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Artificial radionuclides including radiocesium (134 Cs and 137 Cs) and radiosilver (110mAg) were released into the environment following the Fukushima Dai-ichi nuclear power plant accident in March 2011. These particle-bound substances deposited on soils of north-eastern Japan, located predominantly within a ∼3000 km2 radioactive fallout plume and drained by several coastal rivers to the Pacific Ocean. The current dataset (Evrard et al., 2021), which can be accessed at 10.1594/PANGAEA.928594, compiles gamma-emitting artificial radionuclide activities measured in 782 sediment samples collected from 27 to 71 locations across catchments draining ∼6450 km2 during 16 fieldwork campaigns. These campaigns were conducted in Japan between November 2011 and November 2020 in river catchments draining the main radioactive plume. This database may be useful to evaluate and anticipate the post-accidental redistribution of radionuclides in the environment and for the spatial validation of models simulating the transfer of radiocesium across continental landscapes. [ABSTRACT FROM AUTHOR]

A highly valuable database of long-term hydrometeorological measurements is presented, containing in situ observations for a period of 37 years from a well-maintained grassland in the north-east of Japan. The observations include shortwave radiation, net radiation, air and dew point temperatures at three elevations, soil temperature at four depths, sensible heat flux, soil heat flux, wind speed, relative humidity, air pressure and precipitation. The heights of measurements are 1.6, 12.5 and 29.5 m above ground, with the soil-layer observations at depths of 0.02, 0.1, 0.5 and 1 m. This high-quality database includes four temporal resolutions of 10 s, 0.5 h, 1 h and 24 h, with the hourly data presented here. Monthly and annual statistics are presented at the database web page of the Center for Research in Isotopes and Environmental Dynamics and Prediction of the University of Tsukuba, http://doi.org/10.24575/0001.198108. We validated the data by comparing them with published data from the local meteorological agency in Tateno operated by the Japan Metrological Agency, including the average, maximum and minimum values of air temperature, shortwave radiation, wind speed, relative humidity and precipitation. We have generated a daily downward longwave radiation time series with a method developed by Kondo and Xu (1997) based on the observations from the database. This constructed time series agrees well with observations collected between 2002 and 2006, as evaluated based on the values of the Nash–Sutcliffe efficiency (=0.947) and percent bias (=1.486). For the whole database, annually averaged values show a positive trend in precipitation, air temperature, shortwave radiation, net radiation and sensible heat flux over the past 37 years, with a negative trend detected for wind speed, soil heat flux and soil temperature. [ABSTRACT FROM AUTHOR]

The Fukushima Daiichi nuclear power plant (FDNPP) accident in March 2011 resulted in the fallout of significant quantities of radiocesium over the Fukushima region. After reaching the soil surface, radiocesium is quickly bound to fine soil particles. Thereafter, rainfall and snowmelt run-off events transfer particle-bound radiocesium downstream. Characterizing the precipitation regime of the fallout-impacted region is thus important for understanding post-deposition radiocesium dynamics. Accordingly, 10 min (1995-2015) and daily precipitation data (1977-2015) from 42 meteorological stations within a 100 km radius of the FDNPP were analyzed. Monthly rainfall erosivity maps were developed to depict the spatial heterogeneity of rainfall erosivity for catchments entirely contained within this radius. The mean average precipitation in the region surrounding the FDNPP is 1420 mm yr-1 (SD 235) with a mean rainfall erosivity of 3696 MJmmha-1 h-1 yr-1 (SD 1327). Tropical cyclones contribute 22% of the precipitation (422mmyr-1) and 40% of the rainfall erosivity (1462 MJmmha-1 h-1 yr-1 (SD 637)). The majority of precipitation (60 %) and rainfall erosivity (82 %) occurs between June and October. At a regional scale, rainfall erosivity increases from the north to the south during July and August, the most erosive months. For the remainder of the year, this gradient occurs mostly from northwest to southeast. Relief features strongly influence the spatial distribution of rainfall erosivity at a smaller scale, with the coastal plains and coastal mountain range having greater rainfall erosivity than the inland Abukuma River valley. Understanding these patterns, particularly their spatial and temporal (both inter- and intraannual) variation, is important for contextualizing soil and particle-bound radiocesium transfers in the Fukushima region. Moreover, understanding the impact of tropical cyclones will be important for managing sediment and sediment-bound contaminant transfers in regions impacted by these events. [ABSTRACT FROM AUTHOR]

