Introduction

Radiocesium (Cs-137) deposited on forests was intercepted by the canopy, then migrated to the litter layer and eventually to the soil layer, where some of it has been absorbed by roots and circulated through the forest ecosystem for a long time. In other words, the amount of Cs-137 uptake by roots will control the long-term dynamics in the forest ecosystem in the future, temporal changes in Cs-137 in tree roots have rarely been reported. In this study, we investigated the Cs-137 concentration and inventory in the soil and very fine (VF) roots (< 0.5 mm) of Japanese cedar from 2011 to 2020.

Methods

An approximately 3 m x 3 m plot was established in a cedar forest (initial deposition 440 kBq m-2) in the Yamakiya district of Kawamata Town, Fukushima Prefecture. Litter and soil samples were collected twice a year during 2011-2012 and once a year after 2013 using a scraper plate at 0.5 cm intervals for 0-5 cm, 1 cm intervals for 5-10 cm, and 5 cm intervals for 10-20 cm. Root samples were collected by further separating only the roots with tweezers from soil samples in 2012, 2015, 2017, and 2020, and washed by ultrasonic homogenizer to remove soil particles on the root surface. The roots measured were absorptive VF roots of 0.5 mm or less of the current year's growth.

Results and discussions

The Cs-137 concentration in the litter layer was still decreasing exponentially more than 12 years after the accident, its inventory was about 0.2-0.5% of the deposited amount. The depth distribution of Cs-137 concentration in the mineral soil layers was fitted with an exponential equation until 2019, but after 2020, the peak concentration shifted slightly downward and was fitted with a hyperbolic function. The Cs-137 inventory in the soil increased over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots decreased in 2020. Especially, the ^Cs-137 inventory in the VF roots in the 0–2 cm of soil reached 89% in 2012; however, it decreased with time to approximately 43% in 2020. This decrease in the Cs-137 concentration in the VF roots at 0–2 cm was caused by the decrease in Cs-137 concentration in the litter layers. Although the Cs-137 concentration in the VF roots below 2 cm increased with increasing Cs-137 concentration in the soil, the downward migration of Cs-137 within the soil can reduce the amount of Cs-137 absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, Cs-137 can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil without physical decontamination. This natural downward migration process can be regarded as a “self-cleaning” of the forest ecosystem, resulting in a decrease in the air dose rate and the amount of Cs-137 absorbed by roots.

How to cite: Takahashi, J., Iguchi, S., Sasaki, T., and Onda, Y.: Downward migration of Cs-137 in soils reduce root uptake of Japanese cedar in Fukushima, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7677, https://doi.org/10.5194/egusphere-egu24-7677, 2024.

The concentration of dissolved-form ¹³⁷Cs in forested rivers is known to increase during rainstorms, and direct leaching from litter and soil water is considered to be a factor. There have also been many studies showing that competing ions such as K⁺ and NH₄⁺ promote the elution of ¹³⁷Cs. However, there are no examples of detailed measurements of ¹³⁷Cs concentrations and water quality characteristics of stream water and water passing through litter during actual rainstorms. In this study, stream water, throughfall,  water passing through litter, and groundwater were sampled in a small watershed in Fukushima Prefecture, Japan, which was affected by the Fukushima Daiichi Nuclear Power Plant, to measure dissolved ¹³⁷Cs and dissolved organic carbon (DOC), K⁺and NH₄⁺. NH₄⁺ was not detected in stream water. The average concentrations of dissolved¹³⁷Cs, DOC and K⁺ were 6.36 (mBq/L), 0.51 (mg/L) and 0.14 (mg/L), respectively, while the concentrations of ¹³⁷Cs and DOC doubled to 13.38 (mBq/L), 1.13 (mg/L) during rainfall event and the K⁺concentrations remained unchanged (0.15mg/L). The concentrations of ¹³⁷Cs and DOC  in the water passing through the litter were 50 and 30 times higher than in the stream water, respectively, suggesting that the high concentrations of dissolved ¹³⁷Cs at the time of runoff were formed by leaching from the litter rather than by the presence of competing ions. The amount of ¹³⁷Cs and K⁺eluted from the litter increased in the order of near-channel, saturated zone at run-off and slope, while the amount of DOC eluted from the litter was lower near the channel. These results suggest that ¹³⁷Cs, K⁺and DOC release from near-channel litter is lower than that from litter on the slope because of the progress of leaching due to the occurrence of saturated surface flow.

