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We describe an intermediate scale experimental field site located in a coastal alluvial aquifer at the mouth of the Argentona ephemeral stream on the Maresme coastline (Barcelona, Spain). We have been monitoring Seawater Intrusion (SWI) and Submarine Groundwater Discharge (SGD) for several years using geological (lithological description and core samples analyses), geophysical (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemical (major and minor elements), and geophysical methods (cross-hole electrical resistivity. We have found that apparently minor silt layers control the distribution of salinity, with SWI and freshwater SGD occurring at multiple layers. This multiplicity of salinity levels promotes unstable mixing, which is very active and leads to a surprising bio-geochemical activity in the mixing zone. In parallel, instability makes it hard to sample SGD and makes it clear that the traditional SWI-SGD paradigm needs to be revised.

In recent decades, there has been a growing interest in accurately characterizing the process of fresh groundwater discharge into the coastal ocean since it represents a significant pathway for solute from land to sea. Although SGD research has focused on quantifying water and solute fluxes, little is known about the physical forces, mechanisms, and distribution of SGD.Investigating interaction processes between the fresh and saltwater under the sea bottom represents a logistical and technical challenge. Traditionally, electrical geophysical methods have been solely used in the terrestrial part, and during the last decades, the method techniques have been introduced to marine environments. However, these methods are limited by their characteristics in obtaining subsurface resistivity data in the first few meters of the coast. Especially in the land-sea transition zone of microtidal environments, where many of the most critical processes take place, this data is traditionally not available. In this study, we developed a combination of methods to bridge the data gap between the terrestrial and marine realms. This study aims to characterize the FSGD in two aquifers from different geological contexts (alluvial and karstic) on the Mediterranean coast near Barcelona.To study the transition zone of the aquifer with a good spatial resolution, the amphibious electrical tomography method has been chosen, combining a terrestrial and aquatic line fixed in contact with the marine sediment. Profiles perpendicular to the coastline has been made in a shallow water area to obtain electrical resistivity data of the seabed at a local scale. The configuration used is Dipole-Dipole and Wenner-Schlumberger, with an investigation depth of 10 m. Based on the results, designing a 3D model of electrical resistivity in marine sediments was possible. In parallel to getting electrical resistivity data, manual piezometers were used to bring porewater samples at different points.We have been able to measure the presence of freshwater or brine, and we have identified differences in the geometry depending on the geological context, where the karst environment is the one that presents a more significant proportion of freshwater saturating the marine sediments. Therefore, amphibious electrical tomography is a non-invasive method that detects resistive zones of marine sediments associated with discharge processes and can be instrumental in characterizing the presence and distribution of SGD. AcknowledgmentsThis work was partly funded by the Spanish Government (grant no. PID2019-110212RB-C22) and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency.

Submarine Groundwater Discharge (SGD) is recognized as a major source of water and solutes to the coastal ocean, and it is particularly relevant in arid or semi-arid zones. SGD is generally defined as the flow of groundwater from continental margins to the coastal ocean, including thus both fresh groundwater from aquifer recharge and seawater recirculation through the coastal aquifer. Due to its high heterogeneity both in space and time, SGD is difficult to detect and quantify. As a consequence, numerous methods to study SGD have been developed over the last decades. These approaches mainly include hydrogeological approaches, geophysical techniques, direct seepage measurements, and the use of geochemical tracers. Each method presents its challenges, limitations, and advantages and each one works on different spatial and temporal scales, thus targeting different components of SGD. Therefore, comparing SGD studies with estimates derived from different methods is often complex and misleading if the characteristics and assumptions of each quantification technique are not taken into account. This highlights the need to conduct studies comparing SGD derived from different methods, not only to obtain more accurate SGD estimates but also to obtain instrumental information on the characteristics of the estimated fluxes. To this aim, a combined use of different approaches to estimate SGD was applied in a Mediterranean coastal lagoon (Mar Menor, Spain), including direct measurements with seepage meters, radium isotopes, and radon mass balance, 224Ra/228Th disequilibrium in coastal sediments, radon vertical profiles in porewater sediments, and hydrologic modeling. Mar Menor is Europe's biggest saline coastal lagoon, and it is connected to a highly anthropized quaternary aquifer. In this coastal system, SGD is likely playing a major role in the eutrophication of the lagoon. However, despite the economic and biological importance of this lagoon, data about this system is still incomplete, and mostly only hydrological modeling has been performed. Keywords: Submarine Groundwater Discharge, radioactive tracers, seepage meters, porewater exchange, hydrological modeling.

