Seasonality of submarine groundwater discharge pathways in a coastal lagoon revealed by radium isotopes: the importance of porewater exchange in summer

• Coupling a hydrogeological model to a Ra mass balance. • Simultaneous estimation of three SGD pathways. • Recirculated SGD dominates water flows with strong seasonal variation. • PEX rates increase one order of magnitude in summer. • PEX emerges as a major source of nutrients, potentially driving eutrophication. Chemical fluxes supplied by Submarine Groundwater Discharge (SGD) can be relevant for coastal ecosystems, particularly lagoonal environments. An appropriate understanding of SGD requires constraining the magnitude and composition of its different pathways. However, the evaluation of SGD pathways is often limited to independent estimations, lacking a comprehensive understanding of the groundwater processes at play. In this study, we combined a Ra mass balance and hydrogeological modeling to simultaneously estimate different SGD pathways flowing into a coastal lagoon. We assessed water fluxes derived from fresh SGD, long-scale recirculation of seawater, and porewater exchange (PEX) into Mar Menor, a highly anthropized coastal lagoon. The evaluation conducted in multiple monthly samplings (n = 17) in 2021–2022 revealed that recirculation pathways accounted for >96 % of total SGD. The magnitude of fresh groundwater discharge ranged from 5.4x10 3 to 130x10 3 m 3 d -1 , long-scale recirculation ranged from 19x10 5 to 25x10 5 m 3 d -1 , and PEX ranged from 4.8x10 5 to 41x10 5 m 3 d -1 . PEX water inputs were one order of magnitude higher in summer compared to winter. The seasonality observed in summer is likely influenced by high temperature accelerating infaunal activity within the sediments, enhancing bioirrigation across the lagoon. A preliminary estimation of PEX-derived nutrient release from the sediment to the water column is estimated to be comparable to or exceed all other nutrient sources. This could be contributing to eutrophication processes occurring in this lagoon, particularly in summer. Resolving the PEX contribution is critical for predicting water quality and implementing new management strategies in lagoonal ecosystems.