Horizontal and vertical distributions of 137Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

Tadahiko Tsuruta,Yasuyuki Hodotsuka,Hisaya Harada,Toshiyuki Matsuoka,Toshiharu Misonou

doi:10.1007/s10872-017-0439-8

Tadahiko Tsuruta, Yasuyuki Hodotsuka + Show 3 more

Open Access

https://doi.org/10.1007/s10872-017-0439-8

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Abstract

Investigations including a bathymetric survey, sonic prospecting, and vibrocoring were performed to understand the horizontal and vertical distribution of 137Cs in seabed sediments in shallow seas with depths less than 30 m near the Fukushima Daiichi Nuclear Power Plant. Especially, features of 137Cs distributions in deeper sections of the seabed sediments were studied to evaluate the vertical heterogeneity of 137Cs distribution in the seabed sediments in shallow seas. The distribution area of the seabed sediments was less than half of the investigation area, and the locations of the seabed sediments were divided into flat and terrace-like seafloors based on their topographical features. The thicknesses of the seabed sediment layers were mostly <2 m. The 137Cs inventories in the seabed sediments varied from 13 ± 1 to 3,510 ± 26 kBq m−2, and continuous distributions of 137Cs at depths greater than 81 cm were observed. The 137Cs distributions were not uniform; however, the 137Cs inventories tended to be larger near the base of the steeper ascending slopes than in the terrace-like seafloors themselves. Based on the relationship between the 137Cs inventories and mean shear stress, features of the seafloor topography were inferred to be significant control factors governing the horizontal and vertical distribution of 137Cs in the seabed sediments. Rapid changes and multiple peaks in the vertical profile of the 137Cs distributions suggest that they are related to pulse input caused by heavy-rain events. Change in the 137Cs inventories with depth in this study are larger than those reported in previous studies, indicating earlier results of 137Cs inventories per unit in seabed sediments in shallow seas, especially near the river mouth, which drains a radiologically highly-contaminated basin, were underestimated.

Highlights

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, which occurred following the Great East Japan Earthquake and the resulting tsunami in March 2011, resulted in extensive release of radioactive cesium into the Pacific Ocean, especially, 137Cs with a half-life of 30.2 years
The grain size at sampling location No 6, which had a remarkably large 137Cs inventory and was in a semicircular depression with steep sides (Figs. 2, 3), corresponds to silt, and the change in grain size is small (Figs. 5, 6). These results suggest that the semicircular depression area is under a low current, which prompts the settling of fine-grained particles, and that the range of this current velocity is nearly constant compared to velocities in the other areas
Based on the relationship between the 137Cs inventory and the mean shear stress, features of the seafloor topography were determined as significant factors controlling the horizontal and vertical distribution of 137Cs in the seabed sediments

Summary

Introduction

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, which occurred following the Great East Japan Earthquake and the resulting tsunami in March 2011, resulted in extensive release of radioactive cesium into the Pacific Ocean, especially, 137Cs with a half-life of 30.2 years. The numbers of data locations of 137Cs distribution in seabed sediments in regions shallower than 50 m, as described in Kusakabe et al (2013), Otosaka and Kato (2014), and Black and Buesseler (2014), are 4, 3, and 4, respectively These sampling locations were situated in river mouths that had highly contaminated river basins (e.g., Ukedo River Basin; Kitamura et al 2014; Kurikami et al 2014; Yamada et al 2015). Thornton et al (2013) and NRA (2016) used a towed gamma ray spectrometer along with a depth sensor and a bathymetric survey, which indicated a relationship between the 137Cs concentrations in seabed sediments and the features of seafloor topography Their results were insufficient to understand the vertical distribution of 137Cs in seabed sediments in shallow seas

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