The Mar Menor coastal lagoon (Spain) is a critical ecological and socioeconomic ecosystem and the first in Europe to be granted rights of personhood. However, pollution from past and present activities such as mining, agriculture, urbanization, and tourism threatens its health and ecological stability. Previous research has shown the importance of metal contamination in the lagoon and its link to nearby mining activities, but little consideration has been given to historical changes in this industry and in other potential metal sources. In this work, metal concentrations have been analyzed in 12 sediment cores dated with 210Pb, allowing the reconstruction of the recent (last ~150 years) metal contamination in the lagoon. The main metal sources have been identified by using multivariate statistical methods. Metal contamination from mining activities (point-source pollution) peaked in the mid-20th century, whereas nonpoint-source metal contamination reached its highest level in more recent decades. Despite the current decrease in metal deposition trends, concentrations in surface sediments still exceed sediment quality and ecotoxicological thresholds in areas close to former mining sites. Therefore, they need to be considered in future management strategies, which should also include the evaluation of sources and processes that are still supplying them to the lagoon.

Submarine Groundwater Discharge (SGD) is recognized as a fundamental hydrological process that supports many coastal biogeochemical cycles and social-ecological systems. However, very little has been investigated about how SGD affects society and human well-being. Coastal services provided by ecosystems dependent on SGD can be analyzed and clustered into the four main categories of Ecosystem Services (i.e., Provisioning, Supporting, Regulating and Cultural), which are divided into subcategories defined as outcomes. This enables identifying and discussing both benefits and threats to coastal societies resulting from SGD outcomes. Due to the lack of academic literature on this matter, here we explore the academic and local knowledge of the social perception toward SGD and its ecosystem services (ES). This research is conducted through two case studies, the island of Mallorca and the Region of Salento, to unravel the similarities and particularities of each Mediterranean society regarding the SGD-ES identified and their historical evolution. Such evolution transitions from the management of the fresh groundwaters for human consumption to the exploitation by the tourism industry of cultural ecosystem services related to the same discharge. Our review also shows how compiling different search possibilities (e.g., local languages, including paper-based documents; grey literature; local knowledge; academic literature) has resulted in a significant increase in the reported ES and its understanding. In this direction, combing traditional and academic knowledge are key to accessing society's perception of most cultural ES. Therefore, SGD-ES studies are extremely locally-dependent, and thus regional or global require an in-depth understanding of all areas comprehended in the study. Overall, the research presented in this study contributes to a better understanding of the complexity of the SGD and its social implications. Therefore, this research presents to the academic community new insights from traditional knowledge and an opportunity to integrate multidisciplinarity into a study subject that has usually only been looked from the prism of natural sciences.

The use of radiotracer techniques has been a fundamental tool for characterizing fluxes of solutes and water flows into the coastal ocean driven by submarine groundwater discharge (SGD). Indeed, the scientific interest in the use of radionuclides as tracers of SGD started developing in the late 90s when high activities of Ra isotopes and 222Rn in the coastal ocean were associated with groundwater inputs. Since then, the number of articles published about SGD has considerably grown and the technical improvements in radiotracer methods have often been accompanied by concurrent scientific advances in the understanding of the process. Although current research in SGD is conducted through multiple techniques (direct measurements, hydrological, geophysical, and geochemical techniques), the use of tracers such as Ra isotopes and 222Rn continues to be the most used and widespread method. Therefore SGD estimates are likely to be highly dependent on the methodological biases associated with radiotracer techniques. The aim of this study is to evaluate the main biases and assumptions relative to the use of Ra isotopes and 222Rn in SGD studies through a meta-analysis of the published academic literature. The results of this work highlight that a significant number of SGD studies using radionuclides as tracers are based on erroneous assumptions or inaccurate calculations leading to unreliable SGD quantifications, thus preventing its use for comparison with other studies or extrapolating from local to regional-global scale. These results also emphasize that the SGD community should seek comparison, reproducibility, and multiapproach studies that help to understand the complexity of SGD in multiple sites and bridge the gap between different quantification methods.

