Uranium export from a sandy beach subterranean estuary in Australia

Few studies exist on the contribution of subterranean estuaries (STEs) to the oceanic uranium (U) budget. Here, we estimate the dissolved U fluxes out of a quartz sand STE located on the east coast of Australia. Our results indicate that the advective flow of seawater in permeable sands enhances cycling of U in the STE. Dissolved U concentrations ranged from 25 nM in the STE to an effective zero salinity end-member of 3.8 nM in the surface estuary. The dissolved U (salinity corrected) concentrations were positively correlated to Fe (r2 = 0.49 p < 0.001) during a shallow beach time series experiment. These results indicate that reductive dissolution of Fe oxides may be an important process maintaining high concentrations of U in shallow permeable sand STEs. The U export rates from the STE to the surface estuary in this study were estimated to be 1.8 μmol U m−2 day−1 based on shallow saline groundwater exchange pathways and 0.4 μmol U m−2 day−1 based on deep fresh submarine groundwater discharge (SGD). Uranium's behavior in STEs is diverse and site specific. Out of the seven investigations available here and in the literature, three suggested a SGD-derived U source to the coastal ocean, while four suggested a U sink within STEs removing seawater U. Therefore, it remains unclear whether SGD is a source or sink of U to the ocean and additional investigations in contrasting settings are required to resolve the global contribution of SGD to the marine U cycle.