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

Abstract

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 5.4x103 to 130x103 m3 d-1, long-scale recirculation ranged from 19x105 to 25x105 m3 d-1, and PEX ranged from 4.8x105 to 41x105 m3 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.

This work was funded by the Agencia Estatal de Investigaci´on (AEI) of the Spanish Ministry of Science and Innovation through the OPAL project (PID2019-110311RB-CXX) and the WINDERS project (TED2021-130710B-I00). This work contributes to the María de Maeztu Pro gramme (CEX2019-000940-M) for Units of Excellence of the Spanish Ministry of Science and Innovation. J.R-P acknowledges financial port from FPU grant FPU20/01369 and support from Marine and Environmental Biogeosciences Research Group-MERS fundings from the Direcció General de Recerca from Generalitat de Catalunya (2021-SGR 00640). M.D-F acknowledges financial support from grant JDC2022- 050316-I funded by MCIN/AEI/https://doi.org/10.13039/ 501100011033and by the European Union – NextGenerationEU/ PRTR and the support from the Grup d’Hidrologia Subterrània – GHS (Universitat Politècnica de Catalunya) fundings from the Direcció Gen eral de Recerca from Generalitat de Catalunya (2021-SGR 00609). I.A-M acknowledges financial support from FPI grant PRE2020-092343. This study also formed part of the THINKINAZUL programme and was ported by MICIU with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Fundación Séneca with funding from Comunidad Autónoma Región de Murcia (CARM) with additional funds also vided by the D.G. Mar Menor of the Regional Government of Murcia.

Peer Reviewed

Article signat per 11 autors/es: Júlia Rodriguez-Puig a,b,* ; Valentí Rodellas b,* ; Marc Diego-Feliu c , Andres Alcolea d ; Joaquín Jiménez-Martínez e,f ; Irene Alorda-Montiel b,a ; Aaron Alorda-Kleinglass b,g ; Francisco Pereira h,i ; Marisol Manzano j ; Javier Gilabert h ; Jordi Garcia-Orellana a,b,i.

a Institut de Ci`encia i Tecnologia Ambientals, Universitat Aut`onoma de Barcelona, Bellaterra 08193, Spain b Departament de Física, Universitat Aut`onoma de Barcelona, Bellaterra 08193, Spain c Department of Civil and Environmental Engineering, Universitat Polit`ecnica de Catalunya, Barcelona 08034, Spain d HydroGeoModels AG, Seuzach 8472, Switzerland e Department of Water Resources and Drinking Water, EAWAG, Dübendorf 8600, Switzerland f Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, Zürich 8093, Switzerland g Department of Ocean & Earth Sciences, Old Dominion University, 23529 Norfolk, VA, USA h Department of Chemical and Environmental Engineering, Universidad Polit´ecnica de Cartagena, Cartagena 30203, Spain i Centre for Environmental and Marine Studies, Physics Department, University of Aveiro, Aveiro 3810-193, Portugal j Department of Mining and Civil Engineering, Universidad Polit´ecnica de Cartagena, Cartagena 30203, Spain

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