Displays

The highly contaminated Chernobyl exclusion zone (ChEZ) still remains a potential source of the additional atmosphere radioactive contamination due to forest fires there. The possible radionuclide transport outside the ChEZ in the direction of populated regions (including Kyiv, 115 km from the ChEZ borders) and its consequences for people health is a topic of a constant public concern in Ukraine and neighboring countries. The problem of additional radiation exposure of fire-fighters and other personnel within the ChEZ during forest fires is actual too. The reliable models of radionuclide rising and following atmospheric transport, which should be integrated with data of stationary and mobile radiological monitoring, are necessary for real-time forecast and assessment of consequences of wildland fires.

Results of intercomparison of models developed within the set of the national and international projects are presented, including: i) the point source term model of Atmospheric Dispersion Module (ADM) of the real -time online decision support system for offsite nuclear emergency – RODOS, which development was funded by EU; ii) the specialized new tool for modeling radionuclide dispersion from the polygons of the fired areas using the Lagrangian model LASAT incorporated into RODOS system; iii) the Lagrangian-Eulerian atmospheric dispersion model LEDI using a volume source term and including a module for calculation of  parameters of a convective plume  formed over a fire area; iv) the Lagrangian model of Fukushima University. All atmospheric transport models use the results of the numerical weather forecast model WRF as the input meteorological information.

The models evaluation was carried out using the measurement data during large wildland fires occurred in ChEZ in 2015 and June 2018, including the ¹³⁷Cs and ⁹⁰Sr volume activity measured with the monitoring network within the Zone and results due to special measurements with a mobile radiological laboratory outside it.

The sensitivity of atmospheric transport modeling results was estimated to: 1) internal parameterization of different models, first of all, parameterization of the value of the deposited radionuclide fraction re-entering into the atmosphere during forest fires, 2) different parameterization of the source term formed due to the forest fire; 3) quality of input meteorological information, including the space and time step of the used WRF model grid, and the impact of chosen parameterization of some WRF modules (e.g. the atmospheric boundary layer module) on the atmospheric transport model results. Additionally, results of forest fires consequences modeling was compared which were obtained with different sets of input meteorological data: the WRF forecast of metrological fields (on-line calculations) and the similar WRF calculations on the base of objective analysis results.

How to cite: Talerko, M., Kovalets, I., Hirao, S., Zheleznyak, M., Kyrylenko, Y., Lev, T., Bogorad, V., and Kireev, S.: Radionuclide atmospheric transport after the forest fires in the Chernobyl Exclusion zone in 2015-2018: An impact of the source term parameterization and input meteorological data on modeling results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10066, https://doi.org/10.5194/egusphere-egu2020-10066, 2020.

In case of an accidental radioactive release, the Institute for Radiological Protection and Nuclear Safety (IRSN) uses atmospheric dispersion models to assess radiological consequences for human health and environment. The accuracy of the models’ results is highly dependent on the meteorological fields and the source term, including the location, the duration, the magnitude and the isotopic composition of the release.

Inverse modelling methods have proven to be efficient in assessing the source term. Variational deterministic inverse methods have been used on the Fukushima accident and are suitable in operational use since they are able of quickly providing an optimal solution.
However the quantification of the uncertainties of the source term assessed is usually not easily accessible. In contrast, Bayesian inverse methods are developed in order to efficiently sample the distributions of the variables of the source, thus allowing to get a complete characterisation of the source.

In this study, we propose to tackle the Bayesian inference problem through two types of sampling methods: Monte Carlo Markov Chains methods (MCMC) with the parallel tempering algorithm and Stein variational gradient descent. The distributions of the control variables associated to the source and the observations errors are presented. To better quantify observations errors, different approaches based on the definition of the likelihood, the reduction of the number of observations and the perturbation of the meteorological fields and dispersion model parameters are implemented.

These different methods are applied on two case studies: the detection of Ruthenium 106 of unknown origin in Europe in autumn 2017 and the accidental release of Selenium 75 from a nuclear facility in Mol (Belgium) in May 2019. For both of these events, we present a posteriori distributions enable to identify the origin of the release, to assess the source term and to quantify the uncertainties associated to the observations, the dispersion model and meteorological fields. Finally, we show that the Bayesian method is suitable for operational use.

How to cite: Dumont Le Brazidec, J., Bocquet, M., Saunier, O., and Roustan, Y.: Bayesian inference and uncertainty quantification for source reconstruction of radionuclides release: application to recent European radionuclide detection events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15257, https://doi.org/10.5194/egusphere-egu2020-15257, 2020.

Many efforts have been devoted to estimate the release rate of the radionuclide emission in the Fukushima Daiichi nuclear accident using regional scale observations. Because of the radioactive decay, regional scale observations may not provide information of short-lived radionuclides, which contributes the majority of radiation exposure in the early stage. In this study, the local-scale gamma dose rates data were used to estimate the atmospheric release rates of both long- and short-lived radio nuclides.The proposed method uses reactor physics to obtain an a priori radionuclide composition and a reverse source term estimate as an a priori release rate. A weighted additive model is developed, which uses the local-scale gamma dose rates to handle the conflicts between the two priors and to simultaneously incorporate them into the source inversion. The proposed method is validated against both the local-scale gamma dose rates and the regional concentration measurements of Cs-137. The results prove that the retrieved a posteriori source term combines the advantages of both priors and substantially improves the predictions of the on-site gamma dose rates. Given a detailed priori release rate, this approach also improves the regional predictions of both airborne and deposited Cs-137 concentrations.

How to cite: Fang, S., Li, X., and Zhuang, S.: Release rate estimation of both long- and short-lived radionuclides for the Fukushima Daiichi nuclear accident based on local-scale observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-233, https://doi.org/10.5194/egusphere-egu2020-233, 2020.

The study of radionuclides (RNs) retention processes onto the solid phases is a key element for the prediction of their transfer in soils. It allows a better quantification of the persistence of radioactive contaminants on the soil surface, their availability for root uptake and their vertical transfer towards groundwater.

This work addresses the comparison between equilibrium and kinetic hypotheses of sorption processes on real post-accidental soil contamination profiles. The equilibrium-kinetic (EK) sorption model was selected as a non-equilibrium parameterization embedding the K_d approach. It supposes the existence of two types of sorption sites. The first sites are at equilibrium with solution, whereas for the second sites, kinetics of the sorption and desorption are taken into consideration.

We focused our study on four ¹³⁷Cs soil contamination profiles measured in a cedar stand 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant. Profiles were sampled at four different dates (between 2013 and 2018) by measuring ¹³⁷Cs activity in both organic (humus + litter layer) and mineral soil layers reaching a maximum depth of 20cm.

To successfully simulate the ¹³⁷Cs transfer throughout these soil profiles, the input flux at the mineral soil surface was reconstructed from monitored throughfall, stemflow and litterfall fluxes in the same forest stand from July 2011 to November 2016 crossed with initial deposit and dynamic of the organic layer activity.

The EK model reproduced the measured contamination profiles slightly better than the fitted K_d model. While both models were able to reproduce the overall vertical distribution throughout the profiles, the persistent contamination at the surface was closer to the measured value with the EK approach. Additionally, the fitted K_d parameters (2000 L/kg to 6500 L/kg depending on the parcel) were considerably higher than the recommended value by The IAEA for organic soils (270 L/kg). When used, this recommended K_d value produced profiles with considerably faster transfer rate between layers and shorter persistence of the contamination at the surface.

To further distinguish the models behaviors, long term simulations were conducted. EK hypotheses induced much longer residence time of the contamination at the soil surface. For instance, by 2030, the EK approach predicted that 75 % of the contamination still remained in the 0-2 cm layer due to the slow desorption rate, whereas the K_d approach predicted it to be around 51 %. This fraction becomes even smaller (8 %) when using the K_d value (270 L/kg) recommended by the IAEA for organic soils.

These results prove that the choice of the sorption model is critical in post-accidental situations. An equilibrium approach, especially when using recommended parameter values, can result in an underestimation of the RNs residence time in the surface. Whereas a kinetic approach, by distinguishing different sorption and desorption rates, is able to reproduce the slow evolution of ¹³⁷Cs soil profiles with time that is already observed in the case of Chernobyl contaminated areas 30 years after the accident.