How to cite: Nagata, Y., Onda, Y., Takahashi, J., and Sakakibara, K.: Mechanisms of dissolved-form 137Cs runoff from forest source watersheds , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7423, https://doi.org/10.5194/egusphere-egu24-7423, 2024.

Climate change profoundly affects natural water resources by increasing extreme rainfall and persistent drought events. This impact has led to a rising likelihood of over-extraction of groundwater by Taiwanese farmers due to insufficient water resources. Quantifying groundwater pumping activities is challenging, thereby prompting this study to introduce a hybrid AI model combining a Convolutional-based Autoencoder with LSTM. The objective is to explore the spatiotemporal relationship between hydrometeorology and groundwater for providing a quantitative assessment of groundwater resources.

To construct the model, a comprehensive dataset spanning two decades (2000-2019) is utilized, incorporating information from 33 groundwater monitoring wells in the Jhuoshuei River basin of Taiwan. Two types of datasets, observation and simulation, are employed for a robust analysis. The hybrid AI model yields accurate three-month-ahead forecasts for shallow groundwater in the Jhuoshuei River basin, with R² performance ranging from 0.70 to 0.87 for T+1 (short-term forecasts) and from 0.42 to 0.69 for T+3 (long-term forecasts).

The significance of these forecasts lies in their potential to empower farmers to increase crop cultivation efficiency. The long-term forecasts aid in formulating strategic plans for crop cultivation and fallow periods, promoting efficient agricultural management. Simultaneously, the short-term forecasts empower farmers to enhance irrigation efficiency, leading to a reduction in regional water consumption. This proactive approach aligns with Sustainable Development Goals (SDGs) 11 and 12, fostering sustainable water resource management practices. In essence, this hybrid AI model emerges as a valuable tool for proactive and adaptive water resource management, particularly crucial in the context of evolving climate conditions.

Keywords: Groundwater management, AI, Deep Learning, regional forecast, machine learning, SDGs, Taiwan

How to cite: Kow, P. Y., Chang, Y.-W., and Chang, F.-J.: AI-Driven Hydro-Insights: Proactive Water Resource Management for Sustainable Agriculture in the Face of Climate Change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2292, https://doi.org/10.5194/egusphere-egu24-2292, 2024.

The escalating frequency of climate-related disasters underscores the imperative need for robust adaptive strategies to mitigate the impacts of extreme weather events. Crafting effective adaptive solutions, however, presents a formidable challenge. This research investigates the potential of aquavoltaic systems to enhance adaptive capacity and promote low-carbon production in fishing villages grappling with climate change. Focused on clam farming in Yunlin County, Taiwan, our study builds innovative Water-Energy-Food-Land-Climate (W-E-F-L-C) Nexus models using system dynamics (SD) techniques to compare the synergistic benefits and resource utilization efficiency between aquavoltaic systems and conventional aquacultural methods. This study meticulously catalogues factors from SD models and incorporates them into a comprehensive life cycle assessment (LCA) to scrutinize the environmental impacts of both aquavoltaic and conventional systems. Carbon emission data is rigorously calculated by LCA, revealing the carbon emissions flow resulting from interactions between these factors.

