Factors controlling ¹³⁴Cs activity concentrations in sediment collected off the coast of Fukushima Prefecture in 2013–2015

To elucidate the activity concentration and behavior of radiocaesium, we observed the spatial and vertical distributions of radiocaesium in sediment collected at 12 monitoring stations off the coast of Fukushima Prefecture in 2013–2015. In bulk surface-layer sediment (core depth: 0–3 cm), high ¹³⁴Cs activity concentrations were observed at stations around the water depth of 100 m, where the sediment was rich in silt to clay particles and organic matter. The activity concentrations generally decreased with increasing core depth and the extent of the decrease was different between the stations. The difference trend for temporal change of ¹³⁴Cs activity concentrations between the two zones of off Onahama and within 30 km of the FDNPS was partly attributed to the mobility of sediment particles reflecting water content, porosity and permeability. At some stations, shaped peaks for activity concentrations were temporarily observed in upper-layer sediment (core depth: 0–1 cm) or sediment below that. The ¹³⁴Cs activity concentrations in the surface-layer sediment were not always accompanied by an increase in the contribution from fine (i.e., silt to clay) particle-bound ¹³⁴Cs in the sediment. In October 2014, sediment collected at a station about 1.5 km from FDNPS was found to have broad ¹³⁴Cs peaks in the middle-layer sediment (core depth: 5–16 cm). In this middle-layer sediment, both sand and silt to clay fractions contributed to the increased ¹³⁴Cs activity concentrations. On the other hand, such broad peaks were not found in October 2015. These results suggest that the activity concentrations in sediment had changed significantly by a complicated process of sediment mixing, erosion or re-sedimentation that affected the broad peak for the activity concentration in the middle-layer sediment. (author)