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Sediment fingerprinting by using the Ag-110m: Cs-137 ratio along the main rivers draining the Fukushima radioactive pollution plume

Abstract : During the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, large quantities of radionuclides werereleased into the environment between 12 and 19 March 2011. Even though about 80% of these emissions weretransported offshore and out over the Pacific Ocean, 20% were deposited as wet and dry deposits on soils ofFukushima Prefecture on 15–16 March. In particular, 6.4 PBq of Cs-137 were modeled to have deposited onJapanese soils over a distance of 70 km to the northwest of the Fukushima Dai-ichi nuclear power plant. As mostradionuclides are strongly sorbed by fine particles, and their mineralogical clay and organic matter fractions, theyare likely to be redistributed within the landscape in association with soil and sediment particles transported byerosion processes and runoff. Based on a spatial analysis of the gamma-emitting radionuclides present in theenvironment respectively eight and thirteen months after the accident, we aim to provide a radioactive tracer toinvestigate the temporal evolution of the contaminant dispersion across Fukushima Prefecture.For this purpose, sediments were collected along rivers draining the main contamination plume in FukushimaPrefecture (i.e, Rivers Kutchibuto, Mano, Nitta and Ota) in November 2011 and April 2012.These campaignsdirectly followed the main hydro-sedimentary events that occurred in this region, i.e. the typhoon season (Julyand September-October) and the snowmelt (March 2012). The river sediment activities in gamma-emittingradionuclides were then compared to the initial activities measured in soils provided by the Japanese Ministry ofEducation, Culture, Sport, Science and Technology (MEXT).The initial fallout patterns in 110mAg appeared to differ from those of the main contamination plume de-fined mainly by radiocaesium fallout (i.e. Cs-134+137). The Ag-110m:Cs-137 ratio was then used to tracethe spatial origin of contaminated sediment collected in rivers. Sediments collected within the coastal plain inNovember 2011 were locally composed of 50 to 100% of particles originated from inland mountains rangesthat were exposed to the highest initial radionuclide fallout. Typhoons of the summer 2011 have then largelycontributed to the dispersion of the contamination. In addition, the spatial analysis of river sediment contaminationin April 2012 demonstrates that the spring snowmelt amplified significantly the flush of sediment deposited on theriverbed after the summer typhoons.Consequently, export of contaminated particles appears to be particularly fast in those mountainous catch-ments submitted to a very erosive climate. Our results have then important implications suggesting that coastalrivers may have become a perennial source of radioactive contaminants to the Pacific Ocean off FukushimaPrefecture.
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Submitted on : Thursday, May 28, 2020 - 11:15:29 AM
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Caroline Chartin, Olivier Evrard, Yuichi Onda, Jérémy Patin, Irène Lefèvre, et al.. Sediment fingerprinting by using the Ag-110m: Cs-137 ratio along the main rivers draining the Fukushima radioactive pollution plume. EGU General Assembly 2013, Apr 2013, Vienne, Austria. pp.2013 - 2935. ⟨cea-02638976⟩

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