The South China Sea (SCS) is a marginal sea of the Western Pacific Ocean with a broad continental shelf exposed since the last glacial maximum. Five enormous subaqueous deltas were developed in the then river deltaic estuaries and adjacent continental shelves of the SCS where buried paleochannel systems are widely distributed. Saline groundwater with salinity up to 25 g/L has been observed in the terrestrial aquifer system in the Pearl River Delta but freshened groundwater with salinity <1 g/L observed in the offshore part of the aquifer system in the subaqueous delta. Such a co-existence of both saline groundwater inland and freshened groundwater offshore in the same aquifer system is widely observed in other large-river deltaic estuaries and their adjacent shelves, but the mechanism for such a phenomenon is not much addressed in literature. Using the Pearl River Delta and its adjacent continental shelf in the northern margin of the SCS as an example, a sophisticated paleo-hydrogeologic model considering sea-level change, sedimentation processes, and precipitation variation in the past 50 ka is conducted to simulate the evolution of the groundwater system and is further calibrated with present porewater geochemistry data and stable isotopes. The results indicate that the offshore freshened groundwater was formed during the low-stands since the late Pleistocene, whereas the onshore saline groundwater was generated by paleo-seawater intrusion during the Holocene transgression and that the intrusion disconnected the onshore freshwater and offshore freshened groundwater bodies near coastlines. The response of the groundwater system to the paleoclimatic changes was delayed by about 7-8 ka, thus the paleoclimatic forcings still have a dominant influence on the present-day distribution of the groundwater salinity.

How to cite: Jiao, J. J., Sheng, C., Yu, S., and Luo, X.: Evolution of the groundwater system in the northern continental shelf of South China Sea since Late Pleistocene, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10443, https://doi.org/10.5194/egusphere-egu23-10443, 2023.

Freshwater resources in coastal regions are under enormous stress due to population growth, pollution, climate change and political conflicts, and many coastal cities have already suffered extreme water shortages. OFF-SOURCE will address if and how offshore freshened groundwater (OFG) – groundwater stored in the sub-seafloor with a total dissolved solid concentration below that of seawater - can be used as an unconventional source of freshwater in coastal regions. Specifically, the Action will identify where OFG is found in waters of COST Member Countries and in which volumes, delineate the most appropriate approaches to characterise OFG, identify the most cost-effective strategy to utilise this resource, and assess the environmental and legal challenges to sustainable OFG use. These activities will be carried out by a new scientific, gender-balanced and inclusive network of experienced and early-career scientists and stakeholders from diverse and complementary scientific disciplines. Such a network will foster cross­disciplinary and inter-sectoral interaction between currently isolated fields of research to reduce the gap between science, policy making and society. There are five different working groups (WGs) that aim for the critical tasks of this action: Assessment (WG1), Characterization (WG2), and Utilization (WG3) of OFG, the Challenges faced by the community (WG4), as well as Training and Dissemination of up-to-date knowledge about OFG (WG5). This interaction will foster new ideas and concepts that will lead to breakthroughs in OFG characterisation and exploitation, translate into future market applications, and deliver recommendations to support effective resource management. By providing high quality training opportunities for early career investigators, particularly from less research intensive countries, the Action will develop a pool of experts to address future scientific challenges related to OFG.

How to cite: Hong, W.-L., Micallef, A., Bertoni, C., Giustiniani, M., Schwalenberg, K., Thomas, A., Quiroga-Jordan, E., and Wazaz, H.: COST action (OFF-SOURCE, CA21112)- Offshore Freshened Groundwater: An unconventional water resource in coastal regions?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9399, https://doi.org/10.5194/egusphere-egu23-9399, 2023.

Where onshore and offshore groundwater systems are connected, onshore abstraction may access offshore fresh groundwater (Knight et al., 2018; Post et al., 2013). This draws seawater into the offshore system and may eventually lead to onshore salinization and changes to submarine groundwater discharge. The rate of salinization with respect to changes in onshore conditions is understudied. We analysed the salinization of a range of idealized coastal groundwater systems using numerical models, aiming to identify salinization regimes, characteristic timescales, and tipping points. Our cross-sectional semiconfined aquifer models were simulated using FloPy and SEAWAT. We simulated transient conditions leading to the emplacement of offshore fresh groundwater and post-development salinization. We systematically varied geometric properties like aquifer and aquitard thicknesses and slope, and hydraulic properties like hydraulic conductivities, dispersivity, and anisotropy. Our results show the influence of these properties on salinization rates, under a range of levels of onshore abstraction, and interactions between properties. This provides insight into offshore groundwater systems most at risk of salinization and guidance for parameter analysis during modelling studies.