How to cite: Chaif, H., Coppin, F., Bahi, A., and Garcia-Sanchez, L.: Equilibrium and kinetic approaches for modelling sorption processes on radiocesium soil profiles in Fukushima prefecture, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7186, https://doi.org/10.5194/egusphere-egu2020-7186, 2020.

The first results of a detailed study of ¹³⁷Cs and ⁹⁰Sr distribution in elementary landscape-geochemical system (the top-slope-closing depression type, ELGS) are presented on the example of test sites located in the head zone of the East Ural Radioactive Trace formed during the Kyshtym accident in 1957. Field measurements were performed using modified Violinist-III field gamma-spectrometer [1] and the Kolibri spectrometric complex [2]; laboratory determination of ¹³⁷Сs was made by Canberra gamma-spectrometer (HPGe detector). Field measurements were carried out along cross-sections and in a grid manner with a step of 1 and 5 m accompanied by a theodolite survey and soil core sampling at the selected points. The instrumental layer-by-layer determination of ⁹⁰Sr activity in soil samples performed in field conditions by [2] was compared with the radiochemical measurement of the same samples in fractions of less than and more than 1 mm. The correlation between the obtained instrumental and radiochemical values for ⁹⁰Sr activity equaled to r = 0.962 (n = 50). Spatial distribution of both ¹³⁷Cs and ⁹⁰Sr manifested itself in an specifically organized polycentric structure. Against the absence of a pronounced tendency for unidirectional redistribution of radionuclides from the top to the bottom, there was an ordered cyclic change in the activity of both ¹³⁷Cs and ⁹⁰Sr, which in our opinion reflected the unified mechanism of redistribution of substances in ELGS, where the relief is the main controller of water migration. Measurement of ⁹⁰Sr activity in selected meadow plants proved an important role of species in radionuclide accumulation at the ELGS level: the maximum amount of ⁹⁰Sr was found in nettle (Urtica dioica, 86 ± 19 kBq/kg dw, n = 9), the minimum - in bluegrass (Poa sp., 13.8 ± 1.2 kBq/kg dw, n = 19). The revealed features of spatial structure of ¹³⁷Cs and ⁹⁰Sr are believed to mark the general tendencies of substances redistribution in ELGS, which seems important for studying soil formation, environmental monitoring and optimization of soil fertilizing.

The study was supported by the RFBR grant No. 19-05-00816.

References

1. Romanov S.L., Korobova E.M., Samsonov V.L. Experience in using the upgraded VIOLINIST-III instrument in field radioecological research //Yadernyye izmeritel'no-informatsionnyye tekhnologii. 2011, 3 (39), 56-61.

2. Potapov V.N. et al. Development of Portable Beta Spectrometer for Sr-90 Activity Measurements in Field Conditions and Its Application in Rehabilitation Activities at RRC Kurchatov Institute.WM’06 Conference, February 26 - March 2, 2006, Tucson, AZ [on line] http://www.wmsym.org/archives/2006/pdfs/6133.pdf.

How to cite: Korobova, E., Romanov, S., Tarasov, O., Baranchukov, V., Berezkin, V., Dolgushin, D., Mikhailovskaya, L., Modorov, M., and Lukyanov, V.: A study of Cs-137 and Sr-90 distribution in the soil and vegetation cover of elementary landscape-geochemical systems in the zone of the East Ural Radioactive Trace, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5844, https://doi.org/10.5194/egusphere-egu2020-5844, 2020.

Forests provide valuable water and nutrient resources to farming activities as well as places for various leisure activities. However, decontaminating forests in the aftermath of a massive radionuclide contamination event presents challenges because of the topography and the difficulty of collecting a large number of field samples. Achieving accurate remote measurements also can be hindered by the canopy cover. In Fukushima, Japan, where elevated radioactive fallout occurred following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, about 70 percent of the land is covered with forests. Soil samples collected in a forest in Iitate Village, Fukushima, beginning in 2016 through 2018 still contained an average 98200 Bq/kg of Cs-137 in the top 4 cm depth (std. dev. 114100 Bq/kg). Thus, decontamination and identification of Cs-137 distribution patterns in those forests is still a pressing issue. However, soil types in that region, 1000 mm annual precipitation with intense rain during typhoons, and microtopography have presented challenges to understanding how Cs-137 behaves in those forests.  In this study, six topographic parameters were computed from 1-m and 10-m resolution DEMs and the relationships between those parameters and soil water content and bulk density were systematically analyzed for their effects on Cs-137 concentration levels. As the first analytical step, correlation indices and the generalized additive models (GAM) analysis were conducted on those parameters. The results show that not all topographic effects are apparent in the correlation analysis, yet the results can be improved when mixed with other parameters in GAM models. Overall the effect of topographic parameters on Cs-137 levels is DEM resolution-dependent while individual soil properties indicate a strong relationship. Also, it was found that depending on the analysis depth, correlation levels and significance of those parameters in GAM models fluctuate. As the second step, Cs-137 levels were extrapolated to a larger area in the study site to understand further the connections between topography and soil properties. The results, including the limitations and proposals for future forest decontamination, will be presented in the session. Understanding how Cs-137 moves and accumulates in forests, especially immediately after contamination, is critical to avoiding the negative impacts on the environment by decontamination measures and to protecting lowlands from harmful radioactivity levels. This study contributes to the radionuclide research field by presenting an example of data analysis processes using field sampled data.

How to cite: Yasumiishi, M., Nishimura, T., Bittner, T., Aldstadt, J., and Bennett, S.: Predicting Cs-137 Distribution Patterns from Soil Samples - The Relationships Between Topographic Parameters, Soil Properties, and Cs-137 Concentration Levels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2252, https://doi.org/10.5194/egusphere-egu2020-2252, 2020.

Dissolved ¹³⁷Cs discharge represents approximately 30% of the total ¹³⁷Cs discharge from the forested upstream catchment of the Ohta River in Fukushima, Japan [1]. It is thought that a major source of the dissolved ¹³⁷Cs entering the river water may be leaching from forest litter [1]. A watershed simulation based on the distribution coefficient (K_d) that modelled water, sediment, and particulate and dissolved ¹³⁷Cs transport could not reproduce the seasonal variability of the base flow dissolved ¹³⁷Cs concentrations, nor the peaks in concentration that occurred during storms [2].

We developed a combined watershed-biogeochemistry model for simulating dissolved and particulate ¹³⁷Cs discharge from forest catchments to describe the two phenomenon as mentioned above. A compartment model for the forest ecosystem was appended to the General-purpose Terrestrial fluid-Flow Simulator (GETFLOWS) watershed code. The compartment model included compartments for undecomposed and decomposed litter, with transfer from the former into the latter depending on temperature. A pathway for dissolved ¹³⁷Cs input to forest streams was linked from the decomposed litter compartment.

The results from a simulation with the new simulation model reproduced the seasonal variability of dissolved ¹³⁷Cs concentrations and the peaks occurring during storms. Therefore the new modelling results add weight to the theory that leaching from decomposed litter can input dissolved ¹³⁷Cs concentrations in river water in Fukushima Prefecture. The developed model is expected to be useful for further explorations into factors affecting dissolved ¹³⁷Cs input to river water in forested catchments.

[1]Tsuji et al., 2016. J. Geophys. Res. Biogeosci. 121, 2588-2599.

[2]Sakuma et al., 2018. J. Environ. Radioact. 184-185, 53-62.

How to cite: Sakuma, K., Yoshimura, K., Kurikami, H., Malins, A., and Funaki, H.: A coupled watershed-biogeochemical model to simulate dissolved and particulate 137Cs discharge from a forested catchment affected by the Fukushima accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2787, https://doi.org/10.5194/egusphere-egu2020-2787, 2020.

