Estimation of vertical migration velocity of (137)Cs in the Mount IDA/Kazdagi, Turkey

Radioactivity of soil in Croatia II: 137Cs, 40K, and absorbed dose rate

We took samples of uncultivated soil from the surface layer (0–10 cm) at 138 sites from all over Croatia and measured their radionuclide activity concentrations with high-resolution gamma-ray spectrometry. This second part of our report brings the results on ⁴⁰K and ¹³⁷Cs to complement those on the ²³²Th and ²³⁸U decay chains addressed in the first part. Together they give the most complete picture of radioactivity of Croatian soil so far. Activity concentrations of ⁴⁰K were the highest in the Pannonian region, and there was an opposite trend for ¹³⁷Cs. We found that the concentrations of ¹³⁷Cs tended to increase with altitude, annual precipitation, and vegetation density. The concentration ratio of ¹³⁷Cs and K in soil, which indicates the potential for ¹³⁷Cs entering food chains via uptake by plants, was the lowest in agriculturally important areas in the east of the Pannonian region. In addition, we used the obtained results on activity concentrations to calculate the related absorbed dose rate as a measure of external exposure to ionising radiation from soil. The sum of the absorbed dose rates for naturally occurring radionuclides and ¹³⁷Cs showed that external exposure was generally the highest in the Dinaric region and Istrian Peninsula.

Calculations for ambient dose equivalent rates in nine forests in eastern Japan from 134Cs and 137Cs radioactivity measurements

Understanding the relationship between the distribution of radioactive ¹³⁴Cs and ¹³⁷Cs in forests and ambient dose equivalent rates (H˙^∗(10)) in the air is important for researching forests in eastern Japan affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study used a large number of measurements from forest samples, including ¹³⁴Cs and ¹³⁷Cs radioactivity concentrations, densities and moisture contents, to perform Monte Carlo radiation transport simulations for H˙^∗(10) between 2011 and 2017. Calculated H˙^∗(10) at 0.1 and 1 m above the ground had mean residual errors of 19% and 16%, respectively, from measurements taken with handheld NaI(Tl) scintillator survey meters. Setting aside the contributions from natural background radiation, ¹³⁴Cs and ¹³⁷Cs in the organic layer and the top 5 cm of forest soil generally made the largest contributions to calculated H˙^∗(10). The contributions from ¹³⁴Cs and ¹³⁷Cs in the forest canopy were calculated to be largest in the first two years following the accident. Uncertainties were evaluated in the simulation results due to the measurement uncertainties in the model inputs by assuming Gaussian measurement errors. The mean uncertainty (relative standard deviation) of the simulated H˙^∗(10) at 1 m height was 11%. The main contributors to the total uncertainty in the simulation results were the accuracies to which the ¹³⁴Cs and ¹³⁷Cs radioactivities of the organic layer and top 5 cm of soil, and the vertical distribution of ¹³⁴Cs and ¹³⁷Cs within the 5 cm soil layers, were known. Radioactive cesium located in the top 5 cm of soil was the main contributor to H˙^∗(10) at 1 m by 2016 or 2017 in the calculation results for all sites. Studies on the ¹³⁷Cs distribution within forest soil will therefore help explain radiation levels henceforth in forests affected by the FDNPP accident. The merits of this study are that it modelled multiple forests for a long time period, with the important model inputs being informed by field measurements, and it quantified how the measurement uncertainties in these inputs affected the calculation results.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified ¹³⁷Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated ¹³⁷Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the ¹³⁷Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of ¹³⁷Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1-2 years after the accident) for Japanese forest ecosystems.

Prussian blue analogue functionalized magnetic microgels with ionized chitosan for the cleaning of cesium-contaminated clay

