Distribution Of Radiocesium Research Articles

The distribution of radiocesium in the Indian ocean and its relation to the exit passage of the Indonesian Throughflow

The objective of this study was to determine the presence of radiocesium (134Cs and 137Cs) at the monitoring sites and to link its presence to the characteristics and mass water dynamics at the exit of the Indonesian Throughflow (ITF). The main sources of radiocesium are from human activities in the North Pacific Sea, such as from global fallout and release from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) reactor, and are potentially brought to Indonesian waters through the Indonesian Throughflow mechanism. Sea water samples were collected from the surface, thermocline, and deep layers during the expedition. The concentration of 137Cs on the surface was between 0.042–1.003 Bq m−3, the concentration range in the thermocline layer was 0.008–0.795 Bq m−3, and the concentration in the deep layer was 0.046–0.680 Bq m−3. The 134Cs concentration was below the detection limit, which indicates that the 137Cs comes from global fallout. In this research, the measurement of oceanographic parameters was also conducted, and the results showed that temperatures were in the range of 4.982–27.45 °C, salinity was in the range of 34.232–34.979 PSU, and the density was between 22.0979–27.4028 kg m−3. The salinity profile indicates that the eastern part had a lower salinity level than the western part. The Pacific Equatorial Water Mass was found to be the most dominant in the ITF. Furthermore, these oceanographic data were combined with 137Cs data to determine the distribution pattern of 137Cs both horizontally and vertically in the exit passage of the ITF.

Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of 137Cs from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of 137Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of 137Cs in litter and soil layers for 6 y (2011–2017) following the previous study (2011–2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The 137Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80–95% of the deposited 137Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of 137Cs inventory in the litter layer to the total 137Cs inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22–0.44 y−1. The slower migration was observed in the YC site, probably because of higher initial interception of 137Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the 137Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the 137Cs concentration in the litter within 2–3 y of the accident. The 137Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of 137Cs concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y−1. In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of 137Cs and its factors regarding both forest and soil characteristics.

Radiocesium and 40K distribution of river sediments and floodplain deposits in the Fukushima exclusion zone

In this study, radiocesium and 40K analysis were accomplished for samples of riverbed sediments and floodplain deposits collected from five rivers in the vicinity (<20 km) of the damaged Fukushima Daiichi Nuclear Power Plant after seven years of the accident. Sediment particle size distribution and major oxide content were determined also for six selected samples to understand the retention and migration process of radiocesium in river environments. The radiocesium activity concentration varied from 103 ± 6 Bq·kg−1 to 22,000 ± 500 Bq·kg−1 in riverbed sediments and from 92 ± 5 Bq·kg−1 to 117,000 ± 2000 Bq·kg−1 in floodplain deposits. The 134Cs/137Cs ratio (decay corrected to 15 March 2011) was 1.02 in the both samples. Compared to monitoring results in 2011, it was proved that the radiocesium distribution pattern had been changed remarkably during seven years. The radiocesium was primarily attached to fine clay particles but its sorption on sand and coarse sand particles was also considerable. The sorption process of radiocesium was not affected by the presence of water and moreover, after seven years of the Fukushima accident, a significant radiocesium migration cannot be expected without particle migration. Consequently, radiocesium will remain for a long time in the river environments and its redistribution is mainly affected by the erosion process of the sediments. The average 40K activity concentration of riverbed sediment and floodplain deposit samples was 640 ± 152 Bq·kg−1 changing from 319 ± 18 Bq·kg−1 to 916 ± 41 Bq·kg−1. In the river estuary zones, significant activity concentration decrements were observed for both radionuclides. This suggests that seawater intrusion has a decreasing effect on both natural and artificial radionuclides via wash-out of particulate radiocesium and 40K, and desorption of these radionuclides, but to reveal the detail of this process further investigations are required. The analysis of 40K can help in a simple and easy way to reveal the mineral composition differences of sediment samples.

Development of an Analytical Method for Estimating Three-Dimensional Distribution of Sediment-Associated Radiocesium at a Reservoir Bottom.

After the Fukushima Daiichi Nuclear Power Station accident, the distributions of sediment-associated radiocesium were investigated to evaluate the dispersion and accumulation of radiocesium in the reservoir field. To develop an analytical method for measuring the horizontal and vertical distributions of radiocesium on a wide scale, we obtained 253 gamma-ray spectra at the bottoms of 64 ponds in Fukushima during 2014-2016 by using a NaI(Tl) scintillation detector. For visualizing horizontal distribution, the correlation between detector counting rate and radiocesium concentration of the bottom sediment was confirmed. In estimating vertical distribution, the depth profile of sediment-associated radiocesium was found to be correlated to the intensities of scattered and photo peaks. Good agreement was observed between the results of in situ spectrometry and core sampling. These results indicate that the developed method is suitable for understanding the behavior of radiocesium and determining whether decontamination of reservoirs is required.

Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of 137Cs from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of 137Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of 137Cs in litter and soil layers for 6 y (2011–2017) following the previous study (2011–2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The 137Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80–95% of the deposited 137Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of 137Cs inventory in the litter layer to the total 137Cs inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22–0.44 y−1. The slower migration was observed in the YC site, probably because of higher initial interception of 137Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the 137Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the 137Cs concentration in the litter within 2–3 y of the accident. The 137Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of 137Cs concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y−1. In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of 137Cs and its factors regarding both forest and soil characteristics.

Radiocesium distribution in aggregate-size fractions of cropland and forest soils affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (137Cs) contamination in soils in a range of terrestrial ecosystems. It is well documented that the interaction of 137Cs with soil constituents, particularly clay minerals, in surface soil layers exerts strong control on the behavior of this radionuclide in the environment; however, there is little understanding of how soil aggregation—the binding of soil particles together into aggregates—can affect the mobility and bioavailability of 137Cs in soils. To explore this, soil samples were collected at seven sites under different land-use conditions in Fukushima and were separated into four aggregate-size fractions: clay-sized (<2 μm); silt-sized (2–20 μm); sand-sized (20–212 μm); and macroaggregates (212–2000 μm). The fractions were then analyzed for 137Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and 69%–83% of 137Cs was associated with macroaggregates and sand-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and approximately 80% of 137Cs was in the clay- and silt-sized fractions. Across all sites, the 137Cs extractability was higher in the sand-sized aggregate fractions than in the clay-sized fractions. Mineralogical analysis showed that, in most soils, clay minerals (vermiculite and kaolinite) were present even in the larger-sized aggregate fractions. These results demonstrate that larger-sized aggregates are a significant reservoir of potentially mobile and bioavailable 137Cs in organic-rich (forest and orchard) soils. Our study suggests that soil aggregation reduces the mobility of particle-associated 137Cs through erosion and resuspension and also enhances the bioavailability of 137Cs in soils.

Mineralogical control of the size distribution of stable Cs and radiocesium in riverbed sediments

Mineralogical control of the size distribution of stable Cs and radiocesium in riverbed sediments

THE INFLUENCE OF SOIL DENSITY AND THE CHARACTER OF RADIOACTIVE 134Cs And 137Cs POLLUTION’S DISTRIBUTION ON in situ MEASUREMENTS

Periodic radiation monitoring of soils today is a priority task not only for Belarus, but also for Japan, suffered by Fukushima nuclear power plant accident. Use of portable and light spectrometers with ability to perform in situ measurements makes it possible to quickly estimate specific activity of measured radionuclides with required accuracy in particular soil site. Basic information of a gamma radiation source (radionuclides content, effective radius of measurement area and thickness of contaminated layer) can be obtained directly during measurement. The purpose of this research is to test the feasibility of using algorithms for determination of specific activity and thickness of contaminated layer under conditions of soil measurement with variable density parameters and radiocesium distribution in soil.Monte-Carlo simulating allowed to estimate the degree of deviation of the shape of simulated spectra obtained with the use of Monte-Carlo soil model with uniformly distributed radionuclide in it, and for the case when the radionuclide distribution by soil profile can be described by an exponential function. For these cases of natural distribution of radiocesium, the pulse-amplitude spectrum is formed by an effective thickness of the contaminated site, which contains more than 90 % of radionuclides.The developed Monte-Carlo model of a probe and contaminated soil site allows to estimate the effect of the variability of soil density on the total count rate of the pulse-amplitude spectrum. As a result of theoretical estimations, the relationship between the effective radius of contaminated site is determined as a function of soil density.Analysis of the influence of radial zones of the cylindrical gamma source on in situ gamma-spectrometer showed that the main contribution to the total count rate of the pulse-amplitude spectrum is made by the radial zone with radius of up to 40 cm from the center of the probe, regardless of the thickness of the contaminated layer in geometry «Probe is located on the soil surface». A small site facilitates the selection of measurement area of land with a sufficiently flat surface, which is desirable during surveying the territories, especially with complex terrain.

Spatiotemporal distribution and fluctuation of radiocesium in Tokyo Bay in the five years following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident.