We examined the temporal trend of Cs-137 concentration of river sediment and suspended sediment in Eastern Japan areas from September 2011 to January 2017. We used 716 monitoring data by the Ministry of the Environment from 461 sites and applied particle size correction to eliminate the influence of changes in particle size distribution in the concentration of Cs-137.　Also, in some locations, we also compared the activity concentration of suspended sediment and dissolved water in Cs-137 and compared.    The results showed that Cs-137 concentration decreased through the study period in most sites, and the average declining, λ, is about 0.168 in the 2013-2018 period. In some sites increasing trend or larger rate of decline were found, but these locations are limited to lower contaminated catchments (less than 50k Bq/m2). The particle size corrected Kd value of the bottom sediment (Kd ac) shows around 10-4 to 10-5 Kg/L, but varied significantly where the initial catchment inventories are less than 50 kBq/m2. In most sites,  Cs-137 concentration on the particle size corrected Suspended sediment and bottom sediment show similar values, except for some specific sites (such as near the coast, etc).  These data imply that the activity concentration of dissolved Cs is important to control the rate and processes of interaction of dissolved radionuclides with the bottom sediment interface layer in the river environment affected by the Fukushima fallout.

How to cite: Onda, Y., Tang, C., Gao, X., Takeuchi, Y., Taniguchi, K., Kurihara, M., and Hirose, K.: Temporal changes of the radiocesium activity concentration in river bottom sediment and suspended sediment in Eastern Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13220, https://doi.org/10.5194/egusphere-egu2020-13220, 2020.

Processes responsible for long-term changes in environmental radioactivity after the Fukushima accident are currently high on the agenda. Dynamics of particulate and dissolved radiocesium (r-Cs) has been studied on a number of water bodies, namely Abukuma River, Niida River and Maeda River, the dam reservoirs of Yokokawa (Ota River), Sakashita (Kuma River), Ogaki (Ukedo River) and Shinobu (Abukuma River) and four heavily contaminated irrigation ponds in Okuma town (Inkyozaka, Suzuuchi, Funasawa, Kashiramori). Water samples were collected for dissolved and particulate r-Cs analysis at multiple sites for these water bodies. Wash-off from slopes of contaminated catchments and river transport are key long-term pathways for radionuclide dispersal from contaminated areas after the Fukushima accident. The climate and geographical conditions for the Fukushima Prefecture of Japan are characterized by relatively high annual precipitation (1300-1800 mm/year) and steep slopes which promote higher erosion and higher particulate r-Cs wash-off. At the same time, the r-Cs distribution coefficient K_d in Fukushima rivers was found to be at least an order of magnitude higher than the corresponding values for Chernobyl-derived r-Cs and r-Cs resulting from nuclear weapon tests (NWT) in European rivers. The normalized dissolved wash-off coefficient for Fukushima river watersheds, based on the measured dissolved r-Cs activity concentrations was found to be 1-2 orders of magnitude lower than those for Chernobyl and NWT fallout. In the irrigation ponds r-Cs showed a persistent behavior and was characterized by regular seasonal variations: r-Cs concentrations tend to grow during summer and decrease during winter. Speciation analysis for Okuma ponds showed a much higher exchangeability of r-Cs in bottom sediments than catchment soils. Several methodologies to collect water samples and to separate the particulate and dissolved fractions have been used and showed comparable results for all water bodies under study. For all rivers, reservoirs, and ponds higher values of K_d(r-Cs) have been confirmed when compared with Chernobyl-derived r-Cs in European water bodies. Some observations demonstrated remobilization of r-Cs at river mouths compared to upstream sections which could be explained by the change of river water hydrochemistry from upstream to the mouth, specifically a substantial increase in the concentration of major r-Cs competing cations for selective sorption sites on the suspended matter. Some dam reservoirs and ponds were subjected to integrated suspended sediment sampling. For the dam reservoirs, the particulate r-Cs activity concentration has been found to be water depth-dependent. Sediment cores collected at eight sites along the Abukuma river floodplain in 2018 and during October-November 2019, right after Typhoon Hagibis occurred in the middle of October 2019, demonstrated substantial redistribution of r-Cs due to erosion and redeposition during heavy rainfall and extreme flood. Bottom sediments coring in the dam reservoirs allowed estimation of the average sedimentation rate in the reservoirs and the rate of r-Cs accumulation. This research was partially supported by the Japan Society for the Promotion of Science (JSPS), Grant-in-aid for Scientific Research (B) (18H03389), bilateral project No. 18-55-50002 of Russian Foundation for Basic Research (RFBR) and JSPS.

How to cite: Konoplev, A., Wakiyama, Y., Wada, T., Golosov, V., Ivanov, M., Komissarov, M., Kanivets, V., Udy, C., Niida, T., Moritaka, S., Usuki, M., Watanabe, K., Takase, T., Goto, A., Tsukada, H., and Nanba, K.: Radiocesium wash-off, river transport and redistribution in fluvial system after the Fukushima Dai-ichi nuclear power plant accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3245, https://doi.org/10.5194/egusphere-egu2020-3245, 2020.

After the accident of the Fukushima nuclear power plant, decontamination works had been conducted from 2013 to 2017 in the area of heavly contamination by fallout radionuclides. Although decontamination is conductive to decrease the air dose rate, associated disturbances of soil, such as scraping, reversal tillage, and soil dressing. These decontamination works, in turn, could increase the sediment discharge to downstream, but no studies are available on the effect of the decontamination in upstream headwaters that affects sediment discharge in rivers. Furthermore, decontamination has been carried out in the target area, the Yamakiya area, from 2013, and decontamination has been completed in the spring of 2016, decontamination work has been completed in 2017, and the residents have been returned. The sediment runoff due to human activities can be different from the sediment runoff due to decontamination.

The purpose of this study was to study the relationship between sediment dynamics and Cs dynamics due to decontamination, and the changes in sediment dynamics due to the return of residents. Observations and historical data were analyzed at Iboishi mountain in the forest and the middle stream of Kuchibuto. We have been monitoring suspended sediment Cs-137 concentration, water runoff, and suspended sediment runoff since 2014 at the middle point of the Kuchibuto River and since 2013 at Mt.Iboishi. The slope of the approximation line was compared with the LQ curve for comparison of the amount of sediment runoff. In the middle of the Kuchibuto river, it was 1.54 in 2014, 2.28 in 2015, 2.12 in 2016, 0.164 in 2017, and 0.189 in 2018. At Iboishi mountain in the forest, it was 1.72 in 2014, 0.947 in 2015, 1.39 in 2016, 0.219 in 2017, and 1.15 in 2018. The same tendency was shown in the slopes of the LQ curves in the middle part of the Kuchibuto river and the Iboishi mountain in the forest area. The Cs concentration was high until November 2015, but since then, the Cs concentration has decreased. These results suggest that the increased sediment discharge due to decontamination of the forest area affected the sediment discharge in the middle stream of the Kuchibuto River.

How to cite: Makino, F., Onda, Y., Taniguchi, K., Slim, M., Wakiyama, Y., Kozuka, S., Kato, H., and Iwagami, S.: Effect of decontamination on sediment discharge from mountain stream to river and cesium transfer in Yamakiya district, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13965, https://doi.org/10.5194/egusphere-egu2020-13965, 2020.

Regarding with amount of movement of 137Cs from domain to domain for several years after the accident, we also evaluated that the amount of 137Cs transported by the rivers might be 40 TBq which is corresponding to less than 1.3 % of deposited 137Cs. For resuspension, the annual deposition of 137Cs at Okuma during the period from 2014 to 2018 means that 4 TBq year-1to 10 TBq year-1should be amount of resuspension from land to atmosphere and this amount correspond to 0.1 % to 0.3 % of total deposition of 137Cs on land in Japan. The 137Cs activity concentration at 56N canal in 2016-2018 correspond to 137Cs discharge of 0.73 TBq year-1to 1.0 TBq year-1from FNPP1 site to open water. The integrated amount of FNPP1 derived 137Cs that entered the Sea of Japan, SOJ, until 2017 was 0.27 ± 0.02 PBq, which is 6.4 % of the estimated total amount of FNPP1-derived 137Cs in the STMW in the North Pacific. The integrated amount of FNPP1-derived 137Cs that returned to the North Pacific Ocean through the Tsugaru Strait from SOJ was 0.11 ± 0.01 PBq, 42 % of the total amount of FNPP1-derived 137Cs transported to the SOJ. As a result of decontamination works, 134 TBq of 137Cs was removed from surface soil until February 2019 which correspond to 4 % of deposited 137Cs on land in Japan. Therefore, the largest transport amount of 137Cs was 270 ± 2 TBq from STMW in the North Pacific to SOJ until 2017, and the second largest was decontamination work by which work about 134 TBq was removed from surface soil on land until Feb. 2019. Fluvial transport by rivers contributed about 40 TBq since June 2011 until 2016.

