Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf, China

In certain regions, submarine groundwater discharge (SGD) into the ocean plays a significant role in coastal material fluxes and their biogeochemical cycle; therefore, the impact of SGD on the ecosystem cannot be ignored. In this study, SGD was estimated using naturally occurring radium isotopes (223Ra and 224Ra) in a subtropical estuary along the Beibu Gulf, China. The results showed that the Ra activities of submarine groundwater were approximately 10 times higher than those of surface water. By assuming a steady state and using an Ra mass balance model, the SGD flux in May 2018 was estimated to be 5.98×106 m3/d and 3.60×106 m3/d based on 224Ra and 223Ra, respectively. At the same time, the activities of Ra isotopes fluctuated within a tidal cycle; that is, a lower activity was observed at high tide and a higher activity was seen at low tide. Based on these variations, the average tidal pumping fluxes of SGD were 1.15×106 m3/d and 2.44×106 m3/d with 224Ra and 223Ra, respectively. Tidal-driven SGD accounts for 24%–51% of the total SGD. Therefore, tidal pumping is an important driving force of the SGD in the Dafengjiang River (DFJR) Estuary. Furthermore, the SGD of the DFJR Estuary in the coastal zone contributes significantly to the seawater composition of the Beibu Gulf and the material exchange between land and sea.