7; profiles a–b and i–j). They are equipped with dams at 20 km from the outlet for Nitta

River, and at 16 and 12 km from the outlet for the Ota river. Only the finest – and most contaminated – material is exported from learn more their reservoirs, as suggested by the very high 134+137Cs activities measured in sediment collected just downstream of the dams (Fig. 7; profiles a–b and i–j). Those reservoirs stored very large quantities of contaminated sediment, as illustrated by the contamination profile documented in sediment accumulated behind Yokokawa dam (Fig. 8). Identification of a 10-cm sediment layer strongly enriched in 134+137Cs (308,000 Bq kg−1) and overlaid by a more recent and less contaminated layer (120,000 Bq kg−1) shows that Fukushima accident produced a distinct geological record that will be useful for

sediment dating and estimation of stocks of contaminated material in this region of Japan during the next years and decades. The succession of typhoons and snowmelt events during the 20 months that Nutlin-3 mw followed FDNPP accident led to the rapid and massive dispersion of contaminated sediment along coastal rivers draining the catchments located in the main radioactive pollution plume. In this unique post-accidental context, the absence of continuous river monitoring has necessitated the combination of indirect approaches (mapping and tracing based on radioisotopic ratios, connectivity assessment) to provide this first overall picture of early sediment dispersion in Fukushima coastal catchments. These results obtained on riverbed sediment should be compared to the measurements filipin conducted on suspended sediment that are being collected since December 2012. The combination of those measurements with discharge and suspended sediment concentration data will also allow calculating exports of contaminated sediment to the Pacific Ocean. Our

results showing the rapid dispersion of contaminated sediment from inland mountain ranges along the coastal river network should also be compared to the ones obtained with the conventional fingerprinting technique based on the geochemical signatures of contrasted lithologies. Fukushima coastal catchments investigated by this study are indeed constituted of contrasted sources (volcanic, plutonic and metamorphic sources in upper parts vs. sedimentary sources in the coastal plains). This unique combination of surveys and techniques will provide very important insights into the dispersion of particle-borne contamination in mountainous catchments that are particularly crucial in this post-accidental context, but that will also be applicable in other catchments of the world where other particle-borne contaminants are problematic.