How to cite: Aoyama, M., Tsumune, D., Inomata, Y., and Tateda, Y.: Mass balance of radiocaesium derived from Fukushima accident and estimation of latest fluxes among atmosphere, land and ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12301, https://doi.org/10.5194/egusphere-egu2020-12301, 2020.

  Following the previous atmospheric transport model intercomparison project (MIP2: Sato et al, 2018), a new project of model intercomparison (MIP3) has been conducted in which, out of 12 models in MIP2, 9 models are participating. The main aim of MIP3 is to examine the effects of using a refined meteorological data with a finer horizontal resolution of 1 km (Sekiyama et al., 2019). This paper describes outline of the preliminary results of MIP3.

  The horizontal distribution Cs-137 deposition in the eastern part of Honshu Island (the main island of Japan) calculated by the models were compared with the aerial survey results to find that the simple ensemble average of the 9 models was a little worse than that of the 12-model ensemble in MIP2 in terms of the statistical index RANK, which is the combination of the correlation coefficient, the fractional bias, the figure of merit in space and KPS. This slightly poorer performance is tentatively considered to be caused partially by the absence of three models which showed rather broad deposition patterns and by the underestimation in the Nakadori area (the middle part of Fukushima Pref.). However, in the sector in the northwestern direction from the accidental site which had the largest deposition, the deposition pattern simulated by the MIP3 ensemble, if compared with that of MIP2, is more consistent with the survey result. As for the atmospheric concentrations, although the model performance for the plumes that traveled over wider areas was found to be slightly poorer for MIP3 than MIP2, it was found that the MIP3 ensemble generally showed better performance for the plumes that affected the near area in the Hamadori area (the coastal part of Fukushima Pref.). The better performance of the MIP3 in this area can be attributed to the better representation of topography in the meteorological simulation.

How to cite: Yamazawa, H., Sato, Y., Sekiyama, T., Kajino, M., Goto, D., Morino, Y., Kondo, H., Quérel, A., Fang, S., Takigawa, M., Terada, H., Kadowaki, M., and Uchida, J.: Intercomparison of atmospheric transport models using 1 km grid meteorological data for the Fukushima Daiichi Nuclear Power Plant Accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3826, https://doi.org/10.5194/egusphere-egu2020-3826, 2020.

Kajino et al. (2016) (doi:10.5194/acp-16-13149-2016) assessed emission, transport, and deposition of airborne radiocesium from contaminated ground surface after the Fukushima nuclear accident for the entire year of 2013 by using numerical simulation, a field experiment on dust emission flux in a contaminated area (town of Namie, Fukushima prefecture), and air concentration measurements inside (Namie) and outside the contaminated area (Tsukuba, Ibaraki prefecture). In this study, additional comparison of the simulation results has been made against the fallout measurements made in Fukushima (city of Fukushima) and Tsukuba and we found that our previous simulation results substantial underestimated the observed fallout by 2 to 3 orders of magnitude. The reason is, in the previous simulation, we assumed the size distributions of aerosols are in the submicron range, even though recent studies indicated that the aerosol sizes should be much larger (i.g., Kinase et al., 2018, doi:10.1186/s40645-018-0171-z, Igarashi et al., 2019, doi:10.1038/s41598-018-37698-x, and this study). By assuming larger sizes of Cs-bearing aerosols, the simulated concentrations and depositions in both Fukushima and Tsukuba were significantly improved. Consequently, the re-assessed emission flux was modified by several ten times more than that previously assessed by Kajino et al. (2016), which was 0.048% per year.

How to cite: Kajino, M., Watanabe, A., Igarashi, Y., Zaizen, Y., Kinase, T., Ishizuka, M., and Kita, K.: Re-assessment of airborne radiocesium re-suspended from contaminated ground surface after the Fukushima Nuclear Accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19840, https://doi.org/10.5194/egusphere-egu2020-19840, 2020.

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 the additional evidence for rain-enhanced/induced aerosol emissions in a forest environment: the occurrence of radiocaesium-bearing aerosols in a Fukushima 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 players of the resuspension of radiocaesium originally from the nuclear accident in March 2011. We also conducted sampling according to the weather (with and without rain conditions) inside a forest to clarify the atmospheric radiocaesium source from the polluted forest. Thus, we found that rain enhances/induces bursts of radiocaesium-bearing aerosols in forests in Fukushima. With further investigations, we found that the fungal spore sources of resuspended radiocaesium under rainy weather seemed to differ from those under nonrainy weather. Larger fungal particles (possibly conidia-ascospore) are probably emitted during rainy conditions than during nonrainy weather, suggesting that the splash generation by rain droplets is the major mechanism of the suspension of radiocaesium-bearing mould-like fungi. Fungal spores can function as ice and cloud condensation nuclei and cause allergies, and fungal inocula may have a significant impact on agriculture. Therefore, the present findings indicate that radiocaesium could be used as a tracer in research fields such as forest ecology, meteorology, climatology, public hygiene and agriculture.

How to cite: Igarashi, Y., Kita, K., Kinase, T., Hayashi, N., Ishizuka, M., Adachi, K., Koitabashi, M., and Onda, Y.: Rain-enhanced/induced bioecological resuspension of radiocaesium in a polluted forest in Fukushima, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22136, https://doi.org/10.5194/egusphere-egu2020-22136, 2020.

Stemflow takes important role on the hydrological and chemical cycling in the rhizosphere because it brings intensive rainwater input to forest soil and enhances downward infiltration of rainwater along tree root network to deep soil horizon. However, there are few studies on the effects of stemflow in rainwater infiltration mechanisms by collecting of soil water. In this study, stemflow and soil water near the tree roots (Rd : root downslope) and far from the trunk (Bt : between trees) are collected from a cedar forest in Namie Town, Fukushima Prefecture, Japan. Samples were collected from June 24 to December 11, 2019 with a total precipitation of 1100 mm during the period. Water volume and dissolved ¹³⁷Cs concentration drived from the Fukushima Dai-ichi Nuclear Power Plant accident were measured. As a result, Rd which is located in neighbor of the trunk showed greater water infiltration flux and high dissolved ¹³⁷Cs concentration. The average amount of infiltration water which was normalized for open rainfall depth during the whole sampling period was 1.4 times and 3.0 times larger at 5 cm and 20 cm depth for the Rd than the Bt, the average dissolved ¹³⁷Cs concentration was 1.3 times and 1.7 times larger at 5 cm and 20 cm depth, respectively. This suggests that infiltration water flux and dissolved ¹³⁷Cs concentration can be increased due to contribution of stemflow input at the base of tree trunk. To determine the role of stemflow on rainwater infiltration flux and the concentration of dissolved elements in the rhzosphere, further analysis is required to clarify detailed infiltration mechanisms by using multiple tracer techniques such as stable isotopic composition of water and by collecting root oriented preferential flow.

How to cite: Iida, H., Kato, H., Shinozuka, T., Akaiwa, S., Yokoyama, T., Hudson, S., Hudson, J., and Onda, Y.: Effects of stemflow on infiltration flux of rainwater and dissolved Cs-137 to forest soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12465, https://doi.org/10.5194/egusphere-egu2020-12465, 2020.

The 2011 Fukushima-Daiichi nuclear power plant (FDNNP) accident resulted in the atmospheric releases of large quantities of man-made radionuclides. According to [1], Matua Island, located at a distance of more than 1000 km from FDNPP, was also subjected to minor radioactive contamination. Matua Island, 52.6 sq.km, a recent volcano with the highest point of 1446 m a.s.l. is located in the center of the Kuril Islands Arc. Volcanic soils are formed on stratified gravelly-stony tephra more than 60 cm thick, underlain by thin layers of volcanic slags. The latest catastrophic eruption which changed the landscape of the island occurred in 2009.

