, 2010). As we could expect it, the highest contamination levels (total 134+137Cs activities exceeding 100,000 Bq kg−1) Olaparib mouse were measured in sediment collected along the coastal rivers (i.e., Mano and Nitta Rivers) draining the main radioactive plume (Fig. 2). Contamination levels were logically much lower in sediment collected along the Abukuma River that drains less contaminated areas. The analyses conducted by the Japanese Ministry of Environment (MoE) provided an additional temporal insight into contaminated sediment exports in this area. Our samples were collected in November 2011, whereas samples provided by MoE showed that contamination of sediment was systematically the highest
in material collected in September 2011. The presence of contamination hotspots close to Fukushima City and behind a large dam located upstream of the city is likely due to the rapid wash-off of radionuclides on urban surfaces during the first series of rainfall events that followed the accident, to their concentration in urban sewers systems (Urso et al., 2013) and their subsequent export to the rivers. This rapid export of radionuclides learn more shortly after the accident along the Abukuma River is confirmed
by data collected by the MoE (Fig. 2) showing a peak of contamination in sediment collected in September 2011, and then a huge decrease to low activities even during snowmelt. Along the Hirose River, the snowmelt (in March 2012) led in contrast to an increase in sediment contamination. At the light of those first results outlining a very rapid wash-off of radionuclides obtained following the accident in the Abukuma River
basin, we decided to focus the next fieldwork campaigns on the coastal basins where radionuclide activities check in sediment were the highest. We extended sampling to the Ota River catchment, closer to FDNPP, where access was unauthorized during the first campaign (Fig. 1b). Whilst 137Cs and 134Cs gamma-emitting radioisotopes constitute by far the most problematic contaminants (with total activities in soils ranging from 50 to 1,110,000 Bq kg−1), 110mAg was also identified and measured in most samples (with activities ranging from 1 to 3150 Bq kg−1). Because of these low activities, contribution of 110mAg to the global dose rates was considered to be negligible. It appeared from the analysis of the MEXT soil database that the initial fallout pattern of 110mAg displayed significant spatial variations that were not observed for the radiocaesium fallout pattern at the scale of the entire Fukushima Prefecture. Soil activities in 110mAg were the highest within the main radiocaesium contamination plume as well as at several places along the coast located between 40 and 50 km to the north of the power plant (MEXT, 2011b). Most interestingly, the 345 values of 110mAg:137Cs ratio in MEXT soil samples strongly varied across the entire region (0.0004–0.15 with a mean of 0.006; Fig.
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