Additionally, this study conducts a scenario analysis to gain insight into how aquavoltaic and conventional aquacultural systems respond to key influencing factors such as temperature and rainfall. Our findings underscore that elevated temperatures and intensified rainfall significantly impact conventional clam farming compared to the aquavoltaic system. Aquavoltaics emerges as a robust and viable mechanism for aquaculture in the face of capricious weather conditions. Particularly noteworthy is the effectiveness of solar panels in intercepting and diverting rainwater during heavy rainfall in summer, reducing the risk of diluting pond water and thereby stabilize water quality. The shading effect induced by photovoltaic installations also contributes to moderating water temperatures, especially under direct sunlight. By synergizing physical mechanisms with advanced simulation techniques, this study propels toward a more efficient and resilient paradigm in aquaculture. Aquavoltaics demonstrate promising potential for sustainable and low-carbon production as well as promoting the resilience of fishing villages. This study not only illuminates the intricate dynamics of climate-resilient aquaculture but also stands as a milestone for the development of sustainable aquaculture practices.

How to cite: Chen, C.-H., Lee, M.-H., Lin, H.-Y., and Chang, F.-J.: Enhancing Climate-Resilient Aquaculture in Yunlin County, Taiwan: A Comparative Analysis of Aquavoltaic Systems and Conventional Methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2519, https://doi.org/10.5194/egusphere-egu24-2519, 2024.

The fate and reactivity of dissolved organic matter (DOM) in river networks is critical to understanding carbon cycling in inland water systems, and is highly regulated by physio-geographic factors and water residence time (WRT). In this study, we investigate the spatiotemporal variation of DOM concentration and composition in two SMRs in Taiwan with different landscapes and anthropogenic impacts. The WRT for these two rivers, the Keelung and Lanyang River, are around 34 and 23 hours, respectively. Dissolved organic carbon (DOC) concentration measurements and optical analyses (absorbance and fluorescence) were used to examine DOM quantity and quality along the river continuum. The comparative results showed that, along the SMR continuum, the DOC concentrations and optical indexes exhibited slight changes, with significant increases observed only at downstream sites influenced by human activities. Meanwhile, the higher biological index (BIX) and lower humification index (HIX) indicated an increase in autochthonous sources and a decrease in the degree of humic characters. In addition, we observed a positive correlation between WRT and DOC concentration variability, yet not significant for DOM compositions. When comparing the two rivers, the one with steeper topography and less human influence shows lower levels of DOC concentration and degree of humification. Overall, the SMRs seem to have lower DOC concentrations (0.26 - 1.65 mg-C L^-1), lower HIX (0.28 - 0.76), and slightly higher BIX (0.8 - 1.9) on a global scale, which might be attributed to Taiwan's steep landscape and shorter water residence time, limiting soil organic carbon (SOC) production and in-stream processes rates. Through our investigation, DOC concentration and DOM composition across river networks will be better understood and potentially improve the assessment of the global carbon cycle.

How to cite: Lee, L.-C., Huang, J.-C., Weigelhofer, G., Hein, T., Yu, Y.-L., and Chen, P.-H.: Spatiotemporal Variation of DOM Concentration and Composition along the Subtropical Small River Continuum in Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14216, https://doi.org/10.5194/egusphere-egu24-14216, 2024.

Human-induced land-use change has profound effects on both societies and ecosystem services. For example, transitioning from forests to conventional farms using fertilizers can escalate soil nitrogen, degrade groundwater, and impair downstream ecosystems. This study explores the intricate dynamics of human-induced land-use change, focusing on the shift from forests to tea farm-dominated catchments in Taiwan, where conventional farming practices with fertilizers impact soil quality, groundwater, and downstream ecosystems. Utilizing the Soil and Water Assessment Tool (SWAT) for nutrient export analysis, our research reveals that when agricultural land use exceeds 2%, exports of nitrate, phosphate, and potassium spike significantly, ranging from 25% to 150%. Notably, agricultural land use induces a higher impact on nitrate, with concentrations surpassing those by 120% and 233% during the dry season and wet season, respectively. Tea farms, constituting a substantial portion, exhibit a nearly tenfold increase in NO3-N yield compared to forests. Implementing a modified fertilization strategy, involving application during small rainfall events, enhances nitrogen uptake and tea tree harvest yield while reducing nitrogen input by 10%. This research offers actionable recommendations for sustainable agroforestry practices by integrating river and rainwater data with SWAT modeling. By doing so, it advances our understanding of hydrological and biogeochemical processes in subtropical tea farm-dominated catchments, providing valuable insights into hydrology and biogeochemistry.