Coastal aquifers are characterized by their unique land-sea interaction and the main difficulty in studying these systems is the complexity of their biogeochemical cycles. Important processes in the “mixing zone” of coastal systems are associated with the ecosystem’s diversity, and supply and exchange of chemical compounds. This coastal body known as a “subterranean estuary” (STE), is characterized by a free connection to the sea, creating an interface between freshwater and seawater. The distribution and composition of substances flowing from land to sea change because the submarine groundwater discharges (SGD) considerably dilute the seawater that has invaded the aquifer through the free connection to the sea (SWI, Sea Water Intrusion).These processes were studied in an area 100 m long inland from the coastline and 30 m wide, in the alluvial aquifer of Argentona, Mataró, northeast of Barcelona, Catalonia (Spain). This experimental site was established in 2015 and has been equipped for intensive monitoring of coastal aquifer processes such as SWI and SGD. Currently, it is monitored with 25 piezometers (2 meters screened) consisting of 5 nests with 4 piezometers each (at 10m, 20m, 15m, and 25m intervals) and 4 individual piezometers. In this system described as a “multi-aquifer and reactive system”, we are characterizing the physicochemical and hydrogeochemical conditions associated with biogeochemical processes at different depths and seasonal variations. For this presentation, we will show new results of hydrogeochemistry, nitrogen isotopes, and microbiological parameters associated with different periods of monitoring that characterize the dynamics in the subterranean estuary. Acknowledgments: This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22), the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency, and the SENACYT – IFARHU – BID Scholarship.

20 pages, 8 figures, 2 tables, supporting Information https://doi.org/10.1111/mec.16695.-- Data availabitiy statement: DNA sequences and associated metadata: European Nucleotide Archive (ENA) under accession numbers PRJEB52186. The environmental metadata and the nonrarefied ASV and taxonomic tables are provided as Tables S2–S4, Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatially-resolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining bulk heterotrophic activity measurements, flow cytometry, microscopy and 16S rRNA gene sequencing we find large variations in prokaryotic abundances, cell size, activity and diversity at both the horizontal and vertical scales that reflect the pronounced physicochemical gradients. The parts of the transect most influenced by freshwater were characterized by smaller cells and lower prokaryotic abundances and heterotrophic production, but some activity hotspots were found at deep low-oxygen saline groundwater sites enriched in nitrite and ammonium. Diverse, heterogeneous and highly endemic communities dominated by Proteobacteria, Patescibacteria, Desulfobacterota and Bacteroidota were observed throughout the aquifer, pointing to clearly differentiated prokaryotic niches across these transition zones and little microbial connectivity between groundwater and Mediterranean seawater habitats. Finally, experimental manipulations unveiled large increases in community heterotrophic activity driven by fast growth of some rare and site-specific groundwater Proteobacteria. Our results indicate that prokaryotic communities within subterranean estuaries are highly heterogeneous in terms of biomass, activity and diversity, suggesting that their role in transforming nutrients will also vary spatially within these terrestrial–marine transition zones, This work was funded by the Spanish Ministry of Science, Innovation and Universities (MICINN) through the GRAMMI project (RTI2018-099740-J-I00), the Ramon y Cajal contract (RYC2019-026758-I) and partly by the project MEDISTRAES III (PID2019-110311RB-C22 and PID2019-110212RB-C21). Additional financial support was provided by grants SPIP2020-02595 (Ministry for the Ecological Transition and the Demographic Challenge [MITECO]) with funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). JGO acknowledges the financial support of the Spanish Ministry of Science, Innovation and Universities, through the “Maria de Maeztu” programme for Units of Excellence (CEX2019-000940-M). VR acknowledges financial support from the Beatriu de Pinós postdoctoral programme of the Catalan Government (2019-BP-00241). AAK acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015-0552-17-1) and PhD fellowship, BES-2017-080740. The coauthor AF is a Serra Húnter Fellow. MDF acknowledges the economic support from the FI-2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). The authors would like to thank the support of the Generalitat de Catalunya to MERS (2017 SGR-1588), GHS (2017 SGR 1485) and 2017SGR/156, for additional fundin

