4:15 PM - 4:30 PM
[MAG41-10] Particle size distribution and radiocesium concentration in suspended particles within the high turbidity layer at near sea bottom off Fukushima prefecture
Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, Radiocesium, high turbidity layer, particle size distribution
In this study, we focused on the high turbidity layer observed at near sea bottom as a possible source of radiocesium for the benthic organisms in the coastal region of Fukushima. Field observation was conducted in the coastal region off Fukushima and nearby prefectures during June–July 2018. The LISST-200X was used for obtaining the vertical profiles of volumetric concentration of particles and particle size frequency at 28 stations. LISST-HOLO was used concurrently at some stations shallower than 200 m to obtain holographic image of particle. To measure the concentration of 137Cs in suspended materials, large volume in-situ pump was deployed on the sea bottom for collecting suspended materials just above the sea bottom (ca. 80 cm above bottom) at three stations.
The volumetric concentration of particle (especially >100 µm) was increased with depth and formed a peak at pycnocline and was decreased with depth. From holography images, diatom colonies and crustacean plankton (copepod) frequently observed at this peak may represent the sub-surface chlorophyll peak was developed around pycnocline. The increase of volumetric concentration of particle were also observed at near sea bottom at some stations located around continental shelf (ca. 100–200 m depth). The particles of 10–100 µm were abundant at this high turbidity sea bottom boundary layer, on the other hands the particles within this size range were less abundant in the above water column. The holography images of particles represented variable forms including crustacean plankton (copepod), rod shape fragment and amorphous detritus at the high turbidity sea bottom boundary layer. The 137Cs concentration of suspended particle collected at near sea bottom ranged from 0.1 to 0.2 Bq g-dry-1 and was 2 to 13 times higher than that in the sea bottom sediment at same location. These results (10–100µm particles abundant in sea bottom boundary layer and relatively high 137Cs concentration in this particles) revealed the high turbidity layer at near sea bottom could be one of the sources for radiocesium uptake by the benthic organisms. Further studies needed to verify the role of this suspended particles not only the variability of 137Cs concentration, but also portion of organic fraction, particle size distribution, and the horizontal distribution of particles in the continental shelf area off Fukushima.