Knight, A. C., Werner, A. D., & Morgan, L. K. (2018). The onshore influence of offshore fresh groundwater. Journal of Hydrology, 561, 724–736. https://doi.org/10.1016/j.jhydrol.2018.03.028

Post, V. E. A., Groen, J., Kooi, H., Person, M., Ge, S., & Edmunds, W. M. (2013). Offshore fresh groundwater reserves as a global phenomenon. Nature, 504(7478), 71–78. https://doi.org/10.1038/nature12858

How to cite: Cleary, C., Dempsey, D., and Morgan, L.: How long can offshore fresh groundwater support onshore abstractions?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4573, https://doi.org/10.5194/egusphere-egu23-4573, 2023.

Offshore freshened groundwater (OFG) has been proven to exist in continental margins around the world and has been identified as a potential unconventional water resource. In China, freshwater resources are very limited in the developed coastal areas and islands. Buried paleo-channels associated with the ancient Changjiang (Yangtze) river are suspected to be viable hosts of a offshore freshened groundwater reservoir system in the East China Sea, North of Chengsi island. In this study, we used an integrated modeling approach to predict OFG potential and optimize the design of a controlled source electromagnetic survey to image the reservoir. We develop a conceptual 2D geological model of the Quarternary sediments in the region. Porosity and permeability values were assigned based on borehole observations to produce a hydrogeological model. We present the results of a numerical modeling study of groundwater transport and variable density flow as a result of sea-level fluctuation over the past 200,000 years. Based on the bathymetry, the present-day shelf was sub-aerially exposed to meteoric recharge for most of that period. We considered a range of recharge scenarios and the simulation results indicate a high likelihood that freshwater reservoirs would be preserved until present day. Two freshwater intervals were observed between 80 m – 100 m, and 200 m – 300 m below the seafloor. A layered resistivity model was designed based on the observation of two primary freshwater reservoir layers in the flow model. Numerical tests were carried out on the feasibility of controlled- source electromagnetic method in the study area to optimize the survey setup. This approach can be adapted for offshore freshened groundwater prospecting in other siliciclastic shelf environments.

How to cite: Thomas, A., Duan, S., Zou, Z., and Micallef, A.: Numerical investigation of offshore freshened groundwater reservoirs in the East China Sea, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15503, https://doi.org/10.5194/egusphere-egu23-15503, 2023.

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.

How to cite: Alorda-Kleinglass, A., Ruiz-Mallen, I., Rodellas, V., Rossi, S., Belmonte, G., Diego-Feliu, M., and Garcia-Orellana, J.: Ecosystem services derived from SGD: a perspective from traditional and academic knowledge in Mediterranean societies, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13371, https://doi.org/10.5194/egusphere-egu23-13371, 2023.

Coastal ocean acidification is a worldwide marine problem. In this study, a close relationship between submarine groundwater discharge (SGD) and coastal ocean acidification rate in Hong Kong’s coastal waters is discovered. We for the first time evaluated the direct influence of SGD on seawater pH decline. Results show that SGD can contribute to up to 45% of seawater pH decline through direct input of carbonate species. Local air-sea CO₂ exchange has negligible influences on the seawater pH, but the uptake of air CO₂ can alter open ocean pH and indirectly alter coastal seawater pH by bay-open ocean water exchange. Aerobic respiration is the major contributor to the seawater pH decline in most coastal waters, in which SGD plays a significant role as the major nutrient source. The findings highlight the importance of the investigation and management of groundwater to alleviate the fast coastal ocean acidification.

How to cite: Liu, Y., Song, Y., and Jiao, J. J.: Submarine groundwater discharge strengthens acidification in the coastal areas, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2163, https://doi.org/10.5194/egusphere-egu23-2163, 2023.