A monitoring survey was conducted from August 2011 to July 2016 of the spatiotemporal distribution in the 400 km2 area of the northern part of Tokyo Bay and in rivers flowing into it of radiocesium released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The average inventory in the river mouth (10 km2) was 131 kBq⋅m-2 and 0.73 kBq⋅m-2 in the central bay (330 km2) as the decay corrected value on March 16, 2011. Most of the radiocesium that flowed into Tokyo Bay originated in the northeastern section of the Tokyo metropolitan area, where the highest precipitation zone of 137Cs in soil was almost the same level as that in Fukushima City, then flowed into and was deposited in the Old-Edogawa River estuary, deep in Tokyo Bay. The highest precipitation of radiocesium measured in the high contaminated zone was 460 kBq⋅m-2. The inventory in sediment off the estuary of Old-Edogawa was 20.1 kBq⋅m-2 in August 2011 immediately after the accident, but it increased to 104 kBq⋅m-2 in July 2016. However, the radiocesium diffused minimally in sediments in the central area of Tokyo Bay in the five years following the FDNPP accident. The flux of radiocesium off the estuary decreased slightly immediately after the accident and conformed almost exactly to the values predicted based on its radioactive decay. Contrarily, the inventory of radiocesium in the sediment has increased. It was estimated that of the 8.33 TBq precipitated from the atmosphere in the catchment regions of the rivers Edogawa and Old-Edogawa, 1.31 TBq migrated through rivers and was deposited in the sediments of the Old-Edogawa estuary by July 2016. Currently, 0.25 TBq⋅yr-1 of radiocesium continues to flow into the deep parts of Tokyo Bay.

Spatial and vertical distribution of radiocesium in seawater of the East China Sea

The 137Cs activity in surface water of the East China Sea (ECS) was 0.66–1.36 Bq m−3 during May of 2011. The low activities were observed in the Changjiang Estuary and Zhejiang-Fujian coast and high activities were observed in the south offshore and Kuroshio Current pathway, suggesting that the influence from the current system in the ECS. The 134Cs were undetectable (<0.03 Bq m−3) and the contribution of the Fukushima accident to ECS is estimated to be below 3%; hence it is negligible during the investigation period. Using the vertical profiles of 137Cs in the ECS, the mass balance is obtained, which suggests that the oceanic input dominates the 137Cs source in the ECS. 137Cs is potentially useful to trace water mass movement in the ECS. Our study provides comprehensive baseline of 137Cs in the ECS for evaluation of the possible influence of the nuclear power plants in the future.

Vertical distribution of radiocesium affects soil-to-crop transfer coefficient in various tillage systems after the Fukushima Daiichi Nuclear Power Plant accident

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, radiocesium (134Cs and 137Cs) released into the atmosphere contaminated soil across a large part of northeastern Japan. In croplands, moldboard plow and rotary tillage systems redistribute the radiocesium to deeper layers, lowering the level of contamination. However, the long-term effects of tillage on radiocesium contamination of crops and the form and distribution of radiocesium in the soil after continuous tillage remain unclear. We investigated the distribution of radiocesium in soil and changes in plant uptake in organic soybean fields in Ibaraki (∼170 km from FDNPP) to clarify the tillage effect on radiocesium uptake while controlling contamination level. From 2011 to 2015, soybeans and winter cover crops (fallow weeds, rye, and hairy vetch) were cultivated using three tillage systems (moldboard plow/rotary harrow, MP; rotary cultivation, RC; and no tillage, NT). Radiocesium contamination in soil, and plant uptakes were considerably lower in 2012 compared with the first year of the accident in 2011. Four and 5 years after the accident, the NT treatment showed the highest absorption of radiocesium by soybeans and highest radiocesium deposit density in cover crops. Cover crop radiocesium contamination was significantly correlated with aboveground biomass. Soil surface (0–2.5 cm) radiocesium contamination in NT was significantly higher than at other depths and in other tillage treatments. A similar trend was observed in exchangeable radiocesium in the soil profile, although the values were only 23.9–48.0% (2014) and 32.6–71.9% (2015) of radiocesium in the soil. A positive correlation was also observed between exchangeable radiocesium at the soil surface and soybean radiocesium contamination, but this relationship was not observed for cover crop radiocesium contamination. The transfer factor (TF) of soybean grain and residue was highest in the NT treatment. The TF based on exchangeable cesium (eTF) was particularly high. Cover crops showed different TF and eTF values, but rye always had the lowest values. Soybean radiocesium contamination was higher in NT than in MP and RC. TF of soybean was significantly correlated with the depth of the vertical distribution of both radiocesium and exchangeable cesium. This research revealed that the TF value was significantly influenced by plant biomass accumulation and the soil radiocesium distribution.

