Tracing spatial patterns of lacustrine groundwater discharge in a closed inland lake using stable isotopes.

Tracing lacustrine groundwater discharge (LGD) is essential for understanding the hydrological cycle and water chemistry behaviour of lakes. LGD usually exhibits large spatial variability, but there are few reports on quantitatively revealing the spatial patterns of LGD at the whole lake scale. This study investigated the spatial patterns of LGD in Daihai Lake, a typical closed inland lake in northern China, based on the stable isotopes (δ2H and δ18O) of groundwater, surface water, and sediment pore water (SPW). The results showed that there were significant differences between the δ2H and δ18O values of different water bodies in the Daihai Lake Basin: groundwater < SPW < lake water. The LGD through SPW was found to be an important recharge pathway for the lake. Accordingly, stable isotopes of SPW showed that LGD in the northeastern and northwestern of Daihai Lake was significantly greater both horizontally and vertically than that in the other regions, and the proportions of groundwater in SPW in these two regions were 55.53% and 29.84%, respectively. Additionally, the proportion of groundwater in SPW showed a significant increase with profile depth, and the proportion reached 100% at 50 cm below the sediment surface in the northeastern of the lake where the LGD intensity was strongest. The total LGD to Daihai Lake was 1.47 × 107 m3/a, while the LGD in the northeastern and northwestern of the lake exceeded 1.9 × 106 m3/a. This study provides new insights into assessing the spatial patterns of LGD and water resource management in lakes.

Unravelling the signatures of submarine groundwater discharge and seawater intrusion along the coastal plains of Odisha, India: a multi-proxy approach.

Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two contrary hydrological processes that occur across the land-sea continuum and understanding their nature is essential for management and development of coastal groundwater resource. Present study has attempted to demarcate probable zones of SGD and SWI along highly populated Odisha coastal plains which is water stressed due to indiscriminate-exploitation of groundwater leading to salinization and fresh groundwater loss from the alluvial aquifers. A multi-proxy investigation approach including decadal groundwater level dynamics, LANDSAT derived sea surface temperature (SST) anomalies and in-situ physicochemical analysis (pH, EC, TDS, salinity and temperature) of porewater, groundwater and seawater were used to locate the SGD and SWI sites. A total of 340 samples for four seasons (85 samples i.e., 30 porewater, 30 seawater and 25 groundwater in each season) were collected and their in-situ parameters were measured at every 1-2 km gap along ~ 145 km coastline of central Odisha (excluding the estuarine region). Considering high groundwater EC values (> 3000 µS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre- and < 25 ppt in post-monsoons), four probable SGD zones were identified. The identified zones were validated with observed high positive hydraulic gradient (> 10 m) at SGD and negative hydraulic gradient (< 0 m) at SWI sites along with anomalous SST (colder in pre- and warmer in post-monsoon) near probable SGD locations. This study is first of its kind along the Odisha coast and may act as initial basis for subsequent investigations on fresh-saline interaction along the coastal plains where environmental integrity supports the livelihood of coastal communities and the ecosystem.

Fresh Groundwater Discharge as a Major Source of 90Sr into the Coastal Ocean.

The behavior and source of 90Sr in the coastal ocean remain uncertain. Here, we investigated the distributions of 90Sr in coastal fresh groundwater, river water, pore water, and seawater in three bays along the southeastern coast of China between 2019 and 2021 and evaluated the potential of submarine groundwater discharge (SGD) as a source of coastal 90Sr. The 90Sr activity in coastal fresh groundwater was higher than that in river water and seawater, while the 90Sr activity in pore water was comparable to that in adjacent seawater. In addition, nonconservative mixing behavior of 90Sr along the salinity gradient between river water and seawater was observed. These observations indicated that fresh SGD may serve as an additional source of 90Sr in coastal seawater. Combining our groundwater 90Sr data with the reported fresh SGD flux data, the estimated fresh SGD-derived 90Sr fluxes into the three bays were comparable to or even higher than those supplied by riverine sources. These results revealed that fresh SGD is a major but overlooked source of 90Sr in the coastal ocean. This subterranean pathway for transport of 90Sr to the coastal ocean should be considered in the monitoring and risk assessment of coastal areas, especially those near nuclear facilities.