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.

Mining impacts on coastal environments have been extensively studied around the world. However, the role of Submarine Groundwater Discharge (SGD) and Porewater Exchange (PEX) as pathways for pollutants from mining waste deposits into seawater has been largely overlooked. Portmán Bay is located in the Cartagena-La Unión Pb-Zn sulphur mining district in Murcia, SE of Spain. The disposal of about 60 million tons of metal-rich mine tailings from 1957 to 1990 led to the infill of most of the bay. Although the effects of metals on indicator organisms have been shown previously, there is a major lack of knowledge on the release of dissolved metals from the emerged tailing deposit into the sea, more than 25 years after the closure of the mining activities. Samples for Ra isotopes (Ra, Ra, Ra and Ra) and dissolved metals (Ag, Cd, Co, Pb, Zn) were analyzed in porewaters and seawater in order to separately estimate SGD and PEX driven dissolved metal fluxes. Our results show a continuous release of dissolved metals into the sea driven by both PEX and SGD. Most of dissolved metals are remobilized and released into the water column by PEX, which is a ubiquitous mechanism acting along the shoreline. Although SGD only represents 13% of the water flow, it drives large fluxes of dissolved Fe into the sea, mainly restricted to the west side of the bay. Large inputs of dissolved Fe from the anoxic tailings deposit trigger a massive precipitation of iron hydroxides that enables the removal of most dissolved metals from the water column. This study highlights the role of PEX and SGD as significant mechanisms for the land to ocean transfer of dissolved metals from coastal mine tailings deposits., We want to express our gratefulness to “Liga de Vecinos” of Portmán Bay for their help. This project has been funded by the Spanish projects NUREIEV (CTM2013-44598-R) and NUREIEVA (CTM2016-75953-C2-1-R). The authors also acknowledge support from Generalitat de Catalunya autonomous government through its funding schema to excellence research groups (grants 2017 SGR 1588, 2014 SGR 1356 and 2017 SGR 315). This work is a contribution to ICTA ‘Unit of Excellence’ (MinECo, MDM2015-0552). V Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral programme of the Catalan Government (2017-BP-00334).

The naturally occurring Ra isotopes (223Ra; T1/2 = 11.4 d, 224Ra; T1/2 = 3.66 d, 226Ra; T1/2 = 1,600 y, and 228Ra; T1/2 = 5.7 y) and Rn (222Rn; T1/2 = 3.82 d) have been widely applied as environmental tracers. The application of these radioactive tracers has mainly been restricted to the evaluation of oceanographic and land-ocean interaction processes, although in recent years their use has also been extended to the study of groundwater systems. In this context, the activity ratios of 224Ra/228Ra and 222Rn/226Ra can be instrumental in providing key information on groundwater transit times in aquifers and those processes governing groundwater discharge into the coastal sea (often referred to as Submarine Groundwater Discharge or SGD). This work evaluates the potential use of these activity ratios as proxies for investigating groundwater systems through an advective transport model that integrates the radionuclides involved in these activity ratios (224Ra, 228Ra, 226Ra, and 222Rn) and their immediate parents into a single formulation. The results provided by the transport model indicate that the main factors controlling the 224Ra/228Ra and 222Rn/226Ra activity ratios are the alpha recoil supply, the retardation factor of Ra, and the groundwater transit times. The advective transport model and the activity ratios are used to present novel applications that interrelate the disciplines of hydrogeology and coastal oceanography. The main applications include the determination of groundwater transit times and the assessment of pathways and end-members related to submarine groundwater discharge processes. These applications were tested in a Mediterranean coastal aquifer., 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 thank the support of the Generalitat de Catalunya to MERS (2017 SGR-1588) and GHS (2017 SGR 1485) for additional funding. We would like to thank all the colleagues from the Laboratori de Radioactivitat Ambiental (Universitat Autònoma de Barcelona). We would also like to thank Leon Humphries for his detailed English corrections. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. 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 (2017‐BP‐00334). A. Alorda‐Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015‐0552‐17‐1) and PhD fellowship, BES‐2017‐080740. T. Goyetche acknowledges PhD fellowship (BES‐2017‐080028) from the FPI Program by the Spanish Ministry of Economy, Industry and Competitiveness. A. Folch is a Serra Húnter Fellow.