Studies of the vertical distribution of the Cs-137 in soils were carried out on four landscape catenas. The length of the catenas from the sea shore deep into the island ranged from 700 m (maximum height a.s.l.  70 m) to 3.3 km (height a.s.l.  450 m).

Soil core samples were taken in  summer 2017 at a depth of tephra, which was located at a depth of 10 to 25 cm. Soil was sliced into separate layers with a step of 2 to 5 cm.

The measurement activity concentrations of the Cs-137 in the soil samples were carried out on a low-background gamma spectrometer Canberra Industries.  The counting time  was fixed not less than 24 h to ensure that the statistical measurement error is small.

Cs-134, the «Fukushima” fallout marker, due to the decay, was not detected. Therefore, it is difficult to accurately assess the Cs-137 contribution from the FDNNP accident from a global fallout.

The vertical distribution of Cs-137 is characterized by extreme heterogeneity, which reflects both the primary fallout conditions and the landscape conditions of the likely lateral redistribution. For сatena 1 with a length of 1 km and an altitude  of 400 m, the number of pickets (P0, P1, etc. – the numbering of pickets goes downhill), the specific activity of Cs-137 (Bq/kg) and the depth of the layer (cm) are given as follows: P0-27 Bq/kg (2-4 cm); P1 - 64 Bq/kg (6-8 cm); P2 – 70 Bq/kg (8-10 cm); P3 - 53 Bq/kg (4-6 cm); P4 - 15 Bq/kg (0-5 cm).  Similar spatial  heterogeneity of the specific activity of Cs-137 and its depth penetration was also found for catena 3 with a length of 1250 m and a height of 75 m (the numbering of pickets goes up the slope): P1-137 Bq/kg (17-20 cm); P2-76 Bq/kg (0-5 cm); P3 - 35 Bq/kg (0-4 cm); P4 - 43 Bq/kg (3-6 cm); P6 – 24 Bq/kg (5-10 cm).

The distribution of Cs-137 in individual soil layers was used to evaluate the empirically found shapes of the vertical profiles of radionuclide concentration. Cs-137 is believed here to be a  very valuable tracer  that  can be used to test  variability of vertical geochemical migration in Matua  landscapes.

[1]. Ramzaev V.P., Barkovsky A.N., Gromov A.V., Ivanov S.A., Kaduka M.V. Fukushima fallout in Sakhalin Region, Russia, part 1: ¹³⁷Cs and ¹³⁴Cs in grassland soils. Radiation Hygiene, 2018, Vol. 11, No. 1, pp. 25-42.

How to cite: Alexander, B., Ivanov, A., and Linnik, V.: Vertical profiles of Cs-137 in soil of the Matua Island ( the Central Kuril Islands, Russia) in 2017 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6634, https://doi.org/10.5194/egusphere-egu2020-6634, 2020.

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 released significant quantities of radionuclides in the environment. So far, most of the research focused on radio-cesium and rarely on actinides. Until now, most of the studies on uranium and plutonium released by FDNPP were conducted on bulk environmental samples (soil, sediment, biota, etc.) and rarely on individual particles The investigation of individual particles allows working on the FDNPP signature alone compared to studies of bulk material which may also incorporate the signature of global fallout. Accordingly, the objective of the current research is to identify and characterize actinide-bearing particles in soil samples collected in the vicinity of FDNPP to get a better understanding of their formation mechanisms and of their fate in the environment. In order to identify and locate actinide-bearing particles in soil samples, we developed and implemented a method based on alpha-autoradiography (Jaegler et al., 2019), which allows identifying and locating specifically alpha-emitters, including plutonium isotopes 239Pu, 240Pu and 241Pu using a CR39 SSNTD device (Solid State Nuclear Tracks Detector).

Before alpha-autoradiography samples were dried, grinded with an agate mortar and sieved to several cutoffs: 1mm, 700µm, 400µm, 200µm, 100µm and 63µm. Cutoffs were chosen according to the sizes of actinide-bearing particles detected by Satou et al (2018). The absence of significant loss of uranium particles by the sieving process was demonstrated by the analysis of a test soil sample with a ¹³⁷Cs activity below 650 Bq/kg spiked with depleted uranium reference particles. After sieving, the test sample was deposited onto a carbon planchet for secondary ion mass spectrometry analysis in order to determine the particle recovery yield.

Then, the detection of actinide-bearing particles by means of alpha autoradiography was performed on various soil samples collected in the vicinity of FDNPP. Longer exposure times logically improve the detection efficiency of alpha-emitters-bearing particles. However, the exposure time should not exceed two months to limit the impact of emissions from naturally-occurring alpha-emitters.

We will present here the first results of detection of alpha-emitting particles in the analyzed soil samples.  Relative impacts of naturally-occurring alpha-emitters (²³⁴U/²³⁸U, ²³⁵U, ²³²Th and daughter nuclides) and of plutonium isotopes on alpha-radiography trace observation will be discussed.

The next steps of this study will be to develop and implement methods to sample and isolate alpha-emitting particles from the soil matrix and to characterize them in size, morphology, elemental and isotopic compositions. Full characterization of individual particles will be very helpful to determine their origin and to provide an understanding of their formation process and to determine their mobility and life-duration in the environment.

References:

How to cite: Diacre, A., Faure, A.-L., Moureau, A., Marie, O., Griffiths, N., Evrard, O., and Pointurier, F.: Development of a method to identify alpha-emitter-bearing-particles in soil samples collected in the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4197, https://doi.org/10.5194/egusphere-egu2020-4197, 2020.

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) aftermath in 2011, potassium addition has been increasingly valued as the most effective countermeasure for soil remediation of polluted sites. Potassium is a competing cation with caesium during plant root uptake. Recent studies have elucidated that potassium application can increase the Radiocaesium Interception Potential (RIP), a key parameter that determines the radiocaesium selectivity in soil and therefore its phytoavailability. The RIP is determined as the product of the distribution coefficient of caesium and the concentration of potassium in soil solution, considering the occupation in exchange regular sites but especially in the so-called frayed-edged of the 2:1 phyllosilicate layers of clay minerals, that account for most of the high-selectivity sites for caesium. In order to increase soil RIP, mineral amendments -especially zeolite- were applied in Japanese target fields as a major measure for safe agricultural production. In this study, we aimed at the determination of the RIP of Japanese and European soils with different clay mineralogy, as a key parameter for the solid-liquid distribution of radiocaesium in soils. To do so, we analysed the clay mineralogy of soils by X-Ray diffraction (XRD), as well as the solid and soil solution phases of five types of soils with different potassium fixing capacity by atomic absorption spectrometry (AAS) and ionic chromatography (IC), respectively. As potassium fixation varies among soils, we expected very different relationships between their potassium content and RIP. Their RIP was determined by spiking with 1-2 KBq of radiocaesium-134 prior to the use of thallium doped sodium iodine scintillator (NaI(Tl)). Both solid phase exchangeable caesium and soil solution caesium were analysed by inductively coupled plasma mass spectrometry (ICP-MS). Partial findings for Japanese soils showed a potassium fixing rate of approximately 93% for vermiculitic soils, while for imogolitic Andosols with low 2:1 phyllosilicate clay mineral content, only 17% of potassium addition was determined to be fixed. The fixation capacity for smectitic soils reached 57%. Furthermore, additional research is currently being done regarding RIP determination of several agricultural soils and with and without zeolite amendments. The final results will be shown in the EGU General Assembly 2020.

How to cite: Dengra i Grau, F. X., Eguchi, T., Toloza, A., Smolders, E., Tarjan, S., Shinano, T., Gerzabek, M., Bruun Hansen, H. C., and Dercon, G.: Evaluation of the changes in the Radiocaesium Interception Potential (RIP) of Japanese and European soils based on their potassium content, mineralogy and agricultural zeolite amendments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22438, https://doi.org/10.5194/egusphere-egu2020-22438, 2020.

The measurements of  ¹³⁷Cs concentration in the rivers of Fukushima prefecture demonstrate the more significant role of the fluxes of ¹³⁷Cs adherent to the suspended sediments in comparison with the rivers contaminated after the Chernobyl accident. Therefore the forecasting of   ¹³⁷Cs  concentration during the floods requires to use the models of radionuclide wash-off from the watersheds with sediments.