Groundwater Quality and Health: Making the Invisible Visible.

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Quantification of groundwater discharge into a shallow coastal lagoon applying a multi-tracer approach.

In many cases, shallow coastal lagoons are, on the one hand, vulnerable habitats for birds and marine ecosystems and, on the other hand, threatened by discharging nutrient-laden surface waters and groundwater. In particular, the localization and quantification of submarine groundwater discharge (SGD) is of key concern in this regard. The presented study aimed at investigating SGD into a vulnerable coastal lagoon that is strongly impacted by evaporation applying a multi-tracer approach. The joint application of radionuclides (222Rn, 223Ra, 224Ra), stable water isotopes (δ18O, δ2H) and the water salinity as environmental water tracers allowed evaluating the suitability of the individual parameters in this specific type of environment. Whilst stable isotope and salinity data were difficult to construe in terms of SGD occurrence due to the intense impact of evaporation, a radon mass balance allowed localising SGD areas within the lagoon and quantifying the related SGD flux rates. In addition, a 224Ra/223Ra ratio analysis revealed information on the apparent age of the discharged groundwater, and hence on the flushing intensity of the lagoon. Besides these site-specific results, the study allowed the following general conclusions regarding the suitability of the applied tracers: (i) we verified the suitability of a radon mass balance approach for proving/disproving SGD occurrence and quantifying SGD fluxes in shallow coastal lagoons strongly impacted by evaporation; (ii) we showed that the impact of evaporation may impede the use of water stable isotope and salinity data as SGD indicators in such specific environments; (iii) we demonstrated that the tidal impact on a lagoon water body during a sampling campaign can be compensated by adapting sampling schedule and cruise track to the tidal cycle.

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

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.

Benthic Fluxes of Fluorescent Dissolved Organic Material, Salt, and Heat Measured by Multiple-Sensor Aquatic Eddy Covariance.

Aquatic eddy covariance (AEC) is an in situ technique for measuring fluxes in marine and freshwater systems that is based on the covariance of velocity and concentration measurements. To date, AEC has mainly been applied to the measurement of benthic oxygen fluxes. Here, development of a fast multiple-channel sensor enables the use of AEC for measurement of benthic fluxes of fluorescent material, salt, and heat at three distinct sites in Massachusetts, USA, including the Connecticut River, the Concord River, and Upper Mystic Lake. Benthic fluxes of salt, useful as a tracer for groundwater input (submarine groundwater discharge), were consistent with independent measurements made with seepage meters. Eddy fluxes of heat were consistent with the balance of incoming solar radiation and thermal conduction at the sediment surface. Benthic eddy fluxes of fluorescent dissolved organic material (FDOM) revealed a substantial net downward flux in the humic-rich Concord River, suggesting that microbial consumption of dissolved organic carbon in the sediment was significant. Simultaneous measurement of several fluxes expands the utility of AEC as a biogeochemical tool while enabling checks for mutual consistency among data channels.

Radon (222Rn) as tracer for submarine groundwater discharge investigation-limitations of the approach at shallow wind-exposed coastal settings.

Mapping radon (222Rn) distribution patterns in the coastal sea is a widely applied method for localizing and quantifying submarine groundwater discharge (SGD). While the literature reports a wide range of successful case studies, methodical problems that might occur in shallow wind-exposed coastal settings are generally neglected. This paper evaluates causes and effects that resulted in a failure of the radon approach at a distinct shallow wind-exposed location in the Baltic Sea. Based on a simple radon mass balance model, we discuss the effect of both wind speed and wind direction as causal for this failure. We show that at coastal settings, which are dominated by gentle submarine slopes and shallow waters, both parameters have severe impact on coastal radon distribution patterns, thus impeding their use for SGD investigation. In such cases, the radon approach needs necessarily to allow for the impact of wind speed and wind direction not only during but also prior to the field campaign.

