GI2.2

The radioactive cesium (¹³⁴Cs and ¹³⁷Cs), which originated from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, has remained in the soil and on plants as water-insoluble microparticles (termed as CsMPs) to some extent, and maintained relatively high radioactivity levels in the district. However, it has been reported that the radioactive Cs has been absorbed by plants. To interpret this phenomenon, the authors investigated CsMPs to determine if they become soluble during filtration and dialysis experiments. Moreover, other physical properties, such as mechanical properties and thermal stability, were observed during the course of the relevant experiments. These properties can be obtained by using carbonized charcoal litter with CsMPs. And simple and economic decontamination trials of the soil were performed by sieving after drying and roughly crushing.

How to cite: Tanaka, I., Yamaguchi, A., Kikuchi, K., Niimura, N., Saeki, Y., and Sugihara, M.: Dissolution, Mechanical Properties, and Thermal Stability of Microparticles Containing Radioactive Cesium on Plant Litter Derived from the Fukushima Daiichi Nuclear Power Plant Accident,  and Soil Decontamination Trials, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2976, https://doi.org/10.5194/egusphere-egu21-2976, 2021.

The Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011 released significant quantities of radionuclides into the environment. Ten years after the accident, questions still remain, particularly about the processes that led to the partial core meltdown of reactors 1 and 3. So far, some answers have been provided by the investigation of particles containing caesium (Martin et al., 2020) and sometimes uranium (Ochiai et al., 2018). Indeed, the composition of particles, which were produced and spread at the time of the reactor explosion, reflect the conditions that prevailed in the reactor. Accordingly, the objective of the current research is to develop a method for specifically locating actinide-bearing particles in sediment samples collected in the vicinity of FDNPP. To identify and locate such particles, three already existing methods have been upgraded, including 1) the method of fission tracks already used in the field of non-proliferation studies, 2) the autoradiography through the use of imaging plates that are currently employed in the context of the localization of particles containing radio-caesium and the dismantling of nuclear facilities (Haudebourg and Fichet, 2016), and 3) a real time autoradiography method through the use of the BeaQuant® instrument which has been developed for detecting radioactive particles in biology and geosciences.

In this study, a sediment sample collected nearby FDNPP, which may contain particles containing both radio-caesium and actinides, was selected. This sample was dried and sieved to 63 µm before being processed according to the different analysis protocols.  A quality control sample containing only uranium oxide particles was also analysed, as these particles are devoid of gamma-emitters.

The first results of this comparison of autoradiography methods for the detection of actinide-bearing particles in Fukushima samples will be presented. The method of fission tracks was particularly efficient for detecting both natural and anthropogenic uranium.

The next steps of this study will be to implement this method identified as optimal to isolate and characterise a larger number of particles released by FDNPP. The full characterization of these particles (size, morphology, elemental and isotopic compositions) will provide novel insights to determine their origin and to improve our understanding of their formation processes within the reactors and anticipate their fate in the environment.

References:

Haudebourg, R., Fichet, P., 2016. A non-destructive and on-site digital autoradiography-based tool to identify contaminating radionuclide in nuclear wastes and facilities to be dismantled. J. Radioanal. Nucl. Chem. 309, 551–561. https://doi.org/10.1007/s10967-015-4610-7

Martin, P.G., Jones, C.P., Cipiccia, S., Batey, D.J., Hallam, K.R., Satou, Y., Griffiths, I., Rau, C., Richards, D.A., Sueki, K., Ishii, T., Scott, T.B., 2020. Compositional and structural analysis of Fukushima-derived particulates using high-resolution x-ray imaging and synchrotron characterisation techniques. Sci. Rep. 10, 1636. https://doi.org/10.1038/s41598-020-58545-y

Ochiai, A., Imoto, J., Suetake, M., Komiya, T., Furuki, G., Ikehara, R., Yamasaki, S., Law, G.T.W., Ohnuki, T., Grambow, B., Ewing, R.C., Utsunomiya, S., 2018. Uranium Dioxides and Debris Fragments Released to the Environment with Cesium-Rich Microparticles from the Fukushima Daiichi Nuclear Power Plant. Environ. Sci. Technol. 52, 2586–2594. https://doi.org/10.1021/acs.est.7b06309

How to cite: Diacre, A., Fichet, P., Sardini, P., Donnard, J., Fauré, A.-L., Marie, O., Shozugawa, K., Susset, M., Hori, M., Pointurier, F., and Evrard, O.: Localization of actinide-bearing particles in sediment samples from the Fukushima restriction zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3320, https://doi.org/10.5194/egusphere-egu21-3320, 2021.

The 3rd model intercomparison project (MIP) of atmospheric dispersion model targeting on ¹³⁷Cs emitted from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 was conducted (Sato et al. 2020). Nine models participated in the 3rd MIP. All participated models used the identical source term of Katata et al. (2015) and the identical meteorological data (Sekiyama and Kajino, 2020) as in the previous MIP (i.e., 2nd MIP Sato et al. 2018), but finer horizontal grid resolution (1 km) than that of 2nd MIP (3 km) was used for understanding the behavior of atmospheric ¹³⁷Cs measured in the vicinity of FDNPP. Results of the models elucidated that, as in the 2nd MIP, most of the observed high atmospheric ¹³⁷Cs concentrations (plumes) were reasonably well simulated by the models, and the good performance of some models cancelled a bad performance of some models when used as an ensemble, which highlights the advantage of the multimodel ensemble. The analyses also indicated that the use of the finer grid resolution (1 km) improved the meteorological field in the vicinity of FNDPP. As a consequence, the atmospheric ¹³⁷Cs measured near FDNPP was more reasonably reproduced in 3rd MIP than 2nd MIP.

As well as the evaluation of the performance of the model, we examined the usefulness of the results of atmospheric dispersion simulation in an emergency base on the results of 2nd and 3rd MIPs. For the analyses we defined the worst situation as that plume is observed but the model does not simulate it. The analyses reported that the worst situation happened in only 3% of the total calculation period by using the multimodel ensemble, even if the absolute value of the simulated ¹³⁷Cs in each model was different in the range of factor 3-6. The analyses also indicated that from six to eight models are required for making most of the advantages of the multimodel ensemble.

How to cite: Yamazawa, H., Sato, Y., Sekiyama, T., Kajino, M., Fang, S., Quérel, A., Quélo, D., Kondo, H., Terada, H., Kadowaki, M., Takigawa, M., Morino, Y., Uchida, J., Goto, D., Nakamura, M., and Kiriyama, Y.: 3rd model intercomparison projects of atmospheric dispersion model for 137Cs emitted from Fukushima Daiichi Nuclear Power Plant, and application of MIPs' results for usage in an emergency, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14377, https://doi.org/10.5194/egusphere-egu21-14377, 2021.

