Mechanisms of 137Cs leaching based on long-term observations in forested headwater catchments in Yamakiya, Fukushima Prefecture, Japan

Dissolved radiocesium (mainly 134Cs and 137Cs) is thought to be leached mainly from suspended sediment in downstream rivers, while organic matter, such as leaf litter, contributes to catchments in forested headwater streams. It is also known that dissolved 137Cs in headwater streams exhibit seasonal variation with water temperature. Some mechanisms have been proposed as the causes: ionic competition of potassium ion (K+) and ammonium ion (NH4+), leaching associated with the decomposition of organic matter, and thermodynamic adsorption-desorption processes. We investigated the relationship between K+ and the dissolved organic carbon (DOC) concentrations and seasonal changes in dissolved 137Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in 137Cs concentrations in coarse organic matter, suspended sediments, and dissolved forms at four sites (one decontaminated site and three undecontaminated sites) in the Yamakiya area since 2011. The distribution coefficients (Kdcss and Kdorg) of dissolved 137Cs concentrations relative to suspended sediment and coarse organic matter 137Cs concentrations were calculated, and differences in temporal changes due to decontamination were investigated. In addition, we examined the relationship between water temperature and DOC, K+, NH4+ and 137Cs concentrations in the headwater catchments. The suspended sediment 137Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved 137Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH4+ has been detected in headwater streams in our catchments. In the SET and ISH watersheds, where the distance from the groundwater spring is short, a correlation was found between DOC concentration and dissolved 137Cs concentration. In contrast, in the IBO watershed, where the distance from the groundwater spring is long, temperature dependence and a good correlation between K+ and 137Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved 137Cs in forested headwater streams at short distances from the spring, but the influence of competing ions is expected to increase gradually as the water flows downstream.

Changes in air dose rates due to soil moisture content in the Fukushima prefecture forests

Radionuclides released and deposited because of the 2011 Fukushima Dai-ichi Nuclear Power Plant accident caused an increase in air dose rates in Fukushima Prefecture forests. Although an increase in air dose rates during rainfall was previously reported, the air dose rates in the Fukushima forests decreased during rainfall. This study aimed to develop a method to estimate rainfall-related changes in air dose rates, even in the absence of soil moisture data, in Namie-Town and Kawauchi-Village, Futaba-gun, Fukushima Prefecture. Moreover, we examined the relationship between preceding rainfall (Rw) and soil moisture content. The air dose rate was estimated by calculating the Rw in Namie-Town from May to July 2020. We found that the air dose rates decreased with increasing soil moisture content. The soil moisture content was estimated from Rw by combining short-term and long-term effective rainfall using half-live values of 2 h and 7 d and considering the hysteresis of water absorption and drainage processes. Furthermore, the soil moisture content and air dose rate estimations showed a good agreement with coefficient of determination (R2) scores >0.70 and >0.65, respectively. The same method was tested to estimate the air dose rates in Kawauchi-Village from May to July 2019. At the Kawauchi site, variation in estimated value is relatively large due to the presence of water repellency in dry conditions, and the amount of 137Cs inventory was low, so estimating air dose from rainfall remained a challenge. In conclusion, rainfall data were successfully used to estimate soil moisture and air dose rates in areas with high 137Cs inventories. This leads to the possibility of removing the influence of rainfall on measured air dose rate data and could contribute to the improvement of methods currently used to estimate the external air dose rates for humans, animals, and terrestrial forest plants.

Concurrent datasets on land cover and river monitoring in Fukushima decontaminated catchment during 2013-2018

After the Fukushima nuclear accident, the Japanese government implemented extensive decontamination work in 137Cs contaminated catchments for residents' health and local revitalization. Whether dramatic land use changes in the upstream decontaminated regions affected river suspended sediment (SS) and particulate 137Cs discharge downstream remain unknown because of the poor quantification on land cover changes and long-term river SS dynamics. We here introduce a 6-year concurrent database of the Niida River Basin, a decontaminated catchment, including the first available vector decontamination maps, satellite images in decontaminated regions with a spatial resolution of 10 m, and long-term river monitoring datasets spanning decontamination (2013-2016) and subsequent natural restoration stages (2017-2018). These datasets allow us, for the first time, to directly link the transport dynamics of river SS (particulate 137Cs) to land use changes caused by humans in real-time, which provide fundamental data for better understanding the river response of sediment to land use change. Moreover, the data obtained by interdisciplinary methods offer a template for land use change impact assessment in other river basins.

Hydrological setting controls 137Cs and 90Sr concentrations in a headwater catchment in the Chornobyl Exclusion Zone

Concentration-discharge relationships are widely used to understand the hydrological processes controlling river water chemistry. This study investigates how hydrological processes affect radionuclide (¹³⁷Cs and ⁹⁰Sr) concentrations in surface water in headwater catchments within the Chornobyl Exclusion Zone (ChEZ) in Ukraine. In the flat wetland catchments, the depth of the saturated soil layer changes little throughout the year, but changes in the saturated soil surface area during snowmelt and immediately after rainfall affect water chemistry by changing the opportunities for contact between the surface water and the soil surface. On the other hand, in the slope catchments where there are few wetlands, the water chemistry of river water is governed by changes in the relative contributions of "shallow water" and "deep water" due to changes in the catchment water supply pathways feeding the rivers. In this study, no correlations were observed between dissolved or suspended ¹³⁷Cs concentrations and either discharge rates or competitive cations, but the solid-liquid ratio of ¹³⁷Cs was found to be significantly and negatively correlated with water temperature. However, ⁹⁰Sr concentrations in surface water were found to be strongly related to the water pathways for each of the catchments. Moreover, contact between the surface water and the soil surface and changes in the relative contributions of shallow and deep waters to stream water were correlated with changes in ⁹⁰Sr concentrations in surface water in wetland and slope catchments, respectively. The study concludes that ⁹⁰Sr in rivers inside the ChEZ are strongly affected by the water pathways in headwater catchments. Additional studies will be necessary to clarify the details of sorption/desorption reactions.

Combined use of UAV-SfM surveys, soil particle tracking with RFID tags and a sediment connectivity index to study plot-scale sediment transport

To explore the processes of soil erosion at the plot scale, Digital surface model of Differences (DoD) maps (Unmanned Aerial Vehicle - Structure from Motion (UAV-SfM) method) and data from Radio Frequency Identification (RFID) tags were analysed. The comparison of differences in accuracy of UAV-SfM and 3D terrestrial laser scanner (TLS) measurements, and the integration of the UAV-SfM method and soil particle tracing with RFID tag locations were conducted to assess sediment transport in a plot in Fukushima prefecture, Japan. The Universal Soil Loss Equation (USLE) plot was installed and kept with no vegetation and no cultivation. Water and sediment discharges were measured at the outlet of the plot, and the topographic index of runoff and sediment connectivity (IC) -focused on surface roughness- was also estimated. Based on field surveys, four periods were defined. Locations of RFID tags were firstly determined by using orthoimages derived from the UAV-SfM method and then compared with those locations measured with a laser total station. The mean and standard deviation of difference amounts of UAV-SfM were of 1 and 3.3 mm, respectively. On average, the RFID tags were located with an accuracy of 3.1 cm (RMSE). Although data of tags tracing showed short transport distances with rill erosion, the results of the UAV-SfM surveys showed an increase of sediment connectivity (SC) over the study period that may explain the largest sediment discharge, especially of fine soil particles. The concurrence of higher values of SC as well as the development of new and longer rills demonstrated the important activity of net soil loss in our study site. The combination of distinct methods and techniques, all providing accurate measurements, shed light on the sediment transport process at short distances, which affects the net water and sediment discharge at larger scales.