Occurrence of Arsenic in Nearshore Aquifers Adjacent to Large Inland Lakes.

Metal oxides that form near sediment-water interfaces in marine and riverine settings are known to act as a sediment trap for pollutants of environmental concern (e.g., arsenic and mercury). The occurrence of these pollutant traps near sediment-water interfaces in nearshore lake environments is unclear yet important to understand because they may accumulate pollutants that may be later released as environmental conditions change. This study evaluates the prevalence of pollutant sediment traps in nearshore aquifers adjacent to large lakes and the factors that affect the accumulation and release of pollutants, specifically arsenic. Field data from six sites along the Laurentian Great Lakes indicate widespread enrichment of arsenic in nearshore aquifers with arsenic sequestered to iron oxide phases. Arsenic enrichment at all sites (solid-phase arsenic >2 µg/g) suggests that this is a naturally occurring phenomenon. Arsenic was more mobile in reducing aquifers with elevated dissolved arsenic (up to 60 µg/L) observed, where reducing groundwater mixes with infiltrating oxic lake water. Dissolved arsenic was low (<3 µg/L) in all oxic nearshore aquifers studied despite high solid-phase arsenic concentrations. The findings have broad implications for understanding the widespread accumulation of reactive pollutants in nearshore aquifers and factors that affect their release to large lakes.

Evaluation, effect and utilization of submarine groundwater discharge for coastal population and ecosystem: A special emphasis on Indian coastline.

Submarine groundwater discharge (SGD) is an important process driven by marine and terrestrial forces. Low tide affects SGD the most, therefore the ideal time to detect SGD is the low tide, especially during spring tide. Techniques to detect and quantify SGD along with the understanding of the related aquifer characteristics is discussed in this study. Scientific community across the world is realizing the importance of studying and mapping SGD because in the scenario of climate change, this part of the global hydrological cycle is an important process and is known to have a significant effect on the marine ecosystem due to nutrient and metal inputs around the region of discharge. Therefore, understanding the processes governing SGD becomes very important. In this review, various components and processes related to SGD (e.g. Submarine Groundwater Recharge, Deep Porewater Upwelling, Recirculated Saline Groundwater Discharge), along with detailed discussion on impacts of SGD for marine ecosystem is presented. Also, it highlights the future research direction and emphasis is put on more research to be done keeping in mind the changing climate and its impacts on SGD.

Delineation of submarine groundwater discharge and seawater intrusion zones using anomalies in the field water quality parameters, groundwater level fluctuation and sea surface temperature along the Gujarat coast of India.

Nature mediated processes like seawater intrusion (SWI) and its complementary processes like submarine groundwater discharge (SGD) often cause severe water stress on the coastal water resources worldwide. The present work attempts to delineate the SWI and SGD zones along the North-Western coast of India (constituting the entire coastline of Gujarat) which is currently facing severe water crises due to the freshwater salinization and water loss through the coastal aquifer system. In the present work site-specific water characteristics and groundwater level were used as a proxy for identifying the probable zones of SWI and SGD. For the delineation purpose, we have collected 540 water samples distanced at 5-10 km (seawater, porewater and groundwater; 180 each) from the entire coastline of Gujarat. Further, a three-tier validation system has been adopted for delineating the SWI and SGD zones followed by the physical verification of the locations through the integration of (i) Groundwater fluctuation dynamic, (ii) MODIS derived sea surface temperature (SST) anomaly, and (iii) Electrical conductivity (EC) based gradient mapping. The study has identified 9 out of 14 districts being vulnerable to SWI, whereas the remaining five districts from south Gujarat and the Saurashtra coast are suspectable for SGD. The present work will act as a preliminary basis for formulating a framework for the detailed investigation of the mixing process along the coast.

An evaluation of semidistributed-pipe-network and distributed-finite-difference models to simulate karst systems.