Terrestrial water fluxes and nutrient inputs are the major components of marine nutrient budget. Nutrient fluxes through surface and groundwater pathways are especially important for coastal eutrophication and pelagic productivity. Cilician Basin of the Eastern Mediterranean experiences coastal eutrophication, and the roles of surface and underground discharges have not been assessed. We conducted nutrient and ²²⁸Ra monitoring surveys during 2021 and 2022 to elucidate the relative contributions of riverine and submarine groundwater discharges (SGD) to the Cilician basin nutrient/water budget. All major rivers (8 sites) and neighboring groundwater have been monitored for nutrient concentrations and ²²⁸Ra activities monthly throughout 2022. Further, coastal groundwater wells at four different depths (0-50 m) and its neighboring river were sampled twice a week during the same time period as an intensive observation site (METU-IMS) to elucidate high-frequency temporal dynamics in the nutrient concentrations. We also conducted two basin-wide marine surveys to determine ²²⁸Ra activities in the Cilician basin water masses during dry (summer) and wet (spring) seasons. Finally, we constructed a mass-balance box model using ²²⁸Ra activities to estimate the relative flux of SGD in relation to the fluxes from regional rivers. The residence times were calculated to estimate ²²⁸Ra offshore exchange rates using a Lagrangian particle tracking model.

Nutrient monitoring around the catchment revealed very high nutrient concentrations in both river water (PO₄: 0.02-79 µM, TIN: 2.7-1302 µM, SiO₄: 5-542 µM) and groundwater (PO₄: 0.02-11 µM, TIN: 1.91-1187 µM, SiO₄: 6.6-1082 µM).  Concentrations in the river water indicate potential annual riverine N, P, Si loads of 29.1, 0.5, 19.8 Kt/year, respectively. The mean TIN:PO₄ ratio in the groundwater might be as high as c. 400, suggesting that SGD can be one of the main drivers of the Eastern Mediterranean’s phosphate limitation. The high-frequency observations in river and groundwater at METU-IMS site revealed significant variability both in N and P concentrations, which might reflect patterns in extreme weather events as well as agricultural activities. Based on a limited set of samples ²²⁸Ra activities ranged between c.40-174 dpm.m^-³ in the river water and between c. 43-257 dpm.m^-³ in the groundwater during the wet season, indicating lower activities than previous estimations.

The preliminary results of the box model simulations suggested that the total SGD can be comparable or even dramatically larger (max of 60) than the annual riverine flux into the basin. The sensitivity analyses indicated that the variability and potential overestimation of SGD flux were mostly due to the variability in marine water mass residence time estimations, measurements of groundwater ²²⁸Ra activities and the lack of saline end-member activities. We currently measure the remaining set of samples for ²²⁸Ra activities, diversify end-member samplings, and calibrate basin-wide mass-balance model to decrease the uncertainty in our estimations of the SGD budget for the region. Overall, we documented a large N and P load from Cilician Basin catchments with significant temporal intra-annual variability. Furthermore, the ²²⁸Ra activities across the Cilician basin as well as its catchments indicate the predominant role of SGD in the Eastern Mediterranean water and nutrient budget.

How to cite: Özkan, K., Kuyumcu, B., Ertuğrul, S., Kaya, İ. I., Bayari, C. S., Özmen, S. F., Maiti, K., Özhan, K., and Akoglu, E.: Riverine and submarine groundwater discharges into the Mediterranean Cilician Basin and their impact on the marine water and nutrient budget, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15067, https://doi.org/10.5194/egusphere-egu23-15067, 2023.

Freshwater availability at densely populated coastal zones around the world is at risk. The need for freshwater sources will increase the coming decennia as a result of population growth, higher demand of freshwater of good quality, sealing of groundwater systems in urbanized areas, and climate change leading to sea-level rise and increased storm surges (causing saline water overwash). As water quantity and quality requirements for agricultural, industrial and domestic use in many coastal areas around the world are regularly not satisfied by surface waters, coastal fresh groundwater is normally a reliable alternative. But this may change for the worse as salinization processes and overexploitation jeopardizes fresh groundwater resources, negatively affecting health via too salty drinking water and/or food production.

