Quantifying the dilution of the radiocesium contamination in Fukushima coastal river sediment (2011–2015)

O. Evrard (1, 2) , J. Patrick Laceby (1, 2) , Yuichi Onda (3) , Yoshifumi Wakiyama (4) , Hugo Jaegler (1, 2) , Irène Lefèvre (1, 2)

1 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
2 GEDI - Géochimie Des Impacts
3 Université de Tsukuba = University of Tsukuba
4 IER - Institute of Environmental Radioactivity - Fukushima University

Fallout from the Fukushima Dai-ichi nuclear power plant accident resulted in a 3000-km 2 radioactive contamination plume. Here, we model the progressive dilution of the radiocesium contamination in 327 sediment samples from two neighboring catchments with different timing of soil decontamination. Overall, we demonstrate that there has been a ~90% decrease of the contribution of upstream contaminated soils to sediment transiting the coastal plains between 2012 (median-M-contribution of 73%, mean absolute deviation-MAD-of 27%) and 2015 (M 9%, MAD 6%). The occurrence of typhoons and the progress of decontamination in different tributaries of the Niida River resulted in temporary increases in local contamination. However, the much lower contribution of upstream contaminated soils to coastal plain sediment in November 2015 demonstrates that the source of the easily erodible, contaminated material has potentially been removed by decontamination, diluted by subsoils, or eroded and transported to the Pacific Ocean.

Domaines: Océan, Atmosphère; Interfaces continentales, environnement; Science des sols; Géomorphologie; Hydrologie

O. Evrard, J. Patrick Laceby, Yuichi Onda, Yoshifumi Wakiyama, Hugo Jaegler, et al.. Quantifying the dilution of the radiocesium contamination in Fukushima coastal river sediment (2011–2015). Scientific Reports, 2016, 6 (1), ⟨10.1038/srep34828⟩. ⟨hal-01587546⟩