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Fresh and saline submarine groundwater discharge in a large coastal inlet affected by seasonal upwelling

Submarine groundwater discharge is recognized as a major source of chemicals to the global ocean, exerting large control over coastal water composition. Radon and 226Ra are used to evaluate, for the first time, the occurrence and magnitude of submarine groundwater discharge in the Ría de Vigo, a large, highly productive embayment affected by seasonal, wind‐driven upwelling. The system is naturally enriched in 222Rn due to the regional granitic basement geology: high 222Rn activities (up to 106 Bq m−3) are detected in wells and boreholes in the drainage basin of the embayment. High 222Rn activities (>400 Bq m−3) are also measured in certain areas of the embayment. Comparatively lower 226Ra activities (<4 103 Bq m−3) were measured in the freshwater sources to the bay. Mass balances obtained with a box model are used to perform a volumetric estimate of fresh and saline submarine groundwater discharge in the Ría de Vigo under contrasting circulation patterns. Fresh groundwater is shown to be a relevant hydrological component of the Ria de Vigo water balance, equivalent to 9% ± 4% and 23% ± 9% of the volume discharged by tributary rivers during winter and summer, respectively. On the other hand, recirculation of seawater through permeable sediments is capable of filtering the entire upper volume of the Ria de Vigo through its seafloor in <100 days and might thus be a previously overlooked major source of regenerated solutes to the system.