In March 2011, large amount of radionuclides were released into the atmosphere throughout the Fukushima Daiichi nuclear disaster. This massive and very complex release, characterized by several peaks and wide temporal variability, lasted for more than three weeks and is subject to large uncertainties. The assessment of the radiological consequences due to the exposure during the emergency phase is highly dependent on the challenging estimate of the source term.

Inverse modelling techniques have proven to be efficient in assessing the source term of radionuclides. Through Bayesian inverse methods, distributions of the variables describing the release such as the duration and the magnitude as well as the observation error can be drawn in order to get a complete characterization of the source.

For complex situations involving releases from several reactors, the temporal evolution of the release may be as difficult to reconstruct as its magnitude. The source term or function of the release is described in the inverse problem as a vector of release rates. Thus, the temporal evolution of the release appears in the definition of the time steps where the release rate is considered constant. The search for the release variability therefore corresponds to the search for the number and length of these successive time steps.

In this study, we propose to tackle the Bayesian inference problem through sampling Monte Carlo Markov Chains methods (MCMC), and more precisely the Reversible-Jump MCMC algorithm.
The Reversible-Jump MCMC method is a transdimensional algorithm which allows to reconstruct the time evolution of the release and its magnitude in the same procedure.

Furthermore, to better quantify uncertainty linked to the reconstructed source term, different approaches are proposed and applied. First, we discuss how to choose the likelihood and propose several distributions. Then, different approaches to model the likelihood covariance matrix are defined.

These different methods are applied to characterize the ¹³⁷Cs Fukushima source term. We present a posteriori distributions enable to assess the source term and the temporal evolution of the release, to quantify the uncertainties associated to the observations and the modelling of the problem.

How to cite: Dumont Le Brazidec, J., Bocquet, M., Saunier, O., and Roustan, Y.: Bayesian inference and uncertainty quantification for source reconstruction of 137Cs released during the Fukushima accident, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12433, https://doi.org/10.5194/egusphere-egu21-12433, 2021.

In April 2020, the largest forest fire occurred in the Chernobyl Exclusion Zone (ChEZ) in its history. The results of modeling the atmospheric transport of radioactive aerosols released into the atmosphere as a result of wildland fires in the ChEZ and around it are presented. The atmospheric transport model LEDI, developed at the Institute for Safety Problems of NPPs, and the Atmospheric Dispersion Module of the real -time online decision support system for offsite nuclear emergency RODOS, which development was funded by the EU, were used. The ¹³⁷Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The ¹³⁷Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m⁻³ during the period April 3–20. The modeling results are generally consistent with measured data pertaining to radioactive contamination in Kyiv, within the ChEZ, and areas around four operating nuclear power plants in Ukraine.

A method for estimating the radionuclide activity emissions during wildland fires in radioactively contaminated areas is proposed. This method is based on satellite data of the fire radiative power (FRP), the radionuclide inventory in the fire area, and an emission factor for radioactive particles. A method was applied for forest fires in the ChEZ in April 2020. Preliminary estimations of an emission factor are made using FRP values obtained from NASA's MODIS and VIIRS active fire products.

On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional ¹³⁷Cs activity raised by the dust storm from burned areas in the meadow biocenoses was estimated to be about 162 GBq, i.e. up to 20% of the total activity emitted into the air during the entire period of forest fires on April 3-20, 2020. According to the modeling results, during April 16-17, the input of resuspension of radioactive particles due to a dust storm was up to 80-95% of the total ¹³⁷Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%.

The total effective dose to the population of Kyiv during the fire period is estimated to be 5.7 nSv from external exposure and the inhalation of ¹³⁷Cs and ⁹⁰Sr, rising to 30 nSv by the end of 2020. This is about 0.003% of the annual permissible level of exposure of the population. A committed effective dose up to 200-500 nSv is estimated for the personnel of the Chernobyl NPP from the radioactive aerosol inhalation during the 2020 forest fires, which is not more than 0.05% of the established control levels of internal exposure for them.

How to cite: Talerko, M., Lev, T., Kovalets, I., Zheleznyak, M., Igarashi, Y., Paskevych, S., Kashpur, V., and Kireev, S.: Modeling study of the atmospheric transport of radioactivity after wildland fires and a dust storm in the Chernobyl Exclusion Zone in April 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5134, https://doi.org/10.5194/egusphere-egu21-5134, 2021.

The atmospheric release of radionuclides is a crucial potential hazard to public health. Its release rate is vital in assessing the international environmental risk of atmospheric radionuclide leaks and conducting nuclear emergency preparedness. However, according to the radionuclide leaks such as the Fukushima Daiichi accident and the recent iodine-131 and ruthenium-106 releases in 2017, the release rate cannot be directly measured or derived in a forward way, but can only be inversely estimated by comparing the environmental measurements with a model-predicted plume, a technique often referred to as source inversion. However, such inversion is vulnerable to the inevitable plume biases, including the plume range (i.e. the area of positive model predictions) and transport pattern in radionuclide transport modeling, leading to inaccurate source estimates and risk assessment.

This paper describes an automated method that estimates the release rate while comprehensively correcting plume biases. By using the spatial correlation matrix, the predicted plume can spread over a broader area, thus covering the potential range of the true plume. Then, the difficult task of direct plume adjustment is simplified to tuning the predictions inside a correlation-adjusted plume. Based on this, the previous joint method can work efficiently to estimate the release rate while simultaneously refining the predictions inside the adjusted range, correcting both the plume range and the transport pattern. An ensemble-based algorithm is proposed to automatically calculate the spatial correlation in order to execute this method. With this algorithm, SERACT can accomplish realistic and robust source estimation without manual adjustment on any parameters.

The proposed method SERACT is validated with the two wind tunnel experiments based on a real Chinese nuclear power plant site, and the site features highly heterogeneous topography and dense buildings. In this paper, two radionuclide transport models with mild and severe plume biases respectively are used to assess the adjustment efficiency of SERACT, including source estimation and plume distribution. Its performance is compared with that of the standard approach and a recent state-of-the-art method. Its sensitivity to the number and quality of measurements, and the selection of autocorrelation scales is also investigated.

The results demonstrate that SERACT corrects the plume biases with high accuracy (Pearson’s Correlation Coefficient=1.0000, Normalized Mean Square Error≤1.03×10⁻³) and reduces the estimation error by nearly two orders of magnitude at best. In addition, SERACT exhibited stable performance in all the validation tests and gave the lowest error levels with various numbers and quality of measurements. With fully automated parameterization, its performance is close to that obtained with the optimal autocorrelation scale in all test cases. These results indicate that SERACT is robust in various inversion cases and is able to serve as a general remediation to the long-standing imperfect modeling issue in source inversion.