This study seeks to better understand the vertical variation in the transport and fate of radiocesium via branchflow and stemflow in the aftermath of the Fukushima nuclear power plant accident. Working in both a coniferous forest (Cryptomeria japonica, young Japanese cedar stands, average height is 14.0 m) and a mixed deciduous broadleaved forest (Quercus serrata, Japanese oak stands, average height is 13.3 m), we employed isotopic tracers to help determine the fate of radiocesium transported by branchflow and stemflow from the upper and lower portions of the canopy. Branchflow was harvested and examined from the upper canopy layers (including younger foliage, dead foliage, and live branches), whereas stemflow was collected in both the upper and lower portions of the canopy (with varying portions of live and dead branches). Particular attention was paid to the washoff, leaching, adsorption, transport, and storage (stem and bark) of radiocesium. The preliminary results showed radiocesium leaching (Cs-137 concentration) was greater for branchflow that received washoff and leachate from the dead foliage than the branchflow receiving radiocesium inputs from mixed and young foliage. For the tree trunk, radiocesium leached more in stemflow from the lower part of the canopy as compared to the upper canopy. We also found that the isotopic composition of branchflow was generally enriched in δ18O and δD compared to open rainfall and throughfall, however, the differences in enrichment between branchflow and stemflow remains unclear. Further work should examine the effect of tree architecture on the cycling of radiocesium both stemflow and branchflow. [ABSTRACT FROM AUTHOR]

We examined the temporal trend of Cs -137 concentration of river sediment and suspended sediment in Hamadori and Nakadori areas in Fukushima Prefecture from September 2011 to January 2017. We used 716 monitoring data by the Ministry of the Environment from 89 sites, and applied particle size correction to eliminte the influence of changes in particle size distribution in the concentration of Cs-137. Also, we also apply the activity concentration of suspended sediment in Cs-137 and compared. The results showed that Cs-137 concentration decreased through the survey period in most sites, and the rates of decline were faster than previous studies on Chernobyl accident. The trends differed according to areas: in Hamadori area, the concentrations did not converge but showed approximately the same rates of decline (λ) at 1.13/y in during the initial 2 years but then gradually decresed in the Nakadori area. We also we found that particle-size corrected suspended sediment and bottom sediment agrees well in more than half catchment, but some catchment did not agree well. These data imply that the fast processes of interaction of dissolved radionuclides with suspended matter, but, slow processes of interaction of dissolved radionuclides with the bottom sediment interface layer in Fukushima river environment. [ABSTRACT FROM AUTHOR]

