Extreme precipitation events induce high fluxes of groundwater and associated nutrients to the 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.Peer ReviewedPostprint (published version

The social implications of Submarine Groundwater Discharge from an Ecosystem Services perspective: A systematic review

Altres ajuts: Acord transformatiu CRUE-CSICUnidad de excelencia María de Maeztu CEX2019-000940-MSubmarine 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

High spatial heterogeneity and low connectivity of bacterial communities along a Mediterranean subterranean estuary

Unidad de excelencia María de Maeztu CEX2019-000940-MSubterranean 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

Hydrodynamics and hydrogeochemical changes in the mixing zone of a coastal aquifer during a heavy rain event

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.Peer reviewe

Improving the Use of Radium Isotopes and Radon as Tracers of Submarine Groundwater Discharge

Els traçadors naturals són substàncies que es troben presents al medi i que representen una eina central en diferents disciplines científiques, proporcionant informació sobre processos i sistemes. En hidrologia i oceanografia, aquests objectes científics han esdevingut fonamentals per entendre processos invisibles o amb escales temporals que difereixen de la humana. La descàrrega submarina d'aigües subterrànies (SGD, per les seves sigles en anglès) és un procés que ha estat estretament relacionat amb l'ús de traçadors naturals. El procés, que consisteix en la descàrrega d'aigua d'origen terrestre o marí dels aqüífers costaners a l'oceà, ha sigut reconegut com un procés crucial modulant els cicles biogeoquímics costaners, controlant els ecosistemes marins i proporcionant multitud de serveis ecosistèmics a la societat. Els traçadors radioactius dels isòtops de radi i radó representen la tècnica més utilitzada per avaluar la magnitud i les implicacions d'aquest procés en una gran varietat de sistemes costaners, des de petites cales a oceans sencers. No obstant això, l'ús d'aquests radionúclids com a traçadors de la SGD és complex i requereix un coneixement profund en tots els passos necessaris per a la quantificació de la SGD mitjançant aquesta tècnica. Aquesta tesi doctoral explora l'ús dels isòtops de radi i radó com a traçadors de la SGD analitzant algunes de les mancances que presenta l'aplicació d'aquesta tècnica, com ara les relacionades amb (1) les tècniques analítiques per mesurar i quantificar aquests radionúclids, (2) el seu comportament geoquímic en sistemes hidrològics i (3) les seves aplicacions hidrològiques i oceanogràfiques. En relació amb les tècniques analítiques, aquesta tesi inclou una avaluació de la sistemàtica de quantificació del sistema RaDeCC, el detector per quantificar isòtops de radi de vida curta més emprat, aportant límits i pautes de quantificació. Aquesta tesi també presenta una anàlisi detallada del comportament dels isòtops de radi i radó en aigües subterrànies mitjançant el desenvolupament d'un nou model de transport de radionúclids. El model permet l'ús d'aquests traçadors per identificar diferents components de la SGD, delimitar la concentració d'aquests traçadors en l'aigua que descarrega al mar, i avaluar les característiques de l'aigua subterrània (per ex., velocitat, temps de trànsit). Finalment, aquesta dissertació presenta un dels primers treballs avaluant les variacions de la SGD induïts per esdeveniments de precipitació extrema utilitzant els isòtops de radi com a traçadors. Aquest treball emfatitza la importància que aquests esdeveniments episòdics poden tenir pels ecosistemes costaners, la seva rellevància relativa per les estimes anuals de la SGD i les seves implicacions en escenaris futurs de canvi climàtic. Amb tot, els treballs presentats en aquesta tesi doctoral contribueixen a millorar el coneixement actual sobre l'ús dels isòtops de radi i radó com a traçadors de processos ambientals i sobre la magnitud i implicacions de la descàrrega submarina d'aigua subterrània.Los trazadores naturales son sustancias que se encuentran presentes en el medio y que representan una herramienta central en diferentes disciplinas científicas, proporcionando información sobre procesos y sistemas. En hidrología y oceanografía, estos objetos científicos han sido fundamentales para entender procesos invisibles o con escalas temporales que difieren de la humana. La descarga submarina de aguas subterráneas (SGD, por sus siglas en inglés) es un proceso que ha estado estrechamente relacionado con el uso de trazadores naturales. El proceso, que consiste en la descarga de agua de origen terrestre o marino desde los acuíferos costeros al océano, ha sido reconocido como un proceso crucial modulando los ciclos biogeoquímicos costeros, controlando los ecosistemas marinos y proporcionando multitud de servicios ecosistémicos a la sociedad. Los trazadores radioactivos de los isótopos de radio y radón representan la técnica más usada para evaluar la magnitud y las implicaciones de este proceso en una gran variedad de sistemas costeros, des de pequeñas calas a océanos enteros. Sin embargo, la utilización de estos radionúclidos como a trazadores de la SGD es complejo i requiere un conocimiento profundo en todos los pasos necesarios para la cuantificación de la SGD mediante esta técnica. Esta tesis doctoral explora la utilización de los isótopos de radio y radón como trazadores de la SGD analizando algunas de las carencias que presenta la aplicación de esta técnica, como las relacionadas con (1) las técnicas analíticas para medir y cuantificar estos radionúclidos, (2) su comportamiento geoquímico en sistemas hidrológicos y (3) sus aplicaciones hidrológicas y oceanográficas. En relación con las técnicas analíticas, esta tesis incluye una evaluación de la sistemática de cuantificación del sistema RaDeCC, el detector para cuantificar isótopos de radio de vida corta más usado, aportando límites y pautas de cuantificación. Esta tesis también presenta un análisis detallado del comportamiento de los isótopos de radio y radón en aguas subterráneas mediante el desarrollo de un nuevo modelo de transporte de radionúclidos. El modelo permite la utilización de estos trazadores para identificar diferentes componentes de la SGD, delimitar la concentración de los trazadores en agua que descarga al mar, i evaluar las características del agua subterránea (por ej., velocidad, tiempo de tránsito). Finalmente, esta disertación presenta uno de los primeros trabajos evaluando las variaciones de la SGD inducidas por eventos de precipitación extrema empleando los isótopos de radio como trazadores. Este trabajo enfatiza la importancia que estos eventos episódicos pueden tener para los ecosistemas costeros, su relevancia relativa para las estimas anuales de la SGD y sus implicaciones en escenarios futuros de cambio climático. En general, los trabajos presentados en esta tesis contribuyen a mejorar el conocimiento actual tanto sobre el uso de isótopos de radio y radón como trazadores de procesos ambientales, como sobre la magnitud e implicaciones de las descargas submarinas de aguas subterráneas.Natural tracers - any substance present in the environment in small but measurable amounts - are a central tool in many scientific disciplines, providing information about processes and systems. In hydrology and oceanography, these scientific objects have become fundamental for understanding processes that are invisible or have disparate temporal scales, some of them differing greatly from the human time scale. One of the research topics that has been most closely linked to the use of natural tracers is the assessment of submarine groundwater discharge (SGD). This process, which involves the discharge of terrestrial and marine groundwater from coastal aquifers to the coastal ocean, has been recognized as an important process modulating the chemical budgets of the coastal ocean, controlling coastal ecosystems, and providing significant ecosystem services to society. The radioactive tracers of radium isotopes and radon represent the most extensive and widespread tool for investigating the magnitude and implications of this process in a wide variety of environments, from small coves to the entire ocean. However, reporting SGD estimates by means of these tracers is complex and requires profound knowledge regarding fundamental steps in the process of quantifying SGD using these radionuclides, from the tracer measurement techniques to the estimation of groundwater and solute fluxes. This Thesis explores the use of radium isotopes and radon as tracers of SGD by addressing a set of research gaps dealing with (1) the analytical techniques for measuring and quantifying these radionuclides, (2) their geochemical behavior in groundwater systems, and (3) their applications as tracers for both groundwater systems and the coastal ocean. Regarding the analytical techniques, this Thesis includes an assessment of the quantification systematics of the RaDeCC system, the most widely used counter for quantifying short-lived Ra isotopes, providing quantification limits and guidelines. The work represents a significant advance in pursuing better and more precise SGD estimates, as well as for any hydrological and oceanographic application of these tracers. Additionally, the Thesis presents a comprehensive analysis of the behavior of Ra isotopes and Rn in groundwater through a novel transport model of radionuclides. This model enables the use of these tracers for identifying SGD pathways, constraining the tracer concentration in the discharging groundwater, and evaluating the groundwater flow characteristics. Finally, this dissertation presents one of the first works evaluating the variations of SGD and associated nutrient fluxes induced by extreme precipitation events using Ra isotopes as tracers. The work emphasizes the relevance that these episodic events may have for coastal ecosystems, their relative significance for annual SGD estimates, and their implications in future climate change scenarios. Overall, the works presented in this Thesis contribute to improving the current knowledge both about the use of radium isotopes and radon as tracers of environmental processes and about the magnitude and implications of submarine groundwater discharge