Distribution of radiocesium in different density fractions of temperate forest soils in Fukushima

Abstract In forested areas of Fukushima, Japan, radiocesium emitted by the Fukushima Daiichi nuclear power plant accident has accumulated on the forest floor and in surface mineral soils. To estimate and model the mobility of radiocesium in forest soils, the concept of physical variations in soil organic matter is useful. We applied density fractionation to soil samples collected in two different forest stands in Fukushima; a deciduous broadleaf forest (DBF) and an evergreen needleleaf forest (ENF). To determine the relative contributions of the two density fractions to retention of radiocesium in surface mineral soils in a temperate forest region we investigated the concentrations and stocks of radiocesium in the low- and high-density fractions (LF and HF, respectively) of the soils, as well as in the litter, and temporal changes therein between 2012 and 2014. The 137Cs stock in the litter decreased from 2012 to 2014, whereas in the soil, as the sum of the LF and HF, it increased. On average, more than 40% of the 137Cs stock in surface mineral soil (depth of 0–5 cm) was retained in LF particles, which were composed mainly of particulate organic matter. Although the LF accounted for only 10.3% of the dry mass of the soil on average, it was also characterized by an average concentration of 137Cs that was 7.8 times higher than that of the HF, which was composed of organo-mineral particles. The mean increase in the 137Cs stock in the LF from 2012 to 2014 was equivalent to 70.1% and 52.5% of those in the soils in the DBF and ENF, respectively. The relationship between loss on ignition and 137Cs content indicated that the spatial heterogeneity of radiocesium in bulk soil was substantially affected by the heterogeneity in particulate organic matter. The coarse-size (>250 μm) of the LF suggested that it was less physically protected against microbial attack. Accordingly, the LF was considered to act as a large, but temporal reservoir of radiocesium that was originally retained in the litter layer. In contrast, the accumulation of 137Cs in the HF was low, especially in 2013–2014. The HF (or organo-mineral particles) might have a more important role than the LF in 137Cs fixation over a long period, and should be continuously studied as part of the current radiocesium monitoring network.

Downward migration of radiocesium in an abandoned paddy soil after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, continuous monitoring of the detailed vertical distribution of radiocesium in soil is required to evaluate the fate of radiocesium and establish strategies for remediation and management of the contaminated land. It is especially important to investigate paddy soil because little knowledge has been accumulated for paddy soil and wetland rice is a major staple in Japan. Therefore, we monitored the vertical distribution of 137Cs in abandoned paddy soil in a planned evacuation zone from June 2011 to March 2016. The decontamination works (i.e., 5 cm of surface soil removal and re-covering with uncontaminated soil) were conducted by the government in 2015. As a result of monitoring, the 137Cs gradually migrated downward with time and the 137Cs concentration in the 0–10 cm soil was almost homogenous in October 2014, although it was non-cultivated. The liner relationship was obtained between the median depth, which is the thickness of a soil layer containing half of the total 137Cs inventory, and the time after the accident, indicating the migration rate was constant (1.3 cm y−1) before the decontamination works. After the decontamination works, the 137Cs concentration in the uppermost surface layer was reduced by 90%, however the total 137Cs inventory was reduced by only 50–70%. It was shown that the efficiency of 137Cs removal by the decontamination works decrease linearly over time in fields like the studied paddy, in which the homogenization of 137Cs concentration occurred. Conversely, the downward migration of 137Cs to subsurface layers deeper than 10 cm (i.e., plowpan layer) with low permeability rarely occurred. It is expected that these unique trends in distribution and migration of 137Cs would be found in abandoned paddy soils with properties similar to the studied soil, sandy loam but poorly drained because of the low permeable plowpan layer, although further validation is necessary.