Evaluating changes in radionuclide concentrations and groundwater levels before and after the cooling pond drawdown in the Chornobyl Nuclear Power Plant vicinity

In the vicinity of the Chornobyl Nuclear Power Plant (ChNPP), the cooling pond (CP) was an artificially maintained reservoir with water levels regulated to 7 m above the Pripyat River until May 2014, when its pumps stopped operating, resulting in a natural drawdown. To investigate the surface-groundwater system before and after the drawdown, we evaluated the spatial and temporal changes in ⁹⁰Sr and ¹³⁷Cs radionuclide concentrations and groundwater levels in the shallow unconfined aquifer near the ChNPP from 2010 to 2019. Additionally, we compared water levels and ⁹⁰Sr concentrations in Azbuchin Lake, wetlands inside the CP, and the Pripyat River. Using three-year averages before (2011-2013) and after (2017-2019) the drawdown period, we found that ⁹⁰Sr concentrations significantly increased up to 10² kBq/m³ in the Pripyat River floodplain, north of ChNPP, exceeding the WHO drinking water guideline of 10 kBq/m³. In contrast ¹³⁷Cs concentrations ranged consistently between 10 and 100 Bq/m³. The groundwater levels decreased over 50 cm at approximately 65 % of shallow monitoring wells and up to 6 m near the CP. The ⁹⁰Sr concentration increases in some wells at the Pripyat River floodplain were associated with decreased dilution rates from the CP due to the reduced CP leakage, causing changes in groundwater flow direction and decreases in groundwater velocity. From the new finding of this study that the drawdown increased ⁹⁰Sr concentrations near the floodplain, we estimated the ⁹⁰Sr flux and contribution to the Pripyat River and the ⁹⁰Sr contribution did not change significantly after the drawdown. However, radionuclides may accumulate more at the floodplain in the future; therefore, additional monitoring is required to verify ⁹⁰Sr transport from areas of elevated concentrations and its impact on groundwater in the aquifer.

Mode of Atmospheric Deposition in Forests Demonstrates Notable Differences in Initial Radiocesium Behavior

The March 2011 Fukushima Dai-ichi Nuclear Power Plant accident in Japan released 520 PBq of radionuclides compared to a total release of 5300 PBq from the Chornobyl Nuclear Power Plant accident. Both nuclear accidents resulted in deposition of radiocesium throughout the northern hemisphere, and a plethora of studies have been performed regarding radiocesium (¹³⁷Cs) behavior. However, few studies have assessed the impact of precipitation on ¹³⁷Cs deposition in forests. Wide-scale environmental measurements from 2011 and 2016 were used to determine the differences in ¹³⁷Cs deposition because of precipitation following the Fukushima accident. In areas where wet deposition processes were dominant, dense forests generally had lower ambient dose rates and levels of contamination on forest floors than other stands with fewer stems per hectare in 2011. Similar tendencies were not observed in areas that were primarily subject to dry deposition nor were any trends observed in 2016. ¹³⁷Cs was retained in dense forest canopies for an extended period regardless of the deposition mode. Additionally, it was found that the initial retention of radionuclides by forest canopies is in general higher for areas with predominantly dry deposition. Incorporation of radiocesium into wood tissues was the same for both wet and dry deposition.

Radiocesium accumulation in Lake Kasumigaura by riverine input and migration following the Fukushima Dai-ichi nuclear power plant accident

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 (¹³⁷Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the ¹³⁷Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The ¹³⁷Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating ¹³⁷Cs migration. However, the rate of decrease subsequently slowed due to the ¹³⁷Cs supply from the river. The ¹³⁷Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate ¹³⁷Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in ¹³⁷Cs concentrations will be smaller. The ¹³⁷Cs concentration in the Koise River will continue to decrease; thus, the difference in the ¹³⁷Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of ¹³⁷Cs in the entire lake were 3.72 × 10¹² Bq in December 2012 and 4.18 × 10¹² Bq in August 2016. The accumulated amount of ¹³⁷Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate ¹³⁷Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of ¹³⁷Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes.

Distribution of radiocesium and its controlling factors under the Japanese cedar canopies

This study investigated the spatial distribution of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident in a densely planted Japanese cedar stand. Systematic grid sampling was conducted to determine ¹³⁷Cs inventories in the layers of deposited organic material and mineral soil at two different spatial scales (hillslope [60 m²] and small [1 m²]). The results showed that ¹³⁷Cs inventories along the hillslope were heterogeneously distributed, with coefficients of variation for the deposited organic material and mineral soil layers of 46.4% and 48.9%, respectively. The ¹³⁷Cs inventory in each layer tended to show a lognormal distribution. The correlation between the ¹³⁷Cs inventories in deposited organic material and mineral soil in the same sampling grid was weak. The controlling mechanisms of the ¹³⁷Cs inventories in the litter and mineral soil layers differed due to differences in the underlying key processes, such as canopy-forest floor transfer due to hydrological and biological processes. No significant correlation was found between the distance from the nearest tree trunk and the ¹³⁷Cs inventory in the deposited organic layer at each sampling point. In contrast, the ¹³⁷Cs inventory in the soil tended to increase as the distance from the nearest tree trunk increased at both the hillslope and small scales. It was found that the initial spatial patterns of ¹³⁷Cs in the soil layer due to atmospheric deposition were preserved in the cedar stand. Finally, we tested the effects of soil sampling density on the reliability of mean soil ¹³⁷Cs inventory estimations in the cedar stand. The results indicated that a soil sampling area greater than 0.06 m² at the hillslope scale and 0.008 m² at the small scale enabled the mean ¹³⁷Cs inventory to be estimated with an uncertainty of less than 20% in the cedar stand.

