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The contribution of submarine groundwater discharge (SGD) to oceanic metal budgets is only beginning to be explored. Here, we demonstrate that biogeochemical processes in a northern Florida subterranean estuary (STE) significantly alter U and Ba concentrations entering the coastal ocean via SGD. Tidal pumping controlled the distribution of dissolved metals in shallow beach groundwater. Hourly observations of intertidal groundwaters revealed high U and low Ba concentrations at high tide as a result of seawater infiltration into the coastal aquifer. During ebb tide, U decreased and Ba increased due to freshwater dilution and, more importantly, biogeochemical reactions that removed U and added Ba to solution. U removal was apparently a result of precipitation following the reduction of U(VI) to U(IV). A significant correlation between Ba and dissolved organic carbon (DOC) in shallow beach groundwaters …