-life value of 1.2 y in the period 2011–2013 was similar to the corresponding 137Cs biological half-life value. The soil-tolichens aggregated transfer factor, Tag, for 134Cs at time t=0 after the Fukushima accident is calculated as 0.56 m2 kg–1 at Sakhalin and 0.31 m2 kg–1 at Kunashir. In contrast to radiocesium, the natural radionuclides 7Be and 40K did not show clear time-dependent variations in the Usnea lichens. No correlation was found between 7Be and 40K as well as between 40K and cesium radioisotopes. However, positive and statistically significant (P<0.05) correlations were obtained between 7Be and cesium radioisotopes. High abundance of the Usnea sp. lichens in the study area and large values of Tag for radiocesium in the lichens make these organisms suitable candidates for detection of low levels of airborne radioactive contamination of the environment.

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