Characteristics of 137Cs accumulation by Quercus serrata seedlings infected with ectomycorrhizal fungi

ABSTRACT To study the effects of inoculation with ectomycorrhizal fungi on the accumulation and distribution of radiocesium (137Cs) in different plant organs (leaves, stems and roots), we measured the uptake and internal distribution of 137Cs by ectomycorrhizal and non-ectomycorrhizal Quercus serrata seedlings in contaminated forest soil. Contamination was a result of the Fukushima Dai-ichi nuclear Power Plant accident. We also estimated the transfer factor (TF), photosynthetic rate and growth of Q. serrata seedlings during the study period, which lasted a full year. The seedlings were grown in plastic pots under field conditions. The soils used in this study had been contaminated with 7200 Bq kg−1 dm of 137Cs at Fukushima in 2011. The concentration of 137Cs in the plant organs varied between 148, 252 and 294 Bq kg−1 dm in the non-ectomycorrhizal treatment regime for stems, leaves and roots, respectively, and was 47, 69 and 219 Bq kg−1 dm in the ectomycorrhizal treatment regime. The TF was significantly lower (P < 0.01) in the ectomycorrhizal treatment regime EC, and apparent quantum yield (α) and maximum photosynthetic rate at light saturation (P max) was significantly higher (P < 0.01), although the growth parameters (stem diameter, height and dry mass) did not differ significantly.

Reprint of “Vertical migration of radio-caesium derived from the Fukushima Dai-ichi Nuclear Power Plant accident in undisturbed soils of grassland and forest”

The vertical distribution of radio-caesium (137Cs and 134Cs) in undisturbed soil profiles of grassland and forest soils, derived from the Fukushima-Daiichi Nuclear Power Plant (FDNPP) accident that occurred on 11 March 2011, was studied. Surface soil and depth profile soil samples were collected from six locations within the 20km zone of FDNPP, during November 2012 and June 2013. The activity ratio for 137Cs and 134Cs was found to be almost constant about 1 within the soil profiles as well as in the surface soil, indicative of FDNPP accident origin. From soil depth profile distribution of Cs activity, it is observed that Cs is strongly bound to soil materials, which slows Cs migration. >90% of the activity was found to be retained within the upper 5cm layer. Retardation of Cs movement has been quantified by measuring sorption of Cs in soil in terms of distribution coefficient (Kd) using the laboratory batch method. Faster migration has been observed in case of forest land soil compared to grassland soil. The empirical migration velocity of Cs radio isotope was estimated from the depth profile Cs concentration and found to vary from 1.1 to 1.7 and 0.85 to 3.5cmy−1 in grassland and forest soil, respectively. The residential half life for Cs isotopes was found to be 1.03–7.75y and 1.18–4.67y for grassland and forest land respectively using a compartmental model. In addition to the empirical analysis of the profiles, analytical models were fitted to the data which may help elucidate the physical nature of the transport of trace elements.

Shifts of radiocesium vertical profiles in sediments and their modelling in Japanese lakes

Vertical profiles of radiocesium concentrations were measured in sediment cores collected at various times after the 2011 Fukushima nuclear accident in five Japanese lakes (Hinuma, Kasumigaura, Kitaura, Onogawa and Sohara) with different morphological and trophic characteristics in order to investigate the sedimentation-diffusion processes. In lakes where sediments had high porosities and experienced considerable wave action due to shallowness, we observed rapid penetration of radiocesium to a certain depth just after the accident, followed by downward movement of the peak depths. In contrast, gradual downward transfers of distinct peaks were found in other types of lakes. A one-dimensional differential sediment model with water-sediments interaction processes was constructed to describe the vertical shift of radiocesium profiles. Our proposed submodels relating to the length scales of the mixing using wind-induced stress and porosity of sediments were constructed based on one measurement of the vertical distribution of radiocesium in three lakes (Hinuma, Kasumigaura and Sohara). This model was then validated using samples from those lakes in different years, as well as from two other lakes. Good agreement was obtained. We discuss our findings, the limits of model application, and future research targets.

Radioactive and stable cesium isotope distributions and dynamics in Japanese cedar forests

Dynamics of the Fukushima-derived radiocesium and distribution of the natural stable isotope 133Cs in Japanese cedar (Cryptomeria japonica D. Don) forest ecosystems were studied during 2014–2016. For the experimental site in Yamakiya, Fukushima Prefecture, we present the redistribution of radiocesium among ecosystem compartments during the entire observation period, while the results obtained at another two experimental site were used to demonstrate similarity of the main trends in the Japanese forest ecosystems. Our observations at the Yamakiya site revealed significant redistribution of radiocesium between the ecosystem compartments during 2014–2016. During this same period radionuclide inventories in the aboveground tree biomass were relatively stable, however, radiocesium in forest litter decreased from 20 ± 11% of the total deposition in 2014 to 4.6 ± 2.7% in 2016. Radiocesium in the soil profile accumulated in the 5-cm topsoil layers. In 2016, more than 80% of the total radionuclide deposition in the ecosystem resided in the 5-cm topsoil layer.The radiocesium distribution between the aboveground biomass compartments at Yamakiya during 2014–2016 was gradually approaching a quasi-equilibrium distribution with stable cesium. Strong correlations of radioactive and stable cesium isotope concentrations in all compartments of the ecosystem have not been reached yet. However, in some compartments the correlation is already strong. An increase of radiocesium concentrations in young foliage in 2016, compared to 2015, and an increase in 2015–2016 of the 137Cs/133Cs concentration ratio in the biomass compartments with strong correlations indicate an increase in root uptake of radiocesium from the soil profile. Mass balance of the radionuclide inventories, and accounting for radiocesium fluxes in litterfall, throughfall and stemflow, enabled a rough estimate of the annual radiocesium root uptake flux as 2 ± 1% of the total inventory in the ecosystem.