To give better advice to different clients on optimal sustainable freshwater use, we need to increase our understanding and quantification of coastal groundwater salinization, particularly at places where limited data availability and system understanding hinder the proper use of fresh groundwater. This presentation will enlighten some recent developments in the field of global modeling and mapping of coastal groundwater salinity. These developments make it possible to create global-scale groundwater salinity models that can be used to create storylines of, for instance, fresh groundwater availability in the coastal zone, offshore fresh groundwater volumes, submarine groundwater discharge. It supports the achievement of sustainable development goals like SDG6 (clean water and sanitation), and indirectly SDG1 (no poverty), SDG2 (zero hunger) and SDG3 (good health and well-being via drinking water quality and effect groundwater salinization on the risks of heart diseases).

We are currently close to creating a global-scale groundwater salinity model for coastal zones for several reasons. This presentation will elaborate on that. The widely used SEAWAT code for groundwater salinity modelling has been made parallel, which allows for faster and more accurate global projections at high spatial resolutions. Additionally, the number of open source global hydrogeological databases available on web portals is increasing. These databases, along with text and data mining techniques, make it possible to collect hydrogeological data from articles and grey literature. The regional and local data is used to improve the reliability of the model via calibration and validation.

Innovative data collection methods, such as using rapid and cost-effective airborne EM surveys, drones for remote areas, and smartphone apps for citizen-generated data collection, are also being used to map groundwater salinity on a regional scale. Advanced interpolation techniques are available to transform the collected data into 3D groundwater salinity distributions. Initiatives like GROMOPO are working to improve coastal geology (beyond GLHYMPS) and associated flow parameters. Parallel computer power is used to simulate reconstruction of past hydrogeological conditions in data-poor areas to improve understanding of present groundwater salinity.

The model can assess the impact of "compound events" on saltwater intrusion, such as sea level rise and storm surges in subsiding over-exploited coastal areas while freshwater infiltration is reduced by urban development. It can also be used to develop strategies for managing fresh groundwater, such as Managed Aquifer Recharge.

How to cite: Oude Essink, G., Zamrsky, D., King, J., Delsman, J., Verkaik, J., and Bierkens, M.: New developments in the field of global coastal groundwater salinity modelling and mapping, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15232, https://doi.org/10.5194/egusphere-egu23-15232, 2023.

Coastal fresh groundwater reservoirs are often threatened by different salinization processes. Anthropogenic drivers for salinization include groundwater abstraction, land cultivation and artificial drainage. Global climate change, which modifies groundwater recharge patterns and results in sea-level rise, presents another important process enhancing groundwater salinization.

A good understanding regarding the physical behaviour of coastal groundwater systems is needed for sustainable management, including the development of effective counter measures to antagonize future groundwater salinization. At present, however, such knowledge is incomplete.

The aim of this study was to clarify the role of important factors expected to affect future groundwater salinization of low-lying coastal groundwater systems, namely sea-level rise, varying groundwater recharge and abstraction rates, changing drainage and river levels as well as land subsidence. A novel numerical 3-D variable-density groundwater flow and salt transport modeling approach was developed for this purpose, using Northwestern Germany as case study. To systematically evaluate the role of the individual salinization factors, separate model variants were employed.

We found that sea-level rise causes strongest salinization, particularly close to the coastline. Lifting of drainage levels results in freshening of the marsh areas, while a decrease of drainage levels amplifies salinization. Variation of groundwater recharge patterns and abstraction rates have least impact on regional groundwater salinization. As the system is not at steady-state, autonomous salinization will continue into the future and contribute a significant portion of salt loads until the end of the 21^st century. Findings and implications of this study may be relevant to similar low-lying coastal groundwater systems around the world.

How to cite: Seibert, S. L., Greskowiak, J., Karrasch, L., Siebenhüner, B., Oude Essink, G. H. P., van Engelen, J., and Massmann, G.: Assessing the impact of climate change and anthropogenic factors on future salinization of a low-lying coastal groundwater system (Northwestern Germany), EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2859, https://doi.org/10.5194/egusphere-egu23-2859, 2023.