Comprehensive modeling of radionuclide transport processes could be provided on the basis of the physically-based distributed models of hydrological and sediments transport processes. Such distributed models can describe soil erosion and sedimentation processes, as also exchange of the radionuclides between solute, suspended sediment and upper soil level.  We developed such type .model DSHVM-R based on the distributed hydrological- sediment transport model DHSVM of Washington University.  The model implementation for the experimental plots in Fukushima prefecture demonstrated a good possibility of the model for the analyses on the influence of the steepness of the watershed slopes and the intensity of the rainfall in the increased role of particulate ¹³⁷Cs transport.  From another side,  the implementation of such a model for large river watershed required too large computational time and significant efforts for processing of the large sets of the distributed data still not available for all watersheds.

We developed model RETRACE _RS  that combines the simplicity of the watershed empirical models based on the washing -out coefficient approach with the possibility to use geographically distributed data of the radioactive fallout and  GIS layers for rivernets. The model RETRACE_RS is an extension of the model RETRACE _R  (Zheleznyak et al, 2010),  which code is integrated into the Hydrological Dispersion  Module of the Decision Support System RODOS.   RETRACE_R is based on the assumptions that the rate of the radionuclide wash- off from each elementary grid cell of the watershed can be calculated from precipitation rate and density of deposition in this cell through the “wash-off” coefficient Kw; and that the radionuclides washed out from the cell are transported without time delay to the nearest river channel cell - to the grid element of the 1-D river model RIVTOX as lateral inflow. In RETRACE _RS the possibility of RETRACE_R to simulate washing -out of the radionuclides from watershed to river in solute was extended by the fluxes of the particulate radionuclide transport calculated via the “ washing out coefficient for particulate radionuclide transport ” -Kss. The formula to calculate Kss values is based on the empirical relations for the particulate  ¹³⁷Cs transport in the rivers of Fukushima prefecture ( Sakuma et al, 2019). The model was tested on the basis of the measurements of ¹³⁷Cs concentration in Abukuma river during the high floods in 2018-2019. The modeling system RETRACE_RS  - RIVTOX was validated also on the basis of the data sets of radionuclide transport in the Pripyat and Dnieper rivers. The system is testing for the prediction of aquatic radionuclide transport from the Chernobyl NPP area to the  Kyiv region during the extreme floods.

How to cite: Zheleznyak, M., Pylypenko, O., Kivva, S., Sakuma, K., Igarashi, Y., Wakiyama, Y., Konoplev, A., and Nanba, K.: Distributed modeling of radionuclide washing out from the watersheds in solute and with suspended sediments: case studies Abukuma River, Fukushima Prefecture and Pripyat – Dnieper river system, Ukraine , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19250, https://doi.org/10.5194/egusphere-egu2020-19250, 2020.

Long-term behaviors of Cesium-137 (¹³⁷Cs) and Strontium-90 (⁹⁰Sr) have been of great interest in Chernobyl and its downstream area. This study presents plot-scale observations of ¹³⁷Cs and ⁹⁰Sr wash-off in the Chernobyl exclusion zone since 2018 to date. Runoff plots were established on a pine forest in the Kopachi area (PF-KP), an abandoned farmland in the Korogod area (AF-KR) and a post wild fire territory in the Red Forest (WF-RF) in December 2017. Each runoff plot consists of eroding surface of 22.13 m length and 5 m width, a 30° V-notch weir with water level sensor for monitoring surface runoff and tanks for collecting runoff water and sediments. Since February 2018, runoff water and sediment samples trapped in the weir and tanks have been collected after rainfall events and analyzed for particulate ¹³⁷Cs concentration, dissolved ¹³⁷Cs concentration, and dissolved ⁹⁰Sr concentration. Analyses of samples in 2, 4, and 3 wash-off events were completed for PF-KP, AF-KR, and WF-RF, respectively. The ash/litter on soil surface, soil of 0-1 cm depth, soil of 1-2 cm depth, and soil of 2-3 cm depth were sampled with a scraper plate and subject to measurements of ¹³⁷Cs and ⁹⁰Sr concentrations.  Total volume of surface runoff from PF-KP, AF-KR, and WF-RF were 0.97, 0.73, and 3.2 mm, respectively. Total sediment discharge from PF-KP, AF-KR, and WF-RF were 0.29, 0.015, 1.7 g m^-2, respectively. The runoff and sediment discharge from PF-KP and WF-RF were mainly observed in summer and attributed to severe water repellency of the surface soils. Total particulate ¹³⁷Cs wash-off from PF-KP, AF-KR, and WF-RF were 51, 0.082, 270 Bq m^-2, respectively. Total dissolved ¹³⁷Cs wash-off from PF-KP, AF-KR, and WF-RF were 7.4, 0.024, 9.8 Bq m^-2, respectively. Total dissolved ⁹⁰Sr wash-off from PF-KP, AF-KR, and WF-RF were 55, 0.31, 230 Bq m^-2, respectively.  These results indicate that wild fire enhances surface runoff and sediment yield and result in greater wash-off of ¹³⁷Cs and ⁹⁰Sr. In comparisons between PF-KP and WF-RF, apparent Kd value for ¹³⁷Cs at WF-RF was higher than at PF-KP. Ratio of dissolved ¹³⁷Cs and ⁹⁰Sr concentration to those in ash/litter layer at PF-KP was lower than those of WF-RF. The dissolution of these radionuclides into runoff water appeared to be restrained in the post wild-fire site.

How to cite: Wakiyama, Y., Igarashi, Y., Onda, Y., Samoilov, D., Lisovy, H., Demianovych, V., Laptev, G., Konoplev, A., Nanba, K., and Kirieiev, S.: Plot-scale wash-off of Cesium-137 and Strontium-90 after three decades since the Chernobyl accident , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12477, https://doi.org/10.5194/egusphere-egu2020-12477, 2020.

A large amount of radioisotopes of Ceasium (Cs) was released into the surrounding environment by the accident of the Fukushima Dai-ichi Nuclear Power Plant (1F) occurred in 2011, and some of them fell into land and sea. Since Cs is an element inside a group of alkali metals, radioisotopes of Cs are strongly sorbed by soil and other particulate matters, especially clay, organic molecules, and polymers. Hence, full understanding of the movement of them in various environmental zones is strongly demanded. In order to reveal such transport processes of radiocesium in aquatic systems, we have so far developed a code, 3D-Sea-SPEC (3D Sea Simulation for Port and its Environmental Coast).  The code is composed of CFD based simulation solvers using LES scheme and aimed especially for analysis of complex aquatic zones like reservoirs and port areas. In this presentation, we give an overview of 3D-Sea-SPEC and show the technical improvements in realizing the water temperature stratification commonly seen in reservoirs and port areas under strong sun radiation in summer season. Next, we actually apply the improved code to a typical reservoir and a port area around 1F site, and demonstrate characteristic behaviors of the suspended sediment-sorbed radiocesium.

How to cite: Machida, M., Yamada, S., and Kurikami, H.: Characteristic radiocesium transport in temperature-stratified reservoirs and port-areas: Analysis by using a code, 3D-Sea-SPEC, developed for complex aquatic areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22277, https://doi.org/10.5194/egusphere-egu2020-22277, 2020.

Nuclear Power Plants (NPPs) and nuclear fuel reprocessing sites are main producers of anthropogenic radiocarbon. Anthropogenic ¹⁴C can be released into the environment in gaseous forms, with liquid effluents or with spent nuclear fuel [1]. During photosynthesis radiocarbon can be easily assimilated into the plants. As a result, carbon-14 can be transported through the food chain and accumulate in a human body. Therefore, radiocarbon is considered a primary source of increased human radiation dose from industrial nuclear activities [2].

Main goal of this research was to evaluate the influence Ignalina NPP on carbon-14 content in the Lake Druksiai. The sediment core was collected from the Lake Druksiai. The ages of sediment layers were estimated using ¹³⁷Cs and ²¹⁰Pb dating methods. ABA (acid-base-acid) chemical pretreatment procedure was used to extract humin (HM) and humic acid (HA) fractions from the sediments. Chemically pretreated samples were graphitized with the Automated Graphitization Equipment AGE 3 (IonPlus AG). Carbon-14 measurements in prepared samples were performed using the single stage accelerator mass spectrometer (SSAMS, NEC, USA).

