SPATIAL AND TEMPORAL DYNAMICS OF RADIUM ISOTOPES IN COASTAL AQUIFERS (Invited Presentation)

Radium variability within the intertidal aquifer pose significant uncertainties in estimating submarine groundwater discharge (SGD) and associated chemical fluxes when using radium isotopes as tracers. The key issue to reduce the uncertainty is to clarify the spatial temporal characteristics of radium isotopes in the coastal aquifer. This study presents the dynamic variation of radium isotopes within a two-dimensional intertidal aquifer over tidal and seasonal scales. We found the characteristics of radium isotopes are mainly controlled the partitioning of solid-liquid phase, the groundwater residence time, tidal flushing, and the ingrowth rates of different radium isotopes. In the transition zone, radium isotopes exhibit noticeable periodic changes with tidal fluctuations, while in high salinity zone the changes within the tidal cycle are not significant but display clear seasonal variations. One-dimensional reactive transport modeling of radium isotopes and field measurements suggest that continuous flushing of the shallow intertidal aquifer leads to a lower total exchangeable radium isotope inventory in the shallow sediments compared to the deeper aquifer. The two-dimensional groundwater modeling indicates that groundwater residence time has a strong influence on the seasonal variation of radium isotopes. Calculations on the solid-liquid phase distribution of radium isotopes indicate that in the transition zone, radium isotopes are significantly influenced by salinity, whereas in high-salinity areas, the seasonal variations of radium are less affected by salinity. This study provides valuable insights into the short-term and seasonal fluctuations of radium isotopes and highlights the importance of considering multidimensional variations when analyzing SGD across different time scales.