How to cite: Zhuang, S. and Fang, S.: Simultaneous release rate estimation and modeled plume bias correction for atmospheric radionuclide emissions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-114, https://doi.org/10.5194/egusphere-egu21-114, 2021.

Vertical migration of radiocesium is a key issue in soils impacted by Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Among radioactive substances deposited on terrestrial ecosystems, ¹³⁴Cs (with half-life 2.07 years) and ¹³⁷Cs (with half-life 30.2 years) were dominant and have by far the most radiological significance.

This work investigates the importance of non-equilibrium sorption on the vertical migration of ¹³⁷Cs in field conditions. 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 plots measured in a young cedar stand situated around 35 km northwest of the FDNPP. Profiles were sampled at four different dates (2013, 2014, 2016, 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 top of the mineral soil surface was reconstructed from global monitoring data from the forest stand and a first-order compartment model for the organic layer.

Our results showed that the inclusion of non-equilibrium sorption slightly improves the realism of simulated ¹³⁷Cs profiles compared to the equilibrium hypothesis. 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. As a consequence, the K_d model overestimated the contamination into deeper layers and therefore overestimated the migration velocity of ¹³⁷Cs. Fitted sorption parameters suggested a fast sorption kinetic (1 - 7 hours) and a pseudo-irreversible desorption rate (3.2 - 3.4 x 10⁶ years), whereas equilibrium sorption (4.0 x 10^-3 L kg^-1 on average) only affected a negligible portion of ¹³⁷Cs inventory.

To further distinguish the models behaviors, short and long term simulations were conducted. By June 2011, EK parameters fitted on our plots realistically reproduced different profiles measured in the same forest study site. Predictive modeling of ¹³⁷Cs profiles in soil suggested a strong persistence of the surface ¹³⁷Cs contamination by 2030, with exponential profiles consistent with those reported after the Chernobyl accident.

These results prove that the choice of the sorption model is critical in post-accidental situations. An equilibrium approach can result in an underestimation of ¹³⁷Cs 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. Non equilibrium sorption parameters can be partially inferred from in situ measurements. However, further experiments in controlled conditions are required to better estimate the sorption parameters and to identify the processes behind non-equilibrium sorption.

How to cite: Chaif, H., Coppin, F., Bahi, A., and Garcia-Sanchez, L.: Influence of non-equilibrium sorption on the vertical migration of 137Cs in forest mineral soils of Fukushima prefecture, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-97, https://doi.org/10.5194/egusphere-egu21-97, 2021.

To assess the levels of radioactivity, the soils of the Arctic region of Western Siberia were studied. The content of Cs-137 in the soils of the studied region ranges from 2 to 9 Bq/kg, with an average value of 4 Bq/kg.

The average contents of natural radionuclides (U, Th and K-40) in soils are determined by the radioactivity of the parent rocks. The radioanalytical results showed that the average Th content is 2.8 (0.3-6.7) ppm; U – 2.0 (0.5-6.6) ppm, K-40 – 233 (16-473) Bq/kg in the soils.

This work was supported by the Russian Science Foundation grant (project No 18-77-10039). Analytical studies were carried out at the Center for multi-elemental and isotope research SB RAS.

How to cite: Mezina, K., Melgunov, M., and Belyanin, D.: Natural and artificial radioactivity of soil of the Arctic part of Western Siberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16193, https://doi.org/10.5194/egusphere-egu21-16193, 2021.

The ¹³⁷Cs (t_1/2 =30 years) is a principal radioisotope that was artificially introduced into the environment through the atmospheric bomb tests took place from the middle of the 1940s to the 1980s and from the major nuclear accidents (i.e., Chernobyl, 1986 and Fukushima, 2011). From the atmosphere, ¹³⁷Cs easily adsorbs to particles and it returns to lithosphere (pedosphere) by wet and dry deposition as a radioactive fallout component. Due to the Chernobyl nuclear accident, the released contaminated air mass, containing Cs-137, largely propagated, deposited, and distributed across several European countries in the ambient environment (Balonov et al., 1996). These particles also reached houses (e.g. through open windows, cracks, and vents) in an urban environment and deposited inside resulting in the exposition of the habitants to ¹³⁷Cs, especially in areas that are not accessible for a regular cleaning like attics. Following the nuclear accidents, primary attention was drawn to agricultural areas and less attention was paid to urban environments. Accordingly, the goal of this study is to compare the ¹³⁷Cs activity in attic dust as undisturbed samples, and urban soils as disturbed environmental materials to determine the ¹³⁷Cs distribution in urban environment. 

Attic dust (AD) samples were collected from 14 houses, which were built between 1900 and 1990 14 urban soil (US) samples were collected nearby the houses at a depth of 0-15 cm in Salgótarján, a former industrial city. To obtain a representative local undisturbed soil sample, a forest soil sample was collected from the upwind direction (NW) of the city. To check the ¹³⁷Cs content of the local industrial waste material, we also collected fly-ash slag sample from a waste dump.   AD and US samples were analyzed by a well-type HPGe and with an n-type coaxial HPGe detector in a low background iron chamber, respectively.

Cs-137 activity in the studied AD ranges from 5.51±0.9 to 165.9±3.6 Bq kg^-1, with a mean value of 75.4±2.5 Bq kg^-1 (decay corrected in 2016). In contrast, US samples show ¹³⁷Cs activity ranging between 2.3±0.4 and 13.6±0.6 Bq kg^-1.  The brown forest soil sample has elevated ¹³⁷Cs activity concentration (18.5±0.6 Bq kg^-1), compared to the urban soils. The fly-ash slags activity is below the detection limit (0.7±0.5 Bq kg^-1).

The average ¹³⁷Cs activity in AD is ~15 times higher than that of US. This result clearly indicates that attic area provides a protected (hardly or unchanged) environment, therefore physical condition of the dust remains constant in time, and there is a small chance for chemical reaction. Forest soil proves that US were highly disturbed by anthropogenic activity. This is supported by fly-ash slag activity results.  Whereas, ¹³⁷Cs activity concentration of the AD samples shows significantly higher than that of the studied soils in Hungary. This confirms again US cannot show the historical atmospheric ¹³⁷Cs pollution such as attic dust. A statistically significant relationship (p=0.003, r²=0.05) were found between the AD and US samples. Therefore, it can be considered that attic dust remained undisturbed for decades and preserve past record of components of atmospheric pollution.

