title: Depth profiles of radioactive cesium in soil using a scraper plate over a wide area surrounding the Fukushima Dai-ichi Nuclear Power Plant, Japan.
abstract: During the Fukushima Dai-ichi Nuclear Power Plant (NPP) accident, radioactive cesium was released in the environment and deposited on the soils. Depth profiles of radioactive cesium in contaminated soils provide useful information not only for radiation protection and decontamination operations but also for geoscience and radioecology studies. Soil samples were collected using a scraper plate three times between December 2011 and December 2012 at 84 or 85 locations within a 100-km radius of the Fukushima Dai-ichi NPP. In most of the obtained radioactive cesium depth profiles, it was possible to fit the concentration to a function of mass depth as either an exponential or hyperbolic secant function. By using those functions, following three parameters were estimated: (i) relaxation mass depth β (g cm(-2)), (ii) effective relaxation mass depth βeff (g cm(-2)), which is defined for a hyperbolic secant function as the relaxation mass depth of an equivalent exponential function giving the same air kerma rate at 1 m above the ground as the inventory, and (iii) 1/10 depth L1/10 (cm), at which the soil contains 90% of the inventory. The average β value (wet weight) including ones by hyperbolic secant function in December 2012, was 1.29 times higher than that in December 2011. In fact, it was observed that depth profiles at some study sites deviated from the typical exponential distributions over time. These results indicate the gradual downward migration of radioactive cesium in the soils. The L1/10 values in December 2012 were summarized and presented on a map surrounding the Fukushima Dai-ichi NPP, and the average value of L1/10 was 3.01 cm (n = 82) at this time. It was found that radioactive cesium remained within 5 cm of the ground surface at most study sites (71 sites). The sech function can also be used to estimate the downward migration rate V (kg m(-2) y(-1)). The V values in December 2012 (n = 25) were in good agreement with those found by a realistic approach using a diffusion and migration model. Almost all values ranged between 1.7 and 9.6 kg m(-2) y(-1) in this study.
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan. Electronic address: matsuda.norihiro@jaea.go.jp.
authors.forename: Norihiro
authors.initials: N
authors.lastname: Matsuda
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Mikami
authors.affiliation: Center for Nuclear Study, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Shimoura
authors.affiliation: Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.
authors.forename: Junko
authors.initials: J
authors.lastname: Takahashi
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Masakazu
authors.initials: M
authors.lastname: Nakano
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Kiyotaka
authors.initials: K
authors.lastname: Shimada
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Kiichiro
authors.initials: K
authors.lastname: Uno
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Shigetomo
authors.initials: S
authors.lastname: Hagiwara
authors.affiliation: Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda, Tokyo 100-8577, Japan.
authors.forename: Kimiaki
authors.initials: K
authors.lastname: Saito
mesh_terms: D002586:Cesium / Q000032:analysis*; D002588:Cesium Radioisotopes / Q000032:analysis; D062406:Fukushima Nuclear Accident; D007564:Japan; D055848:Nuclear Power Plants; D011834:Radiation Monitoring; D012621:Seasons; D012990:Soil Pollutants, Radioactive / Q000032:analysis*
chemical_list: D002588:Cesium Radioisotopes; D012990:Soil Pollutants, Radioactive; D002586:Cesium
keywords: Cesium; Depth profile; Radioactive; Relaxation mass depth