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Nakanishi, T.; Tsuruta, T.; Misonou, T.; Shiribiki, T.; Urabe, Y., and Sanada, Y., 2023. Behavior of radiocesium (137Cs) on the coastal seafloor near the Fukushima Daiichi Nuclear Power Plant inferred from radiocesium distributions in long cores. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.-S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 161-165. Charlotte (North Carolina), ISSN 0749-0208. Continuous data of 137Cs concentrations in surface seabed sediment at several monitoring points around the Fukushima Daiichi Nuclear Power Plant (FDNPP) from 2012 to 2022 were compiled, and exponential functions fitted its temporal change. The 137Cs concentrations are gradually declining with time. However, at some monitoring points in shallow regions with a water depth of 30 m or less, long half-lives and/or large deviations of 137Cs concentrations were noticed. Long sediment cores (∼1 m) were collected at shallow regions near the FDNPP to gain insight into radiocesium dynamics in shallow seafloor, and the vertical profiles of 137Cs concentrations and particle size distribution were determined. In the sediment cores collected in the onshore zone, the 137Cs concentrations and particle size distribution were homogenized from several tens of cm to more than 1 m depth, indicating strong vertical mixing by wind waves. Therefore, onshore sediments have significant 137Cs reserves in deeper layers at present. The 137Cs supply from the deep layers and the land area might suppress this zone's surface 137Cs concentration decline. However, in sediment cores at the offshore zone, the vertical 137Cs concentration distributions were highly heterogeneous, showing the particle size dependency. Moreover, 137Cs profiles obtained at the same point for several years differed significantly, indicating local spatial distribution and temporal deposition. Therefore, it might cause a large fluctuation in 137Cs concentrations at surface sediment over time. [ABSTRACT FROM AUTHOR]