How to cite: Tserendorj, D., Szabó, K. Z. S., Völgyesi, P. V., Abbaszade, G., Tan, D. L. T., Salazar, N., Zacháry, D., Nguyen, T. C., and Szabó, C.: Comparative study of Cs-137 activity concentration between attic dust and urban soil from Salgotarjan city, Hungary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16233, https://doi.org/10.5194/egusphere-egu21-16233, 2021.

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the largest quantity of radiocaesium into the terrestrial environment since the Chernobyl nuclear accident. The surrounding land received 2.7 PBq of radiocaesium to forests, agricultural lands, grasslands, and urban areas, from which the radionuclides migrated through soil and waterways. In this presentation, the deposition and distribution of radionuclides, especially radiocaesium, in the terrestrial environment as a result of the FDNPP accident are discussed based on the past 10 years' intensive dataset. Anthropogenic activities such as rice and vegetable cultivation and residential activities in the upstream area have led to a rapid decline in the activity concertation of 137Cs of suspended sediment (SS) transport in the river network, and these declines directly control the dissolved 137Cs concentration in the river water. We outline the environmental and anthropogenic factors that influenced the subsequent transport and impacts of radionuclides through the environment. The environmental aftermath of the accident at Fukushima is compared to Chernobyl, and the relatively rapid remediation of the Fukushima region relative to the region surrounding Chernobyl will be explained.

How to cite: Onda, Y., Taniguchi, K., Yoshimura, K., and Wakiyama, Y.: The impact and fate of fallout radionuclides by Fukushima Daiichi Nuclear Power Plant Accident in hydrological systems and post-accident environmental recovery, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14671, https://doi.org/10.5194/egusphere-egu21-14671, 2021.

Radioiodine is one of the most important radionuclides released by the accidents of Fukushima Dai-ichi nuclear power plant (FDNPP). Iodine-131 elevated ambient radiation dose rate immediately after the accident, but it is extinguished due to its short half-life. The long-lived ¹²⁹I can be used as a tracer to retrospectively infer the level of ¹³¹I. Understanding of environmental behavior of ¹²⁹I is important for preparedness against nuclear disaster. This study presents sampling campaigns on the Abukuma River during two high flow events, and discusses riverine ¹²⁹I behavior based on comparisons with that of ¹³⁷Cs. The study site was the Kuroiwa site locating at midstream of the Abukuma river. Its catchment area was 2886 km² and mean ¹²⁹I inventory in the catchment was 0.041 Bq m^-2. Five and seven river water samples were taken during high-flow events in July 2018 (JUL18) and October 2018 (OCT18), respectively. Suspended sediment and filtrate samples were obtained by decantation and subsequent filtration with 0.45 μm-mash membrane filter, respectively. The suspended sediment and filtrate samples were measured for ¹³⁷Cs activity concentrations with HPGe detector and then measured for ¹²⁹I/¹²⁷I ratio with accelerator mass spectrometer and for ¹²⁷I concentration with ICP-QQQ-MS. Mean ¹²⁹I activity concentration in suspended sediment during JUL18 and OCT18 were 1.0 and 0.43 mBq kg^-1, respectively. In terms of temporal trends, ¹²⁹I activity concentration in suspended sediment lowered in the peak water discharge phase, contrastive to those of ¹³⁷Cs which were the highest at peak water discharge phase. Mean dissolved ¹²⁹I activity concentrations during JUL18 and OCT18 were 0.18 and 0.067 μBq L^-1, respectively. Both ¹²⁹I and ¹³⁷Cs activity concentrations in dissolved form tended to decrease with time during two high-flow events. Mean apparent distribution coefficient (Kd) during JUL18 and OCT18 were 4.3 ×10⁴ and 6.1 ×10³ L kg^-1, respectively. The Kd values of ¹²⁹I were lower than those of ¹³⁷Cs and it reveal relatively-high solubility of ¹²⁹I. Total exportation of ¹²⁹I from the catchment during JUL18 and OCT18 were estimated as 1.0 × 10⁴ and 2.3 × 10⁴ Bq, respectively. Exported ¹²⁹I in dissolved form accounted for 80 and 27% of total exportations, respectively. By contrast, more than 95% of ¹³⁷Cs was exported in particulate form in the events. These results indicated an importance of dissolved form for understanding environmental behavior of radioiodine.

How to cite: Wakiyama, Y., Matsumura, M., Matsunaka, T., Hirao, S., and Sasa, K.: Behavior of 129I in the Abukuma River water during two high-flow events in 2018, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14420, https://doi.org/10.5194/egusphere-egu21-14420, 2021.

In Fukushima Prefecture, radiocesium as the particulate and dissolved form has been discharging from the mountains and forests since the Fukushima Dai-ichi nuclear power plant (FDNPP) accident in 2011. In particular, in October 2019, the watershed around the FDNPP was subjected to extensive flooding due to Typhoon Hagibis, resulting in significant changes in the hydrological environment. In this study, we investigated the characteristics of changes in particulate/dissolved ¹³⁷Cs concentrations in the main 3 rivers in the north region of FDNPP 3–9 years after the nuclear accident and the impact of the typhoon on ¹³⁷Cs dynamics in river water.

Monthly observations of river waters in baseflow conditions showed a decrease in dissolved ¹³⁷Cs concentration with an environmental half-life of 2–10 years, and seasonal fluctuation such as increasing in summer and decreasing in winter. The annual amplitude of the dissolved ¹³⁷Cs concentration in water released from dams was smaller and the peak of the concentration was observed later than that in river sites where the influence of dams is small. The ¹³⁷Cs concentrations in the suspended solids did not show any significant seasonal variation, and the environmental half-life of 1–8 years was relatively faster than the dissolved forms observed at the same site. Immediately after Typhoon Hagibis in 2019, the dissolved ¹³⁷Cs concentration decreased significantly compared to the previous years, especially at two dam lake discharge sites. At the two sites, the dissolved ¹³⁷Cs concentration did not recover to the level predicted by the pre-typhoon data even one year after the typhoon event, but no significant decrease in ¹³⁷Cs concentration in suspended solids was observed. These differences in the environmental behavior of ¹³⁷Cs in different forms suggest that there are limitations in predicting particulate and dissolved ¹³⁷Cs concentrations with a fixed parameter such as partition coefficient.

How to cite: Tsuji, H., Ozaki, H., and Hayashi, S.: Mid-long term change of particulate/dissolved 137Cs concentration in river water and the impact of Typhoon Hagibis in 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14063, https://doi.org/10.5194/egusphere-egu21-14063, 2021.

The impact of freeze-thaw processes and subsequent runoff affecting the ¹³⁷Cs flux and concentration in sediment discharge were revealed in bareland erosion plot following the Fukushima nuclear power plant 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 bareland 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.