Understory biomass measurement in a dense plantation forest based on drone-SfM data by a manual low-flying drone under the canopy

Forest is an important part of the environmental system, which has a significant impact on soil hydrological characteristics and forest landscapes, because these processes are influenced by forest management and understory vegetation. Quantitative understory vegetation biomass (UVB) measurement and estimation are vital processes in forest ecology and environmental management. However, these estimations are difficult to make on a large scale especially dense planted forest. Here, we applied catchment-scale Structure from Motion and a manually operated ultralow-flying drone under the canopy of a dense planted (2000 stems/ha) Japanese cedar/cypress plantation forest to reconstruct the understory. An understory drone survey was performed over a 1.1-ha sub-catchment to generate a canopy height model based on dense point cloud data. A biomass survey of three 16-m² harvesting plots was conducted to compute understory vegetation volume data based on point clouds. Combined with harvested biomass data in the field, quantitative models were developed between the understory vegetation volume and biomass. Subsequently, the models were used to map spatial understory vegetation biomass distribution in the sub-catchments. Aerial photos taken by the ultralow-flying drones under the canopy yielded a high-resolution catchment-scale understory with point cloud density >10/cm². Strong cubic model coefficients of determination (R² = 0.75) predicted the understory vegetation biomass based on the canopy height model. The mean understory vegetation biomass was 0.82 kg/m² and dominated by low ferns. In the present study, we successfully reconstructed the multilayered forest structure and generated understory vegetation biomass distribution models. This results also will be essential to evaluate the erosion and evapotranspiration in dense plantation forests and future environmental management.

Pre- and post-accident environmental transfer of radionuclides in Japan: lessons learned in the IAEA MODARIA II programme

An international review of radioecological data derived after the accident at the Fukushima Daiichi nuclear power plant was an important component of activities in working group 4 of the IAEA Models and data for radiological impact assessment, phase II (MODARIA II) programme. Japanese and international scientists reviewed radioecological data in the terrestrial and aquatic environments in Japan reported both before and after the accident. The environmental transfer processes considered included: (a) interception and retention radionuclides by plants, (b) loss of radionuclides from plant and systemic transport of radionuclides in plants (translocation), (c) behaviour of radiocaesium in soil, (d) uptake of radionuclides from soil by agricultural crops and wild plants, (e) transfer of radionuclides from feedstuffs to domestic and wild animals, (f) behaviour of radiocaesium in forest trees and forest systems, (g) behaviour of radiocaesium in freshwater systems, coastal areas and in the ocean, (h) transport of radiocaesium from catchments through rivers, streams and lakes to the ocean, (i) uptake of radiocaesium by aquatic organisms, and (j) modification of radionuclide concentrations in food products during food processing and culinary preparation. These data were compared with relevant global data within IAEA TECDOC-1927 'Environmental transfer of radionuclides in Japan following the accident at the Fukushima Daiichi Nuclear Power Plant'. This paper summarises the outcomes of the data collation and analysis within MODARIA II work group 4 and compares the Japan-specific data with existing radioecological knowledge acquired from past and contemporary radioecological studies. The key radioecological lessons learned are outlined and discussed.

Vertical distribution and transport of radiocesium via branchflow and stemflow through the canopy of cedar and oak stands in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident

Aiming to fill a need for data regarding radiocesium transport via both branchflow and stemflow through forests impacted by radioactive fallout, this study examined the vertical variation of radiocesium flux from branchflow and stemflow through the canopies of young Japanese cedar (Cryptomeria japonica (L. f.) D. Don) and Japanese oak (Quercus serrata Murray) trees in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident. In forested areas approximately 40 km northwest of the location of the Fukushima Dai-ichi accident, the ¹³⁷Cs concentration varied significantly among sampling periods and between the two forests, with the oak stand exhibiting higher ¹³⁷Cs concentrations and depositional fluxes than the cedar stand. Expressed per unit trunk basal area, the depositional flux of ¹³⁷Cs generated from the cedar and oak stands was 375 and 2810 Bq m^-2 year^-1, respectively. Of this total, 71% and 48% originated from the cedar and oak canopy, respectively, while the remainder originated from the trunk. Accordingly, the origin of radiocesium was more balanced for the oak stand with almost half of the flux coming from the canopy (48%) and the other half from the trunk (52%). Only about a quarter (29%) of the radiocesium flux originated from the trunk in Japanese cedar. Results from this work provide needed data that can enable a more thorough conceptualization of radiocesium cycling in forests. Coupling these empirical results with a physically-based model would likely lead to better forest management and proactive strategies for rehabilitating radioactively-contaminated forests and reducing the exposure risk of radiation dose rate for those that utilize forest products.

A storm-induced flood and associated nearshore dispersal of the river-derived suspended 137Cs

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment.

Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of 137Cs derived from the FDNPP accident in a cedar forest soil

The vertical distribution of ¹³⁷Cs in forest soil is important for predicting air dose rates and future cycling in forest ecosystems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the ¹³⁷Cs flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of ¹³⁷Cs has been conducted since 2011. As a result, the ¹³⁷Cs concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the ¹³⁷Cs inventory in the litter layer has been exponentially decreasing, the annual ¹³⁷Cs fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14-0.17% of the deposition density of ¹³⁷Cs. Comparing these ¹³⁷Cs fluxes with the apparent amounts of downward migration of ¹³⁷Cs estimated from the change in the vertical distribution of ¹³⁷Cs, the contribution rate of the infiltrated water to downward migration of ¹³⁷Cs from litter to soil was calculated to be 8.5-17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6-0.8% on a measured basis and estimated to be 3.0 ± 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of ¹³⁷Cs, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots.

Radiocesium leaching from litter during rainstorms in the Fukushima broadleaf forest

Forests are important sources of dissolved radiocesium (¹³⁷Cs) discharge downstream. To improve understanding of dissolved ¹³⁷Cs discharge processes during rainstorms, we investigated the relationship between rainfall-runoff hydrological processes and the discharge of ¹³⁷Cs leached from litter. Leaching tests were conducted with broadleaf litter collected in the area where saturated overland flow was generated during rainstorms in a broadleaf-tree-dominated forest. According to the leaching test results, the ¹³⁷Cs leaching rate was higher in the early stage of the test and decreased afterward. There was no significant difference in the overall results between the agitation and non-agitation cases. The ¹³⁷Cs leaching rate from litter after the 24-h test was up to 33.7%. A large proportion of the original ¹³⁷Cs activity was present even after the tests, as leaching from litter during rainstorms in the headwater area could be an additional source of dissolved ¹³⁷Cs in the stream water. If mixing of ¹³⁷Cs originating from groundwater, soil water, and rainfall with the hydrological processes is assumed, differences between the observed and estimated ¹³⁷Cs in the surface runoff water became larger under high flow conditions. This analysis indicates additional ¹³⁷Cs loading on surface runoff water during rainstorms, where saturated surface area can expand as the surface runoff rate increases. Contact area between surface runoff and litter accumulated on the forest floor should increase and accelerate ¹³⁷Cs leaching from the litter. Therefore, ¹³⁷Cs leaching in the saturated surface area that is temporarily formed during rainstorms can play a principal role in dissolved ¹³⁷Cs discharge during rainfall-runoff events. Contaminated litter in the temporally saturated region of forested headwaters is an important factor contributing to elevated levels of dissolved ¹³⁷Cs during rainstorms in the Fukushima area.