Groundwater discharge as a driver of methane emissions from Arctic lakes

Lateral CH4 inputs to Arctic lakes through groundwater discharge could be substantial and constitute an important pathway that links CH4 production in thawing permafrost to atmospheric emissions via lakes. Yet, groundwater CH4 inputs and associated drivers are hitherto poorly constrained because their dynamics and spatial variability are largely unknown. Here, we unravel the important role and drivers of groundwater discharge for CH4 emissions from Arctic lakes. Spatial patterns across lakes suggest groundwater inflows are primarily related to lake depth and wetland cover. Groundwater CH4 inputs to lakes are higher in summer than in autumn and are influenced by hydrological (groundwater recharge) and biological drivers (CH4 production). This information on the spatial and temporal patterns on groundwater discharge at high northern latitudes is critical for predicting lake CH4 emissions in the warming Arctic, as rising temperatures, increasing precipitation, and permafrost thawing may further exacerbate groundwater CH4 inputs to lakes. CH4 inputs to Arctic lakes via groundwater discharge are an important pathway that links CH4 production in thawing permafrost to emission via lakes. Here the authors unravel the role and drivers of groundwater inflows for CH4 emissions from Arctic lakes.Funding Agencies|Umea University</p

Remobilization of dissolved metals from a coastal mine tailing deposit driven by groundwater discharge and porewater exchange

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)

Hydrodynamics and hydrogeochemical changes in the mixing zone of a coastal aquifer during a heavy rain event

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Facing geological heterogeneity impact on reciprocal coastal systems

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Combining fiber optic DTS, cross-hole ERT and time-lapse induction logging to characterize and monitor a coastal aquifer

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand submarine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and compare different methods of characterization and monitoring, a new experimental site has been constructed in a coastal alluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from the seashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and 25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed monitoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatial scales before and after the dry season of 2015. At the site scale, fibre optic distributed temperature sensing (FO-DTS), for the first time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has been applied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in a repeated manner (“time lapse”), but also for the first time at relatively high frequency (1 sample every 10 min). CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data obtained from piezometers. The combination of techniques has allowed improving the understanding of the system by: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3) identifying preferential flow paths over different time and spatial intervals. Future challenges and the application 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.Peer ReviewedPostprint (author's final draft