Spatial Characteristics Reveal the Reactive Transport of Radium Isotopes (224Ra, 223Ra, and 228Ra) in an Intertidal Aquifer

This study presents high-resolution two-dimensional distributions of radium isotopes in the shallow intertidal aquifer of Tolo Harbor, Hong Kong, illustrating the importance of salinity, groundwater residence time, and tidal flushing and mixing in controlling radium behavior in the system. The activities of radium isotopes are low in the fresh groundwater zone with activities increasing in the transition zone (1 < salinity < 25) due to desorption of radium from sediment surface coatings. In the high salinity zone (salinity > 25), the activities of radium isotopes increase with depth due to tidal flushing and mixing of the recirculating seawater in the shallow aquifer as well as increasing residence time with depth. The dissolved radium isotopes are identified to be in disequilibrium in shallow intertidal aquifer based on radium isotopic ratios and observed depletion of dissolved radium isotopes compared to the theoretical mixing line between the equilibrium activity and fresh groundwater. The influence of continuous tidal flushing of the shallow intertidal aquifer on radium is revealed, and shallow sediments are observed to have less total exchangeable radium isotopes than deeper in the aquifer. A new one-dimensional reactive transport model that considers a depth-dependent production rate is applied to simulate the vertical distribution of radium isotopes in the intertidal aquifer as seawater infiltrates the beach. Using least squares fitting of the model to field data, the vertical infiltration seepage velocity is estimated to be similar to 0.5 m/day, and the dimensionless adsorption partition coefficient of radium isotopes (K) is 350. This agrees well with K values (202-365) calculated using an adsorption/desorption model as well as measured values (266 in February and 205 in April). Based on the spatial distribution of radium isotopes and flow patterns in the intertidal aquifer, groundwater in Ra disequilibrium zone is recommended as the endmember for tide- or wave-driven submarine groundwater discharge (SGD) that has shallow flow paths, and groundwater in Ra equilibrium zone is recommended as endmember for seasonally driven or density-driven SGD that has deep flow paths.