Utility of Radium Isotopes for Evaluating Residence Time and Submarine Groundwater Discharge

It's very difficult to measure submarine groundwater discharge (SGD) directly, but naturally occurring radium isotopes and radon tracers make it possible to estimate SGD fluxes indirectly. In order to evaluate the residence time of seawater and SGD fluxes in Wuyuan Bay, Xiamen, China, we measured the ²²⁴Ra and ²²⁶Ra activities in the bay water, open ocean seawater and groundwater, calculated the water parcel ages and average residence time in the Bay based on the differences in half-life between ²²⁴Ra and ²²⁶Ra, and estimated the SGD fluxes via the mass balance on ²²⁴Ra and ²²⁶Ra. The water parcel ages range from 0.6 to 2.4 d among thirteen sampling stations, with an average residence time of 1.4 d. The water parcel ages in the bay head relatively get older than those in the bay mouth. The calculated ²²⁴Ra and ²²⁶Ra fluxes driven by SGD are 5.17×10⁶ Bq/d and 5.28×10⁶ Bq/d, respectively. Both radium fluxes are further converted into SGD fluxes by dividing the activities of groundwater end-member to be 0.21 m³/m²/d (²²⁴Ra mass balance model) and 0.23 m³/m²/d (²²⁶Ra mass balance model), respectively. The calculated results from two models are so close that their average of 0.22 m³/m²/d can be considered as SGD fluxes in Wuyuan Bay.