Specific Inhibitor of Placental Alkaline Phosphatase Isolated from a DNA-Encoded Chemical Library Targets Tumor of the Female Reproductive Tract

Placental alkaline phosphatase (PLAP) is an abundant surface antigen in the malignancies of the female reproductive tract. Nevertheless, the discovery of PLAP-specific small organic ligands for targeting applications has been hindered by ligand cross-reactivity with the ubiquitous tissue non-specific alkaline phosphatase (TNAP). In this study, we used DNA-encoded chemical libraries to discover a potent (IC₅₀ = 32 nM) and selective PLAP inhibitor, with no detectable inhibition of TNAP activity. Subsequently, the PLAP ligand was conjugated to fluorescein; it specifically bound to PLAP-positive tumors in vitro and targeted cervical cancer in vivo in a mouse model of the disease. Ultimately, the fluorescent derivative of the PLAP inhibitor functioned as a bispecific engager redirecting the killing of chimeric antigen receptor-T cells specific to fluorescein on PLAP-positive tumor cells.

Scots pine stands biomass assessment using 3D data from unmanned aerial vehicle imagery in the Chernobyl Exclusion Zone

Thirty-five years after the accident, large forest areas in the Chernobyl Exclusion Zone still contain huge amounts of radionuclides released from the Chernobyl Nuclear Power Plant Unit 4 in April 1986. An assessment of the radiological and radioecological consequences of persistent radioactive contamination and development of remediation strategies for Chernobyl forests imply acquiring comprehensive data on their contamination levels and dynamics of biomass inventories. The most accurate forest inventory data can be obtained in ground timber cruises. However, such cruises in radioactive contaminated forest ecosystems in the Chernobyl Exclusion Zone result in radiation exposures of the personnel involved, which means the need for development of the remote sensing methods. The purpose of this study is to analyze the applicability and limitations of the photogrammetric method for the remote large-scale monitoring of aboveground biomass inventories. Based on field measurements, we estimated the biomass inventories in 31 Scots pine stands including both artificial plantations and natural populations. The stands differed significantly in age (from a few years in natural populations to 115 years in the oldest plantation), productivity (from 0.4 to 19.8 kg m^-2), mean height (from 4.1 to 36 m), and other parameters. Photogrammetric data were obtained from the same stands using unmanned aerial vehicle (UAV). These data were then processed using two approaches to derive the canopy height model (CHM) parameters which were tested for correlation with the aboveground biomass inventories. In the first approach, we found that the inventories correlated well with the mean value of CHM of the site (R² = 0.79). In the second approach, the total aboveground biomass was approximated by a function of the average height of trees detected at the site and the total crown projection area (R² = 0.78). Among other local parameters, the total crown projection area was identified as the major factor impacting the accuracy of the aboveground biomass inventory estimates from the UAV survey data in both approaches. In the dense stands with the high total crown projections areas (more than 0.90), the average relative deviations of the UAV-based aboveground biomass estimates from the results of the field measurements were close to 0, which means the adequate accuracy of the UAV surveys data for radioecological monitoring purposes. The relative deviations of the UAV-based estimates in both approaches increased in the stands consisting of separated groups of trees, which indicates potential limitation of the approaches and need for their further development.

Impacts of freeze-thaw processes and subsequent runoff on 137Cs washoff from bare land in Fukushima

The deposited ¹³⁷Cs is one of the long-lived radionuclides, that was released following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, has been hydrologically transported as particulates in the terrestrial environment of the Fukushima region. The impact of freeze-thaw processes and subsequent runoff affecting the ¹³⁷Cs flux and concentration in sediment discharge were revealed in bare land erosion plot following the FDNPP accident by detailed monitoring and laser scanner measurement on the soil surface. We found that surface topographic changes due to the frost-heaving during the winter-spring period, and rill formation during the summer. We also found the evident seasonal changes in ¹³⁷Cs concentration; high during the early spring and gradually decreased thereafter, then surface runoff from the plot frequently occurred during spring and autumn when rainfall was high and reached a maximum in summer. From these results, the higher ¹³⁷Cs concentration in spring was caused by a mixture of unstable surface sediment following freeze-thaw processes and then transported in the early spring, but erosion amount is not significant because of the less rainfall event. The sediment with a lower ¹³⁷Cs concentration, which was supplied from the rill erosion and its expansion, was wash-offed during the summer, contributing most of the flux from erosion in bare land in Fukushima region. In case, heavy rainfall occurs in the early spring, caution is required because high concentrations of cesium may flow down into the river.

A DNA-Encoded Chemical Library Based on Peptide Macrocycles

The synthesis and characterization of a novel DNA-encoded library of macrocyclic peptide derivatives are described; the macrocycles are based on three sets of proteinogenic and non-proteinogenic amino acid building blocks and featuring the use of copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction for ring closure. The library (termed YO-DEL) which contains 1 254 838 compounds, was encoded with DNA in single-stranded format and was screened against target proteins of interest using affinity capture procedures and photocrosslinking. YO-DEL selections yielded specific binders against serum albumins, carbonic anhydrases and NKp46, a marker of activated Natural Killer cells.

Soil and vegetation sampling during the early stage of Fukushima Daiichi Nuclear Power Plant accident and the implication for the emergency preparedness for agricultural systems

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, immediate soil and vegetation sampling were conducted according to the action plan of nuclear emergency monitoring; however, analysing the monitoring dataset was difficult because the sampling protocols were not standardised. In this study, the sampling protocols applied just after the FDNPP accident were reviewed, and the monitoring data were analysed. The detailed protocols and results can provide a sound basis for guidelines of soil and vegetation sampling for nuclear emergency monitoring. The activity concentrations of ¹³⁷Cs and ¹³¹I in weed samples measured immediately after the FDNPP accident were related to the air dose rate at 1 m. Consequently, vegetation sampling is recommended when the additional dose rate (above background) is higher than 0.1 μSv/h. To enhance the efficiency of a protective response in the case of a nuclear accident, predetermined sampling points for soil and vegetation sampling should be considered in the preparedness plan for nuclear emergencies. Furthermore, sampling and analytical measurement capacities (time, people, cost) during the early phase after nuclear emergencies need to be considered in the preparedness and action plan, and sampling and measurement exercises are highly recommended.

