title: Cumulative history recorded in the depth distribution of radiocesium in sediments deposited on a sandbar.
abstract: We collected sediments deposited on a sandbar from the surface to 20 cm in depth in the Abukuma River to clarify the history of radiocesium derived from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. We analyzed the (137)Cs concentration in the sediments from size-fractioned samples as well as bulk samples. The depth distribution of (137)Cs showed the highest concentration in the deepest sediment layer (18-20 cm) studied, which indicates that sediments with a lower (137)Cs concentration were transported and deposited on sediments having a higher (137)Cs concentration. At the same time, the depth distribution suggests a decrease in radioactivity in provenance areas of the sediments. Analysis of the size-fractioned sediments indicated that the three sediment layers at 4-6 cm, 16-18 cm and 18-20 cm intervals had similar size distribution of (137)Cs and grain size composition although the concentration levels of (137)Cs were different according to their bulk concentrations. The size distribution of (137)Cs also supported the possibility that the decrease in (137)Cs concentration in bulk sediments above 18 cm is due to a decrease in the level of radioactivity in the catchment area. A comparison of the size distribution of (137)Cs between the sediment layers above and below 18 cm suggested that the (137)Cs concentration in the transported fine sediment particles decreased more with time than the (137)Cs concentration in the coarse particles, reflecting the selective transport of the finer particles. The results of this study demonstrated that sediment layers deposited on a sandbar retained the cumulative history of the fluvial transport of radiocesium after the FDNPP accident.
authors.affiliation: Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8530, Japan. Electronic address: tanaka.kazuya@jaea.go.jp.
authors.forename: Kazuya
authors.initials: K
authors.lastname: Tanaka
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Kondo
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan; Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8577, Japan.
authors.forename: Aya
authors.initials: A
authors.lastname: Sakaguchi
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan; Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan.
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Takahashi
mesh_terms: D002588:Cesium Radioisotopes / Q000032:analysis*; D062406:Fukushima Nuclear Accident; D019015:Geologic Sediments / Q000032:analysis*; D007564:Japan; D011834:Radiation Monitoring; D045483:Rivers; D012990:Soil Pollutants, Radioactive / Q000032:analysis*; D014875:Water Pollutants, Radioactive / Q000032:analysis*
chemical_list: D002588:Cesium Radioisotopes; D012990:Soil Pollutants, Radioactive; D014875:Water Pollutants, Radioactive
keywords: Depth distribution; Fukushima; Radiocesium; Sediment