Several different approaches have been developed to model the specific characteristics of karst aquifers, taking account of their inherent complex spatial and temporal heterogeneities. This paper sets out the development of a semidistributed modelling approach for applications in an Irish karst context using urban drainage software. The models have proven to be very useful for different studies, with examples given for the ecohydrology of ephemeral karst lakes, extreme groundwater-flood alleviation, karst network investigation, submarine groundwater discharge, and quantification of different recharge and flow components. The limitations of the approach are also highlighted, in particular not being able to simulate diffuse infiltration and flow paths explicitly across the groundwater catchment. Hence, a more distributed, finite-difference modelling approach using MODFLOW Unstructured Grid (USG) with the newly developed Connected Linear Network (CLN) process is then compared against the semidistributed approach on the same karst catchment. Whilst it has proven difficult to achieve the same levels of model performance in simulating the spring flows in the distributed model compared to the semidistributed model, the ability to interrogate the flow paths at any point on the three-dimensional aquifer is demonstrated, which can give new insights into flows (and potential contaminant transport) through such complex systems. The influence of the proximity of highly transmissive conduits on the flow dynamics through the much-lower transmissive matrix cells in which the network is embedded has been particularly investigated.

Plusieurs approches différentes ont été développées pour modéliser les caractéristiques spécifiques des aquifères karstiques, prenant en compte leurs hétérogénéités spatiales et temporelles associées à leur complexité intrinsèque. Cet article présente le développement d'une approche de modélisation semi-distribuée appliquée à un contexte karstique irlandais en utilisant un logiciel de drainage urbain. Ces modèles se sont avérés très utiles pour différentes études, avec des exemples appliqués à l'écohydrologie de lacs karstiques éphémères, l'atténuation de crues extrêmes liées aux eaux souterraines, à l'étude du réseau karstique, à la décharge des sources sous-marines et à la quantification des différentes composantes aussi bien de la recharge que du débit. Les limites de cette approche sont aussi mises en évidence, avec en particulier l'incapacité de simuler l'infiltration diffuse et les voies d'écoulement de manière explicite au sein du bassin d'alimentation d'eaux souterraines. Par conséquent, une approche de modélisation plus distribuée aux différences finies utilisant le logiciel MODFLOW avec une grille non structurée (USG) intégrant le processus récemment développé de réseau linéaire connecté (CLN) est ensuite comparée à l'approche semi-distribuée appliquée au même bassin karstique. Alors qu'il s'est avéré difficile d'atteindre les mêmes niveaux de performance du modèle concernant la simulation du débit à la source à l'aide du modèle distribué en comparaison au modèle semi-distribué, la possibilité d'examiner les voies d'écoulement en tout point de l'aquifère en 3D est démontrée, ce qui peut donner de nouvelles connaissances sur les écoulements (et le transport potentiel de contaminants) au sein de tels systèmes complexes. L'influence de la proximité de conduits hautement transmissifs sur les écoulements dynamiques au travers des cellules de transmissivité plus faible de la matrice au sein de laquelle le réseau est intégré, a été particulièrement étudiée.

Se han elaborado varios enfoques diferentes para modelar las características específicas de los acuíferos kársticos, teniendo en cuenta sus complejas y propias heterogeneidades espaciales y temporales. En el presente documento se expone la elaboración de un enfoque de modelización semidistribuido para aplicaciones en un contexto kárstico de Irlanda utilizando programas informáticos de drenaje urbano. Los modelos han demostrado ser muy útiles para diferentes estudios, y se dan ejemplos para la ecohidrología de los lagos kársticos efímeros, la mitigación de las inundaciones extremas de aguas subterráneas, la investigación de redes kársticas, la descarga submarina de aguas subterráneas y la cuantificación de diferentes componentes de la recarga y el flujo. También se ponen de relieve las limitaciones del enfoque, en particular el hecho de no poder simular explícitamente la infiltración difusa y las trayectorias de flujo a través de la cuenca de captación de aguas subterráneas. Por lo tanto, un enfoque de modelización de diferencias finitas distribuidas utilizando la red no estructurada (USG) de MODFLOW con el proceso de la Red Lineal Conectada (CLN) recientemente desarrollado se compara entonces con el enfoque semidistribuido en la misma cuenca kárstica. Si bien se ha demostrado que es difícil lograr los mismos niveles de rendimiento del modelo en la simulación de los flujos de manantiales en el modelo distribuido en comparación con el modelo semidistribuido, se demuestra la capacidad de interrogar las trayectorias de los flujos en cualquier punto del acuífero tridimensional, lo que puede dar nuevos conocimientos sobre los flujos (y el transporte de contaminantes potenciales) a través de sistemas tan complejos. Se ha investigado en particular la influencia de la proximidad de conductos altamente transmisivos en la dinámica de los flujos a través de las células de la matriz transmisiva, mucho más bajas, en las que está incorporada la red.