Fluxes of nutrients, metals, contaminants, among others dissolved compounds transported from land to oceans have a direct impact on coastal biogeochemical cycles. One of these land-ocean interaction mechanisms, recently recognized as an important source of these compounds, is the flow of groundwater from continental margins to the coastal ocean, commonly known as Submarine Groundwater Discharge (SGD). The Mediterranean coastline is characterized by the presence of ephemeral streams, geomorphological settings that act as preferential flow paths for both runoff and groundwater towards the sea. The Mediterranean Basin is highly influenced by strong precipitation events (>50 mm) that commonly occur during fall and spring seasons such as isolated depression at high levels events, locally named ‘gota-freda’. In such situations, runoff causes several social and environmental impacts along ephemeral streams that have been long recognized. However, when the surface water flow ceases, the subterranean part of ephemeral streams may continue supplying water and solutes to the coastal ocean via SGD. This process and its effects for coastal ecosystems have been largely overlooked. In this study, we evaluated the influence of a ‘gota-freda’ event on both the role of ephemeral streams as preferential areas for groundwater discharge and on the magnitude of SGD and SGD-derived nutrient fluxes. To do so, three seawater sampling campaigns were performed in a Mediterranean coastal region dominated by ephemeral streams (Maresme, Catalunya) after heavy rainfall events (~50 mm) and in baseflow conditions. Results of this study indicate that SGD flows are between 5 and 7 times higher after a strong precipitation event than in baseflow conditions indicating that the supply of nutrients and other dissolved compounds to the Mediterranean Sea is highly dependent on these events. This study highlights that this mechanism is a relevant process for coastal biogeochemical cycles of semi-arid regions such as the Mediterranean basin.

Mar Menor (SE, Spain) is one of the largest coastal lagoons in the Mediterranean basin. Its ecological and economic significance has led to its inclusion in several protection programmes, both on a national and international level. In the last decades, this semiconfined habitat has been under high anthropogenic pressure from agricultural, mining and tourism activities, which have resulted in significant changes, such as eutrophic events and their cascading ecological effects. Previous research suggests that this degradation is linked to the introduction of nutrients and contaminants to this ecosystem, which are accumulated in the sediments of the lagoon. In this work, sediment cores from key locations of the Mar Menor were collected in order to estimate the amount of accumulated chemical compounds, such as metals and organic compounds. The results of this study are used to reconstruct the historical record of contaminants, which can fuel future contamination episodes in the lagoon.