Temporal changes in the radiocesium distribution in forests over the five years after the Fukushima Daiichi Nuclear Power Plant accident

To elucidate the temporal changes in the radiocesium distribution in forests contaminated by the Fukushima Daiichi Nuclear Power Plant accident, we monitored the 137Cs concentration and inventory within forests from 2011 to 2015 across nine plots containing variable tree species and different contamination levels. The 137Cs concentrations in needles and branches decreased exponentially at all coniferous plots, with effective ecological half-lives of 0.45–1.55 yr for needles and 0.83–1.69 yr for branches. By contrast, the 137Cs concentration in deciduous konara oak leaves did not change over the five years. The concentration of 137Cs in oak wood increased by 37–75%, whereas that in Japanese red pine decreased by 63% over the five years. In Japanese cedar and hinoki cypress, the 137Cs concentration in wood showed an increasing trend in half of the plots. The changes in 137Cs in the organic and mineral soil layers were not strongly related to the tree species or contamination level. Our multi-site, multi-species monitoring results revealed that the pattern of temporal changes in radiocesium in the 9 forest plots was similar overall; however, changes in 137Cs in needles/leaves and wood differed among tree species.

Estimation of the Vertical Distribution of Radiocesium in Soil on the Basis of the Characteristics of Gamma-Ray Spectra Obtained via Aerial Radiation Monitoring Using an Unmanned Helicopter.

After the Fukushima Daiichi Nuclear Power Plant accident, the vertical distribution of radiocesium in soil has been investigated to better understand the behavior of radiocesium in the environment. The typical method used for measuring the vertical distribution of radiocesium is troublesome because it requires collection and measurement of the activity of soil samples. In this study, we established a method of estimating the vertical distribution of radiocesium by focusing on the characteristics of gamma-ray spectra obtained via aerial radiation monitoring using an unmanned helicopter. The estimates are based on actual measurement data collected at an extended farm. In this method, the change in the ratio of direct gamma rays to scattered gamma rays at various depths in the soil was utilized to quantify the vertical distribution of radiocesium. The results show a positive correlation between the abovementioned and the actual vertical distributions of radiocesium measured in the soil samples. A vertical distribution map was created on the basis of this ratio using a simple equation derived from the abovementioned correlation. This technique can provide a novel approach for effective selection of high-priority areas that require decontamination.

Interpreting the deposition and vertical migration characteristics of 137Cs in forest soil after the Fukushima Dai-ichi Nuclear Power Plant accident.

We investigated the deposition and depth distributions of radiocesium in the Takizawa Research Forest, Iwate Prefecture, in order to understand the behavior of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant. The deposition distribution and vertical depth distribution of radiocesium in the soil were compared between topographically distinct parts of the forest where two different tree species grow. The results for all investigated locations show that almost 85% of the radiocesium has accumulated in the region of soil from the topmost organic layer to a soil depth of 0-4cm. However, no activity was detected at depths greater than 20cm. Analysis of the radiocesium deposition patterns in forest locations dominated by either coniferous or deciduous tree species suggests that radiocesium was sequestered and retained in higher concentrations in coniferous areas. The deposition data showed large spatial variability, reflecting the differences in tree species and topography. The variations in the measured 137Cs concentrations reflected the variability in the characteristics of the forest floor environment and the heterogeneity of the initial ground-deposition of the Fukushima fallout. Sequential extraction experiments showed that most of the 137Cs was present in an un-exchangeable form with weak mobility. Nevertheless, the post-vertical distribution of 137Cs is expected to be governed by the percentage of exchangeable 137Cs in the organic layer and the organic-rich upper soil horizons.