Coastal communities around the world face an increasing risk from surge-driven inundation because of rising sea levels and intensifying storm conditions. During storm surges, inland propagation of ocean water drives infiltration of saltwater into the fresh groundwater, jeopardizing coastal water resources. The magnitude, shape, and duration of these ocean surges vary both between storm events and spatially during a given storm, resulting from differences in bathymetry, coastline shape, and the path and characteristics of the storm. Our study aims to understand how these temporal and spatial variabilities in ocean surge affect groundwater salinization and recovery in the Delaware (DE) Inland Bays. The DE Inland Bays are composed of two adjoining, shallow bays in Eastern Delaware connected to the Atlantic Ocean via a single inlet. The geometry of the bays results in complex surface water hydrodynamics. To study the effect of spatial variability in surge levels we used  the output from a surface water hydrodynamic model, NearCOM-TVD, simulated for past storm surge events, as boundary conditions for 2D Hydrogeosphere simulations of groundwater flow and salt transport. During Hurricane Sandy (2012), the largest surge event in the past decade, there was a 0.7 m range in the maximum surge height within the bays. Corresponding groundwater simulations showed that for this range, there was 33% more surge-induced aquifer salinization for the highest surge level within the bays relative to the lowest. This elevated salt mass persisted over the ten year recovery period. Results from Hurricane Sandy will be compared with more moderate storm surge events. The resulting salinized volumes and recovery times from all the storm simulations were used to develop a salinization vulnerability metric for the Delaware Inland Bays. The goal of these simulations is to identify the surge conditions that present the greatest salinization risk and the locations in the Inland Bays that have the highest salinization vulnerability, as well as to improve understanding of the feedbacks between ocean and groundwater hydrodynamics.

How to cite: Housego, R., Paldor, A., Frederiks, R., Shi, F., and Michael, H.: Using  a surface water hydrodynamic model to understand how spatial variability in ocean surge affects groundwater salinization in Delaware Inland Bays, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9457, https://doi.org/10.5194/egusphere-egu23-9457, 2023.

Groundwater resources on barrier islands sustain both human lives and ecosystem functioning worldwide. A critical threat to these vital freshwater resources is the projected increase in frequency of storm-surges with climate change. These extreme events have the potential to salinize the aquifer, which increases the mortality of stabilizing vegetation and potentially causes substantial erosion. It is important to understand which types of systems are most vulnerable to better plan mitigation efforts. To that end, we collected data on water level and salinity at three different barrier islands along the east coast of the US. These study sites span a range of barrier island typologies, including differences in topography, vegetation, overwash frequency and connection to the ocean. The relationship between the various processes and the vulnerability to salinization was quantified using transfer function noise models (Pastas) and cross correlation. It was found that changes in groundwater head can be well represented with bay water levels, ocean water levels, recharge (both precipitation and evapotranspiration), and overwash events (periods of time when bay or ocean water levels exceed the land surface elevation of the wells). The data-based models indicate that ocean/bay levels are the primary driver of water level change close to the shore while in the center of the barrier islands precipitation is a more significant driver. Substantial differences were found between the different sites in terms of the dominating factors and the overall vulnerability. A sheltered maritime forest site showed minimal impact from storm surge overwash while a less sheltered marsh and maritime forest site showed a clear relationship between the height and duration of overwash events and the total amount of salinity observed in the wells. Finally, at a barren high energy beach site, storm surge appeared to both salinize the aquifer from the top and raise the freshwater-saltwater interface from below. These findings have important implications for management of barrier island groundwater resources, which is a vital resource that is compromised by future changes in storm surge frequency and intensity.

How to cite: Frederiks, R. S., Paldor, A., Donati, L., and Michael, H. A.: Groundwater resources in barrier islands are vulnerable to storm-surge salinization through various dominating processes, as revealed by data-based modeling, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9687, https://doi.org/10.5194/egusphere-egu23-9687, 2023.

How to cite: Farias, I., Oude Essink, G. H. P., de Louw, P., and F.P. Bierkens, M.: Effects of surface water boundary condition scaling on modelled groundwater salinity and salt fluxes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-17249, https://doi.org/10.5194/egusphere-egu23-17249, 2023.

