sediment mobilized from the coastal plains. This investigation is particularly crucial in the case of coastal rivers in Fukushima Prefecture to guide the implementation of appropriate soil and river DAPT management measures. Nitta

River drains mountainous areas characterized by a high initial contamination to the Pacific Ocean, by flowing across coastal plains that were relatively spared by initial continental fallout but that are still currently densely populated (e.g. in Minamisoma town). The relative contribution of each source in the composition of riverbed sediment collected during the three sampling campaigns in the Nitta catchment was then quantified through the application of a binary mixing model. As an example, the relative contribution of ‘western’ source area Xw was determined from Eq. (3): equation(3) XW=Ag110mCs137S−Ag110mCs137EAg110mCs137W−Ag110mCs137E × 100,where XW is the percentage fraction of the western source area, (110mAg:137Cs)W

and (110mAg:137Cs)E are the median values of 110mAg:137Cs ratio measured in MEXT soil samples collected in the ‘western’ and the ‘eastern’ source areas of the Nitta catchment, i.e. 0.0024 and 0.0057 respectively ( Table 2), and (110mAg:137Cs)S is the isotopic ratio measured in the river sediment sample. We did not include initial river sediment as a third end-member as the Hydroxychloroquine price violent typhoons that occurred between the accident (March 2011) and our first fieldwork campaign Thiamine-diphosphate kinase (November 2011) likely flushed the fine riverbed sediment that was already present in the channels before the accident. Application of the mixing model illustrates the very strong reactivity of this catchment and

the entire flush of sediment stored in the river network during a one-year period only (Fig. 5). In November 2011, following the summer typhoons (i.e., Man-On on 20 July and Roke on 22 September that generated cumulative precipitation that reached between 215 and 310 mm across the study area), contaminated soil was eroded from upstream fields and supplied to the upstream sections of the rivers (Fig. 5a). Then, this sediment was exported to the coastal plains during the discharge increase generated by the snowmelt in March 2012, as illustrated by the measurements conducted on material sampled in April 2012 (Fig. 5b). Finally, sediment deposited within the river network was flushed by the typhoons that occurred during summer in 2012. Those typhoons were less violent than the ones that happened in 2011, and led to less intense erosion than during the previous year, but they were sufficiently powerful to increase river discharges, to export the sediment stored in the river channel and to replace it with material originating from closer areas (Fig. 5c).