Coastal environments have experienced a rapid transformation due to the expansion of tourism. This growth may enhance problems as over-saturation of spaces or environmental pollution. One of the main problems is associated with the collapse of environmental infrastructures, which may become saturated during high seasons. Indeed, wastewater treatment plants (WWTP) can be located in coastal areas delivering high concentrations of nutrient effluents into the marine environment. Alternatively, WWTP effluents are introduced into coastal aquifers via injection wells, given that the geological matrix is used to filter naturally the transported effluent solutes. However, the injection of significant amounts of WWTP effluents can modify the hydrogeological dynamics and enrich substantially the solute concentrations in groundwaters. Zones with a hydraulic connection between the coastal aquifer and the sea, these contaminated groundwaters may be transferred to coastal environments via Submarine Groundwater Discharge (SGD). Thus, SGD may act as a pathway delivering part of the WWTP-derived nutrients and pollutants into the marine environment, which may lead to eutrophication or harmful algal blooms. More importantly, such process may become threatening for society when the discharge occurs into bathing waters, affecting the ecosystem and perception of stakeholders.In this study, we evaluate the role of SGD as a conveyor of nutrients from a karstic coastal aquifer affected by the injection of WWTP effluents to the Deià cove in Mallorca (Balearic Islands). Results show that the tourism seasonality changes the coastal aquifer natural dynamics during the dry season, delivering via SGD, nutrients concentrations above the maximum limits established by the Spanish and European water framework directives. Due to those enriched nutrient fluxes, the coastal water ecosystem has registered the highest values of ∂15N in Posidonia oceanica in the Balearic Islands and suffers periodic algal blooms, creating a conflict among stakeholders.

The assessment of the biogeochemical cycles in coastal environments often relies on riverine inputs as the main source of nutrients and other dissolved compounds from land to the ocean. However, the discharge of groundwater through continental margins, commonly known as Submarine Groundwater Discharge (SGD), is also recognized as relevant sources of nutrients to the coastal ocean, particularly in oligotrophic and semi-arid environments, such as the Mediterranean Sea. In this study, we use radioactive tracers (radium isotopes and radon) to i) quantify the magnitude of SGD-driven nutrient fluxes to a Mediterranean cove (Cala Pudent, Menorca, Balearic Islands) and ii) characterize the nutrient transformations occurring in the beach before groundwater discharges to the sea. Cala Pudent is a limestone coastal cove with a restricted connection to the open sea. In this system, groundwater from a permanent spring infiltrates through an organic substrate dominated by thick deposits of seagrass (Posidonia oceanica) leaf litter and flows into the sea. This substrate, together with the dynamic groundwater-seawater mixing, are chiefly influencing the nutrient enrichment and transformation occurring in the beach and thus modulating the SGD-derived nutrient input to the sea. The ecological implications of these inputs are also assessed, particularly for the Posidonia oceanica and Cymodocea nodosa meadows located near the study site.

Altres ajuts: acords transformatius de la UAB Unidad de excelencia María de Maeztu CEX2019-000940-M Submarine Groundwater Discharge (SGD) is recognized as a fundamental hydrological process that supports many coastal biogeochemical cycles and social-ecological systems. However, very little has been investigated about how SGD affects society and, specifically, human well-being. This study systematically examines the published scientific literature on the social implications of SGD by using an Ecosystem Service (ES) perspective. Coastal services provided by ecosystems dependent on SGD are analyzed and clustered in the four main categories of Ecosystem Services (i.e., Provisioning, Supporting, Regulating and Cultural), which are in turn divided into subcategories defined as outcomes. This allows identifying and discussing both benefits and threats to coastal societies resulting from SGD outcomes. From the 1532 articles initially reviewed, the most frequently mentioned category was the supporting services (835) due to the mainstream trend in scientific literature to focus on the role of SGD as a process influencing coastal biogeochemical cycles. Conversely, cultural ES were mentioned in only 49 cases, which should not necessarily be interpreted as a lack of research or interest in this topic, but that this type of references are often not found in the scientific literature but in the grey literature. A detailed publication review was additionally conducted, identifying 114 case studies from 96 different locations worldwide that reported cases in which SGD had social implications on the well-being. Our review also shows how the different types of Ecosystem Services can have multiple synergies and trade-offs between them, resulting in unequal impacts among stakeholder groups. Overall, this study identifies research gaps related to Ecosystem Services provided by SGD as well as opportunities for further studies, while developing an analytical framework that relies on the Ecosystem Services approach to guide future research on the social implications of SGD.