Diversas abordagens diferentes foram desenvolvidas para modelar as características específicas dos aquíferos cársticos, levando em consideração suas complexas heterogeneidades espaciais e temporais inerentes. Este artigo apresenta o desenvolvimento de uma abordagem de modelagem semidistribuída para aplicações em um contexto cárstico irlandês usando software de drenagem urbana. Os modelos provaram ser muito úteis para diferentes estudos, com exemplos dados para a ecohidrologia de lagos cársticos efêmeros, alívio de inundações subterrâneas extremas, investigação de rede cárstica, descarga de água subterrânea submarina e quantificação de diferentes componentes de recarga e fluxo. As limitações da abordagem também são destacadas, em particular não ser capaz de simular infiltração difusa e caminhos de fluxo explicitamente através da captação de água subterrânea. Portanto, uma abordagem de modelagem de diferenças finitas, mais distribuída, usando MODFLOW Unstructured Grid (USG) com o processo de Rede Linear Conectada (RLC) recém-desenvolvido é então comparada com a abordagem semidistribuída na mesma bacia cárstica. Embora tenha se mostrado difícil alcançar os mesmos níveis de desempenho do modelo na simulação dos fluxos de nascente no modelo distribuído em comparação com o modelo semidistribuído, a capacidade de interrogar os caminhos de fluxo em qualquer ponto do aquífero tridimensional é demonstrada, o que pode dar novas percepções sobre os fluxos (e o transporte potencial de contaminantes) por meio de tais sistemas complexos. A influência da proximidade de condutos altamente transmissivos na dinâmica do fluxo através das células da matriz transmissiva muito inferior nas quais a rede está inserida foi investigada particularmente.

Contribution of groundwater discharge and associated contaminants input to Dongting Lake, Central China, using multiple tracers (222Rn, 18O, Cl-).

Lacustrine groundwater discharge (LGD) can play an important role in water and contaminant mass balance of lakes. Dongting Lake is the second largest fresh lake in China which is connected to Yangtze River and has quite prominent ecological status and function within Yangtze River basin. However, the effect of groundwater discharge on the balance of water and contaminant in Dongting Lake has long been overlooked. This study estimated the groundwater discharge and associated contaminants input into Dongting Lake during the dry season using multiple tracers (222Rn, 18O, Cl-). After sensitivity analysis of different models, it is found that the result of 222Rn mass balance model is the most reliable. Based on the 222Rn mass balance model, the groundwater discharge rate is estimated to be 73.94 mm/d and the contribution of LGD to water balance is 10.94%. As the main nutrient components, NH3-N, P and Si from groundwater input account for 23.65%, 5.12% and 30.15%  % of the total input, respectively. As the main heavy metal components, Fe, Mn and As from groundwater input all account for more than 50% of the total input. Although the LGD rate is relatively small, the contaminant input from LGD is significant enough, which may be a potential threat to the ecological stability of Dongting Lake. In this study, the mass balance models of multiple tracers were integrated to understand the role of groundwater in maintaining the water balance and pollution status of Dongting Lake, which has certain reference significance for the LGD study in plain lakes or reservoirs with complex water systems in humid regions.

Submarine groundwater discharge alters coral reef ecosystem metabolism.