A substantial quantity of radionuclides was deposited on the surrounding environment of the Fukushima prefecture, after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Radionuclides are discharged through river systems resulting in downstream contamination of irrigation water, paddy fields, ponds, and sea. In order to reduce the effects of such contamination, the Japanese government has engaged in decontamination works. In farmland and grassland, decontamination was performed by removing the contaminated topsoil and vegetation and recovering the area with uncontaminated soil. Along with the significant reduction of radiation levels, decontamination works have contributed to a critical change in the land-use and soil properties of decontaminated areas. Several decontaminated grassland and farmland are used now as temporal storage sites for the contaminated soils. Such land-use change can influence multiple parameters that control the hydrological, soil erosion, and sediment transport processes like soil infiltration capacity, soil erodibility, surface roughness, and the local capacity to store water and sediment. In this study, we monitored the sediment, water and radioactive Cesium (137Cs) discharges, at plot and basin scale, in order to assess the impacts of land cover and land-use change on soil erosion, hydrological processes and 137Cs discharge. Two USLE-type plots with a length of 22.13 m and a width of 5 m, were installed in 2011 and 2013. Those plots present the use of a grassland area before and after the decontamination works. Four forested headwater catchments with different grassland fractions (0 %, 1.5 %, 19 %, and 23%) were selected to investigate the impacts of land cover on 137Cs discharge. In one of those catchments (the Iboishi-yama catchment), decontamination works were conducted in the grassland area in December 2013. This catchment was served as a case of study to investigate the impacts of land-use change at the basin scale. The results of the present study provide clear evidence of the sensitivity of the hydrological process, soil erosion and 137Cs discharge to the land cover and the land-use change. First, the declining trend of the dissolved (in stream water) and particulate (attached to suspended sediment or coarse organic matter) 137Cs, in the four headwater catchments, was demonstrated with a two-component exponential and a one-component exponential model, respectively. The 137Cs was mainly discharged by suspended sediments (96 ~ 99 %) and the decline rate of the dissolved 137Cs was related to the grassland area extent in the different catchments. Second, we found that the decontamination works resulted in a radical decrease in the 137Cs discharge, a change in the critical radio-cesium source areas, and an increase in sediment yield and surface runoff in the Iboishi-yama catchment and the decontaminated plot. The event-based analysis of sediment flux from the Iboishi-yama catchment showed an increase in sediment yield by 5 to 8.5 times after the decontamination works. Third, the plot experiments showed a significate increase in sediment discharge (~ 40 times) after the decontamination works. Finally, the relationship between sediment and 137Cs discharge fluxes suggested the potential use of 137Cs as a good tracer of sediment source. [ABSTRACT FROM AUTHOR]

A substantial quantity of radiocesium was deposited on the Fukushima prefecture, Japan in 2011 because of the Fukushima Daiichi nuclear power plant accident. On farmland, contaminated surface soil was removed to reduce the radiation dose rate because radiocesium is strongly attached to fine soil particles. Instead of the removed soil, decomposed granite, which has larger soil particles and less organic materials, was added. Such large-scale land use change can affect the sediment transport system. Soil erosion plot is generally applied to quantify the soil erosion, but this method cannot reveal how the sediment is detached, transported and deposited. In order to understand soil particle movement and soil redistribution, Unmanned Aerial Vehicle – Structure from Motion (UAV-SfM) method, which enables to obtain 3D data of the soil surface and detect soil surface changes, can be used. Radio Frequency Identification (RFID) tag can also be applied to trace soil particle movement (Parsons et al., 2014). It can show how the sediment transport system is different in the plot. However, it takes so much time to locate the RFID tag with a total station. This issue would be solved by means of high-resolution orthomosaic image derived with UAV-SfM method.This research has two objectives: (1) assessing the accuracy of UAV-SfM measurement and the survey of the RFID tag location with the orthomosaic image. (2) applying UAV-SfM method and soil particle tracing with the RFID tag to study the sediment transport processes on decontaminated slope farmland. Study sites were located in Kawamata town (decontaminated) and Date city (not decontaminated), Fukushima prefecture, Japan. USLE plots (22.13 m × 5 m) were installed on each slope farmland. These plots were kept with no vegetation and no cultivation. Water and sediment discharge were measured at the outlet of each plot. The accuracy of 3D data derived from the UAV-SfM method was assessed by comparing the data with those derived from 3D laser scanning. Locations of RFID tags were determined by using Orthoimage derived from UAV-SfM method and were compared with those measured with a total station. We found that the accuracy of UAV-SfM can be minimized as 3.3 mm (S.D.). RFID tags also can be located with an accuracy of 28 mm (RMSE). The results of UAV-SfM surveys showed no rill erosion in the decontaminated plot (DP) and the development of micro-terraces, and the result of RFID tracing showed a short transport distance in inter-rill zones at DP. After the decontamination works, the runoff and sediment yield decreased in DP. Our results suggest that the combination of these methods can provide new insights to understand sediment transport processes, and the decontamination works affected the water and sediment discharge. [ABSTRACT FROM AUTHOR]