BOOK OF ABSTRACTS 251 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Hall A Poster Session / Environment, Agriculture, and Forestry P4.21 Characterization of Solid Organic Matter in Sedimentary Rocks from Horonobe URL and Its Implications for Radionuclide Migration Zezu Chen*, Huiyun Xue1, Kanako Toda2, Takumi Saito2 1 Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN 2 Nuclear Professional School, School of Engineering, The University of Tokyo, 2-22 Shirakatashirane, Tokai-mura, Naka-gun, Ibaraki 319-1188, JAPAN * chenzezu@g.ecc.u-tokyo.ac.jp The role of Solid Organic Matter (SOM) in the geological disposal of high-level radioactive waste (HLW) is a critical yet under-researched area. In sedimentary formations, SOM constitutes a significant fraction of the total organic carbon, potentially acting as a long-term sink or a secondary source for radionuclides through complexation and redox reactions. In the context of geological disposal in deep sedimentary formations, understanding the chemical speciation of key long-lived radionuclides is essential for constructing reliable safety assessment models [1]. However, the molecular characterization of SOM is frequently impeded by its inherent structural complexity and the high susceptibility of lipid or plasticizer contamination during extraction and filtration. To address these challenges, this study investigates sedimentary rock samples from the Horonobe Underground Research Laboratory (URL), focusing on the siliceous mudstones of the Wakkanai and Koetoi formations. While dissolved organic matter (DOM) in groundwater has been characterized, the SOM embedded within these rocks has primarily been studied through bulk analytical methods, leaving its molecular-level heterogeneity and chemical role in radionuclide sequestration largely unexplored. This research establishes a robust analytical framework to characterize the molecular composition and heterogeneity of the target SOM. The innovation of this study lies in its focus on the immobile organic phase, which governs the long-term geochemical stability of the repository, moving beyond the traditional emphasis on mobile organic fractions. To study the complex SOM in these samples, a systematic approach was adopted, beginning with optimized Soxhlet extraction using high-purity methanol to maximize recovery while minimizing external contamination (Fig. 1). This is followed by high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) to achieve molecular-level identification, a technique proven effective in deciphering the
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