Submarine groundwater discharge (SGD) influences near-shore coral reef ecosystems worldwide. SGD biogeochemistry is distinct, typically with higher nutrients, lower pH, cooler temperature and lower salinity than receiving waters. SGD can also be a conduit for anthropogenic nutrients and other pollutants. Using Bayesian structural equation modelling, we investigate pathways and feedbacks by which SGD influences coral reef ecosystem metabolism at two Hawai'i sites with distinct aquifer chemistry. The thermal and biogeochemical environment created by SGD changed net ecosystem production (NEP) and net ecosystem calcification (NEC). NEP showed a nonlinear relationship with SGD-enhanced nutrients: high fluxes of moderately enriched SGD (Wailupe low tide) and low fluxes of highly enriched SGD (Kupikipiki'o high tide) increased NEP, but high fluxes of highly enriched SGD (Kupikipiki'o low tide) decreased NEP, indicating a shift toward microbial respiration. pH fluctuated with NEP, driving changes in the net growth of calcifiers (NEC). SGD enhances biological feedbacks: changes in SGD from land use and climate change will have consequences for calcification of coral reef communities, and thereby shoreline protection.

Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan.

There are 440 operational nuclear reactors in the world, with approximately one-half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 (137Cs) levels (up to 23,000 Bq⋅m-3) outside of the FDNPP site not in the ocean, rivers, or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave- and tide-driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137Cs source (0.6 TBq⋅y-1) is of similar magnitude as the ongoing releases of 137Cs from the FDNPP site for 2013-2016, as well as the input of Fukushima-derived dissolved 137Cs via rivers. Although this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in nuclear power plant monitoring and scenarios involving future accidents.

A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland.

Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.

La connaissance des connexions hydrauliques entre les zones de sorties d'eaux souterraines sous-marines et le bassin terrestre côtier attenant est pertinente pour la gestion des eaux marines et côtières en zones karstiques. Cette étude applique différentes approches méthodologiques et de suivi pour tracer la sortie d'eaux souterraines sous-marines entre le plateau calcaire de Burren et la baie de Galway en Irlande de l'Ouest, via une perte et un conduit profond. Les zones de sorties potentielles d'eaux souterraines sous-marines ont été délimitées dans un premier temps sur la base d'anomalies de température des eaux de mer en utilisant des images satellites Landsat. Deux traceurs fluorescents et des copeaux de bois ont été utilisés comme traceurs. Les copeaux de bois ont été utilisés comme moyen potentiel pour contourner les problèmes de dispersion élevée dans la mer, qui impactent les traceurs fluorescents et engendrent des concentrations sous les limites de détections. L'échantillonnage a été menée sur dix sites différents à terre et en mer dans la baie de Galway. L'échantillonnage en mer e a été mené selon des profils sur une période de quatre jours successifs à bord d'un navire muni d'un fluorimètre de terrain automatique et d'un capteur de conductivité-température-profondeur. Aucun copeau de bois n'a été récupéré en mer, mais les deux traceurs fluorescents ont été échantillonnés avec succès. Les temps de séjour estimés sont de l'ordre de 100 à 354 m/h, et les valeurs de traceurs localisées corrèlent bien en espace et en temps avec les basses valeurs de conductivité électrique. En confirmant les connexions hydrauliques entre les deux phénomènes karstiques et la baie de Galway, l'étude corrobore l'hypothèse de l'importance des veines du cycle varisque sur l'organisation des écoulements régionaux d'eau souterraine dans la région.