How to cite: Igarashi, Y., Onda, Y., Wakiyama, Y., Yoshimura, K., Kato, H., Kozuka, S., and Manome, R.: Impacts of freeze-thaw processes and subsequent runoff on 137Cs washoff from bareland in Fukushima, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8553, https://doi.org/10.5194/egusphere-egu21-8553, 2021.

The Chornobyl accident in 1986 led to radioactive contamination of surface water bodies (SWB) in the Chornobyl Exclusion Zone (ChEZ), including lakes (Azbuchyn, Glyboke, Yanov crawl etc) and the Cooling Pond (CP). An abundance in fallout the dispersed fuel particles (FPs) was a specific feature of the accident, comprising the debris of irradiated nuclear fuel in different states of uranium oxidation mixed with construction materials.

Contamination of SWB by 90Sr and transuranic isotopes was mainly because of FPs. Experimental studies on the behaviour of FPs in soils and aquatic systems have shown that main factors controlling release of radionuclides outside FPs are: composition of the matrix, state of initial oxidation and oxidation properties of the environment.  

FPs behavior in SWB has not been sufficiently studied, though limited data suggest contrasting differences to terrestrial environments. Mainly is because of the different oxidation properties of soil and bottom sediment, creating better conservation conditions for FPs in sediment. In case when in SWB bed become dry and exposed, as is the case in the CP after decommissioning, an intensive processes of primary soil formation begin on exposed areas. This later forced dissolution of FPs, and hence radionuclides transition into more mobile forms followed by release to groundwater and surface runoff. 

We have developed convenient method for identifying Chornobyl FPs based on radiography, which comprises the exposure of X-ray film by spreading over a thin-layered dry solid sample. Processing X-ray films and image analysis makes it possible to estimate the size of FPs, as well as dispersion and distribution of radionuclide activity within the FPs of different size fractions. This also facilitates picking up single FPs to carry out extended individual analysis by EDXRF, SEMs etc. The radiography method was used to estimate the chemical resistance of FPs after sequential leaching to predict the behaviour of radionuclides in natural field conditions.

Overall, more than 120 samples were collected from bottom sediments cores taken from different SWB in ChEZ and analyzed by radiography. The results obtained show that from 70 to 90% of activity of radionuclides in sediments are associated with FPs. One gram of sediment contains several 10s to several 100s of individual FPs, while there is significant spatial heterogeneity of FPs density over the territory of ChEZ. The majority of FPs are less than 3 microns and their contribution to total activity was estimated as minor.

The main contribution to activity (>70%) comes from particles with a diameter of more than 10 microns and, accordingly, mobility of radionuclides will be determined by processes of destruction and leaching of radionuclides from particles of these size. Chemically resistant FPs are of 3-5 microns, and the highest concentration of fuel particles is typical for SWB located in close proximity to the ChNPP.

Up to 7% of the activity remains associated with FPs after being treated with strong 8M nitric acid indicating that 90Sr, transuranic and, partly, 137Cs are confined in chemically very stable particles and may not be mobilized under natural conditions for many decades. 

How to cite: Protsak, V., Laptev, G., Derkach, G., Korychenskyi, K., Prokopchuk, N., Nanba, K., Igarashi, Y., Konoplev, A., Kireev, S., and Smith, J.: Recent developments of the radiography method for characterisation of Fuel particles in Surface water bodies of Chornobyl Exclusion zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15221, https://doi.org/10.5194/egusphere-egu21-15221, 2021.

Since the 1986 Chornobyl accident transport of radionuclides by Pripyat River shares more than 90% of the annual total flux of radioactivity coming out the Chornobyl Exclusion Zone (ChEZ).  90Sr was the main contributor to this flux. In course of time destruction of the accidentally dispersed "fuel particles" leads to increase of mobile, e.g. water-soluble, forms of 90Sr  on territories affected by, while fixation of 137Cs in soil is reflected by significant reduction in 137Cs aquatic transport outside the ChEZ.

Heavily contaminated floodplain of the Pripyat River, located in vicinity of ChNPP upstream of Yaniv Bridge up to Ukrainian - Belorussian border, was considered as a “hotspot” with highest risks to the Pripyat and Dnipro water contamination due to recurrent flooding of these territories. This was evidently elucidated after the 1991 ice jam event when drastic increase of 90Sr in water was observed. The dikes splitting leftbank floodplain from the Pripyat river channel were constructed in 1992-1993. Yet, it is still important to quantify the amount of 90Sr that can be washed off the floodplain due to potential dike breakage caused by the extreme floods. 

Key parameters used to describe status of radionuclide in reaching equilibrium in soil-water system are the distribution coefficient (Kd) and kinetic rate that is reciprocal of typical time scale of desorption processes. These parameters subsequently were estimated in 1991 on the basis of batch experiment carried out with the soil monoliths sampled from the Pripyat floodplain ( Laptev and Voistekhovich, 1991). Results were used in the 2D model COASTOX for justification the construction of protecting dikes (Zheleznyak at al., 1992).

To analyze current ability of 90Sr to be washed off the floodplain, soils monoliths were collected in 2020. The experimental studies of the soil cores collected from same location as the monoliths allowed to estimate mobile speciation of 90S and calibrate mass-exchange parameters. Amount of the readily exchangeable forms of 90Sr in soils significantly increased from 10-30% in the first years after the accident up to 65-75% as to 2020. Results of field and laboratory  studies were used for simulation the scenarios of 90Sr washing off the floodplain during the dikes breaks on the basis of contemporary version of COASTOX model, that includes the parallel algorithms for numerical solution of the model equations on the unstructured computational grids for multi CPU and GPU systems. Approaches for the modelling of the secondary release of 90Sr due the rapid destruction of “fuel particles” are considered. Taking into account two concurrent processes - decrease of amount of 90Sr in uppermost soil layer due to decay and downward vertical migration (1), amid increased amount of exchangeable forms of 90Sr (2), one could project subsequent increasing of 90Sr  in Pripyat and Dnipro river waters downstream the source in case of the dike breakage scenarios. On the other hand, computer simulation suggests that the maximal values of the 90Sr concentrations expected to be not higher than the measured ones during the high floods events after the accident.

How to cite: Laptev, G., Voitsekhovych, O., Protsak, V., Zheleznyak, M., Nanba, K., Konoplev, A., Igarashi, Y., Wakiyama, Y., Bezhenar, R., Kivva, S., Pylypenko, O., Sorokin, M., Kireev, S., and Veremenko, D.: Temporal changes of mobile forms of 90Sr on Pripyat River floodplain in vicinity of Chornobyl NPP: measurements and risk assessments for river water contamination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10934, https://doi.org/10.5194/egusphere-egu21-10934, 2021.

