Tracking the early dispersion of contaminated sediment along rivers draining the Fukushima radioactive pollution plume

Caroline Chartin (1, 2) , O. Evrard (1, 2) , Yuichi Onda (3) , Jérémy Patin (3) , Irène Lefevre (1, 2) , Catherine Ottle (1, 4) , Sophie Ayrault (1, 2) , Hugo Lepage (1, 2) , Philippe Bonté (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 MOSAIC - Modélisation des Surfaces et Interfaces Continentales

Soil erosion and subsequent sediment transport in rivers play a major role in the global biogeochemical cycles and on the dispersion of contaminants within the natural environment. As other particle-borne pollutants, fallout radionuclides emitted after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident are strongly sorbed by fine particles, and they are therefore likely to be redistributed by hydro-sedimentary processes across catchments. Although regrettable, the Fukushima catastrophe and the associated massive radionuclide release provide a unique opportunity to track the dispersion of sediment in catchments exposed to typhoons and to better understand the anthropogenic impacts on particle-borne transfers within the natural environment. Fieldwork around FDNPP and subsequent lab work on Fukushima samples required the compliance with very demanding radioprotection and security rules. Here, we collected exposed riverbed sediment (n=162) along rivers and in reservoirs draining the catchments contaminated by the main radioactive pollution plume that extends across Fukushima Prefecture in November 2011, April 2012 and November 2012. We measured their gamma emitting radionuclide activities and compared them to the documented surveys in nearby soils. We show that the 110mAg: 137 Cs ratio provided a tracer of the dispersion of contaminated sediment in one specific catchment draining the most contaminated area. Our results demonstrate that the system was very reactive to the succession of summer typhoons and spring snowmelt. We identified a partial export of contaminated sediment from inland mountain ranges – exposed initially to the highest radionuclide fallout – to the coastal plains as soon as in November 2011, after a series of violent typhoons. This export was amplified by the spring snowmelt, and remaining contaminated material temporarily stored in the river channel was flushed by the typhoons that occurred during summer in 2012. This catchment behaviour characterized by the high transmissivity of paddy fields strongly connected to the river network in upland mountain ranges and the potential storage in the coastal plains that are successively filled with contaminated sediment and then flushed was illustrated by the calculation of an index of hydro sedimentary connectivity and the construction of river longitudinal profiles. We thereby suggest that coastal rivers have become a perennial supply of contaminated sediment to the Pacific Ocean. Our findings show that Fukushima accident produced original tracers to monitor particle borne transfers across the affected area shortly after the catastrophe. Furthermore, we outlined that this accident generated a distinct geological record that will be useful for sediment dating behind reservoirs in Japan during the next decades.

Mots clés: erosion; sediment; river catchments; contamination; transfer; Panoply

Domaines: Sciences de l'environnement; Milieux et Changements globaux; Interfaces continentales, environnement; Géochimie; Hydrologie; Géomorphologie

Caroline Chartin, O. Evrard, Yuichi Onda, Jérémy Patin, Irène Lefevre, et al.. Tracking the early dispersion of contaminated sediment along rivers draining the Fukushima radioactive pollution plume. Anthropocene, 2013, 1, pp.23-34. ⟨10.1016/j.ancene.2013.07.001⟩. ⟨cea-02614734⟩