[A-CG39] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG39-P02] Characteristics and distribution of radium isotopes in Ise Bay: Examination as a land-derived water tracer

*Yuitsu Tochio¹, Tomohiko Kobayashi², Ryo Sugimoto¹ (1.Fukui prefectural university, 2.Mie Prefecture Fisheries Research Institute)

There are two pathways for water supplied from land to coastal seas: river water and groundwater. Groundwater is also divided into two types: fresh groundwater, which contains little or no salt, and recirculated saline groundwater, which is discharged back into the ocean after seawater has been submerged under sandy beaches. Recirculated saline groundwater (RSGD) supplies a massive amount of dissolved materials into coastal seas, and thus, understanding the role of RSGD has been the focus of recent attention. Radium isotopes (²²³Ra,²²⁴Ra,²²⁶Ra,²²⁸Ra) are effective dissolved tracers when assessing the inflow of water and materials from land areas. Radium isotopes released into seawater exhibit conservative behavior, unaffected by biological or chemical actions. The activity of radium isotopes decreases depending on the half-life of each isotope, and the ratio of these isotopes can be used as a tracer by itself, and the radium age (≒ water mass age) can also be calculated by using the difference in half-lives. In this study, we took surface seawaters, river waters, and sandy groundwater in Ise Bay were sampled in September and October 2022 and May and August 2023, and measured four radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra). At the presentation, the distribution characteristics of radium and its usefulness as a tracer will be discussed.