International audience; The Radium Delayed Coincidence Counter (RaDeCC) is one of the most extensively used equipment for measuring 223Ra and 224Ra activities in water and sediment samples. Samples are placed in a closed He-circulation system that carries the Rn produced by the decay of Ra to a scintillation cell. Each alpha decay recorded in the cell is routed to an electronic delayed coincidence system which enables the discrimination of 223Ra and 224Ra. In this study, the measurement and quantification methods using the RaDeCC system are assessed through analyses of registered data in different RaDeCC systems worldwide and a set of simulations. Results of this work indicate that the equations used to correct for 223Ra and 224Ra cross-talk interferences are only valid for a given range of activities and ratios between isotopes. Above certain limits that are specified in this study, these corrections may significantly overestimate the quantification of 223Ra and 224Ra activities (up to ~40% and 30%, respectively), as well as the quantification of their parents 227Ac and 228Th. High activities of 226Ra may also produce an overestimation of 224Ra activities due to the buildup of 222Rn, especially when long measurements with low activities of 224Ra are performed. An improved method to quantify 226Ra activities from the buildup of 222Rn with the RaDeCC system is also developed in this study. Wethus provide a new set of guidelines for the appropriate quantification of 223Ra, 224Ra, 227Ac, 228Th, and 226Ra with the RaDeCC system.

Coastal aquifers are affected by seawater intrusion (SWI), which causes their salinization, and yield submarine groundwater discharge (SGD), which feeds marine ecosystems. Characterizing groundwater dynamics in coastal aquifers is fundamental for understanding both processes and their interaction. In order to gain insights into SWI and SGD, we developed a 100 m-scale experimental field site located in a coastal alluvial aquifer at the mouth of an ephemeral stream on the Maresme coastline (Barcelona, Spain). Given the complexity of coastal aquifers and the dynamism of the processes occurring therein, understanding of the coupled processes can be achieved by combining methods and approaches across different hydrogeological disciplines. In this study, we conduct a detailed aquifer characterization based on the four pillars of hydrogeology: geology (lithological description and core samples analyses), geophysics (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemistry (major and minor elements, together with stable and Ra isotopes). Each discipline contributed to the characterization of the aquifer: (1) geological characterization revealed that the aquifer consists of fluvial sediments, organized in fining upwards sequences with alternating layers of gravel, sand and silt; (2) geophysics helped in identifying silt layers and their continuity, which play a segmenting role in the aquifer hydrodynamics; (3) hydraulics tests, specifically tidal response tests, evidenced that tidal loading, rather than hydraulic connection to the sea, drives the tidal response; and (4) hydrochemistry revealed a surprising high reactivity, as most ions reflect some reaction, beyond the expected cation exchange. The summary is that the aquifer, which initially looked like a homogeneous unconfined aquifer 22 m thick, effectively behaves as a multi-aquifer and reactive system with freshwater discharging beneath saltwater at several depths. The fact that thin silt layers caused such a significant impact opens new paths beyond this study both for coastal aquifer management (the possibility of transient pumping for freshwater resources) and marine ecology (expect diffuse groundwater discharge)., We acknowledge the contribution of all the members of the MEDISTRAES I, II and III project, from the Groundwater Hydrology Group of the Technical University of Catalonia (UPC) and the Spanish National Research Council (CSIC), the Autonomous University of Barcelona (UAB), the University of Barcelona (UB), the Geosciences Montpellier Laboratory, CNRS and the University of Rennes, for their support during experimental laboratory tests, the Argentona site creation, the setup of the electrodes during boreholes installation and for ensuring site maintenance. The final manuscript was significantly improved by critical, but constructive comments by two anonymous reviewers. This work was funded by the project PID2019-110212RB-C21, CGL2016-77122-C2-1-R/2-R and PID2019-110311RB-C22 of the Spanish Government and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. This study contributes to the work carried out by the GHS (2017 SGR 1485) and MERS research group (2017 SGR 1588) for additional funding. 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) The co-author Albert Folch is a Serra Húnter Fellow.

