[M-AG33] Basic study on the estimation of the "Black Rain" area caused by the atomic bomb

[MAG33-P04] Results of soil survey for estimation of the "Black Rain" area (Vertical profiles of radionuclides in surface soil)

*Satoshi FUKUTANI¹, Hiroshi YASHIMA¹, Koichi TAKAMIYA¹, Naoki TOE¹, Tomonori ADACHI², Satoru ENDO³, Naoko FUKUDA⁴, Naoki MATSUDA⁴, Yuichi KURIHARA⁵, Yayoi INOMATA⁶, Yasuhito IGARASHI¹, WG Meteorology and Soil (1.Kyoto University, 2.ATOX Co., Ltd., 3.Hiroshima University, 4.Nagasaki University, 5.Nagaoka University of Technology, 6.Kanazawa University)

Keywords:Atomic bomb, Black Rain, Distribution in Soil

[Introduction] In January 2021, we started a research project to accurately estimate the deposition area of radioactive fallout from the atomic bombings of Hiroshima and Nagasaki in 1945, the so-called "Black Rain". The research consists of two main components: the area reconstruction calculation by meteorological model simulation and the soil survey to obtain the validation data (Igarashi et al., 2024 JpGU this session). The soil profiles of the two types of products were determined based on the profiles of the time-dependent accumulation of ¹³⁷Cs, a fission product from A-bomb and atmospheric nuclear tests (GF), and ²¹⁰Pb (referred to as excess ²¹⁰Pb to distinguish it from ²¹⁰Pb originally contained in the soil), which is constantly deposited and accumulated as a decay product of ²²²Rn generated from the soil (Yashima et al., 2024JpGU this session). This presentation will present a listing of specific data.
[Methods] Soil samples were collected from sites estimated to be unaltered since 1945, using a 5 km grid in Hiroshima and a 2.5 km grid in Nagasaki, taking into account the topography. We aim to confirm traces of the atomic bomb as close-in fallout (CiF) from the depth-directed distribution of ¹³⁷Cs preserved in the stratigraphy of the surface layer of 30 cm collected from shrine pavilions and ruins of old castles, etc. In other words, we aimed to obtain information accumulated in the time direction. We obtained concentration distribution profiles of (1) fission-derived radionuclide ¹³⁷Cs and (2) atmospheric-derived excess ²¹⁰Pb accumulated in the soil layer: a) profiles in the surface soil determined to be undisturbed in Hiroshima and Nagasaki, b) profiles that are difficult to determine (to be determined by analysis results of other indicators such as particulate carbon), and c) profiles in the soil layer determined to be undisturbed in Nagasaki. (to be determined based on the results of analysis of other indicators such as particulate carbon), and c) examples of profiles determined to be disturbed are listed.

Acknowledgments: This study was funded by the Ministry of Health, Labour and Welfare of Japan, to which we express our gratitude.