Cooling Pond (CP) of the Chornobyl Nuclear Power Plant (ChNPP) is one of the most radioactively contaminated large water bodies over the globe. During the active phase of the ChNPP accident, radionuclides got into the CP in result of atmospheric deposition, release of highly contaminated water from system of accidental cooling, and water used to extinguish the fire. In the years after the accident, the contamination was distributed in the CP due to currents. For this period, three types of hydrological conditions dominated in the CP. Initially, the currents were forced by the cooling system of the ChNPP, which caused a circular movement of water. After the decommissioning of the ChNPP, the natural circulation took place in the CP. Starting from the end of 2014, when pumps that continuously fed the CP with water from the Prypiat River were shutdown, a gradual decrease of water level began. Now the water level has dropped by about 6 m leading to the transformation of the whole reservoir into several small lakes and redistribution of radionuclides in them. The objectives of the study were to calibrate models, which were customized for the CP, using data for the whole post-accident period including data collected during the drawdown period by the joint efforts of Ukrainian and Japanese researchers, and then to provide model based predictions of the future radionuclide concentrations in new water bodies.

During field studies that were carried out in November 2020, the current state of radioactive contamination of the CP was investigated. Samples of water, suspended and bottom sediments and biota were taken in 9 closed or semi-closed water bodies formed after partial drying of the CP. Concentrations of Cs-137 and its distribution in dissolved and particulated forms were measured in the laboratory. For simulations, the modeling system that consists of the 3D model of thermohydrodynamics and radionuclide transfer THREETOX and the box model POSEIDON-R was created. The THREETOX model was used for the obtaining currents in the CP for each type of hydrological conditions. The POSEIDON-R model was applied for the long-term simulations of the changes of activity concentration in the water, bottom sediments and biota starting from the 1986. The system of boxes in the POSEIDON-R model includes shallow and deep-water boxes. It was built in such a way that after the water level in the CP fell, the calculations were performed only in deep-water boxes. Fluxes of water between boxes were calculated based on currents from the THREETOX model. Seasonal changes in distribution coefficient K_d describing the partition of Cs-137 concentration between water and sediments were also taken into account. Calculated concentrations of Cs-137 in water and bottom sediments agree well with measurements for all boxes and for entire modeling period. It has been shown that POSEIDON-R model is able to reproduce changes in the concentrations of Cs-137 in freshwater fish occupying different levels the food chain. Scenarios for the potential changes of Cs-137 concentrations were considered by the variation of basic parameters.

How to cite: Bezhenar, R., Zheleznyak, M., Gudkov, D., Kanivets, V., Laptev, G., Protsak, V., Sakaguchi, A., Nanba, K., Wada, T., Kanasashi, T., Kireev, S., Veremenko, D., Nasvit, O., and Uematsu, S.: Model & data based assessment of the impacts of drawdown of the Chornobyl NPP Cooling Pond on the Cs-137 concentrations in water, sediments and biota, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12329, https://doi.org/10.5194/egusphere-egu21-12329, 2021.

In Japan's forests, field data on the distribution and migration of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident, from the initial dynamics to the distribution in the forest over ten years, have been vigorously collected. The results have been published in an IAEA technical report (IAEA- TECDOC-1927, 2020) as a database of migration parameters to be compiled by an international project (MODARIA II: Modelling and Data for Radiological Impact Assessments) promoted by the International Atomic Energy Agency (IAEA). Radiocesium's transfer mechanisms in the forest by hydrological processes and the runoff through the water system were summarized based on investigations in experimental forests and watersheds in Fukushima Prefecture. Besides, we outlined the missing links that need to be clarified by re-analysis of existing data and additional experiments based on previous studies on Fukushima and Chernobyl. Finally, we discussed the direction of future monitoring surveys.

How to cite: Kato, H., Shinozuka, T., Akaiwa, S., Iida, H., Saidin, Z. H., and Onda, Y.: Transfer of the Fukushima accident-derived radiocesium by hydrological processes in Japanese forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14511, https://doi.org/10.5194/egusphere-egu21-14511, 2021.

The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011 led to the uncontrolled release of a significant amount of radioactive materials into the marine environment. To prevent the further release of highly contaminated water, which was used for cooling the overheated nuclear reactor cores, and groundwater, which was continuously pumped out the reactor buildings, a large number of tanks was installed in the area around NPP to collect all this water. However, at the moment the capacity of tanks is almost exhausted. The contaminated water was processed to decrease the activity stored in the tanks, but any decontamination system cannot remove all radionuclides from the water. According to TEPCO (2020) data, about 1.2 million m³ of contaminated water were stored in tanks in March 2020 containing radionuclides with long and moderate half-life, among which 10 radioisotopes (H-3, C-14, Co-60, Sr-90, Tc-99, Ru-106, Sb-125, I-129, Cs-134, Cs-137) are dominant (Buesseler, 2020). Therefore, it is important to estimate the impact on human health of potential release of contaminated water from tanks to the ocean. This impact significantly depends on the ability of radionuclides to concentrate in the marine organisms, which are in the human diet, and the values of dose coefficient. The compartment model POSEIDON-R was applied for calculation the concentration of activity in the water, bottom sediments and biota at different distances from the FDNPP. The area of interest was covered by the system of compartments with specification around FDNPP. The exchanges of activity between compartments were governed by average currents in the region. The maximal concentrations and doses were conservatively estimated for coastal box 4x4 km around the FDNPP. Accumulation of activity in the organisms was calculated by dynamical model taking into account chemical properties of the element, its role in metabolic processes and the positions of organisms in the pelagic and benthic food webs. The potential individual doses of radiation were estimated using average consumption rates of marine products in Japan based only on domestic production. The conservative scenario, when a whole volume of contaminated water will be released into the marine environment at a constant rate during 10 years, was chosen. According to results of modelling for 50 years, the obtained dose even in the coastal box turned out to be significantly lower than the maximum annual effective dose commitment for the public equal to 1 mSv (IAEA, 2011). The main contribution into the dose is expected from I-129 and C-14. Although the activity of tritium (H-3) far exceeds activities of other radionuclides in tanks, its contribution to the total dose is only third due to low ability to concentrate in organisms and low dose coefficient. The dose factors and activity factors for 10 radionuclides at different distances from the FDNPP were obtained to be used for estimation of doses to human and concentration of activities in marine organisms for any long-lasting release scenario.

How to cite: Maderich, V. and Bezhenar, R.: Modelling study of the potential release of contaminated water from storage tanks at the Fukushima Dai-ichi NPP into marine environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10690, https://doi.org/10.5194/egusphere-egu21-10690, 2021.