Most density-dependent flow and transport models assume homogeneity of natural aquifers, a strong simplification with respect to reality, while subsurface formations are known to have spatially variable properties (e.g. Freeze, 1975). Previous studies of saltwater intrusion in heterogeneous aquifers have considered mainly macro-scale geological structures, but the effects of local heterogeneities on density-dependent flow and transport are known to be highly affected by spatially correlated distributions of hydraulic conductivities (e.g., Dagan and Zeitoun, 1998, Prasad and Simmons, 2003, Li et al., 2022). Moreover, investigations have been performed mainly via numerical modeling and, to the best of our knowledge, only in one case numerical results have been compared with physical evidence from laboratory reproduction of a heterogeneous media (Koch and Starke, 2001).

The present work describes the design and the realization activities developed to reproduce a controlled heterogeneous porous media in a laboratory flume, aimed at defining the influence of the hydraulic conductivity spatial variability on the density-dependent transport in coastal phreatic aquifers.

The sandbox measures 500 cm long by 30 cm wide by 60 cm high, with 3 cm thick plexiglass walls. Two tanks are located upstream and downstream of the sandbox, with volumes of approximately 0.5 m³ and 2.0 m³, respectively. The upstream tank is filled with fresh-water and is continuously supplied by a small pump, providing fresh-water recharge. The downstream tank is filled with salt-water, previously prepared by adding salt to fresh-water till a proper density is reached, and it represents the sea. In both tanks the level is maintained constant via two spillways, whose height can be adjusted. The discharge through the downstream spillway can be measured.

The flume has been used in previous works (Bouzaglou et al., 2018, Crestani et al., 2022), but here the homogeneous porous media has been substituted by three different nominal size ranges of glass beads, equal to 0.3-0.4, 0.4-0.8 and 1.0-1.3 mm respectively, organized in 250 cells, each of size 20x30x5 cm³ to reproduce a prescribed statistical anisotropic structure (Figure 1).

Figure 1- Sandbox 3D view from upstream (left side) to downstream (right side)

After a preliminary analysis carried out by constant head permeameter tests on each glass beads nominal size range, the hydraulic characterization of the whole heterogeneous formation has been developed considering the filtration process that affects different thicknesses of the aquifer (10, 20, 30, 40 cm) forced by three upstream-downstream head differences (2, 4 and 6 cm).

During the saltwater intrusion experiment a water level difference upstream - downstream of 2 cm has been maintained for 8 days, introducing two separate drought periods (about 8 and 10 hours) at the end of the second and of the third days respectively.

The findings from the heterogeneous media characterization and the seawater advance-retreat phenomenon are discussed in comparison with the results of a numerical model.

This study has been co-funded by the Interreg Italy–Croatia CBC Programme 2014–2020 (Priority Axes: Safety and Resilience) through the ERDF as a part of the projects MoST (AID: 10047742) and SeCure (AID: 10419304).

How to cite: Salandin, P., Belluco, E., Bottegal, L., Camporese, M., Crestani, E., Darvini, G., Giaretta, P., and Trentin, T.: A large-scale laboratory experiment of seawater intrusion in heterogeneous aquifers affected by drought periods, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12120, https://doi.org/10.5194/egusphere-egu23-12120, 2023.

