Environmental mobility of $^{110m}$Ag: lessons learnt from Fukushima accident (Japan) and potential use for tracking the dispersion of contamination within coastal catchments

Abstract : Silver-110 metastable ($^{110m}$Ag) has been far less investigated than other anthropogenic radionuclides, although it has the potential to accumulate in plants and animal tissues. It is continuously produced by nuclear power plants in normal conditions, but emitted in much larger quantities in accidental conditions facilitating its detection, which allows the investigation of its behaviour in the environment. We analysed $^{110m}$Ag in soil and river drape sediment (i.e., mud drapes deposited on channel-bed sand) collected within coastal catchments contaminated in Fukushima Prefecture (Japan) after the Fukushima Dai-ichi Nuclear Power Plant accident that occurred on 11 March 2011. Several field experiments were conducted to document radiosilver behaviour in the terrestrial environment, with a systematic comparison to the more documented radiocesium behaviour. Results show a similar and low mobility for both elements in soils and a strong affinity with the clay fraction. Measurements conducted on sediment sequences accumulated in reservoirs tend to confirm a comparable deposition of those radionuclides even after their redistribution due to erosion and deposition processes. Therefore, as the $^{110m}$Ag:$^{137}$Cs initial activity ratio varied in soils across the area, we justified the relevance of using this tool to track the dispersion of contaminated sediment from the main inland radioactive pollution plume generated by FDNPP accident.