How to cite: Chen, P.-H., Naqvi, H. R., Lin, G.-Z., Lee, T.-Y., Lee, L.-C., and Huang, J.-C.: Assessment of nutrient export in agroforestry catchments dominated by tea farms in subtropical small mountainous rivers, Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17762, https://doi.org/10.5194/egusphere-egu24-17762, 2024.

In the region affected by the Fukushima nuclear accident in 2011, some freshwater fish shipments continue to be suspended owing to radioactive contamination (mainly ¹³⁷Cs) of the aquatic environment. In predicting the future ¹³⁷Cs contamination of aquatic organisms, investigations must focus on the dynamics of ¹³⁷Cs in dissolved form, which is highly bioavailable and abundant in the environment. In particular, dam lakes that deposit large amounts of sediment contaminated with ¹³⁷Cs and have a long residence time of water can substantially influence the dynamics of ¹³⁷Cs in rivers, as suggested by prior research. This study focuses on the effect of desorption of ¹³⁷Cs from lake sediment on the formation of dissolved ¹³⁷Cs concentrations in dam discharge water, using the results from monitoring surveys at two dam lakes located near the Fukushima Daiichi Nuclear Power Plant.

We collected inflow and discharge water from the Matsugabo and Yokokawa dams in Fukushima Prefecture every month from 2014 and measured the concentration of dissolved and particulate ¹³⁷Cs in the water using the cartridge filter method. On the basis of these results, combined with flow data from the dam lakes, we estimated the annual budgets of ¹³⁷Cs (inflow/outflow) in the dam lakes. For the particulate form, annual ¹³⁷Cs inflow into the lakes decreased by more than 80% in most years, indicating that most of the inflow particles sedimented. For the dissolved form, the annual discharge of ¹³⁷Cs was higher than the annual inflow of ¹³⁷Cs, concurring with results from a neighboring dam lake. This increment suggests ¹³⁷Cs desorption from the sediment.

According to the monthly monitoring data, the dissolved ¹³⁷Cs concentration in the dam discharge water at some periods showed a higher value than the peak value from the previous year. This phenomenon was observed when the reservoir storage rate of the dam lake fell below approximately 30%. To determine the main source of dissolved ¹³⁷Cs in the dam lake, we investigated the horizontal distribution of the dissolved ¹³⁷Cs concentrations at several points in Yokokawa dam lake and the vertical distribution of the dissolved ¹³⁷Cs concentration at the center of the lake in August 2023, when the water level was very low. The concentration of dissolved ¹³⁷Cs in the lake water was found to increase in the inlet part of the lake, while the concentration remained almost the same in the downstream direction from the site. The concentration of dissolved ¹³⁷Cs at the center of the lake was almost unchanged vertically. This trend was different from the increase in the concentration of dissolved ¹³⁷Cs in bottom water, previously observed at the same location (Tsuji et al., 2022). These results indicate that ¹³⁷Cs desorption from sediment in the inlet area mainly led to the increase in the dissolved ¹³⁷Cs concentrations in the lake water, in part owing to the low volume of flowing water.

How to cite: Tsuji, H., Funaki, H., and Hayashi, S.: Effect of 137Cs desorption from sediment on the formation of dissolved 137Cs concentrations in dam discharge water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7140, https://doi.org/10.5194/egusphere-egu24-7140, 2024.