El conocimiento de las conexiones hidráulicas entre la descarga submarina de aguas subterráneas (SGD) y su cuenca costera terrestre es relevante para la gestión de las aguas marinas y costeras en las zonas kársticas. Este estudio aplica diferentes métodos y enfoques de monitoreo para el seguimiento de la SGD entre la meseta de caliza de Burren y la bahía de Galway en el oeste de Irlanda, a través de un pozo excavado y un conducto profundo. Las áreas de SGD potenciales fueron delineadas primero basándose en anomalías de la temperatura de la superficie del mar usando imágenes satelitales Landsat. Dos tintes fluorescentes y astillas de madera maciza se utilizaron como trazadores. Las astillas de madera maciza se probaron como medio potencial para eludir el problema de la alta dispersión en el mar, impactando en los colorantes fluorescentes para obtener lecturas por debajo de los límites de detección. El muestreo se llevó a cabo en 10 lugares terrestres diferentes y en el mar en la Bahía de Galway. El muestreo en alta mar se llevó a cabo en transectas durante un período de cuatro días consecutivos a bordo de un buque utilizando un fluorómetro de campo automatizado y un sensor de conductividad-temperatura-profundidad. No se recuperaron astillas de madera en el mar, pero se tomaron muestras exitosas de ambos tintes fluorescentes. Los tiempos de tránsito estimados son del orden de 100 a 354 m/h, y las lecturas del trazador localizadas se correlacionan bien en el espacio y el tiempo con las lecturas de baja conductividad. Al confirmar las conexiones hidráulicas entre las dos características kársticas y la Bahía de Galway, el estudio corrobora la importancia hipotética de las vetas de Variscan con respecto al flujo regional de agua subterránea en la región.

O conhecimento sobre as conexões hidráulicas entre a descarga submarina de águas subterrâneas (DSAS) e sua captação terrestre na costa é relevante no que diz respeito ao gerenciamento de águas marinhas e costeiras em áreas cársticas. Este estudo aplica diferentes métodos e abordagens de monitoramento para rastrear a DSAS entre o platô de calcário de Burren e a baía de Galway, no oeste da Irlanda, por meio de um poço escavado e um conduto profundo. Áreas de potencial DSAS foram primeiramente delineadas com base em anomalias de temperatura da superfície do mar usando imagens do satélite Landsat. Dois corantes fluorescentes e lascas de madeira maciça foram então utilizados como marcadores. Lascas de madeira maciça foram testadas como um meio potencial para contornar o problema de alta dispersão no mar, impactando nos corantes fluorescentes para produzir leituras abaixo dos limites de detecção. A amostragem foi realizada em 10 locais terrestres diferentes e no mar na Baía de Galway. A amostragem fora da costa foi realizada em transectos durante um período de quatro dias sucessivos a bordo de uma embarcação usando um fluorômetro de campo automatizado e um sensor de condutividade-temperatura-profundidade. Nenhuma lasca de madeira foi recuperada no mar, mas os dois corantes fluorescentes foram amostrados com sucesso. Os tempos de viagem estimados são da ordem de 100 a 354 m/h, e as leituras localizadas do traçador se correlacionam bem no espaço e no tempo com as leituras de baixa condutividade. Ao confirmar as conexões hidráulicas entre as duas formações cársticas e a Baía de Galway, o estudo confirma a hipótese da importância das veias variscas em relação ao fluxo regional de águas subterrâneas na região.

Major and trace elements in the surface water of Tonle Sap Lake, Mekong River, and other tributary rivers in Cambodia.

To evaluate the seasonal water circulation of Tonle Sap Lake and its tributary rivers in Cambodia, the spatial distribution patterns of major and trace elements in surface water were investigated. Based on the similarity of the dissolved elemental concentrations, the water samples were mainly divided into the three groups: samples with relatively high percentages of Ca, Mo, and Sb (Subcluster B1); samples with high Si, Al, and Fe (B2); and samples with high Na, K, and Mg (B3). During the rainy season, the elemental composition of lake water (B1) appeared to be greatly influenced by the intrusion of water from the Mekong River (B1) through the Tonle Sap River (B1). During the dry season, the type of lake water shifted to B3, suggesting that the lake water stored during the rainy season was replaced by inflow from other tributaries and groundwater in its vicinity. Thus, the seasonal changes in the elemental composition of the lake water were largely controlled by surface water and groundwater circulation. The dissolved As concentration was higher in the lake water and during the dry season than that in the river water and during the rainy season, indicating the discharge of As from the lake's bottom sediment during the dry season. Although the redox cycling of Fe and Mn appeared to be less important due to the shallow water depth in the lake, there are potential risks of As poisoning induced by the formation of an anoxic water mass and increment in the concentration of phosphorus if eutrophication continues to progress.

