Vertical distribution of 137Cs in grassland soils disturbed by moles (Talpa europaea L.)

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F_l/t), the migration center (X_c) and the relaxation depth (α) using 99 soil inventory data sets. F_l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F_l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X_c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X_c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high ¹³⁷Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of ¹³⁷Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of ¹³⁷Cs from organic to surface mineral soil layer after the second year. These results indicate that ¹³⁷Cs retention capacity of the organic layer can affect the apparent vertical migration of ¹³⁷Cs in the underlying mineral soil layer.

Calculation of the effective external dose rate to a person staying in the resettlement zone of the Vetka district of the Gomel region of Belarus based on in situ and ex situ assessments in 2016-2018

The aim of this study was to perform a preliminary assessment of the expected effective dose rate from external exposure to an adult individual staying at that part of the radioactively contaminated territory of the Vetka district of the Gomel region of the Republic of Belarus, from where residents had been resettled after the Chernobyl accident. For this assessment, in summer 2016 and 2018 soil samples were taken from 19 sites located in forests (7 plots), virgin meadows (4 plots), cultivated meadows (6 plots) and vegetable gardens (2 plots), with the subsequent estimation of the inventory and vertical distribution of ¹³⁷Cs in the soil. The values of ¹³⁷Cs inventory in the soil ranged from 452 to 1620 kBq m^-2 (mean = 904 kBq m^-2, median = 964 kBq m^-2). The results of the measurement of soil samples were used to calculate values of the air kerma rate, normalized to the inventory of radioactive caesium in the soil. On average, the normalized indicator of the air kerma rate from the man-made source was higher in forests (1.13 nGy h^-1 per kBq m^-2) compared to virgin meadows (0.95 nGy h^-1 per kBq m^-2). Normalized air kerma rate in cultivated meadows and vegetable gardens was approximately two times lower than the corresponding indicator for virgin meadows. Using a field gamma spectrometer-dosemeter, ambient dose equivalent rate of gamma radiation in the air was measured at the surveyed sites and the contributions of the technogenic and natural components to the dose rate were estimated. Additionally, such measurements were performed on asphalted surfaces (5 sites) and inside two wooden houses. The measured values of the total ambient dose equivalent rate at a height of 1 m above the ground, asphalted surface or house floor varied from 160 to 2260 nSv h^-1. The lowest levels were recorded over asphalted surfaces and inside houses, and the highest ones at forest and virgin meadow sites. The contribution of the technogenic component to the total dose rate varied from 61.9% to 98.8% (mean = 88.9%; n = 26). The effective dose of anthropogenic radiation calculated from the results of in situ measurements in a forest, virgin meadow, cultivated meadow, kitchen garden, asphalted area and house was 0.59, 0.80, 0.34, 0.29, 0.06 and 0.06 μSv h^-1, respectively. Similar values for land plots were calculated based on ex situ analysis of soil samples. It can be expected that, starting from 2020, the average effective external dose of a person staying in the resettlement zone of the Vetka district will not exceed 1 mSv year^-1.

Correlation between calculated and measured values of gamma dose rate in air in forests contaminated with 137Cs: the remote period after the Chernobyl accident

In 2015–2016, 13 forest and 7 virgin grassland plots located in the south-western districts of the Bryansk region were surveyed. The aim of the work was to experimentally test the possibility of using a method for calculating the dose rate of gamma radiation in air in radioactively contaminated forests in a remote period after the Chernobyl accident. According to the results of gamma-spectrometric analysis of soil samples obtained at the sites in another study, the values of inventory and vertical distribution of 137Cs in the upper 20 cm layer were established. In this paper, these data were used to calculate the air kerma rate using a method taken from literature. In addition, at the sites of soil sampling, ambient dose equivalent rate in air was measured, and the contribution of 137Cs to the total gamma dose rate was determined with a field gamma spectrometer-dosemeter. The measured values of the ambient dose equivalent rate from 137Cs correlated positively and statistically significantly with the calculated values of the air kerma rate. The Spearman correlation coefficient was 0.989 (P < 0.01) for the location “forest” and 0.893 (P < 0.05) for the location “grassland”. There was no statistically significant difference between the “forest” and “grassland” locations when analyzing the ratio of the measured dose rate values to the calculated dose rate values (the Mann-Whitney U test, P > 0.05). Results of this work show that, when calculating gamma radiation dose rate in air in forests at a remote stage after the Chernobyl accident, it is enough to know the 137Cs inventory in the upper 20 cm soil layer and a detailed picture of vertical distribution of the radionuclide in this layer. The presence of woody biomass can be neglected. This dose rate estimate is conservative. However, a degree of overestimation of the dose rate in air is small, within +10%, which is quite acceptable for determining the external effective dose rate for an individual in the radioactively contaminated forest.

