[U-02] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U02-01] Investigation of Groundwater Dynamics in Yoron Island, the Amami Islands, based on Radiocarbon

*Shoko Hirabayashi¹, Yusuke Yokoyama¹, Naoto Fukuyo², Toshihiro Miyajima¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo , 2.National Institute of Advanced Industrial Science and Technology)

Keywords:Radiocarbon, Groundwater dynamics, oxygen isotopes

Groundwater, the most extensive source of freshwater on Earth, is increasingly threatened by aquifer overexploitation and depletion in the 21st century. Understanding subsurface water systems is especially critical for small island aquifers, which serve as essential but vulnerable freshwater reservoirs. In particular, carbonate islands composed of sandy or gravel deposits overlying coral reef platforms present unique challenges for water resource management. To enhance regional and global insights into groundwater behavior, it is essential to improve our understanding of spatial and temporal groundwater movement through hydrogeological, geochemical, and isotopic analyses. Multi-isotope tracer methodologies have been demonstrated as powerful tools for investigating groundwater dynamics across diverse hydrogeological settings.
A recent study by Tsujino et al. (2024) provided valuable insights into groundwater dynamics on Kikai Island, demonstrating that radiocarbon (¹⁴C) effectively traces groundwater movement with a high dynamic range and increased sensitivity, offering deeper insight into the various carbon sources influencing the groundwater system.
In this study, we investigate groundwater dynamics on Yoron Island to validate the methodology proposed by Tsujino et al. (2024). Yoron Island, a carbonate island within the Amami Archipelago, is smaller and has a lower elevation than Kikai Island. Groundwater sampling has been conducted since July 2022. This presentation will introduce radiocarbon concentrations and oxygen isotope ratios (δ¹⁸O) in the groundwater, elucidating groundwater evolution along flow paths and recharge–discharge relationships.

Reference:
Tsujino et al. (2024) Groundwater dynamics on small carbonate islands: Insights from radiocarbon and stable isotopes in Kikai Island, Southwest Japan, Science of The Total Environment 921, 171049. doi: 10.1016/j.scitotenv.2024.171049.