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi-site dual nitrate isotopes.

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO3- ) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well-drained hillslopes, as has been shown for the subtropical headwater catchment "TieShanPing", where dual NO3- isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO3- isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH4+ to NO3- on well-drained hillslopes, and subsequent interflow of NO3- over the argic B-horizons to groundwater discharge zones. Depletion of 15 N- and 18 O-NO3- on hillslopes suggested nitrification as the main source of NO3- . In all catchments, except one of the northern sites, which had low N deposition rates, NO3- attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous 15 N and 18 O enrichment in residual NO3- . By contrast to the southern sites, the northern catchments lack continuous and well-developed groundwater discharge zones, explaining less efficient N removal. Using a model based on 15 NO3- signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha-1 year-1 for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO3- attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region.

Porewater exchange drives trace metal, dissolved organic carbon and total dissolved nitrogen export from a temperate mangrove wetland.

Porewater exchange is usually the least quantified process in delivering dissolved material from wetlands to coastal waters, although it has been recognised as an important pathway for the transport of trace metal, carbon and nutrient to the ocean. Here, surface water fluxes of dissolved manganese (Mn), iron (Fe), dissolved organic/inorganic carbon (DOC/DIC), total dissolved nitrogen (TDN) and phosphorous (TDP) were estimated from a temperate mangrove wetland (Kooragang Island, Newcastle, Australia). Radon (222Rn, a natural groundwater tracer) was used to develop a mass balance model to quantify porewater exchange rates and evaluate the contribution of porewater-derived dissolved material to the overall wetland surface water export. A 25-h time series dataset depicted a clear peak of Mn, Fe, TDN, DOC and radon during ebb tides which related to porewater discharge. Porewater exchange rates were estimated to be 14.0 ±â€¯6.3 cm/d (0.18 ±â€¯0.08 m3/s), mainly driven by tidal pumping, and facilitated by a large number of crab burrows at the site. Results showed that the wetland was a source of Mn, Fe, TDN and DOC to the adjacent river system and a sink for TDP and DIC. Surface water Mn, Fe, TDN and DOC exports were 4.0 ±â€¯0.6, 6.6 ±â€¯1.6, 23.9 ±â€¯3.6 and 197.7 ±â€¯29.7 mmol/m2 wetland/d, respectively. Porewater-derived Mn, Fe, TDN and DOC accounted for ~95, 100, 89 and 54% of the wetland surface water exports demonstrating its significant contribution. Our study indicates that temperate mangrove wetlands can be a major source of dissolved metal, carbon and nutrient delivery to coastal waters and that mangrove porewater exchange significantly contributes to this process.

Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea.

The Mediterranean Sea (MS) is a semienclosed basin that is considered one of the most oligotrophic seas in the world. In such an environment, inputs of allochthonous nutrients and micronutrients play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to the MS, whereas the role of submarine groundwater discharge (SGD) has been largely ignored. However, given the large Mediterranean shore length relative to its surface area, SGD may be a major conveyor of dissolved compounds to the MS. Here, we used a (228)Ra mass balance to demonstrate that the total SGD contributes up to (0.3-4.8)⋅10(12) m(3) ⋅ y(-1) to the MS, which appears to be equal or larger by a factor of 16 to the riverine discharge. SGD is also a major source of dissolved inorganic nutrients to the MS, with median annual fluxes of 190⋅10(9), 0.7⋅10(9), and 110⋅10(9) mol for nitrogen, phosphorous, and silica, respectively, which are comparable to riverine and atmospheric inputs. This corroborates the profound implications that SGD may have for the biogeochemical cycles of the MS. Inputs of other dissolved compounds (e.g., iron, carbon) via SGD could also be significant and should be investigated.