Occurrence and partition ratios of radiocesium in an urban river during dry and wet weather after the 2011 nuclear accident in Fukushima

Interactions between micaceous minerals weathering and cesium adsorption

The environmental behavior of radioactive cesium (RCs) in contaminated areas is generally governed by soil and sediment components and natural weathering conditions. In this study, desorption tests and spectroscopic approaches were used to explore the interaction between the weathering of micaceous minerals (i.e., biotite and phlogopite) and the adsorption of Cs⁺ and the critical role of weathering in the environmental behavior of RCs. Results showed that the reaction sequence between weathering and Cs⁺ adsorption significantly affected the surface species of Cs⁺ and the structure of biotite and phlogopite. Regardless of whether it occurred before, after, or during Cs⁺ adsorption, weathering generated more high-affinity adsorption sites, namely, interlayer sites (ITs) and frayed edge sites (FESs), to different extents, and then facilitated the uptake of Cs⁺ at FESs and ITs on micaceous minerals in a poorly exchangeable state. Cs⁺ stabilized the micaceous mineral structure once it was absorbed within collapsed interlayers by hindering cation exchange and preventing further destruction during weathering. As important weathering factors, high temperature and Ca²⁺ content promoted the binding of Cs⁺ in the interlayers of biotite and phlogopite by enhancing interlayer cation exchange. These findings are beneficial for a better understanding of the environmental behaviors of RCs in the hydrosphere and pedosphere.

A comparative study of riverine 137Cs dynamics during high-flow events at three contaminated river catchments in Fukushima

This study presents the temporal variations in riverine ¹³⁷Cs concentrations and fluxes to the ocean during high-flow events in three coastal river catchments contaminated by the Fukushima Daiichi Nuclear Power Plant accident. River water samples were collected at points downstream in the Niida, Ukedo, and Takase Rivers during three high-flow events that occurred in 2019-2020. Variations in both the dissolved and particulate ¹³⁷Cs concentrations appeared to reflect the spatial pattern of the ¹³⁷Cs inventory in the catchments, rather than variations in physico-chemical properties of water and suspended solid. Negative relationships between the ¹³⁷Cs concentration and δ¹⁵N in suspended solid were found in all rivers during the intense rainfall events, suggesting an increased contribution of sediment from forested areas to the elevation of particulate ¹³⁷Cs concentration. The ¹³⁷Cs flux ranged from 0.33 to 19 GBq, depending on the rainfall erosivity. The particulate ¹³⁷Cs fluxes from the Ukedo River were relatively low compared with the other two rivers and were attributed to the effect of the Ogaki Dam reservoir upstream. The percentage of ¹³⁷Cs desorbed in seawater relative to ¹³⁷Cs in suspended solids ranged from 2.8% to 6.6% and tended to be higher with a higher fraction of exchangeable ¹³⁷Cs. The estimated potential release of ¹³⁷Cs desorbed from suspended solids to the ocean was 0.022-0.57 GBq, and its ratio to the direct flux of dissolved ¹³⁷Cs was 0.12-6.2. Episodic sampling during high-flow events demonstrated that the particulate ¹³⁷Cs flux depends on catchment characteristics and controls ¹³⁷Cs transfer to the ocean.

Importance of desorption process from Abukuma River's suspended particles in increasing dissolved 137Cs in coastal water during river-flood caused by typhoons

Desorption of radiocesium (¹³⁷Cs) from riverine particles into seawater strongly influences ¹³⁷Cs concentrations in coastal seawater. This process is important for quantifying the input of radionuclides to marine environments. Here we quantify the particulate ¹³⁷Cs flux from the Abukuma River, Japan, during typhoon Hagibis and following typhoons in 2019 and estimate the resulting increased dissolved ¹³⁷Cs levels in coastal seawater. Particulate ¹³⁷Cs export flux, 1.1 × 10¹² Bq, from the Abukuma River during the 4-day period of typhoon Hagibis (12-15 October 2019) equaled two-thirds of the annual flux during 2012-2015, the period of high ¹³⁷Cs levels following the Fukushima Daiichi Nuclear Power Plant accident. The flux of the desorbed fraction from the Abukuma River during typhoon Hagibis was 0.061-0.12 × 10¹²Bq, and its daily flux to the surrounding coastal seawater (1.5-3.0 × 10¹⁰ Bq/d) was one to two orders of magnitude greater than the estimated input to the coastal seawater during the pre-typhoon period (1.3× 10⁸-1.0 × 10⁹ Bq/d). Simulated results suggest that the massive influx of riverine particles and subsequent desorption of ¹³⁷Cs increased dissolved ¹³⁷Cs levels in the coastal seawater by an order of magnitude, from 3.3 mBq/L (pre-typhoon level) to 45-126 mBq/L. This found pathway opens up new scenarios involving radionuclide dynamics in the boundary area of river-sea system.