The anthropogenic radionuclides such as caesium-137 (¹³⁷Cs), strontium-90 (⁹⁰Sr), ³H, ¹⁴C, and plutonium (Pu) were released into the global ocean as results with large scale weapon tests in the late 1950s and early 1960s. Because these anthropogenic radionuclides have been still existed in the ocean, it is necessary to investigate the behavior of these anthropogenic radionuclides due to investigate the effects of human health. In this study, the spatiotemporal variations in the ¹³⁷Cs and ⁹⁰Sr activity concentrations in global ocean surface seawater from 1956 to 2021 using the HAMGlobal2021: Historical Artificial radioactivity database in Marine environment, Global integrated version 2021. The global ocean was divided into 37 boxes. The 0.5-yr average value of ⁹⁰Sr in the northern North Atlantic Ocean and its marginal sea, decreased exponentially in 1970–2010, just before the F1NPS accident. Estimated apparent half residence time of ¹³⁷Cs and ⁹⁰Sr ranged from 4.1-34.1 years and 3.6-25.2 years, respectively. Considering that longer Tap occurs larger inflow and shorter Tap occurs larger outflows/smaller inflow of radionuclide from the upstream region, ¹³⁷Cs and ⁹⁰Sr were inflowed into the Eastern China Sea from the subtropical western North Pacific Ocean. Inflow of ⁹⁰Sr into the Sea of Japan from the Eastern China Sea were relatively smaller than those of ¹³⁷Cs. Although ⁹⁰Sr were decreased exponentially, these trends tended to be larger than those of ¹³⁷Cs, which was investigated by our previous study (Inomata and Aoyama, 2023). This might be caused by the different behavior of ⁹⁰Sr and ¹³⁷Cs such as particulate form for ⁹⁰Sr in the seawater.

Keywords: ⁹⁰Sr, ¹³⁷Cs, Database, surface seawater, global ocean

Reference: Inomata and Aoyama, Evaluating the transport of surface seawater from 1956 to 2021 using ¹³⁷Cs deposited in the global ocean as a chemical tracer. Earth Syst. Sci. Data, 15, 1969–2007, 2023.

How to cite: Inomata, Y. and Tsumune, D.: Spatiotemporal variations of 137Cs and 90Sr in the global ocean based on the historical data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14837, https://doi.org/10.5194/egusphere-egu24-14837, 2024.

Radionuclides including radioactive Cs were released into the environment due to the Fukushima Daiichi Nuclear Power Plant accident in 2011. Two years after the accident, glassy water-resistant particles incorporating radioactive Cs were first reported. Such glassy particles are called cesium-bearing microparticles (CsMPs). CsMPs have been studied because (i) they have information on the condition in the reactor at the time of the accident, and (ii) there is concern about the exposure to the humans and the other organisms.

Several types of CsMPs have been reported, which is assumed to reflect the difference in the accidental progress of each unit. It is also known that CsMPs were transported in the atmospheric plume at the time of emission and therefore have different deposition regions. Type-A CsMPs, are presumed to originate from Unit 2, deposited over a wide area including the Kanto region due to their small size (~0.1–10 µm). Type-B CsMPs, are presumed to originate from Unit 1, deposited in a limited area in the north direction because of their large size (50–400 µm). Matrix of Types-A and -B CsMPs is SiO₂ but Type-A CsMPs have higher concentration of volatile elements including Cs than Type-B CsMPs due to the difference in forming process. Type-A CsMPs were formed through gas condensation, whereas Type-B CsMPs were formed through melt solidification.

The presence of CsMPs emitted from Unit 3 in the ocean was confirmed by our research. The plume at the time of the emission of radionuclides from Unit 3 was in the ocean direction, which suggests that many CsMPs from Unit 3 deposited directly into the ocean. We will report the comparison of CsMPs from marine and terrestrial sources. In addition, we reported Type-A CsMPs from suspended particles in rivers and marine samples, such as plankton net and suspended particle samples. This fact suggests that Type-A CsMPs deposited on land and transported to the ocean through rivers. The presence of CsMPs may be the cause of the overestimation of solid–water distribution coefficient for marine sediments and particulate matters and apparent high concentration factor of marine biota of radioactive Cs.

How to cite: Miura, H., Ishimaru, T., Kanda, J., Ito, Y., and Kubo, A.: Comparison of cesium-bearing microparticles from marine and terrestrial sources, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7685, https://doi.org/10.5194/egusphere-egu24-7685, 2024.