Differences in leaching characteristics of dissolved radiocaesium and potassium from the litter layer of Japanese cedar and broadleaf forests in Fukushima, Japan

Cesium is an element that belongs to the same group as K, and is known to show similar behaviour to that of K in plants. In this study, we conducted a serial leaching test for 120 h to compare the leaching characteristics of dissolved ¹³⁷Cs and K in forest litter, obtained from Japanese cedar and deciduous broadleaf forests located 40 km from the site of the 2011 Fukushima Dai-ichi nuclear power plant accident. The litter was collected in 2018 and was divided into three groups according to the decomposition level. The cumulative leachable fraction of ¹³⁷Cs at 120 h ranged from 0.3% to 3.3%, suggesting that most of the ¹³⁷Cs in the litter was hardly leachable in water. The leachable fraction of ¹³⁷Cs generally decreased with the decomposition level of the litter, implying that the easily leachable ¹³⁷Cs eluted into the water during the first stage of decomposition. Meanwhile, the cumulative leachable fraction of K at 120 h was approximately 10 times greater than that of ¹³⁷Cs and ranged from 22.7% to 54.8%. The leaching speeds of ¹³⁷Cs and K decreased suddenly with elapsed time regardless of the tree species, decomposition degree, or element. Our findings contribute to the long-term understanding of the ¹³⁷Cs cycle in forest ecosystems.

Dynamics of radionuclide activity concentrations in weed leaves, crops and of air dose rate after the Fukushima Daiichi Nuclear Power Plant accident

Data on reduction of radioactivity in plants are highly important for making decision on emergency response and remediation of contaminated areas. Dynamics of the ¹³¹I and ¹³⁷Cs concentrations in the weed leaves sampled in the areas affected by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in Japan was assessed for the period March-December 2011. The effective half-lives for ¹³¹I and ¹³⁷Cs of 3.8-4.2 days and 7.1-13.3 days in the weed leaves were calculated for the first two months after the deposition. The approach for assessment of the aggregated transfer factors based on the ambient dose equivalent rate was suggested and validated. The geometric means of the soil to the weed leaves aggregated transfer factors were estimated for June-December 2011. Soil to crop ¹³⁷Cs- concentration ratios (buckwheat, brown rice and soybean) were estimated for 2011-2016. Soil to crop concentration ratios were found to decrease in the order of soybean > buckwheat > brown rice. The effective half-lives for ¹³⁷Cs in these crops were estimated to be between 1 and 2.5 years for the period 2011 2016, and longer than 5-7 years after 2016. It was found that these data comply with the Chernobyl related data obtained for similar conditions and complement of international documents on radionuclide transfer in agricultural environment such as the IAEA TRS 472.

Rain-induced bioecological resuspension of radiocaesium in a polluted forest in Japan

It is the conventional understanding that rain removes aerosols from the atmosphere. However, the question of whether rain plays a role in releasing aerosols to the atmosphere has recently been posed by several researchers. In the present study, we show additional evidence for rain-induced aerosol emissions in a forest environment: the occurrence of radiocaesium-bearing aerosols in a Japanese forest due to rain. We carried out general radioactive aerosol observations in a typical mountainous village area within the exclusion zone in Fukushima Prefecture to determine the impacts and major drivers of the resuspension of radiocaesium originating from the nuclear accident in March 2011. We also conducted sampling according to the weather (with and without rain conditions) in a forest to clarify the sources of atmospheric radiocaesium in the polluted forest. We found that rain induces an increase in radiocaesium in the air in forests. With further investigations, we confirmed that the fungal spore sources of resuspended radiocaesium seemed to differ between rainy weather and nonrainy weather. Larger fungal particles (possibly macroconidia) are emitted during rainy conditions than during nonrainy weather, suggesting that splash generation by rain droplets is the major mechanism of the suspension of radiocaesium-bearing mould-like fungi. The present findings indicate that radiocaesium could be used as a tracer in such research fields as forest ecology, meteorology, climatology, public health and agriculture, in which fungal spores have significance.

Simulating dissolved 90Sr concentrations within a small catchment in the Chernobyl Exclusion Zone using a parametric hydrochemical model

Strontium-90 (⁹⁰Sr) is the major long-lived radionuclide derived from the Chernobyl accident, and is still being detected in the heavily contaminated catchments of the Chernobyl Exclusion Zone. This study examines the long-term decrease in the dissolved-phase ⁹⁰Sr concentration and the concentration–discharge (⁹⁰Sr-Q) relationship in stream water since the accident. We show that the slow decline in ⁹⁰Sr follows a double-exponential function, and that there is a clear relationship between ⁹⁰Sr and Q. This study is the first to reveal that the log(⁹⁰Sr)-log(Q) slope has been gradually decreasing since the accident. This trend persists after decay correction. Thus, it is not caused by the physical decay of ⁹⁰Sr and environmental diffusion, but implies that the concentration formation processes in stream water have been changing over a long period. We propose a hydrochemical model to explain the time-dependency of the ⁹⁰Sr-Q relationship. This paper presents a mathematical implementation of the new concept and describes the model assumptions. Our model accurately represents both the long-term ⁹⁰Sr trend in stream water and the time-dependency of the ⁹⁰Sr-Q relationship. Although this paper considers a small catchment in Chernobyl, the conceptual model is shown to be applicable to other accidental releases of radionuclides.

Impact of wildfire on 137Cs and 90Sr wash-off in heavily contaminated forests in the Chernobyl exclusion zone

Wildfires may play a role in redistributing radionuclides in the environment in combination with hydrological processes such as surface runoff and soil erosion. We investigated plot-scale radionuclide wash-off at forest sites affected by wildfires in the Chernobyl Exclusion Zone (CEZ). We also compared speciation of the washed-off radionuclides with those in previous studies conducted just after the accident in 1986. We observed the surface runoff and the radionuclide wash-off with a soil erosion plot at forest and post-fire sites during May-September 2018. In the post-fire site, 2.81 mm of surface runoff was observed in at least three flow events resulting from 285.8 mm total rainfall. The fluxes of dissolved and particulate ¹³⁷Cs were estimated as 4.9 and 161 Bq m^-2, respectively. The dissolved phase ⁹⁰Sr flux was estimated as 214 Bq m^-2. At the forest site, a single surface runoff (0.67 mm) event was generated by rainfall of 182.2 mm. The fluxes of dissolved and particulate ¹³⁷Cs wash-off values were 6.2 and 8.6 Bq m^-2, respectively. The flux of dissolved ⁹⁰Sr wash-off from the forest was estimated as 45.1 Bq m^-2. The distribution coefficient, which indicates the dissolved-particulate form of radionuclides, in the post-fire site was 30 times higher than that in the forest site, indicating the importance of particulate ¹³⁷Cs wash-off after fire in the CEZ. The entrainment coefficients for dissolved and particulate ¹³⁷Cs concentrations were around 50 times lower than those obtained in the corresponding position within the CEZ immediately after the accident in 1987. The effect of downward migration of ¹³⁷Cs over 30 years led to decreased entrainment coefficients for dissolved and particulate ¹³⁷Cs. The effect of downward migration of radionuclides was considered sufficient to indicate changes in normalized liquid and solid radionuclides wash-off entrainment coefficient and the distribution coefficient in this study.