This investigation uses the tracer information provided by the 2011 direct ocean release of radio-isotopes, (¹³⁷Cs, ~30-year half-life and ¹³⁴Cs, ~2-year half-life) from the Fukushima Dai-ichi nuclear power plant (FDNPP) together with hydrographic profiles to better understand the origins and pathways of mode waters in the North Pacific Ocean. While using information provided by radionuclide observations taken from across the basin, the main focus is on the eastern basin and results from analyses of two data sets 2015 (GO-SHIP) and 2018 (GEOTRACES) along the 152°W meridian. The study looks at how mode waters formed in the spring of 2011 have spread and mixed, and how they have not. Our radiocesium isotope samples tell a story of a surprisingly confined pathway for these waters and suggest that circulation to the north into the subpolar gyre occurs more quickly than circulation to the south into the subtropical gyre. They indicate that in spite of crossing 6000 km in their journey across the Pacific, the densest 2011 mode waters stayed together spreading by only a few hundred kilometers in the north/south direction, remained subsurface (below ~200 m) for most of the trip, and only saw the atmosphere again as they followed shoaling density surfaces into the boundary of the Alaska Gyre. The more recent data are sparse and do not allow direct measurement of the FDNPP specific ¹³⁴Cs, however they do provide some information on mode water evolution in the eastern North Pacific seven years after the accident. 

How to cite: Macdonald, A., Yoshida, S., and Rypina, I.: A 2011-2018 Fukushima Perspective on North Pacific Mode Water Pathways, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10397, https://doi.org/10.5194/egusphere-egu21-10397, 2021.

Artificial radionuclide ¹³⁷Cs has been supplied into the ocean by global fallout due to atmospheric nuclear weapons tests since 1945, releases from reprocessing plants since 1952, and most recently by fallout and discharge due to the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) accident since 2011.¹³⁷Cs 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. We used the Parallel Ocean Program version 2 (POP2) of the Community Earth System Model version 2 (CESM2). The horizontal resolution is 1.125 degrees in longitude and 0.28 to 0.54 degrees in latitude. The simulation period was from 1945 to 2030, and the atmospheric conditions were forced to cycle through repeating normal years. The purposes of this study are to investigate the effect of the release from the reprocessing plants on the distribution of ¹³⁷Cs activity by global fallout in the Atlantic Ocean, and the effect of the release derived from the 1F NPP accident on the one by global fallout in the Pacific Ocean.

The simulated ¹³⁷Cs activities were in good agreement with the observed data in the database in the Atlantic Ocean and the Pacific Ocean. The simulated ¹³⁷Cs activity immediately after each release event in the North Pacific were inconsistent with the observed one because of the inadequate reproduction of the Kuroshio Current in this quasi-resolution ocean model. However, the influence of the dilution effect is expected to become smaller as the time after the release increases. The influence of the ¹³⁷Cs activity by release from the reprocessing plant on the one by global fallout in the Atlantic Ocean is limited to the northeast coast of the European continent and the Marginal Seas. It was also suggested that ¹³⁷Cs activity by global fallout has made detection difficult since the 1990s.The influence of the ¹³⁷Cs activity by the 1F NPP on the one by global fallout was found to be broadened by the Kuroshio extension area and extended to the California coast. This distribution was similar to that of the one by global fallout. However, there are few observed data off the California coast after 2011. It was also suggested that ¹³⁷Cs activity by global fallout has made detection difficult since the 2020 in the Pacific Ocean.

Even after 2020, it is still possible to detect ¹³⁷Cs activity by global fallout in the global ocean. The difference in the vertical distribution between the Pacific and Atlantic oceans reflects the ocean circulation, which is useful for the validation of ocean general circulation models. There is still room for improvement in setting the input conditions to the ocean for each event.

How to cite: Tsumune, D., Bryan, F., Lindsay, K., Misumi, K., Tsubono, T., Tateda, Y., Inomata, Y., and Aoyama, M.: Estimation of 137Cs inventories by a global ocean general circulation model for the global database interpolation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14483, https://doi.org/10.5194/egusphere-egu21-14483, 2021.

A process of the removal of dissolved elements in the ocean by adsorption onto sinking particulate matters (scavenging) is studied analytically and using Lagrangian and Eulerian numerical methods. The generalized model of scavenging in a multicomponent reactive medium with first-order kinetics consisting of water and multi-fraction suspended particular matter has developed. Two novel numerical schemes were used to solve advection-diffusion-reaction equations for advection-dominated flows. The particle tracking algorithm based on the method of moments was developed. It is free on time step limitation necessary for an application of a standard method to the equations with reaction kinetics. The modified flux-corrected transport method for the Eulerian equations is a flux-limiter method based on a convex combination of low-order and high-order schemes. The similarity solutions of the model equations for an idealized case of instantaneous release of reactive radionuclide on the ocean surface were obtained. It was found that the dispersion of reactive contamination caused by reversible phase transition can be much greater than caused by diffusion. The solutions using both numerical methods are consistent with the analytical similarity solution even at zero diffusivity. The scavenging of the ^239,240Pu that was introduced to the ocean surface due to the fallout from past nuclear weapon testing was simulated. The results of the simulation agreed with observation data in the north-western Pacific Ocean. The importance of the scavenging by both the large fast-sinking particles and small particles slowly sinking and dissolving with depth due to the biochemical processes was shown.

How to cite: Brovchenko, I., Maderich, V., Kivva, S., Kim, K. O., Kim, H., and Kovalets, K.: Scavenging of radionuclides in multicomponent medium with first-order reaction kinetics: Lagrangian and Eulerian modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8664, https://doi.org/10.5194/egusphere-egu21-8664, 2021.

Radiocesium-bearing microparticles (CsMPs), which are insoluble, Cs-bearing, silicate glass particles, have been found in terrestrial and freshwater environments after the TEPCO's Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in Japan. Few studies have investigated the distribution of CsMPs in freshwater ecosystems and their uptake by aquatic organisms. In this study, we determined the uptake of CsMPs by aquatic insects in the Ota River in Fukushima. Although aquatic insects are usually measured for radioactivity in bulk samples of several tens of insects, we investigated the variability of ¹³⁷Cs concentration in individual aquatic insects, and the influence of CsMPs on them. Measurement of ¹³⁷Cs concentrations in detritivorous caddisfly (Stenopsyche marmorata) larvae and carnivorous dragonfly larvae showed that 3 of 47 caddisfly larvae had considerably higher radioactivity, whereas no such outliers were observed in dragonfly larvae. These caddisfly larvae were confirmed to contain the CsMPs emitted from Unit 2 of the FDNPP, using a scanning electron microscope and radioactivity measurements after isolation of the CsMPs. CsMPs were also found in potential food sources of caddisfly larvae, such as periphyton and drifting particulate organic matter, indicating that larvae may ingest CsMPs along with food particles of similar size. Our study demonstrated that CsMPs could be taken up by aquatic insects and possibly by the fish consuming them. The existence of CsMPs can result in sporadic, extremely high ¹³⁷Cs concentrations, and large variations in samples, and consequently obscure the actual transfer and temporal trends of ¹³⁷Cs in freshwater ecosystems.