Reproduction of sediment deposition and prediction of 137Cs concentration in the major urban rivers of Tokyo

Radioactive caesium- 137 (¹³⁷Cs) can be used as a tracer to infer sediment dynamics due not only to its long radioactive half-life but also its affinity for fine sediment. A novel advanced interpolation assessment was conducted to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan, which had a time-dependent input function and incorporated the effects of mixed-sediment particle dynamic behaviour on radioactive decay. In addition, transport parameters derived from Chernobyl measurements were applied as predictors of the long-term contamination of the cardinal urban rivers by the fallout from the Tokyo Electric Power Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. The behaviour of suspended sediment substances, incorporating the effects of deposition and pickup, was assessed using a mixed-sediment particle dynamics model. The concentrations of ¹³⁷Cs adsorbed on fine sediment particles of each size fraction were determined. Removal of ¹³⁷Cs from the cardinal urban river channel had significant effects on both long-term decline, including extreme flash flood events, and the dynamic and time-dependent behaviours of interspersed ¹³⁷Cs and sediment activity. A novel advanced interpolation assessment method was used to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan. The assessment procedure has a time-dependent input function and incorporates the effects of mixed-sediment particle dynamics on this time dependence. The results indicated that sediment and ¹³⁷Cs concentrations could decline more rapidly than observed in the Fukushima and Chernobyl regions. This rate of decrease depended on terraces covered with volcanic ash soil, which incorporated the effects of fine sediment behaviour for particle adsorption. In addition, comparatively large impacts were observed during extreme flash flooding events, which were associated with the land cover of the major urban river catchments in Tokyo. This work provides a new perspective for understanding ¹³⁷Cs behaviour associated with reproduction of sediment deposition and prediction of ¹³⁷Cs concentration in the major urban rivers of Tokyo, incorporating the effects of baseline ¹³⁷Cs behaviour with the impact of sediment particle adsorption in a volcanic ash soil-covered terrace.

Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant

¹³⁷Cs activity concentration in the water of four ponds, Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), that are within 10 km of the Fukushima Dai-ichi nuclear power plant were observed from April 2015 to August 2016. ¹³⁷Cs inventories in soils surrounding SU, FS, IZ, and KM were 6.4, 2.9, 2.1, and 0.9 MBq m^-2, respectively. ¹³⁷Cs inventories in the bottom sediments of SU, FS, IZ, and KM were 13, 8.9, 1.6, and 1.1 MBq m^-2, respectively. Higher ¹³⁷Cs inventories in bottom sediment than those of soil in SU and FS suggest that ¹³⁷Cs was delivered to and accumulated in these ponds. Mean total ¹³⁷Cs activity concentrations in SU, FS, IZ, and KM were 41, 13, 9.5, and 1.4 Bq L^-1, respectively. Particulate ¹³⁷Cs concentration accounted for 71-90% of total ¹³⁷Cs in the water samples, on average. The mean distribution coefficient, K_d, in SU, FS, IZ, and KM was 1.3 × 10⁵, 2.1 × 10⁵, 1.7 × 10⁵, and 6.2 × 10⁵ L kg^-1, respectively. These K_d values were higher than the K_d values observed in the Chernobyl area by 1-2 orders of magnitude. Although no significant decreasing trends were found, dissolved ¹³⁷Cs activity concentration tended to be low during winter in all four ponds. Dissolved ¹³⁷Cs activity concentrations were proportional to K⁺ and DOC concentrations in all the ponds. The results from principal component analysis performed for ¹³⁷Cs activity concentration and water chemistry data sets suggested that there were different mechanisms behind variability of dissolved ¹³⁷Cs activity concentrations for each pond. Continuous monitoring is required to reveal temporal trends in ¹³⁷Cs activity concentrations of these waters and controlling factors of such in closed water systems in Fukushima.

Source analysis of radiocesium in river waters using road dust tracers

Following the Fukushima Dai-ichi Nuclear Power Station accident, regional road dust, heavily contaminated with radiocesium, now represents a potential source of radiocesium pollution in river water. To promote effective countermeasures for reducing the risk from radiocesium pollution, it is important to understand its sources. This study evaluated the utility of metals, including Al, Fe, and Zn as road dust tracers, and applied them to analyze sources of ¹³⁷Cs in rivers around Fukushima during wet weather. Concentrations of Zn in road dust were higher than agricultural and forest soils, whereas concentrations of Fe and Al were the opposite. Concentrations of Zn were weakly but significantly correlated with benzothiazole, a molecular marker of tires, indicating Zn represents an effective tracer of road dust. Al, Fe, and Zn were frequently detected in suspended solids in river water during wet weather. Distribution coefficients of these metals and ¹³⁷Cs exceeded 10⁴, suggesting sorptive behavior in water. Although concentrations of Al, Fe, Zn, and ¹³⁷Cs were higher in fine fractions of road dust and soils than in coarse fractions, use of ratios of ¹³⁷Cs to Al, Fe, or Zn showed smaller differences among size fractions. The results demonstrate that combinations of these metals and ¹³⁷Cs are useful for analyzing sources of radiocesium in water. These ratios in river water during wet weather were found to be comparable with or lower than during dry weather and were closer to soils than road dust, suggesting a limited contribution from road dust to radiocesium pollution in river water.

Behavior of radioactive materials and safety stock of contaminated sludge

The radioactive fallout from the Fukushima Dai-ichi nuclear power plant disaster in 2011 has flowed into and accumulated in many wastewater treatment plants (WWTPs) via sewer systems; this has had a negative impact on WWTPs in eastern Japan. The behavior of radioactive materials was analyzed at four WWTPs in the Tohoku and Kanto regions to elucidate the mechanism by which radioactive materials are concentrated during the sludge treatment process from July 2011 to March 2013. Furthermore, numerical simulations were conducted to study the safe handling of contaminated sewage sludge stocked temporally in WWTPs. Finally, a dissolution test was conducted by using contaminated incinerated ash and melted slag derived from sewage sludge to better understand the disposal of contaminated sewage sludge in landfills. Measurements indicate that a large amount of radioactive material accumulates in aeration tanks and is becoming trapped in the concentrated sludge during the sludge condensation process. The numerical simulation indicates that a worker's exposure around contaminated sludge is less than 1 µSv/h when maintaining an isolation distance of more than 10 m, or when shielding with more than 20-cm-thick concrete. The radioactivity level of the eluate was undetectable in 9 out of 12 samples; in the remaining three samples, the dissolution rates were 0.5-2.7%.