Radiocarbon content was measured in the sediment core which covers all phases of the NPP exploitation (commissioning, operation and decommissioning). These measurements in HM and HA fractions showed that after the start of the operation of the Ignalina NPP in 1983, the ¹⁴C concentration in these organic fractions increased by 4 pMC and 3 pMC, respectively. In addition, a sharp increase of radiocarbon content (concentration almost doubled) in HA fraction was observed in the year 1999. Similar increase in ¹⁴C activity in fish samples from Lake Druksiai was measured. In HM fraction such drastic changes in radiocarbon concentration were not observed. These results suggest that ¹⁴C enriched effluents were released from the Ignalina NPP in 1999.

[1] Z. Ezerinskis et al., Annual Variations of 14C Concentration in the Tree Rings in the Vicinity of Ignalina Nuclear Power Plant, Radiocarbon 60, 1227–1236 (2018).

[2] IAEA, Generic Models for Use in Assessing the Impact of Discharges of Radioactive Substances to the Environment (2001).

How to cite: Butkus, L., Barisevičiūtė, R., Ežerinskis, Ž., Šapolaitė, J., Maceika, E., Garbaras, A., Pabedinskas, A., Mažeika, J., and Remeikis, V.: The impact of Ignalina Nuclear Power Plant on Carbon-14 content in Lake Druksiai, Lithuania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18812, https://doi.org/10.5194/egusphere-egu2020-18812, 2020.

The world's oceans act as a sink for artificial radionuclides as well as for other anthropogenic pollutants released into the environment. Owing to physical and biogeochemical processes in the ocean, artificial radionuclides in the ocean are redistributed from their initial entry points which depend on the various sources. Long range transport of radiocaesium in the ocean interior were investigated and presented. Radiocaesium were derived from global fallout which occurred mainly late 1950s and early 1960s and the Fukushima accident occurred in 2011. In the ocean interior, main factor is subduction of mode water formation from surface to two mode waters, STMW and CMW. Radiocaesium then stayed long in both STMW and CMW, but relatively first recirculation and southward movement were observed in STMW for decadal time scale.

We establish database for artificial Radionuclides in the marine environment as HAM global 2018, doi: 10.34355/CRiED.U.Tsukuba.00001, and we reconstruct 137Cs activity concentration sections for 1965-1968 and 1970-1973 to understand initial conditions of 137Cs activity concentration in ocean interior just after large atmospheric fallout in early 1960s and 5 years after injection. We also carried out observations at stations between 49 deg. N and 60 deg. S along 165 deg.  E in 2002, 2012 and 2015. After that, we also observed vertical profiles in the western North Pacific Ocean.　

Basic feature of radiocaesium distribution along 165 deg. E section in 1963-1965 was dome shape distribution of which deepest places were around 30-40 deg. N and of which maximum depth were around 600- 800 meter depths. The penetration of 137Cs is found less than 800 m depth, associated with the bowl shape of isopycnals in the midlatitude region. In general, the 137Cs activity concentrations in the subsurface and intermediate water of the mid latitude region of the western North Pacific were higher than those in surface waters of the subtropical and equatorial Pacific. In 2002, we observed two 137Cs activity concentration maxima at 250 m and at 400 to 500 m depth at around 20 deg. N. The 137Cs activity concentration at the core at 400 to 500 m depth in 2002 was around 2 – 3 Bq m-3 and the start of moving in 1963-1965 was 16 Bq m-3 which indicates only one thirds of dilution occurred during about 40 years travel in the ocean interior as CMW. In 2012, we also observed two 134Cs activity concentration maxima at 150 m, 30 deg. N and at 300 m depth at 40N, while we observed a Fukushima derived at 300 m, 30 deg. N with southward movements. Basic feature of 137Cs distribution derived from atmospheric weapons test along 165 deg. E section in 2012 still keep dome shape distribution of which deepest places were around 30-35 deg. N and of which maximum depth were around 400 meters depths, while deepest places were around 20-30 deg. N in 2015. These findings strongly suggest that radiocaesium has been transporting in the ocean interior by subduction of mode waters from subarctic region to subtropical region and tropical region.

How to cite: Aoyama, M., Hamajima, Y., Inomata, Y., Kaeriyama, H., Kumamoto, Y., Nakano, T., and Oka, E.: Long range transport of radiocaesium derived from global fallout and the Fukushima accident in the ocean interior of the Pacific Ocean since 1960s through 2017, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3175, https://doi.org/10.5194/egusphere-egu2020-3175, 2020.

We investigated spatial and temporal variations in 137Cs concentrations in the surface waters of the global ocean for the period from 1957 to 2018. In order to study the distribution of 137Cs concentrations in surface seawater, we divided the global ocean into 37 latitudinal boxes on the basis of known ocean current systems, latitudinal and longitudinal distributions of 137Cs concentrations, the distribution of global fallout, locations of nuclear reprocessing plants, fallout from the Chernobyl accident, and release from Fukushima Nuclear Power Plant accident. Based on the 0.5-y average value of 137Cs concentrations in the surface water in each box, we classified the temporal variations into four types. In the North Pacific Ocean where there was high fallout from atmospheric nuclear weapons tests, the rates of decrease in the 137Cs concentrations changed over the five decades: the rate of decrease from the 1950s to the 1970s was much faster than that after the 1970s, and the 137Cs concentrations were almost constant after the 1990s. Latitudinal differences in 137Cs concentrations in the North Pacific Ocean became small with time. After March 2011, extremely high concentrations (3.26×107 Bq/m3) were observed in the western North Pacific Ocean based on the direct release and atmospheric deposition of FNPP1-derived 137Cs. In the equatorial Pacific and Indian Oceans, the 137Cs concentrations varied within a constant range in the 1970s and 1980s, due to the advection of 137Cs from areas of high global fallout in the mid-latitudes of the North Pacific Ocean. In the eastern South Pacific and Atlantic Oceans (south of 40°S), the concentrations decreased exponentially over the six decades. In the Arctic and North Atlantic Oceans, including marginal seas, 137Cs concentrations were strongly controlled by discharge from nuclear reprocessing plants after the late 1970s. The 137Cs concentrations were rapidly decreased after the early 1980s, and advected into the Arctic Ocean. 
The averaged 137Cs concentrations in each box in the year of 1970 were 1-716 Bq/m3, and those were decreased to 0.2-28 Bq/m3 in the year of 2010. The apparent half-residence times of 137Cs in the surface waters of the global ocean from 1970 to 2010 ranged from 4.2 to 48.1 years for each box. 

(Reference)
Inomata et al. (2009) Analysis of 50-y record of surface 137Cs concentrations in the global ocean using the HAM-global database. Journal of Environmental Monitoring, DOI: 10.1039/b811421h. 

How to cite: Inomata, Y. and Aoyama, M.: Analysis of 60-yr record of surface 137Cs concentrations in the global ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11813, https://doi.org/10.5194/egusphere-egu2020-11813, 2020.

