Tracing the sources and dynamics of contaminated sediment in coastal rivers draining the main Fukushima radioactive fallout plume using gamma-emitting radionuclides and carbon/nitrogen stable isotopes (2011-2017).

International audienceFieldwork was conducted every 6 months between 2011 and 2017 to monitor the dispersion of sediment contaminated with radionuclides in rivers draining the main Fukushima radioactive pollution plume (~500 km²). Fine sediment was systematically collected at the same locations (n=47) and analysed for gamma-emitting radionuclides (including 137Cs and 110mAg). A selection of samples was also analysed for carbon/nitrogen concentrations and isotopes. Organic matter analyses showed that paddy fields provided the main source of contaminated sediment to the rivers shortly after the accident. The spatial variations of the 110mAg:137Cs activity ratio in soils were used to demonstrate the very rapid export of sediment to the Pacific Ocean. However, this tool could only be used during the first campaigns, because of the quick decay of 110mAg. Overall, radiocesium concentrations measured in sediment deposits decreased by ~90% between 2011 and 2017. This may be explained by remediation works, by a massive export of material during typhoons and by the occurrence of landslides or channel bank erosion that supply material sheltered from the Fukushima fallout to the river network. Consequently, 6 years after the accident, most of the residual radioactive contamination is found in forests and in dam reservoirs. The analysis of sediment cores collected in a reservoir confirmed the significant storage of contaminated material in these lakes and that paddy fields provided their main source. Ongoing research focuses on the development of a soil erosion model and on the quantification of the impact of remediation works