How to cite: Ishii, Y., Miura, H., Jo, J., Tsuji, H., Saito, R., Koarai, K., and Hayashi, S.: Radiocesium-bearing microparticles cause a large variation in 137Cs concentration in the aquatic insect, Stenopsyche marmorata, in the Ota River, Fukushima, Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13886, https://doi.org/10.5194/egusphere-egu21-13886, 2021.

Elimination of the powerful radiation accidents consequences (i.e. Chernobyl and Fukushima accidents) has undoubtedly provided mankind with great experience in implementing both practical and fundamental knowledge about the radiation safety of society and the environment. In general, the practical application of scientific knowledge accumulated in the pre-accident period has led to significant positive successes of post emergency measures. The advantage of practical needs in scientific studies has narrowed the scope of fundamental work to the impact of radiation on biological objects in the affected area at the Chernobyl NPP, so the progress in this direction is modest so far.

To date, there is no unambiguous answer to the problem of the small radiation doses impact on biota, namely under such conditions people live today in areas contaminated with artificial radionuclides after Chernobyl and Fukushima accidents. Despite the accumulated experience in the elimination of radiation accidents, it is premature to consider the problem of environmental radionuclide pollution solved. This calls us to expand basic research question at identifying patterns in the state of cellulose-destroying soil microflora on contaminated areas in Ukrainian Polissya and assess their soil-forming activity.  

The study of the microbiota state on territories contaminated with radionuclides (including high level of contamination) is at an early stage, despite the intensive development of such studies after the radiation accident in Fukushima, Japan by a team of researchers from the University of Tokyo led by Professor T. Takahashi.

The aim of our work was to study the cellulose-destroying activity of the soil microflora of Ukrainian Polissya under conditions of elevated radionuclides contamination. We selected two locations - one outside the exclusion zone and the second in the exclusion zone. Both locations were characterized by a significant gradient of radionuclide contamination. At the first location, three points with soil ¹³⁷Cs activity of 0.6±0.045, 2.9±0.08, 4.6±0.11 kBq×kg^-1 soil and ⁹⁰Sr activity of 0.033±0.004, 0.18±0.015, 0.27±0.012 kBq×kg^-1 soil were selected. At the location in the exclusion zone, the ¹³⁷Cs activity at the sampling points 25±2, 170±1.5, 490±1 kBq×kg^-1 soil were selected. Since the experimental sites are located in a relatively small area, the physico-chemical soil properties between the points at each location do not have a significant difference.

To determine the rate of organic matter decomposition by soil microorganisms at all micro-sites, the standardized Tea Bag Index (TBI) method was applied. We used two types of tea bags TM Lipton - green tea (EAN8722700055525 or EAN8714100770542) and rooibos (EAN8722700188438) as a standardized plant material. Therefore, the obtained results can be compared between the microsites with different contamination level, as well as with similar data obtained by researchers for all ecosystems and many soil types from more than 2000 places around the world.

We acknowledge the National Research Foundation of Ukraine for the financial support of this research (Project №2020.01/0489).

How to cite: Klepko, A., Illienko, V., Lazarev, M., and Bilyera, N.: Study of cellulose-destroying activity of soil microflora on the radionuclide contaminated territories of Ukraine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15011, https://doi.org/10.5194/egusphere-egu21-15011, 2021.

Sludge and slurry uranium repositories of ore processing wastes built in the middle of the 20th century lost their waterproofing properties over time, which leads to the upper aquifers pollution with uranium, heavy metals, sludge macrocomponents: anions of nitrates, sulfates, carbonates, calcium and iron. It is known that the uranium behavior in the environment depends on its oxidation-reduction potential, which determines its oxidation state and solubility. High nitrate content in aquatic systems leads to the uranium migration in the highly oxidized form and minimizes its sorption on the rocks minerals. 

Biogenic elements solution in waters can lead to the microbiota activity stimulation and biogeochemical uranium immobilization. Nitrate consumption leads to the redox potential decreasing and uranium reduction to insoluble forms. It is known that the most stable form of microorganisms in the environment is microbial biofilms. Their development on rock surface changes its physicochemical characteristics and sorption properties. This paper focused on changing the physicochemical and mineralogical parameters of rocks after microbial growth and its effect on the uranium immobilization. It should be noted that in situ bioremediation is one of the promising and inexpensive methods of groundwater remediation. Therefore, an assessment of the role of microbial biofilms in the immobilization of uranium will provide important information for predicting the effectiveness of the bioremediation.  

Angarsk Electrochemical Combine AECC (Irkutsk Region, Russia) is engaged in the processing of uranium ores and concentrates; has sludge storage facilities on the territory, which for a long time have contaminated the upper aquifers with nitrate ions, ammonium, uranium and other components. The main minerals of upper aquatic horizons weathered sandstone are: quartz, plagioclase, K-feldspar, kaolinite, smectite, specular stone, illite-smectite, vermiculite, chlorite, amphibole and apatite in trace amounts. The clay component is more than 20% with a kaolinite predomination. 

In laboratory experiments, the modeling of the growth of microbial biofilms on rocks from contaminated and uncontaminated areas of the formation was carried out by adding organic substrates. Samples were dominated by representatives of the family Pseudomonadaceae, known for their ability to form biofilms and wide range of metabolic capabilities. An uneven distribution of biofilm on the sand was established, presumably in areas containing an increase in the amount of biogenic elements - Ca, Fe, etc., as well as organic carbon. On average, after 15 days, the coverage area of the polysaccharide matrix was 20-30%. The appearance of a polysaccharide matrix can lead to a change in the sorption capacity of rocks and to formation of local zones of uranium accumulation in organic matter. 

As a result of microbial action, the dissolution of carbonate minerals and a number of changes in the composition of clay sandstones is noted. The microbial effect on rocks leads to a multidirectional change in their sorption capacity in relation to uranium. The formation of an exopolysaccharide matrix increases the sorption capacity of rocks due to the appearance of new functional groups. Moreover, with strong microbial fouling, it can lead to a decrease in uranium sorption.

This work was supported by a grant from RFBR 20-05-00602 A.

How to cite: Popova, N., Alexey, S., Grigoriy, A., and Anatoly, B.: Uranium immobilization at microbial biofilms on upper water horizons loams nearby sludge repository, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13193, https://doi.org/10.5194/egusphere-egu21-13193, 2021.