The March 11, 2011, Great East Japan Earthquake triggered accidents at the Fukushima Daiichi Nuclear Power Plant (1F NPP), releasing radioactive substances into the ocean. Sparse observational data on ¹³⁷Cs in the ocean led to interpolation and simulation for a comprehensive understanding. The primary focus was on direct release, emphasizing the need for a suitable source.

The direct release rate (Bq/day) was calculated by multiplying the seawater exchange flow rate (m³/day) and observed ¹³⁷Cs concentration (Bq/m³). Using a mesh size of 735 m x 929 m x 8 m on the model, the seawater exchange flow rate at the release point was simulated. The ¹³⁷Cs concentration relied on average observed radioactivity at 5, 6, and the south discharge canals near the 1F NPP. Direct release was estimated at 2.2x10¹⁴ Bq/day from March 26 to April 6, 2011, aligning with rates derived from other methods.

The seawater exchange flow rate's dependency on the model's mesh size was acknowledged. For this estimation, a 735 m x 929 m mesh size encompassing key points was considered reasonable for the seawater exchange flow rate, given the complex transport process from the release source (Unit 2 intake) to observation points (5, 6, and the south discharge point) due to port structures.

A higher resolution model with a 147 m x 186 m mesh (1/5) was used for a detailed analysis of direct release rates. The size of the sea area for determining the volume of seawater exchange flow rate can now be changed. Despite challenges in setting due to damaged ports, using the seawater exchange flow rate in a similar area as the previous resolution was deemed appropriate. The results of the validation of the release rate and the observed results by the relationship equation confirmed the consistency with the amount of seawater exchange obtained by the results of the dye tracer release experiments in the 1970s.

The release of ¹³⁷Cs from the 1F NPP site persists. Estimating direct release rates up to 2016, a long-term simulation with a higher resolution model was conducted for validation. Results showed the oceanic ¹³⁷Cs concentration distribution influenced by coastal currents, eddies, and the Kuroshio Current, leading to spatio-temporal variability. Validation with observed annual mean concentrations revealed good agreement. The higher resolution improved coastal transport reproducibility, addressing ¹³⁷Cs radioactivity underestimation at the Fukushima 2 NPP, 10 km south of the 1F NPP.

How to cite: Tsumune, D., Tsubono, T., and Misumi, K.: Verification of direct release rate of oceanic 137Cs from Fukushima Daiichi Nuclear Power Plant Accident by higher resolution ocean dispersion model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14451, https://doi.org/10.5194/egusphere-egu24-14451, 2024.

To cope with the increasing threat of radioactivity release accidents in the Yellow Sea a Lagrangian radionuclide transport model in the region was recently developed coupled in off-line manner with current-wave-suspended sediment modeling system (Brovchenko et al, 2022).  The radionuclide model included as an essential feature the fast adsortion-desorption processes of dissolved and particulate radionuclides in the presence of multi-ftactional sediments. Upgrade is made in this work by including fast and slow adsorption-desorption processes of radionuclides and a novel approach for lagrangian simulation of the radionuclide exchange between near-bottom water-layer and bed sediments. Lagrangian particles in the model can possess several states: dissolved in the water column, adsorbed on suspended sediment of particular size, dissolved in the pore water, adsorbed on the bed sediments of particular size. Note that, If particles are adsorbed on the sediments then it can be in two different states, namely fast and slow reversible forms; if there are Nsed sediment size classes then we have N_tot =2+4N_sed  total states of the radionuclide. Throughout the numerical integration the model calculates the probabilities to transfer into each possible state (that depends on the current state and time step) during the next time step and then chooses the new particular state by comparing with the generated uniformly distributed random number. Hypothetical accident at the Haiyang NPP in China, which is located at the coast of Yellow Sea close to Korea is considered as a scenario of accident. The atmospheric transport and deposition of radionuclides on the sea surface was simulated by the FLEXPART model. The obtained deposition fluxes were used as a source term in the Lagrangian radionuclide transport model. 3D fields of currents, suspended sediment concentration and turbulent diffusion coefficient as well as bed sediment fractional composition are identical to the previous results of the Yellow Sea (Brovchenko et. al. 2022).  Computational domain of the FLEXPART model includes bigger outer area, which covers Yellow and East China Sea, with spatial resolution of 0.15 deg, and inner area, which covers only Yellow Sea with better spatial resolution of 0.05 deg. The source term of ¹³⁷Cs released due to hypothetical accident at the Haiyang NPP was obtained from the 6-day simulations of the FLEXPART model. The total amount of radioactivity that deposited on the calculation area is approximately 55 PBq. The radioactivity budget analysis reveals that almost near 50% of the ¹³⁷Cs was deposited to the bottom sediments and approximately half remained in the dissolved form. About 4% of the total amount remains on the suspended sediments in one-step modelling and about 9% with the use of two-step model. The total bed contamination changed only 1% because for this period bottom contamination fluxes dominated over the bed cleaning process. More differences are expected for simulation with duration of several years when dissolved ¹³⁷Cs concentration in water will decrease and bed cleaning process become more significant.

