[M-AG34] ラジオアイソトープ移行：福島原発事故環境動態研究の新展開

[MAG34-02] High-resolution reconstruction of sediment and radiocesium source contributions in a lake draining an early decontaminated catchment impacted by the main Fukushima radioactive pollution plume.

*Thomas Chalaux Clergue¹、Seiji Hayashi²、Hideki Tsuji²、Yoshifumi Wakiyama³、Sylvain Huon⁴、Anthony Foucher¹、Pierre-Alexis Chaboche^3,5、Atsushi Nakao⁶、Olivier Evrard¹ (1.French Atomic Energie Commission - Univ. Paris-Saclay、2.National Inst. for Environmental Science、3.Inst. of Environmental Radioactivity of Fukushima Univ.、4.Paris Inst. of Ecology and Environmental Sciences、5.Japan Society for the Promotion of Science、6.Kyoto Pref. Univ.)

キーワード：Sediment fingerprinting, Soil erosion, Source-to-sink, Fukushima, 137Cs, environmental DNA

Significant quantities of radionuclides including a majority of ¹³⁷Cs have been deposited onto Fukushima landscapes following the accident of Fukushima Dai-ichi Power Plant in March 2011. Starting from late 2013, the Japanese authorities initiated a large-scale decontamination programme in cultivated and residential areas. The surface layer of soil (about 5 cm) concentrating radiocesium (¹³⁴Cs, ¹³⁷Cs) was removed and replaced with a saprolite layer mined at local quarries. Twelve years after the accident, questions remain regarding the fate of particle-bound ¹³⁷Cs across terrestrial environments in response to heavy rainfall and the associated erosion events. In particular, there is a need to identify and quantify the sources delivering sediment and associated ¹³⁷Cs to the water bodies, in order to evaluate the impact of decontamination policy on sediment and radiocesium transfers. To conduct this project, two sediment cores were collected in June 2021 at midstream and downstream locations of the Mano Dam reservoir, also referred to as Hayama lake, draining an early decontaminated catchment (2014 - 2016). Elemental geochemistry, organic matter and stable isotopes, visible colorimetry, particle size, and radiocesium isotope spectrometry analyses were conducted at a very high resolution, with depth increments of 1 cm, on both sediment cores. In addition, 17 samples were collected for environmental DNA (eDNA) analyses on the midstream core. These analyses were used to provide multiple lines of evidence to define and interpret the flood event sequence recorded by the sediment deposits, and rainfall records were used for relative dating. The sediment tracing approach allows to identify changes in sediment sources, with variable contributions from forest, cropland, and subsoil (i.e. channel bank collapse and landslides) throughout time. Following the Fukushima accident, during the abandonment period (2011 - 2014), there was a sharp decrease in cropland contribution. However, as soon as the decontamination work began (2014 - 2016), cropland contribution started increasing until it returned again to the pre-accident level during the remediation phase (2018 - 2019). Finally, we investigate the added value of eDNA measurements to study the temporal evolution of biological communities in response to these changes.