Factors controlling Cs activity concentrations in sediment collected off the coast of Fukushima Prefecture in 2013-2015

Fukuda, Miho*; Aono, Tatsuo*; Yamazaki, Shinnosuke*; Ishimaru, Takashi*; Kanda, Jota*; Nishikawa, Jun*; Otosaka, Shigeyoshi  

To elucidate recent activity levels and the behavior of radiocesium in the coast off Fukushima Prefecture, lateral and vertical distributions of Cs in sediment were observed in 2013-2015 at 12 monitoring stations. At stations around the water depth of 100 m, relatively higher Cs activities were observed from surface sedimentary layer (0-3 cm). In these stations, sediments had high content of silt to clay particles and organic matter. The high Cs activities in the surface sediments were attributed to accumulation of highly mobile sediment particles. In October 2014, at some coastal stations, sharp peaks of Cs activities were observed in the subsurface (5-16 cm) sediments, whereas such broad peaks were not found in October 2015. These results suggest that the Cs activities in sediment had largely changed by lateral transport and re-sedimentation on the surface, as well as sediment mixing in the middle-layers.