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

Investigations including a bathymetric survey, sonic prospecting, and vibrocoring were performed to understand the horizontal and vertical distribution of Cs-137 in seabed sediments in shallow seas with depths less than 30 m near the Fukushima Daiichi Nuclear Power Plant. Especially, features of Cs-137 distributions in deeper sections of the seabed sediments were studied to evaluate the vertical heterogeneity of Cs-137 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 Cs-137 inventories in the seabed sediments varied from 13 +/- 1 to 3,510 +/- 26 kBq m(-2), and continuous distributions of Cs-137 at depths greater than 81 cm were observed. The Cs-137 distributions were not uniform; however, the Cs-137 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 Cs-137 inventories and mean shear stress, features of the seafloor topography were inferred to be significant control factors governing the horizontal and vertical distribution of Cs-137 in the seabed sediments. Rapid changes and multiple peaks in the vertical profile of the Cs-137 distributions suggest that they are related to pulse input caused by heavy-rain events. Change in the Cs-137 inventories with depth in this study are larger than those reported in previous studies, indicating earlier results of Cs-137 inventories per unit in seabed sediments in shallow seas, especially near the river mouth, which drains a radiologically highly-contaminated basin, were underestimated.