Impacts of direct release and river discharge on oceanic 137Cs derived from the Fukushima Dai-ichi Nuclear Power Plant accident

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the ¹³⁷Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of ¹³⁷Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured ¹³⁷Cs activities adjacent to the 1F NPP. River discharge rates of ¹³⁷Cs were estimated by multiplying simulated river flow rates by the dissolved ¹³⁷Cs activities, which were estimated by an empirical function. Inflow of ¹³⁷Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of ¹³⁷Cs activity was large, the simulated results were compared with the annual averaged observed ¹³⁷Cs activity distribution. Normalized annual averaged ¹³⁷Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated ¹³⁷Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted ¹³⁷Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic ¹³⁷Cs activity was small compared to the effect of directly released ¹³⁷Cs.

Impact of forest thinning on the dynamics of litterfall derived 137Cs deposits in coniferous forest floor after Fukushima accident

The effects of a 50% forest thinning intensity on Fukushima-derived ¹³⁷Cs deposition by litterfall and its discharge by runoff in hillslope coniferous forest were monitored using four litterfall traps and a hillslope erosion plot. The observation was underway during the pre-and post-thinning periods. Results demonstrated that during the pre-thinning period a total 150 ± 13 g m^-2 of litterfall deposited about 924 ± 69 Bq m^-2 of ¹³⁷Cs. This accounts for 11% of the local ¹³⁷Cs fallout recorded for the study site in the aftermath of the accident. After thinning, both litterfall and ¹³⁷Cs increased by more than six- and two-fold, respectively. This is possibly owing to the slow individual tree recovery rate assisted by the change on the running space provided by canopy openings, which can accelerate even the normal gust wind to gain damaging power on the unshielded mechanically injured parts of the contaminated residual trees. In both cases, litterfall generally transferred about 37% (3 ± 0.2 kBq m^-2) of the local ¹³⁷Cs fallout onto the forest floor over the observation period. The eroded litter-associated ¹³⁷Cs increased by about a factor of two after thinning, which only accounted for less than 1% of ¹³⁷Cs deposited by litterfall. This implies that the forest floor retains ¹³⁷Cs and remains contaminated regardless of the size of the eroded litter material. But this could become a potential secondary contamination source for the downstream resources such as water bodies and villages, especially at the time of flooding, which in turn calls a serious attention in designing decontamination schemes.

Factors controlling dissolved 137Cs concentrations in east Japanese Rivers

To investigate the main factors that control the dissolved radiocesium concentration in river water in the area affected by the Fukushima Daiichi nuclear power plant accident, the correlations between the dissolved ¹³⁷Cs concentrations at 66 sites normalized to the average ¹³⁷Cs inventories for the watersheds with the land use, soil components, topography, and water quality factors were assessed. We found that the topographic wetness index is significantly and positively correlated with the normalized dissolved ¹³⁷Cs concentration. Similar positive correlations have been found for European rivers because wetland areas with boggy organic soils that weakly retain ¹³⁷Cs are mainly found on plains. However, for small Japanese river watersheds, the building area ratio in the watershed strongly affected the dissolved ¹³⁷Cs concentration. One reason for this would be because the high concentrations of solutes, such as K⁺ and dissolved organic carbon, discharged in urban areas would inhibit ¹³⁷Cs absorption to soil particles. A multiple regression equation was constructed to predict the normalized dissolved ¹³⁷Cs concentration with the topography, land use, soil component, and water quality data as explanatory variables. The best model had the building land use as the primary predictor. When comparing two multiple regression models in which the explanatory variables were limited to (1) the land use and soil composition and (2) the water quality, the water quality model underestimated the high normalized dissolve ¹³⁷Cs concentration in urban areas. This poor reproducibility indicates that the dissolved ¹³⁷Cs concentration value in urban areas cannot be solely explained by the solid-liquid distribution of ¹³⁷Cs owing to the influence of the water quality, but some specific ¹³⁷Cs sources in urban areas would control the dissolved ¹³⁷Cs concentration.

Reconstruction of a Fukushima accident-derived radiocesium fallout map for environmental transfer studies

Ascertaining the initial amount of accidently released radiocesium is fundamental for determining the extent of radioactive contamination following nuclear accidents, and is of key importance to environmental transfer models. A series of the airborne monitoring surveys of radioactivity have conducted by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), and provide basic information on radioactive contamination following the accident. However, there are no clear guidelines regarding the selection of airborne monitoring survey results for estimating the initial fallout input in studies of the environmental transfer of radiocesium. This study reconstructed a fallout map of Fukushima accident-derived radiocesium based on a comparison of the radiocesium deposition densities (D_l) derived from the third and fifth airborne monitoring surveys. The D_l derived from the fifth airborne monitoring survey were adjusted for variation in the measured radioactivity associated with the influence of radioactive decay, natural weathering processes, variation in the calibration procedure, and other, undefined mechanisms. The calibrated deposition density of the fifth airborne monitoring survey for each land use type (A'_5th*l) were used to establish the initial fallout map in the East Japan area. Furthermore, the airborne monitoring surveys which were independently conducted in each prefecture area were used to complement the lack of data in the South Kanto region and the mountainous area in the North Kanto region due to snow cover during the measurement period of the fifth airborne monitoring survey. The reconstructed initial fallout map of the Fukushima accident derived ¹³⁷Cs was opened to the public via the database of the Center for Research in Isotopes of Environmental Dynamics, University of Tsukuba, Japan (www.ied.tsukuba.ac.jp/∼fukushimafallout/). Finally, the total atmospheric deposition of Fukushima Dai-ichi Nuclear Power Plant accident-derived radiocesium onto each prefecture and land uses was estimated based on the reconstructed map in this study.

Land use types control solid wash-off rate and entrainment coefficient of Fukushima-derived 137Cs, and their time dependence

Understanding solid ¹³⁷Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid ¹³⁷Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of ¹³⁷Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in ¹³⁷Cs activity concentration in sediment and solid wash-off processes of ¹³⁷Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized ¹³⁷Cs solid entrainment coefficient, defined as Sc (m² kg^-1), was calculated by dividing ¹³⁷Cs activity concentration in sediment by initial ¹³⁷Cs deposition. A particle size-corrected Sc, defined as Sc_correct (m² kg^-1), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sc_correct, ranged from 0.0072 to 0.084 m² kg^-1 and 0.0052-0.078 m² kg^-1, respectively. Annual wash-off rates of solid ¹³⁷Cs were 0.0029-12% year^-1. There was no significant decreasing trend in Sc or Sc_correct on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sc_correct significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in ¹³⁷Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity.

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.

Six-year monitoring study of 137Cs discharge from headwater catchments after the Fukushima Dai-ichi Nuclear Power Plant accident

Since headwater catchments are the source areas of ¹³⁷Cs for downstream river systems, ¹³⁷Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and ¹³⁷Cs concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed. The normalized ¹³⁷Cs concentrations (¹³⁷Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10^-6 m²/L, 10^-2 m²/kg, and 10^-1 m²/kg, respectively, before 2013 and declined to around 10^-8 m²/L, 10^-4 m²/kg, and 10^-2 m²/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas ¹³⁷Cs concentrations in SS declined significantly and the total flux of ¹³⁷Cs decreased. Although the clear effect of land use on decline trend in normalized ¹³⁷Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them.

