Vertical Profiles of Radioactive Cs Distributions and Temporal Changes in Seabed Sediments near River Mouth in Coastal Area of Fukushima Prefecture

Understanding the features of radioactive Cs (Cs-137) in seabed sediments in coastal areas linked with rivers, which are major settlement areas for particulate (137C)s, is a key issue in evaluating the supply of Cs-137 from river discharges. The vertical profile of Cs-137 distribution in seabed sediments is an important clue for elucidating the settlement process of particulate Cs-137 near river mouth. Therefore, we sampled seabed sediments using a long core sampler (vibrocoring; the maximum lengths of the core was approximately 100 cm) to clarify the entire vertical profile of the Cs-137 distribution and changes in seabed sediments near the river mouth in the coastal area of Fukushima Prefecture. From 2014 through 2019, the Cs-137 concentration along the entire vertical profile did not increase, and there was no remarkable migration of the Cs-137 toward the deeper layers of the seabed sediments. On the contrary, the amount of Cs-137 decreased by more than 70% throughout the entire depth, and the Cs-137 distributions in the depth direction become uniform. These results suggest that the supply of Cs-137 from the river discharge is not a major concern near the river mouth in the coastal area of Fukushima Prefecture. Resuspension of particle-bound Cs-137 and offshore transport are the main factors that reduce the Cs-137 concentration in the coastal seabed sediments.