International audience; Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3- was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2- and NH4+ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m3 s-1 for the entire investigated coastline, determined from simultaneous 224Ra and 226Ra mass balances. The contribution of groundwater PO43-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand sub-marine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and comparedifferent methods of characterization and monitoring, a new experimental site has been constructed in a coastalalluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from theseashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed mon-itoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatialscales before and after the dry season of 2015. At the site scale,fibre optic distributed temperature sensing (FO-DTS), for thefirst time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has beenapplied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in arepeated manner (¿time lapse¿), but also for thefirst time at relatively high frequency (1 sample every 10 min).CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data ob-tained from piezometers. The combination of techniques has allowed improving the understanding of the systemby: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3)identifying preferentialflow paths over different time and spatial intervals. Future challenges and the appli-cation of these techniques in other areas are also discussed., This work was funded by the projects CGL2013-48869-C2-1-R/2-R and CGL2016-77122-C2-1-R/2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. The authors want to thank the support of the Generalitat de Catalunya to MERS (2018 SGR-1588). This work is contributing to the ICTA ‘Unit of Excellence’ (MinECo, MDM2015-0552). Part of the funding was provided by the French network of hydrogeological observatories H+ (hplus/ore/fr/en) and the ANR project EQUIPEX CRITEX (grant ANR-11-EQPX-0011). V Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya (2017-BP-00334). M. Diego‐Feliu acknowledges the economic support from the FI‐2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). This project also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 722028.

Coastal zones are increasingly demanded spots for human settlements and economic development. In these areas, aquifers are subjected to the threat of landward entrance of seawater caused by overexploitation of fresh groundwater resources. In some contexts, a coeval groundwater flow of freshwater seaward can also take place, often linked to the surficial discharge of ephemeral streams. Understand the interactions between this two reciprocal flows in coastal aquifers is needed to be able to characterize the freshwater-saltwater interface and the effects on rate of submarine groundwater discharge, This work was funded by the project CGL2016-77122-C2-1-R/2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site.

Submarine groundwater discharge (SGD) is now recognized as an important process of the hydrological cycle worldwide and plays a major role as a conveyor of dissolved compounds to the ocean. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) are widely employed geochemical tracers in marine environments. Whilst Ra isotopes were initially predominantly applied to study open ocean processes and fluxes across the continental margins, their most common application in the marine environment has undoubtedly become the identification and quantification of SGD. This review focuses on the application of Ra isotopes as tracers of SGD and associated inputs of water and solutes to the coastal ocean. In addition, we review i) the processes controlling Ra enrichment and depletion in coastal groundwater and seawater; ii) the systematics applied to estimate SGD using Ra isotopes and iii) we summarize additional applications of Ra isotopes in groundwater and marine studies. We also provide some considerations that will help refine SGD estimates and identify the critical knowledge gaps and research needs related to the current use of Ra isotopes as SGD tracers.

The coastal aquifers are the place where meet fresh water and seawater. This confluence between both of them is called mixing zone which is in equilibrium due to the different density of the two masses and where various biogeochemical reactions takes place. We know that this equilibrium can be canceled during an important recharge event. Within the aquifer, such events displace the mixing zone over a short time period. This study is aims at the identification and description at high frequency of spatial and temporal response of the mixing zone and its impact on geochemical processes during one such fast water inflow event. We choose the experimental site of Argentona as the place of this study. Located in the northeast of Spain this site is subjected to a Mediterranean climate characterized by heavy rainfall amount recorded during Fall and early Spring period, concentrated during a few hours. Furthermore, this site is located on an alluvial aquifer, equipped with 16 shallow boreholes over a 100 m scale (Fig.1). The geological cross section presented in Fig.2 shows that the aquifer is multilayered and we suspect an effect of a clay/silt layer located at around 12 m., This work was funded by the projects CGL2013-48869-C2-1 y CGL2013-48869-C2-2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site.