No abstract

The caesium-134 and caesium-137 radionuclides released into the atmosphere as a result of the Fukushima accident were dispersed over the entire Northern Hemisphere. To assess the risks associated with the exposure due to Fukushima fallout, a comprehensive radiological survey was performed in the Russian Far East. One of the objectives of the project was to determine the densities of ground contamination by 137 Cs and 134 Cs on Sakhalin and Kuril Islands that constitute the Sakhalin oblast, an administrative region of Russia. In 2011, soil samples were collected at grasslands on Sakhalin, Kunashir and Shikotan Islands and results of the 2011 survey were published earlier. In the present study, activities of 137 Cs and 134 Cs were measured in soil samples obtained on Kunashir, Iturup, Urup and Paramushir Islands in 2012. From the studies carried out in 2011-2012, it was estimated that the Fukushima-derived 134 Cs inventory at 37 undisturbed grassland sites in the Sakhalin oblast varied from 8 Bq m −2 to 345 Bq m −2 (as of 15 March 2011). For this date, the inventory of the 137 Cs radionuclide originated from the Fukushima NPP was assumed to be the same as that of the 134 Cs radionuclide. The southern Kuril Islands were the most contaminated due to Fukushima fallout. In 2011 and 2012, Fukushima-derived radiocaesium was detected only in the top 5 cm layer of soil at all sites, excluding one, where ~20% of the 134 Cs inventory was found at a depth of 5-10 cm. In the period September 2011-September 2012, the inventory of 134 Cs declined by ~26% at four plots selected for long-term observations. The decline in the 134 Cs inventory closely corresponded to the reduction (29%) of 134 Cs activity due to radioactive decay. Pre-accidental inventory of 137 Cs in the top 20 cm layer of soil ranged from 53 Bq m −2 to 3630 Bq m −2. The mean reference inventory of pre-accidental 137 Cs for 13 representative sites was amounted as 2600 Bq m −2. Hence, the Fukushima accident added relatively small quantities of radioactivity to the reference preaccidental inventory of 137 Cs in grassland soils in the Sakhalin region: about 3% (~80 Bq m −2) on the average and 15% (~350 Bq m −2) at the maximum. Such small additional radioactive contamination is absolutely safe from a radiological point of view.

Samples of vegetation (vascular plants)

No abstract

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)(2012)(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 134 Cs and 137 Cs and the natural radionuclides 7 Be and 40 K 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 134 Cs and 137 Cs 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 137 Cs and <1 Bq kg -1 for 134 Cs. A decline in the annual median 137 Cs 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 137 Cs and 134 Cs in lichens were strongly correlated (r=0.978, P<0.01) and the 134 Cs biological half-life value of 1.2 y in the period 2011-2013 was similar to the corresponding 137 Cs biological half-life value. The soil-tolichens aggregated transfer factor, T ag , for 134 Cs at time t=0 after the Fukushima accident is calculated as 0.56 m 2 kg -1 at Sakhalin and 0.31 m 2 kg -1 at Kunashir. In contrast to radiocesium, the natural radionuclides 7 Be and 40 K did not show clear time-dependent variations in the Usnea lichens. No correlation was found between 7 Be and 40 K as well as between 40 K and cesium radioisotopes. However, positive and statistically significant (P<0.05) correlations were obtained between 7 Be and cesium radioisotopes. High abundance of the Usnea sp. lichens in the study area and large values of T ag for radiocesium in the lichens make these organisms suitable candidates for detection of low levels of airborne radioactive contamination of the environment.