Freshwater demand will increase in the coming years due to climate change and socio-economic developments. One approach to help combat this is through aquifer storage and recovery (ASR), whereby excess fresh water is stored in the subsurface and recovered later as required. Furthermore, brackish groundwater extraction (BWE) can also be used to combat salinization and produce fresh groundwater. COASTAR is a Dutch research consortium that focuses on the use of ASR and BWE to secure sustainable access to freshwater in coastal areas. Previous research within this program produced geohydrological suitability maps at a national level through the assessment of several geohydrological factors using existing data. In this study, we test previous results using numerical validation by ‘blindly’ placing ASR and BWE well systems into the centre of 10 x 10km sub models, using 3D variable-density groundwater flow and coupled salt transport modelling. In total, 12 scenarios were simulated for approximately 170 locations in Dutch coastal areas, resulting in over 2000 model simulations. The scenarios were implemented in two different aquifers (shallow or deep) with extraction rates of 1200, 6.000, and 12.000m³/d. The resulting suitability of BWE and ASR systems at a given location was decided by how the system performs and affects the surrounding environment. A sensitivity analysis provided insights into the main geohydrological parameters and threshold values ​​applicable to ASR and BWE. Overall, results were like the pre-existing geohydrological suitability maps but offered further quantitative insights. On an international level, this knowledge can help to better understand suitability in other areas with similar subsurface characteristics. Additionally, a quick-scan analysis was performed to quantify the total potential extractable volumes for ASR and BWE. The results of this are based on maximum possible extraction/infiltration rates for each model area, by estimating the summed environmental effects of multiple wells. The method used an extrapolation approach based on the numerical model results. For this approach two factors were considered: 1) the effect of multiple extractions on the environment (changes in phreatic groundwater head), and 2) the optimal number of wells given the width of the freshwater ‘bubble’ for ASR scenarios. The quick-scan analysis showed that ASR and BWE systems have the potential to fulfill the increase in freshwater demand in the Netherlands in 2050.

How to cite: America - van den Heuvel, I., King, J., Bootsma, H., Delsman, J., Oude Essink, G., and de Groot - Wallast, I.: A study on the suitability and quantitative potential of aquifer storage and recovery and brackish water extraction in Dutch coastal areas., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7607, https://doi.org/10.5194/egusphere-egu23-7607, 2023.

High-resolution three-dimensional variable-density groundwater flow and coupled salt transport models (abbreviated 3D-VD-FT models) are useful instruments to support coastal groundwater management strategies and to forecast impacts of climate change. However, the ability of 3D-VD-FT models to provide accurate groundwater salinity predictions depends on computational capabilities, availability of sufficient and adequate high-resolution data and understanding of coastal groundwater salinity processes in the subsurface. Often, local aquifer heterogeneities are simplified in numerical models. In doing so, flow and transport are simplified, and consequently, local groundwater salinity changes become difficult to predict accurately.

New avenues in data acquisition and computational methods have opened up the possibility to greatly improve the accuracy of predictions. Recent developments in innovative geophysical monitoring methods are able to observe salinity and (indirect) flow velocities in detail. For instance, one can use automated measurements with Electrical Resistivity Tomography (abbreviated ERT) to monitor salinity changes. In addition, new parallelization methods are able to overcome computational challenges that plague 3D-VD-FT models.

In this research, we are examining the ability of 3D-VD-FT models to reproduce observed groundwater salinity changes during multi-level groundwater extractions and the impact of these extractions on upconing and subsequent downconing of brackish and saline groundwater. To achieve this, we are developing a 3D-VD-FT model that is able to simulate groundwater salinity changes at high resolution that occur in response to multi-level groundwater extractions during the brackish groundwater extraction pilot project FRESHMAN in Scheveningen. The FRESHMAN project allows for a unique view in the subsurface during groundwater extractions due to close monitoring by innovative geophysical monitoring methods such as ERT.

Preliminary results show that heterogeneity of the aquitard in the study area can affect the ability of the 3D-VD-FT model to reproduce observed groundwater salinity changes. For instance, accounting for potential high conductivity conduits in the aquitard can improve the fit of the observed upconing to the simulated upconing. To account for heterogeneity of the aquitard, sequential indicator simulation will be applied to generate multiple realizations of the aquitard. For each realization, the 3D-VD-FT model will be run and results subsequently evaluated in terms of fit and the best-fitting realizations selected. In addition, for the selected best fit realizations, hydrogeological model parameters will be further optimized using PEST in combination with chloride measurements.

How to cite: Hendrikx, T., Oude Essink, G., Karaoulis, M., and Bierkens, M.: Detailed monitoring and simulation of groundwater salinity in response to extractions in a coastal aquifer system, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15557, https://doi.org/10.5194/egusphere-egu23-15557, 2023.