Temporal changes of the ambient dose rate in the forest environments of Fukushima Prefecture following the Fukushima reactor accident

Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011-2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014-2016 and 2011-2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10-20% faster than that induced solely by physical decay of radiocesium for the observation period 2011-2016.

Six-year monitoring study of radiocesium transfer in forest environments following the Fukushima nuclear power plant accident

The study investigated temporal changes in the ¹³⁷Cs concentrations in vegetal and hydrological samples collected from various forests in Yamakiya District, Kawamata Town of Fukushima prefecture over six years following the Fukushima Dai-ichi nuclear power plant accident. Cesium-137 was detected in all forest environmental samples. However, the concentration in most samples decreased exponentially with time. The ¹³⁷Cs concentrations in throughfall samples exhibited a double-exponential decreasing trend with time. Temporal changes in the ¹³⁷Cs concentration in vegetal samples and stemflow were approximated by using a single-exponential equation. A comparison of the decline coefficient for the latter observation period (>2 y since the accident) revealed that the declining trend of ¹³⁷Cs concentrations varied between foliage and the outer barks of the Japanese cedar and Japanese konara oak trees. The ¹³⁷Cs concentration in cedar needles decreased exponentially while that in konara oak leaves was constant over the last six years. Conversely, the declining trend of ¹³⁷Cs concentration in the outer bark of konara oak exceeded that of cedar. The results suggested that self-decontamination processes and internal recycling of ¹³⁷Cs varied among tree species and different tree parts. The results indicated that the leaching of ¹³⁷Cs in the throughfall in Japanese cedar was dependent on the ¹³⁷Cs concentration in needles. However, a comparison of ¹³⁷Cs concentrations in vegetal and hydrological samples from each sampling year showed that the leaching rate decreased with time. Conversely, the ¹³⁷Cs concentrations in the stemflow were independent of the concentrations in the outer bark. The declining trend of ¹³⁷Cs concentrations in litterfall (λ: 0.31-0.33 y^-1) was similar to that of the mean of new/old needles (λ: 0.26-0.33 y^-1) for cedar stands. With respect to the hydrological components, the ¹³⁷Cs concentration in the stemflow (λ: 0.32-0.33 y^-1) decreased at a slightly slower rate than that in the throughfall (λ: 0.36-0.54 y^-1) for the cedar forest. The decline coefficients of ¹³⁷Cs concentration in the aforementioned types of hydrological components slightly exceeded that for the vegetal samples. The results suggest that monitoring of ¹³⁷Cs concentrations in hydrological components and vegetal samples can aid in further understanding the leaching mechanisms of ¹³⁷Cs from trees to rainwater.

Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of ¹³⁷Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ₁) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of ¹³⁷Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of ¹³⁷Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported ¹³⁷Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ₂) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

Factors controlling the variability of 137Cs concentrations in 5 coastal rivers around Fukushima Dai-ichi power plant

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to the contamination by radiocesium (¹³⁷Cs) of large drained areas. Cesium-137 concentrations in rivers result from complex transfer processes, depending on multiple forcings. Better knowledge of the factors controlling these concentrations is therefore a prerequisite to improve predictions of ¹³⁷Cs transfers within river catchments. This study aimed at analyzing the spatial and temporal variability of ¹³⁷Cs concentrations in rivers and identifying the key factors controlling their variability. Published values of ¹³⁷Cs concentrations in rivers in the north of FDNPP were collected, characterizing 122 sampling sites from May 2011 to October 2014. It resulted in three datasets: dissolved concentrations C_W (Bq/L), concentrations in suspended sediment C_SS (Bq/kg) and total concentrations C_T (Bq/L). The resulting database reflected a large variety of catchments and hydrological conditions. Observed ¹³⁷Cs concentrations varied by 2-4 orders of magnitude and were poorly explained (R² = 0.13-0.38) by the average contamination density. Indices summarizing the complex spatial and temporal properties of the catchments were proposed as candidate explanatory variables of concentrations in rivers. They were selected by stepwise regression for each dataset (C_W, C_SS, C_T). For the three datasets, the selection and combination of 5-10 indices significantly better explained this variability (R² = 0.69-0.83). Deposit indices were identified as first drivers of concentrations in rivers. A deposit index was selected for each dataset, indicating no effect of the contamination distribution for C_W, whereas C_T and C_SS required considering the distribution of contamination and connectivity, as well as the presence of dams for C_SS. The others selected variables significantly contributed to explain the concentration variability. This meta-analysis emphasizes the importance of structural (e.g. slope, land-cover) and functional (e.g. delay, season, rainfall) properties in the dissimilarities of catchments responses, stressing that assessments could be improved by including more these properties in models.

Assessing spatially distributed infiltration capacity to evaluate storm runoff in forested catchments: Implications for hydrological connectivity

We assessed spatial distribution of infiltration capacity under forest canopies to simulate runoff processes in a steep small catchment in Japan. High resolution LiDAR data were used to assess light availability to understory vegetation, which in turn was converted into an understory vegetation cover ratio. Spatially distributed infiltration capacities based on data from field rainfall-runoff experiments were incorporated into the TopoTube model using a relationship between infiltration capacity and vegetation cover. Effects of spatial distribution of infiltration capacity and its influence on hydrological connectivity and runoff processes were examined based on numerical simulations in the following three infiltration scenarios for seven natural storms: (1) spatially distributed infiltration capacity; (2) uniform infiltration capacity using the weighted mean of the spatial distribution in the drainage basin; and (3) uniform infiltration capacity using the arithmetic mean of field measurements. Although saturated subsurface flow dominated storm runoff in all simulations, Hortonian and saturated overland flow were significant for storm runoff response in smaller catchments (headwaters and zero-order basins). Simulations using spatially distributed infiltration capacities showed more Hortonian overland flow compared to using uniform infiltration because of areas with low infiltration located along channels. On the other hand, no differences in runoff were apparent between spatially-distributed and uniform infiltration scenarios in larger catchments. Our study also indicates that differences in contributions of subsurface and groundwater flow at the catchment scale need to be examined based on observations and simulations in nested catchments. Our findings suggest that including detailed patterns of vegetation ground cover are important for understanding hydrogeomorphic connectivity within catchments.