We investigate the oceanic dispersion of ¹³⁷Cs (half-life of 30.1 years) and its impact on the environment. ¹³⁷Cs has been released into the ocean due to the atmospheric weapons tests, discharge from nuclear reprocessing plants, the Chernobyl accident, and most recently due to Fukushima Daiichi Nuclear Power Plant (1F NPP) accident. ¹³⁷Cs activities measured for scientific purposes as well as environmental health and safety monitoring have been summarized in a historical database by IAEA. The spatio-temporal density of the observations varies widely, therefore simulation by an ocean general circulation model (OGCM) can be helpful in the interpretation of these observations. Although simulations of ¹³⁷Cs activity by OGCMs have been carried out previously, the input condition of ¹³⁷Cs still has large uncertainties due to a lack of observations of global fallout. The horizontal resolution of the previously available estimated global fallout of ¹³⁷Cs was 10 degree longitude x latitude. We have produced a new estimate of the global fallout of ¹³⁷Cs with 2.5-degree resolution using the Global Precipitation Climatology Project (GPCP) data, and investigated the impact of the revised input condition on the simulation of distribution of ¹³⁷Cs in the ocean. In addition, discharges of ¹³⁷Cs from the reprocessing plants (Sellafield and La Hague) were also considered. We used the Parallel Ocean Program version 2 (POP2) of the Community Earth System Model version 2 (CESM2). The horizontal resolution is 1.125 degree of longitude, and from 0.28 degree to 0.54 degree of latitude. There are 60 vertical levels with a minimum spacing of 10 m near the ocean surface, and increased spacing with depth to a maximum of 250 m. The simulated period was from 1945 to 2010 with the circulation forced by repeating (“Normal Year”) atmospheric conditions. We estimated the global distribution of ¹³⁷Cs deposition from 1945 to 2010 by using these geographical distribution data, the observed time-series data of annual ¹³⁷Cs deposition at the MRI from 1958 to 2010, and time-series data of ¹³⁷Cs deposition from 1945 to 1958 estimated from ice-core data. Simulated ¹³⁷Cs activities derived from the 2.5-degree deposition data were in good agreement with observations, particularly in the Pacific Ocean. Simulated ¹³⁷Cs activities were strongly influenced by the discharge of ¹³⁷Cs from the reprocessing plants. Transport processes were also investigated in the simulated results.

How to cite: Tsumune, D., Bryan, F., Lindsay, K., Misumi, K., Tsubono, T., Inomata, Y., and Aoyama, M.: Comparison of 137Cs activity between an ocean general circulation model and the global database, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12627, https://doi.org/10.5194/egusphere-egu2020-12627, 2020.

We conducted the ensemble simulation of Cs-134 activity in the North Pacific Ocean (NPO) water after the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) by setting four different passive tracers corresponding to the fluxes of the Cs-134 activity; 1. Cs_DD for Cs-134 activity directly discharged from the coast of the 1F NPP (Tsumune et al., 2013), 2. Cs_ADN for the activity derived from the atmospheric deposition (Aoyama et al, 2015) northern from 36°N, 3. Cs_ADKE for that in the Kuroshio Extension area from 32°N or 36°N, 4. Cs_ADS for that southern from 32°N. The totals of the Cs_DD, Cs_ADN, Cs_ADKE and Cs_ADS in the NPO in May 2011 are 5.6, 8.7, 1.0 and 0.6 PBq respectively, suggesting that the impact was dominant northern from 36°N in the NPO. The sum of four tracers showed comparable to the Cs-134 activity from all the fluxes in previous study with the correlation coefficient of 0.99 and the RMS of 5 Bq m$^{-3}$ in 2011 and 0.99 and 0.1 Bq m$^{-3}$ in 2012, except for the area of Japanese coast near the 1F NPP in which the rapid increase in the direct discharge flux produced the different negative values due to the dispersive error of the difference scheme. Since the Cs-134 activity diminishes in time due to the short half life of about 2 years, the abundance ratio was calculated for the investment of the meridional and vertical transport. The abundance ratio of the whole Cs-134 activities showed that although almost all the Cs-134 activity existed in the surface layer above 200m depth after the accident, the ratio in the intermediate layer from 200m to 600m depth increased and exceeded 50 percent since 2017. Moreover the ratio in the intermediate layer southern from 32°N exceeded the 25 percent since 2017, suggesting that more than 25 percent of the Cs-134 activity in the surface layer northern to the 36°N in early period after the accident were transported to the southern and deep in 2017. While the abundance ratio of Cs_DD and Cs_ADN in the intermediate layer showed an increase like a logarithmic function shape, the ratio of the Cs_DD, 60 percent, was larger than that of the Cs_ADN, 50 percent, in 2021. Moreover, the abundance ratio in 2011 showed the 70 percent of Cs_DD and Cs_ADN existed in the intermediate layer southern to the 32 °N, suggesting a large amount of both the Cs_DD and Cs_ADN were transported southern and deep in 2021.

How to cite: Tsubono, T., Misumi, K., Tsumune, D., Aoyama, M., and Hirose, K.: Transport of the Cs-134 activity derived from Fukushima Dai-ichi Nuclear Power Plant in the North Pacific Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4451, https://doi.org/10.5194/egusphere-egu2020-4451, 2020.

Just after Fukushima Daiichi Nuclear Power Plant (1F) accident occurred in March 2011, the direct discharges of highly-contaminated water from reactor Unit 2 and 3 into the 1F port followed. After the suppressions of the direct discharges, Kanda (Biogeosci. 10, 6107–6113, 2013) suggested that relatively small amounts of run-off of a radionuclide (Cs-137) from 1F port into Fukushima coastal region has continued based on his estimation method. However, Kanda’s estimation period was limited up to September 2012. Therefore, we expand the estimation period of the discharged inventory up to very recently, March 2020 with significant accuracy improvements by the present authors. As a result, we find that totally, in the period over 9 years, the discharged inventory has gradually diminished together with various characteristic fluctuations. In this presentation, we analyze the observed diminishing trends with temporal fluctuations and discuss their relationships with various suppression measures and constructions toward decommissioning of 1F. Furthermore, we estimate the annual discharged amount of Cs-137 and evaluate its impacts on the coastal area in terms of seawater concentrations.

How to cite: Yamada, S., Machida, M., Iwata, A., Otosaka, S., Kobayashi, T., Watanabe, M., Funasaka, H., and Morita, T.: Analysis of temporal variations of Cs-137 discharge inventory from the port of Fukushima Daiichi Nuclear Power Plant over 9 years after the accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21319, https://doi.org/10.5194/egusphere-egu2020-21319, 2020.

Introduction: A large amount of radioactive Cs was emitted into the environment by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Adachi et al. (2013) first reported radiocesium-bearing microparticles (CsMPs) from aerosol filters. Subsequent researches showed that the CsMP is SiO₂ glass with Cs, Cl, K, Fe, and Zn mainly contained in the particle. Diameter of CsMP is ~1-10 μm and ¹³⁷Cs radioactivity is ~0.5 to 10² Bq. It has been suggested that the CsMP was mainly emitted from Unit 2 or Unit 3 of FDNPP based on the ¹³⁴Cs/¹³⁷Cs activity ratio in the samples. Miura et al. (2018) reported CsMPs from the suspended particles in river water and their effect on K_d value, which suggested CsMPs may exist in the ocean transported through rivers. Kubo et al. (2018) and Ikenoue et al. (2018) reported hot spots in the ocean samples by autoradiography but they did not separate CsMPs from these spots. In this presentation, we first report CsMPs separated from marine suspended particles, sinking particles, and sediments in coastal area of Fukushima and compare them with CsMPs from the terrestrial samples.

Method: We collected suspended particles (2011, 2013, 2015), sinking particles (2014), sediment cores (2011) from coastal area of Fukushima. By a wet separation method (Miura et al., 2018), we separated CsMPs from these samples. After measurement of radioactivity with a high-purity germanium semiconductor detector, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses were performed for separated CsMPs. Using autoradiography, we calculated ¹³⁷Cs activity of unseparated hot spots over 0.1 Bq, which may be CsMPs.

Results and discussion: We separated 5 CsMPs from marine samples. The results of SEM-EDS analyses showed that these CsMPs have almost similar characteristics to the reported CsMPs because they mainly consist of Si, Cs, Fe, and Zn.  Their ¹³⁴Cs/¹³⁷Cs showed that the CsMPs were from Unit 2 or 3 of FDNPP. ¹³⁷Cs radioactivity per volume is also similar to reported CsMPs from Unit 2 or 3. In this presentation, we will show the effect of CsMPs on K_d values. CsMPs in the ocean samples will make apparent K_d values be higher than intrinsic K_d values related to the adsorption-desorption reaction to the clay minerals, which may explain the large variation of Cs concentration in marine samples.

How to cite: Miura, H., Ishimaru, T., Ito, Y., Kanda, J., Kubo, A., Otosaka, S., Kurihara, Y., Tsumune, D., and Takahashi, Y.: Discovery of radiocesium-bearing microparticles from ocean samples emitted from the Fukushima Daiichi Nuclear Power Plant accident , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6987, https://doi.org/10.5194/egusphere-egu2020-6987, 2020.