To deal with regeneration of nuclear-waste-contaminated soil, it is important to develop new materials and techniques for effective removal of radioactive cesium ions from clay. We report herein a synergistic remediation method for cleaning cesium-contaminated clay by Prussian blue analogue-functionalized magnetic microgel along with ionized chitosan. The magnetic microgels were prepared by surface polymerization of 4-vinyl pyridine and styrene on magnetite nanoparticles and attachment of Prussian blue analogues by ligand exchange reaction. The adsorption of cesium ions by magnetic microgels in aqueous solution follows the second-order kinetics process. And the maximum adsorption capacity was determined to be 149.70 mg/g by Langmuir adsorption model. When ionized chitosan hydrochloride was mixed with cesium-contaminated clay, we found that 200 mg/g clay of chitosan hydrochloride can realize 87.6 % of cesium release from clay within 2 h. Further use of magnetic microgel adsorbents can adsorb 95.5 % free cesium ions in solution, achieving an overall 83.7 % cleaning efficiency from cesium-contaminated clays. The microgels can be regenerated effectively and recycled magnetically while keeping the adsorption capacity constant after multiple times of use. The underlying principle demonstrated in this work can be extended to remediation of other types of radionuclides or heavy-metal ions in contaminated soil.

Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with ¹³⁷Cs. It is important to characterize the behavior of ¹³⁷Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, ¹³⁷Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and ¹³⁷Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of ¹³⁷Cs could be suppressed rapidly in Japanese forests. At all sites, ¹³⁷Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous ¹³⁷Cs supply from the organic layer. The diffusion coefficients of ¹³⁷Cs in the mineral soil were estimated to be 0.042-0.55 cm² y^-1, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited ¹³⁷Cs will migrate to the mineral soil but only less than 10% of the total ¹³⁷Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the ¹³⁷Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

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.

Vertical distribution and radiological risk assessment of 137Cs and natural radionuclides in soil samples

The aims of this study were to investigate the vertical distributions of natural radionuclides 232Th, 226Ra and 40K as well as anthropogenic radionuclide 137Cs in soil samples and to analyze the correlation among the radioactivity of these radionuclides and the physiochemical characteristics of soil samples namely pH, grain size, carbonate content and organic matter. Risk assessment of the radiological hazard has also been estimated. Forty-four soil samples were collected from eleven locations in Qatar at four depth levels from 0 to 16 cm. The average concentrations of 232Th, 226Ra, 40K and 137Cs in the soil depth of 16 cm were 10, 17, 201 and 4 Bq/kg, respectively, which were within the reported world mean. The external absorbed gamma dose rate, the annual effective dose, the mean radium equivalent activity, the external hazard index and the lifetime cancer risk were 22 nGy/h, 0.027 mSv/y, 47 Bq/kg, 0.125 and 0.096 × 10-3, respectively. These values were far below the minimum recommended international values. The level of radioactivity concentrations in the soil was affected by the physiochemical characteristics of the soil. The positive correlation with highest R2 value was found among the radioactivity concentrations of 232Th and 40K and the soil clay content. Total organic carbon was also positively correlated for 226Ra and 137Cs activity concentrations, whereas, carbonate content was negatively correlated with the radioactivity concentrations of 232Th and 40K. As far as soil moisture content is concerned, the positive correlation with highest R2 value was obtained for 226Ra activity concentrations.

Stable and radioactive cesium: A review about distribution in the environment, uptake and translocation in plants, plant reactions and plants' potential for bioremediation

Radiocesium in water, soil, and air represents a severe threat to human health and the environment. It either acts directly on living organisms from external sources, or it becomes incorporated through the food chain, or both. Plants are at the base of the food chain; it is therefore essential to understand the mechanisms of plants for cesium retention and uptake. In this review we summarize investigations about sources of stable and radioactive cesium in the environment and harmful effects caused by internal and external exposure of plants to radiocesium. Uptake of cesium into cells occurs through molecular mechanisms such as potassium and calcium transporters in the plasma membrane. In soil, bioavailability of cesium depends on the chemical composition of the soil and physical factors such as pH, temperature and tilling as well as on environmental factors such as soil microorganisms. Uptake of cesium occurs also from air through interception and absorption on leaves and from water through the whole submerged surface. We reviewed information about reducing cesium in the vegetation by loss processes, and we extracted transfer factors from the available literature and give an overview over the uptake capacities of 72 plants for cesium from the substratum to the biomass. Plants with high uptake potential could be used to remediate soil and water from radiocesium by accumulation and rhizofiltration. Inside plants, cesium distributes fast between the different plant organs and cells, but cesium in soil is extremely stable and remains for decades in the rhizosphere. Monitoring of contaminated soil therefore has to continue for many decades, and edible plants grown on such soil must continuously be monitored.