Numerical study of sediment and 137Cs discharge out of reservoirs during various scale rainfall events

Remobilisation of radiocaesium from bottom sediments to water column in reservoirs in Fukushima, Japan

Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilise in bioavailable form from sediments to water columns, which may contribute to the long-term contamination of aquatic products. This study systematically investigated the ¹³⁷Cs activities of sediment-pore water, providing a direct evidence of the remobilisation of bioavailable ¹³⁷Cs from sediments in two highly contaminated reservoirs affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. We observed that the dissolved ¹³⁷Cs activity concentration of pore water (3.0-65.8 Bq L^-1) was one to two orders of magnitude higher than that of reservoir water. Moreover, the distribution coefficient (K_d) values for the ¹³⁷Cs of sediment-pore water (2.6-14 × 10³ L kg^-1) decreased with depth. The K_d values were significantly and negatively correlated with the concentration of the major ¹³⁷Cs competing cation NH₄⁺. Our results strongly indicate a competitive ion exchange process between ¹³⁷Cs and NH₄⁺ via a highly selective interaction with the frayed edge sites of phyllosilicate minerals, which is the major reason for the variability of K_d values of sediment-pore water, even in the Fukushima case. Additionally, the sediment accumulation rates were relatively high, and the annual depositional rate of exchangeable ¹³⁷Cs prevailed over the annual diffusive flux of ¹³⁷Cs from the sediment to the overlying water. This finding indicates that even after 10 years since the FDNPP accident, the bioavailable ¹³⁷Cs is still continuously supplied from the catchment covered by mountainous forests, and reservoir sediments are a long-term important source of bioavailable ¹³⁷Cs in the riverine system. Our findings provide important parameter values for mid- and long-term assessments of the radiation impact of radionuclide discharges to freshwater environments.

Reservoir sediments as a long-term source of dissolved radiocaesium in water system; a mass balance case study of an artificial reservoir in Fukushima, Japan

Because of their large mobility and high bioavailability, it is necessary to elucidate the origins and dynamics of dissolved radionuclides in river and reservoir systems to assess the transfer of those radionuclides from water to crops and aquatic organisms. Elution from contaminated reservoir sediments, a potential source of dissolved radionuclides, presents a long-term concern, particularly for long-lived radionuclides. In this study, we systematically investigated caesium-137 (¹³⁷Cs) concentrations using a time-series suite of input and output water samples collected from 2014 to 2019 from the Ogaki Dam Reservoir, which has a catchment with a high ¹³⁷Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved ¹³⁷Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved ¹³⁷Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved ¹³⁷Cs in the reservoir to elucidate how much dissolved ¹³⁷Cs from the rivers and production from reservoir sediments contribute to ¹³⁷Cs in the reservoir output. The annual output of dissolved ¹³⁷Cs was significantly higher than the total input of dissolved ¹³⁷Cs, with approximately 32%-40% of the dissolved ¹³⁷Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved ¹³⁷Cs fluxes from reservoir sediments to overlying water were 0.57-1.3 × 10⁴ Bq m^-2 y^-1. This implies that approximately 0.04%-0.09% of ¹³⁷Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high ¹³⁷Cs levels may thus become even more important as sources of bioavailable dissolved ¹³⁷Cs in the future.

Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles

The time and size dependencies of particulate ¹³⁷Cs concentrations in a reservoir were investigated to evaluate the dynamics of ¹³⁷Cs pollution from a mountainous forested catchment. Sediment and sinking particle samples were collected using a vibracorer and a sediment trap at the Ogaki Dam Reservoir in Fukushima, which is located in the heavily contaminated area that formed as a result of the Fukushima Dai-ichi Nuclear Power Plant accident of 2011. The inventory of ¹³⁷Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 10¹²-3.9 × 10¹² Bq, which is equivalent to 1.1%-1.4% of the initial estimated catchment inventory. The particulate ¹³⁷Cs concentration showed a decline with time, but the exponent value between the specific surface area and the ¹³⁷Cs concentration for the fine-sized (<63 μm) particle fraction remained almost constant from the immediate aftermath of the accident. These quantitative findings obtained by reconstructing the contamination history of particulate ¹³⁷Cs in reservoir sediments and sinking particles have important implications for the evaluation of ¹³⁷Cs dynamics in mountainous forested catchments.

Applicability of Kd for modelling dissolved 137Cs concentrations in Fukushima river water: Case study of the upstream Ota River

A study is presented on the applicability of the distribution coefficient (K_d) absorption/desorption model to simulate dissolved ¹³⁷Cs concentrations in Fukushima river water. The upstream Ota River basin was simulated using GEneral-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) for the period 1 January 2014 to 31 December 2015. Good agreement was obtained between the simulations and observations on water and suspended sediment fluxes, and on particulate bound ¹³⁷Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved ¹³⁷Cs in the river water were much harder to reproduce with the simulations. By tuning the K_d values for large particles, it was possible to reproduce the mean dissolved ¹³⁷Cs concentrations during base flow periods (observation: 0.32 Bq/L, simulation: 0.36 Bq/L). However neither the seasonal variability in the base flow dissolved ¹³⁷Cs concentrations (0.14-0.53 Bq/L), nor the peaks in concentration that occurred during storms (0.18-0.88 Bq/L, mean: 0.55 Bq/L), could be reproduced with realistic simulation parameters. These discrepancies may be explained by microbial action and leaching from organic matter in forest litter providing an additional input of dissolved ¹³⁷Cs to rivers, particularly over summer, and limitations of the K_d absorption/desorption model. It is recommended that future studies investigate these issues in order to improve simulations of dissolved ¹³⁷Cs concentrations in Fukushima rivers.

