Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean

Current Submarine Groundwater Discharge (SGD) studies are commonly conducted under aquifer baseflow conditions, neglecting the influence of episodic events that can significantly increase the supply of nutrients and water. This limits our understanding of the social, biogeochemical, and ecological impacts of SGD. In this study, we evaluated the influence of an extreme precipitation event (EPE) on the magnitude of both the terrestrial and marine components of SGD. To do so, three seawater sampling campaigns were performed at a Mediterranean ephemeral stream-dominated basin after an extreme precipitation event (~90 mm in few hours) and in baseflow conditions. Results indicate that the groundwater flows of terrestrial and marine SGD after the extreme precipitation event were 1 order of magnitude higher than those in baseflow conditions. SGD induced by extreme precipitation events, which only take place a few days per year, represented up to one third of the annual discharge of groundwater and associated nutrients at the study site. This work accentuates the need to account for episodic increases in the supply of water and nutrients when aiming at providing reliable annual SGD estimates, particularly in the current context of climate change, since the occurrence of such events is expected to increase worldwide. This work was partly funded by the projects PID2019-110212RB- C22, CGL2016-77122-C2-1-R/2-R and PID2019-110311RB-C21 of the Spanish Government and the project TerraMar ACA210/18/00007 of the Catalan Water Agency. The authors want to express their thanks for the support of the Generalitat de Catalunya for MERS (2017 SGR-1588) and GHS (2017 SGR 1485) for additional funding. The authors would like to thank Maravillas Abad from ICM-CSIC for the analysis of nutrients. M. Diego-Feliu acknowledges the economic support from the FI-2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). V. Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya autonomous government (2019-BP-00241). A. Alorda-Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015‐440 0552‐17‐1) and PhD fellowship, BES‐2017‐080740. Albert Folch is a Serra Hunter Fellow. We would like to thank all colleagues from the Grup de Recerca en Radioactivitat Ambiental de Barcelona - GRAB (Universitat Autònoma de Barcelona). We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme for the construction of the research site.