Behavior of radiocesium (Cs) on the coastal seafloor near the Fukushima Daiichi Nuclear Power Plant inferred from radiocesium distributions in long cores

Nakanishi, Takahiro   ; Tsuruta, Tadahiko  ; Misono, Toshiharu   ; Shiribiki, Takehiko; Urabe, Yoshimi*; Sanada, Yukihisa   

Continuous data of Cs-137 concentration in surface seabed sediment around the Fukushima Daiichi Nuclear Power Plant (FDNPP) from 2012 to 2022 was compiled and its temporal change was fitted by exponential function. In general, Cs-137 concentrations were gradually declining with time. However, at some monitoring points in shallow region, long half-lives and/or large deviations of Cs-137 concentrations were noticed. To gain insight into cesium dynamics in shallow seafloor, long sediment cores were collected at shallow region near the FDNPP, and the vertical profiles of Cs-137 concentration and particle size distribution were determined. At the beach, Cs-137 concentration and particle size distribution were very homogenized from several tens of cm to more than 1 m deep, probably due to strong vertical mixing by wind waves and tides. Therefore, beach sediments have significant Cs-137 reserves in deeper layers at present. It was suggested that the Cs-137 supply from deep layers as well as the land area might suppress the decline of surface Cs-137 concentration in this area. At sampling points located at the bases of cliffs and depressions at the offshore zone, the vertical distributions of Cs-137 concentration were highly heterogeneous, showing the particle size dependency. Moreover, Cs-137 profiles obtained at the same point for several years were quite different. Therefore, it might cause the large fluctuation of Cs-137 concentration at surface sediment over time.