Research and management challenges following soil and landscape decontamination at the onset of the reopening of the Difficult-To-Return Zone, Fukushima (Japan)

Abstract. Twelve years after the nuclear accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant in March 2011, radiocesium contamination (with a large dominance of ¹³⁷Cs, with a 30-years half-life) remains a major concern in various municipalities of Northeastern Japan. The Japanese authorities completed an unprecedented decontamination programme in residential and cultivated areas affected by the main radioactive plume (8953 km²). They implemented a complex remediation programme scheme relying on different decision rules depending on the waste type, its contamination level and its region of origin, after delineating different zones exposed to contrasted radiation rates. The central objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr^-1 in addition to the natural levels. At the onset of the full reopening of the Difficult-to-Return Zone in Spring 2023, the current review provides an update of a previous synthesis published in 2019 (Evrard et al., 2019). Although this ambitious remediation and reconstruction programme is almost completed, in the 12 municipalities of Fukushima Prefecture in which an evacuation order was imposed in at least one neighbourhood in 2011, from the 147,443 inhabitants who lived there before the accident, only 29.9 % of them had returned by 2020. Waste generated by decontamination and tsunami cleaning/demolition work is planned to have been fully transported to (interim) storage facilities by the end of 2023. The cost of the operations conducted between 2011–2020 for the so-called ‘nuclear recovery’ operations (including decontamination) was estimated by the Audit Board of Japan in 2023 to 6122.3 billion yen (~44 billion euro). Decontamination of cropland was shown to have impacted soil fertility, and potassium fertilization is recommended to limit the transfer of residual radiocesium to new crops. In forests that cover 71 % of the surface area of the Fukushima Prefecture and that were not targeted by remediation, radiocesium is now found in the upper mineral layer of the soil in a quasi-equilibrium state. Nevertheless, ¹³⁷Cs concentrations in forest products (including wood for heating and construction, wild plants, wildlife game, mushrooms) often keep exceeding the threshold values authorized in Japan, which prohibits their exploitation in the area affected by the main plume. Radionuclides from forest were shown to be exported in dissolved and particle-bound forms to downstream river systems and floodplains, although multiple monitoring records showed the continuous decrease in radiocesium concentrations in both river water and sediment across the main plume between 2011–2021. Fish contamination is now generally found below the threshold limits although reputational damage remains a major concern for local fishing communities. The remobilisation of radiocesium from sediment accumulated in reservoirs of the region is also of potential concern as it may lead to secondary contamination of fish or irrigation waters supplied to decontaminated fields. Overall, this synthesis demonstrates the need to continue monitoring post-accidental radiocesium transfers in these environments and to keep sharing data in order to refine our predictive understanding of radiocesium mobility and consolidate the tools available to model contaminant transfers in ecosystems. In forests in particular, novel countermeasures and wood uses remain to be developed and tested. Furthermore, the hydrologic connectivity between ecosystems is of great influence on long term radiocesium transport. The consequences of extreme phenomena (e.g., typhoons, forest fires) that may become more frequent in the future as a result of global change in these contaminated environments should be further anticipated.