# Comparing radiation dose rates in soils and riverine sediment to track the dispersion of radioactive contamination in Fukushima coastal rivers

Olivier Evrard, Yuichi Onda, Hugo Lepage, Caroline Chartin, Irène Lefèvre, Olivier Cerdan, Philippe Bonté, Sophie Ayrault

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## To cite this version:

Olivier Evrard, Yuichi Onda, Hugo Lepage, Caroline Chartin, Irène Lefèvre, et al.. Comparing radia-
tion dose rates in soils and riverine sediment to track the dispersion of radioactive contamination in
Fukushima coastal rivers. EGU General Assembly 2014, Apr 2014, Vienne, Austria. pp.EGU2014-
1702. cea-02640917

HAL Id: cea-02640917

https://cea.hal.science/cea-02640917

Submitted on 28 May 2020

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# Comparing radiation dose rates in soils and riverine sediment to track the dispersion of radioactive contamination in Fukushima coastal rivers

Olivier Evrard (1), Yuichi Onda (2), Hugo Lepage (1), Caroline Chartin (1), Irène Lefèvre (1), Olivier Cerdan (3), Philippe Bonté (1), and Sophie Ayrault (1)

(1) Laboratoire des Sciences du Climat et de l'Environnement, Transfert Continents - Océans, Gif-sur-Yvette, France (olivier.evrard@lsce.ipsl.fr), (2) CRIED, Tsukuba University, Japan, (3) BRGM, Orléans, France

The Fukushima Dai-ichi nuclear power plant (FDNPP) accident that occurred in March 2011 led to the formation of a 3000-km² radioactive pollution plume on soils located up to 70 km to the northwest of the damaged site. Forests and paddy fields are the dominant land uses in this mountainous region drained to the Pacific Ocean by several rivers that flow across densely inhabited coastal plains. It is then crucial to track the dispersion of radioactive material conveyed by those rivers to estimate the continental supply of radionuclides to the Ocean and to assess redistribution of radioactive sediment in those catchments. Radiations emitted by this contaminated material may indeed lead to an external exposure threat for local populations. As river discharge and sediment concentration data were not available during the first two years that followed the accident, alternative methods had to be developed to track this dispersion.

We therefore organized field campaigns every six months and conducted local ground dose rate measure-
ments to estimate whether fresh sediment drape deposits were more or less contaminated compared to local soils.
Overall, our results showed that, in those regions exposed to violent typhoons and spring snowmelt, transfers of
sediment are massive and episodic, and that they followed a seasonal cycle in 2011-2012. Then, in May 2013,
contamination levels measured in sediment found in the upper parts of the catchments were almost systematically
lower than the ones measured in nearby soils, whereas their contamination was higher in the coastal plains. This
could have indicated a drying-up of the upstream sources of contamination. However, after the violent typhoons
that occurred during summer in 2013, dose rates measured in fresh sediment deposits in November 2013 increased
again systematically across the region. We thereby suggest that remobilization of contaminated sediment by
typhoons and their storage in reservoirs and in coastal sections of the river channels now represent the most crucial
issues to protect the local populations and manage the most contaminated catchments.