Presentation Information
[AOS26-P13]Environmental drivers of temporal variations in groundwater discharge to the coastal region off the Tone River estuary: Insight from radium isotopes
*Toshimi Nakajima1, Eiji Masunaga2, Anne Takahashi1, Takashi Murakami1, Shigeyoshi Otosaka1, Naoki Furuichi3, Ryo Sugimoto4, Yu Umezawa5, Sachihiko Itoh1 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Ibaraki University, 3.Fisheries Technology Institute, Japan Fisheries Research and Education Agency, 4.Fukui Prefectural University, 5.Tokyo University of Agriculture and Technology)
Keywords:
Groundwater,Pore water,Radioisotopes,Open coast
Coastal groundwater discharge, including fresh and saline groundwater, is recognized as an important phenomenon for delivering water and nutrients to coastal seas. While groundwater discharge is driven by various forcing effects, the environmental drivers of long-term variations beyond the seasonal-scale remain poorly understood. In this study, we evaluated the temporal variations of radium (Ra) isotopes as a tracer of groundwater and their driving factors in the open coastal waters near the Tone River estuary based on monthly oceanographic surveys conducted over a period of more than one year.
Throughout the study period, short-lived Ra (223Ra, 224Ra) concentrations in seawater are higher than those in river water, indicating a consistent Ra supply from coastal sediments at the land-ocean interface. Temporal variations showed subtle peaks in early spring and autumn, with lower concentrations during winter and summer. Correlation analysis between short-lived Ra concentrations in surface seawater and multiple environmental factors revealed no significant relationship with tidal range or terrestrial groundwater levels. In contrast, short-lived Ra concentrations in surface waters were positively correlated with significant wave height and onshore wind speed. These results suggest that in the open coastal environment of the Tone River estuary, waves generated by onshore winds promote seawater recirculation within nearshore sediments, acting as a primary external driver of groundwater discharge.
Throughout the study period, short-lived Ra (223Ra, 224Ra) concentrations in seawater are higher than those in river water, indicating a consistent Ra supply from coastal sediments at the land-ocean interface. Temporal variations showed subtle peaks in early spring and autumn, with lower concentrations during winter and summer. Correlation analysis between short-lived Ra concentrations in surface seawater and multiple environmental factors revealed no significant relationship with tidal range or terrestrial groundwater levels. In contrast, short-lived Ra concentrations in surface waters were positively correlated with significant wave height and onshore wind speed. These results suggest that in the open coastal environment of the Tone River estuary, waves generated by onshore winds promote seawater recirculation within nearshore sediments, acting as a primary external driver of groundwater discharge.