Open Access

Thomas Chalaux-Clergue

• Significant quantities of radionuclides (mainly radiocesium) were deposited on the soils of Northeastern Japan after the accident of the Fukushima Daiichi Power Plant in March 2011. Starting from late 2013, the Japanese authorities initiated an unprecedented large-scale decontamination programme in cultivated and residential areas affected by the main radioactive plume (8953 km2). The main objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr−1 in addition to the natural levels, in order to allow a rapid return of the local inhabitants and the resumption of agricultural activities. In these areas, decontamination consisted of removing the uppermost 5-cm layer of soil that concentrated radiocesium (90-99%) and then replacing it with saprolite sand extracted from local quarries opened for this purpose. Decontamination targeted a minor proportion of the landscape, as farmland covers less than 10% of this region. Furthermore, as agricultural land started toSignificant quantities of radionuclides (mainly radiocesium) were deposited on the soils of Northeastern Japan after the accident of the Fukushima Daiichi Power Plant in March 2011. Starting from late 2013, the Japanese authorities initiated an unprecedented large-scale decontamination programme in cultivated and residential areas affected by the main radioactive plume (8953 km2). The main objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr−1 in addition to the natural levels, in order to allow a rapid return of the local inhabitants and the resumption of agricultural activities. In these areas, decontamination consisted of removing the uppermost 5-cm layer of soil that concentrated radiocesium (90-99%) and then replacing it with saprolite sand extracted from local quarries opened for this purpose. Decontamination targeted a minor proportion of the landscape, as farmland covers less than 10% of this region. Furthermore, as agricultural land started to be re-cultivated, many questions were raised regarding the impact of this decontamination work on the transfer of radiocesium into rivers and across landscapes. A multi-proxy sediment tracing approach was designed to quantify the evolution of sources of sediment and particle-bound radiocesium transiting river systems draining the main contamination plume since the accident, through the combined analysis of sediment cores and river flood deposits collected across the region. In 2021, Japanese authorities announced the potential reopening of parts of the Difficult-to-Return zone without obligatory decontamination from Spring 2023 onwards. The current research reconstructed and compared the sources and transfers of sediment and radiocesium in three contrasted catchments (i.e., Mano Dam, Yokokama Dam and Ogaki Dam) where different management modes (and schedules) were implemented. This will shed light into the impact of different management decisions and their timing (early-decontamination or late-decontamination) on sediment and radiocesium transfers in these unique post-accidental conditions.…