A modeling approach to estimate the 137Cs discharge in rivers from immediately after the Fukushima accident until 2017

We developed a simple model to evaluate and predict the ¹³⁷Cs discharge from catchments using a tank model and the L-Q equation. Using this model, the ¹³⁷Cs discharge and discharge ratio from the Abukuma River and 13 other rivers in the Fukushima coastal region were estimated from immediately after the Fukushima accident up to 2017. The ¹³⁷Cs discharge (and discharge ratio to the deposition inventory in the catchment) of the Abukuma River and 13 other rivers in the Fukushima coastal region during the initial six months after the accident were estimated to be 18 TBq (3.1%) and 11 TBq (0.79%), respectively. These values of ¹³⁷Cs discharge ratio were 1-2 orders of magnitude higher than those observed after June 2011 in previous studies (Ueda et al., 2013; Tsuji et al., 2016; Iwagami et al., 2017a), indicating that the initial ¹³⁷Cs discharge from the catchments through the rivers was significant. The simulated initial ¹³⁷Cs discharge rates for the initial six months after the Fukushima accident were about 9-30 times larger in each catchment than those after that point until 2017, though initial ¹³⁷Cs concentration in river water was derived from an extrapolation of data based on a two exponentially decreasing fitting. However, it was found that the impact on the ocean from the initial ¹³⁷Cs discharge through the rivers can be limited because the ¹³⁷Cs discharge from the Abukuma River and the 13 other rivers in the Fukushima coastal region (29 TBq) was two orders of magnitude smaller than the direct release from Fukushima Dai-ichi Nuclear Power Plant (FDNPP) into the ocean (3.5 PBq) and from atmospheric deposition into the ocean (7.6 PBq) (Kobayashi et al., 2013). This model is expected to be useful to evaluate and predict ¹³⁷Cs discharge from catchments in future water management and in the estimation of ¹³⁷Cs discharge into reservoirs and the ocean.

Development of an Analytical Method for Estimating Three-Dimensional Distribution of Sediment-Associated Radiocesium at a Reservoir Bottom

After the Fukushima Daiichi Nuclear Power Station accident, the distributions of sediment-associated radiocesium were investigated to evaluate the dispersion and accumulation of radiocesium in the reservoir field. To develop an analytical method for measuring the horizontal and vertical distributions of radiocesium on a wide scale, we obtained 253 gamma-ray spectra at the bottoms of 64 ponds in Fukushima during 2014-2016 by using a NaI(Tl) scintillation detector. For visualizing horizontal distribution, the correlation between detector counting rate and radiocesium concentration of the bottom sediment was confirmed. In estimating vertical distribution, the depth profile of sediment-associated radiocesium was found to be correlated to the intensities of scattered and photo peaks. Good agreement was observed between the results of in situ spectrometry and core sampling. These results indicate that the developed method is suitable for understanding the behavior of radiocesium and determining whether decontamination of reservoirs is required.

Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles

The time and size dependencies of particulate ¹³⁷Cs concentrations in a reservoir were investigated to evaluate the dynamics of ¹³⁷Cs pollution from a mountainous forested catchment. Sediment and sinking particle samples were collected using a vibracorer and a sediment trap at the Ogaki Dam Reservoir in Fukushima, which is located in the heavily contaminated area that formed as a result of the Fukushima Dai-ichi Nuclear Power Plant accident of 2011. The inventory of ¹³⁷Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 10¹²-3.9 × 10¹² Bq, which is equivalent to 1.1%-1.4% of the initial estimated catchment inventory. The particulate ¹³⁷Cs concentration showed a decline with time, but the exponent value between the specific surface area and the ¹³⁷Cs concentration for the fine-sized (<63 μm) particle fraction remained almost constant from the immediate aftermath of the accident. These quantitative findings obtained by reconstructing the contamination history of particulate ¹³⁷Cs in reservoir sediments and sinking particles have important implications for the evaluation of ¹³⁷Cs dynamics in mountainous forested catchments.

Applicability of Kd for modelling dissolved 137Cs concentrations in Fukushima river water: Case study of the upstream Ota River

A study is presented on the applicability of the distribution coefficient (K_d) absorption/desorption model to simulate dissolved ¹³⁷Cs concentrations in Fukushima river water. The upstream Ota River basin was simulated using GEneral-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) for the period 1 January 2014 to 31 December 2015. Good agreement was obtained between the simulations and observations on water and suspended sediment fluxes, and on particulate bound ¹³⁷Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved ¹³⁷Cs in the river water were much harder to reproduce with the simulations. By tuning the K_d values for large particles, it was possible to reproduce the mean dissolved ¹³⁷Cs concentrations during base flow periods (observation: 0.32 Bq/L, simulation: 0.36 Bq/L). However neither the seasonal variability in the base flow dissolved ¹³⁷Cs concentrations (0.14-0.53 Bq/L), nor the peaks in concentration that occurred during storms (0.18-0.88 Bq/L, mean: 0.55 Bq/L), could be reproduced with realistic simulation parameters. These discrepancies may be explained by microbial action and leaching from organic matter in forest litter providing an additional input of dissolved ¹³⁷Cs to rivers, particularly over summer, and limitations of the K_d absorption/desorption model. It is recommended that future studies investigate these issues in order to improve simulations of dissolved ¹³⁷Cs concentrations in Fukushima rivers.