How to cite: Seo, S., Brovchenko, I., Maderich, V., Kovalets, I., Kovalets, K., and Jung, K. T.: Lagrangian radionuclide transport modeling with fast and slow adsorption-desorption processes: application to the Yellow Sea with a hypothetical atmospheric deposition , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13825, https://doi.org/10.5194/egusphere-egu24-13825, 2024.

After accidents at the Chornobyl NPP in 1986 and Fukushima Daiichi NPP in 2011, it became clear that there are many causes that can lead to a nuclear accident, including techno-genic and natural disasters. There is a danger of damage to the Zaporizhzhia NPP, with the subsequent release of radioactivity into the environment, as a result of the Russian invasion of Ukraine. The coastline of the Yellow Sea and East China Sea(YSECS) is a place where 9 NPPs are in operation in China and Korea. Since they are semi-enclosed seas with a very high density of population, any potential nuclear accident in the region can significantly contaminate the marine environment and affect the health of many people.

In the current study, a set of numerical models for the first time was applied to simulate the spreading of radionuclides in the environment as a result of the hypothetical accident at the Haiyang Nuclear Power Plant in China. The scenario of accidental release with containment-bypass was considered in this work. The atmospheric transport and deposition of radionuclides on the sea surface were simulated by the FLEXPART model. The set of 1450 dispersion scenarios following hypothetical accidental releases with different start dates were calculated for the next 120 h after release start, thus covering meteorological conditions from 1 Mar 2020 to 28 Feb 2021. Scenario with the heaviest deposition densities on the Yellow Sea was selected. These results were used as a source term for three different marine dispersion model simulating the transfer and fate of Cs-137 in YSECS: the grid-based Eulerian model THREETOX, Lagrangian radionuclide transport model and compartment model POSEIDON-R. Such approach emulates the application of various models with their own settings in the event of an unexpected accidental release, similar to the Fukushima accident. For THREETOX model setup, 3D current velocities with 30 vertical layers were extracted from the KIOST-MOM model, results of which are monthly averaged and cover North Pacific. The Lagrangian radionuclide transport model used regional currents and suspended sediments concentrations from circulation model adopted for the YSECS taking into account tides and multi-fractional sediments. These two models were applied for emergency and post-emergency phases for the period from half a year to one year after deposition. The POSEIDON-R model already had a system of boxes for the North-Western Pacific covering the YSECS, East/Japan Sea and Eastern coastal area of Japan. It was applied for a long-term assessment of several decades. Obtained concentrations of Cs-137 in water, bottom sediments and partly in marine organisms were compared and the differences were analysed. Application of three marine dispersion models provides the possible ranges of radionuclide concentrations on the one hand and increases the reliability of results on the other.

How to cite: Kim, K. O., Bezhenar, R., Kovalets, I., Brovchenko, I., Maderich, V., and Kwon, K.: Multi-model simulation of the radionuclide transfer in the Yellow Sea as a result of hypothetical atmospheric deposition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13386, https://doi.org/10.5194/egusphere-egu24-13386, 2024.