Vertical distribution of 137Cs in soddy-podzolic sandy soil in grasslands and forests of the Bryansk region in 2015–2016

Vertical distribution of natural and man-made radionuclides in the soil profile is a decisive parameter when calculating the dose rate of gamma radiation in the air above the ground and the effective dose of external human exposure. The main purpose of this work was to determine vertical distribution of137Cs in soddy-podzolic sandy and sandy-loam soils in forests and grasslands in the south-western districts of the Bryansk region in the remote period after the Chernobyl accident. In 2015–2016, soil cores were sampled in 7 virgin meadows and 13 forested areas to a depth of 20 cm. The cores were cut into horizontal layers 2 cm thick. The137Cs activity in the samples was determined using a semiconductor gamma spectrometer. The activity concentration in samples of dry soil (n = 200) ranged from 6.35 Bq/kg to 83300 Bq/kg with an average of 4550 Bq/kg. In the meadows in three cases, the maximum activity concentration was determined in the uppermost layer. With increasing depth, the activity concentration decreased and reached a minimum in the deepest layers. A difference between the surface layer and the deepest layer was two to three orders of magnitude. The three other meadows showed a relatively uniform distribution of137Cs in the upper 4–6 cm, followed by a decrease in activity concentration with an increase in depth. In one meadow area, a peak of the137Cs activity was found at a depth of 4–6 cm. In the forest, the most typical (in 10 cases) was the presence of a pronounced maximum activity concentration of137Cs in the uppermost layer. The experimentally obtained values of the137Cs inventory in the upper 20 cm of soil at the surveyed sites ranged from 42 to 1940 kBq/m2. The values of137Cs inventory positively and statistically significantly correlated with officially established levels of137Cs surface ground contamination for the territory of nearby settlements. Vertical migration of137Cs in the soil in the surveyed areas was mainly limited to the upper 10 centimeters. The layer on average contained 94% of the total137Cs inventory. Less than 1% of the total137Cs inventory was found in the deepest soil layer sampled (18–20 cm). The obtained137Сs activity distributions were used to calculate kerma rate in the air at a height of 1 m above the ground at the surveyed sites. The air kerma rate ranged from 52 to 2240 nGy/h (on average, 807 nGy/h). The caesium-137 deposit in the upper 6 cm of soil determined about 95% and 90% of the air kerma rate in the forests and in meadows, respectively. Radioactive caesium, which migrated into the soil to a depth of more than 10–12 cm, gave a negligible contribution (less than 1%) to the gamma-radiation dose rate in the air. In practical terms, it indicates that the depth of soil sampling equal to 20 cm is currently quite sufficient to estimate the dose rate of gamma radiation in the air in virgin grasslands and in forests.

Estimation of the air kerma rate from 137Cs and 134Cs deposited on the ground in the Sakhalin region of Russia after the Fukushima accident

Previous studies conducted by the Federal Service for Surveillance on Consumer Rights Protection and Human Well-Being (Rospotrebnadzor) in Sakhalin Region, Russia in 2011–2012 have shown that soils on the southern Kuril Islands were contaminated with 137Cs and 134Cs due to the accident at the Fukushima-1 NPP. In this study, we evaluated the air kerma rate from 137Cs and 134Cs using earlier published data on the vertical distribution of Fukushima-derived radiocaesium in the soil at grasslands on the islands of Kunashir, Shikotan and Iturup. The air kerma rate due to gamma-rays from pre-Fukushima 137Cs was estimated as well. In the period from May 2011 to September 2012, the air kerma rate due to the fresh fallout of radioactive caesium varied from 0.31 to 0.84 nGy h−1. On average, the contribution of the radiation from Fukushima-derived 134Cs and 137Cs to the total air kerma rate from radioactive cesium was 35% in the fall of 2011 and 25% in the fall of 2012. The mean normalized kerma rate from Fukushima-derived 137Cs was estimated at a level of 1.63 nGy h−1 per kBq m−2 in mid-May 2011, 1.53 nGy h−1 per kBq m−2 in autumn 2011 and 1.45 nGy h−1 per kBq m−2 in autumn 2012. The mean normalized air kerma rate from pre-Fukushima 137Cs at reference undisturbed grasslands was estimated as 0.77 nGy h−1 per kBq m−2. The results of our calculations of the air kerma rate from Fukushima-derived 137Cs were in good agreement with the forecast of the 2013 UNSCEAR model, which was used to estimate the external doses of the Japanese population after the Fukushima accident. For pre-Fukushima 137Cs, the UNSCEAR model underestimated the calculated normalized air kerma rates in Sakhalin Region by approximately 40%. Our calculations showed that, overall, the aboveground biomass of herbaceous plants had practically no effect on the air kerma rate from radioactive cesium contained in the soil. The fallout of 134Cs and 137Cs after the accident at the Fukushima-1 NPP did not lead to any significant increase in the gamma dose rate in the air at grasslands on Kuril Islands. In 2019–2020, the contribution of radiocaesium to the total gamma dose rate in air will not exceed 5%.