Groundwater age and mixing process for evaluation of radionuclide impact on water resources following the Fukushima Dai-ichi nuclear power plant accident

Radionuclide contamination of groundwater causes critical impacts on water resources, human lives, and ecosystems. The intrusion of radionuclides into the groundwater flow system in Fukushima, Japan, could be illuminated by determining groundwater age and mixing processes. To do this, periodical field surveys were conducted in catchments contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Sampling began in May 2011, which was 2 months after the disaster, and continued through June 2012. Chlorofluorocarbon (CFCs), tritium, and oxygen and hydrogen stable isotopes were used as environmental tracers. The observed tritium concentrations suggested that the water contained accident-derived radionuclides that exceeded the natural background baseline. Groundwater ages in the selected two headwater catchments were estimated to be between 10 and 26 years by combined use of multiple CFCs concentrations. In addition, the governing groundwater flow system was mostly approximated by a piston flow model; however, modern water fraction was also suggested based on the relationship between CFC-11 and CFC-12. The estimated water age and isotopic signals among stream water, spring water, and groundwater revealed that the intrusion of radionuclides into the groundwater was caused by the mixing between groundwater and modern water sources such as soil water and precipitation with relatively high radionuclide concentrations. This mixing was facilitated by a weathered and fractured granite bedrock and a thin unsaturated subsurface layer in the study area. Continued long-term monitoring of radionuclides in the groundwater will be necessary for water resources management in the future. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration.

Reconstruction of uranium and plutonium isotopic signatures in sediment accumulated in the Mano Dam reservoir, Japan, before and after the Fukushima nuclear accident

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with ¹³⁷Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios (²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu) and for ¹³⁷Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, ²³⁶U/²³⁸U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U.

Environmental DNA provides information on sediment sources: A study in catchments affected by Fukushima radioactive fallout

An excessive supply of sediment is observed in numerous rivers across the world where it leads to deleterious impacts. Information on the sources delivering this material to waterbodies is required to design effective management measures, and sediment tracing or fingerprinting techniques are increasingly used to quantify the amount of sediment derived from different sources. However, the current methods used to identify the land use contributions to sediment have a limited discrimination power. Here, we investigated the potential of environmental DNA (eDNA) to provide more detailed information on the plant species found in sediment source areas as a next generation fingerprint. To this end, flood sediment deposits (n = 12) were collected in 2017 in two catchments impacted by the Fukushima radioactive fallout along differing river sections draining forests, cropland or a mix of both land uses. Conventional fingerprints (i.e. fallout radionuclides and organic matter properties) were also measured in these samples. The conventional fingerprint model results showed that most sediment samples contained a dominant proportion of subsoil material. Nevertheless, the eDNA information effectively discriminated the three above-mentioned groups of sediment, with the dominance of tree, shrub and fern species in sediment sampled in rivers draining forests versus a majority of grass, algae and cultivated plant species in sediment collected in rivers draining cropland. Based on these encouraging results, future research should examine the potential of eDNA in mixed land use catchments where the contribution of topsoil to sediment dominates and where the cultivation of land has not been abandoned in order to better characterize the memory effect of eDNA in soils and sediment.

Spatial and temporal variation in vertical migration of dissolved 137Cs passed through the litter layer in Fukushima forests

We examined spatial variation in vertical ¹³⁷Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug-4 Oct), autumn (5 Oct-30 Nov) and winter (1 Dec-27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m² areas with 3-5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m²/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m²/month. The ratio of ¹³⁷Cs flux during the survey period to litter ¹³⁷Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The ¹³⁷Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the ¹³⁷Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20-25 times. The spatial variations in the ¹³⁷Cs flux were affected positively by those in the litter ¹³⁷Cs inventory and negatively by canopy openness.

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.

Temporal changes of the ambient dose rate in the forest environments of Fukushima Prefecture following the Fukushima reactor accident

Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011-2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014-2016 and 2011-2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10-20% faster than that induced solely by physical decay of radiocesium for the observation period 2011-2016.

Plutonium isotopic signatures in soils and their variation (2011-2014) in sediment transiting a coastal river in the Fukushima Prefecture, Japan

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a significant release of radionuclides that were deposited on soils in Northeastern Japan. Plutonium was detected at trace levels in soils and sediments collected around the FDNPP. However, little is known regarding the spatial-temporal variation of plutonium in sediment transiting rivers in the region. In this study, plutonium isotopic compositions were first measured in soils (n = 5) in order to investigate the initial plutonium deposition. Then, plutonium isotopic compositions were measured on flood sediment deposits (n = 12) collected after major typhoon events in 2011, 2013 and 2014. After a thorough radiochemical purification, isotopic ratios (²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu) were measured with a Multi-Collector Inductively Coupled Mass Spectrometer (MC ICP-MS), providing discrimination between plutonium derived from global fallout, from atmospheric nuclear weapon tests, and plutonium derived from the FDNPP accident. Results demonstrate that soils with the most Fukushima-derived plutonium were in the main radiocaesium plume and that there was a variable mixture of plutonium sources in the flood sediment samples. Plutonium concentrations and isotopic ratios generally decreased between 2011 and 2014, reflecting the progressive erosion and transport of contaminated sediment in this coastal river during flood events. Exceptions to this general trend were attributed to the occurrence of decontamination works or the remobilisation of contaminated material during typhoons. The different plutonium concentrations and isotopic ratios obtained on three aliquots of a single sample suggest that the Fukushima-derived plutonium was likely borne by discrete plutonium-containing particles. In the future, these particles should be isolated and further characterized in order to better understand the fate of this long-lived radionuclide in the environment.

Radiocesium concentrations in soil and leaf after decontamination practices in a forest plantation highly polluted by the Fukushima accident

Owing to the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident a vast amount of radiocesium was released polluting the land. Afterwards, a variety of decontamination practices has been done, reducing the ambient dose rates. In this study we evaluated the effectiveness of eight forest decontamination practices by means of monitoring the radiocesium (¹³⁷Cs) concentration in soil and leaf samples, and the daily discharge rates in ten plots during 27 months (May 2013-July 2015). A forest plantation located 16 km southwest to the FDNPP and within the exclusion area was selected. Radiocesium concentrations were analysed using a germanium gamma ray detector. The differences in radiocesium activities between the different plots were statistically significant (p < 0.05) and four homogeneous groups were distinguished. Tree thinning and litter removal greatly reduced the radioactivity and the two plots devoted to these practices presented the highest discharge rates of ¹³⁷Cs (Th + LR; 350-380 Bq/m² day), followed by the two Th plots (163-174 Bq/m² day). The clearcutting with LR and the LR plots (104 and 92 Bq/m² day) also had higher rates than those rates in the control plots (51 Bq/m² day). We only observed low rates in the two plots with matting (19-25 Bq/m² day). The temporal variability was explained by (i) the different rainfall depths registered during the measurement intervals (accumulated precipitation from 14 to 361 mm); and (ii) the fluctuations of the total surface coverage. The decrease trend in radiocesium concentration was high in 2013, moderate in 2014 and low in 2015 owing to the vegetation recovery after the countermeasures, thus reducing the possibility of the second pollution of the neighbouring areas. The average proportions of contribution of ¹³⁷Cs discharge by soil and leaf fraction were 96.6% and 3.4%.