Coastal communities in dry and semiarid areas confront a severe scarcity of potable water. To meet the requirements of a growing population by increased inland groundwater pumping, led to saltwater intrusion, which eventually rendered the available water unfit for human use. According to studies, desalination plants using saline groundwater as feed water (subsurface intake) might alleviate the problems with coastal water supply and quality. In this context, it becomes imperative to understand the saltwater transport and the associated flow and mixing process when saline groundwater is pumped from beneath the freshwater-saltwater interface. Hence to ascertain the effect of saline groundwater pumping on the hydrodynamics, a standard test aquifer was simulated using the finite difference model SEAWAT. The hydrodynamics is quantified by the entrained saltwater which will influence width of mixing zone, length of intrusion of toe and discharge of Darcy flux back to sea. Entrained water is the amount of saltwater entrained through freshwater - saltwater transition zone, for a steady and stable (lighter freshwater above heavier saltwater) flow system in an aquifer. Performance analysis were carried out to infer the influence of pumping rates on the velocity of entrainment. The results indicate that the mixing zone changed from having a sloping shape to one that was partly vertical and partially sloping, resulting in the establishment of a mixed water interface adjacent to the saline groundwater well. It is likely that there exist modest density gradients between the two fluids, but they quickly attained equilibrium due to the localized velocity change that was observed. Pumping induced more saltwater into the aquifer, which get pumped out restricting the further inland invasion of wedge. Understanding this interaction is crucial in order to determine the method's mitigation potential prior to adopting subsurface input for desalination. 

How to cite: Narayanan, D. and t i, E.: Numerical investigations of change in hydrodynamics of a coastal aquifer due to saline groundwater pumping , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13948, https://doi.org/10.5194/egusphere-egu23-13948, 2023.

The salt water intrusion (SWI) and submarine groundwater discharge (SGD) communities have traditionally approached coastal aquifers in different ways, but both communities agree on the importance of defining the mixing zone between freshwater and saltwater in coastal aquifers. The problem is that this mixing zone extends offshore in many aquifers, where it is very difficult to map. Laboratory and modeling studies of the freshwater-saltwater interface have generated very useful insights into fundamental processes but commonly struggle to incorporate the heterogeneity and temporal variability of real systems. This talk reviews current conceptual models and diverse evidence for offshore mixing zones, with recommendations for possible approaches moving forward.

How to cite: Wilson, A.: Where is the freshwater-saltwater interface in offshore coastal aquifers?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8357, https://doi.org/10.5194/egusphere-egu23-8357, 2023.

In coastal areas, the natural groundwater flow is affected by human activities, such as managed aquifer recharge (MAR) and groundwater extraction. They can induce saltwater intrusion and impact the fresh submarine groundwater discharge (FSGD). Resistivity methods, such as electrical resistivity tomography (ERT) and continuous resistivity profiling (CRP) are easy to use and very effective to assess the distribution of salt and freshwater in coastal environments. The Western Belgian coast, De Panne and Koksijde, was already investigated with ERT and CRP by Paepen et al. (2022; 2020). In this area, the source of FSGD is a sandy dune ridge of around 2.5 km wide. Now, we compare the FSGD footprint in front of De Panne and Koksijde to other Belgian coastal sites (Raversijde, Wenduine, Knokke-Heist, and Zwin), which have a different structure of the phreatic aquifer and a much smaller dune belt.

The quantitative interpretation of ERT and CRP is not straightforward, but image appraisal tools - such as the model resolution matrix (R), cumulative sensitivity matrix (S), and depth of investigation index (DOI) - can aid (Caterina et al., 2013). To be able to quantitatively assess the resistivity inversion models, five synthetic models were created (Paepen et al., 2022). These models reflect the present situation of the Western Belgian coast, where we find freshwater outflow on the lower beach or below the low water line. Based on the inversion models of the synthetic cases, the model resolution matrix, cumulative sensitivity matrix, and DOI (Oldenburg & Li, 1999) were calculated. The image appraisal tools were then compared to the error on the salinity for each cell in the inversion model (we find an error below 0.05 acceptable). This allows to define a threshold of the different image appraisal tools for which the model can be quantitatively assessed and to apply them to the field data. The thresholds reveal that no quantitative interpretation is possible for the zones of FSGD and that the FSGD resistivity is underestimated by the inversion process, so the salinity of the outflow is overestimated. Nevertheless, lateral qualitative changes can be deduced from the inversion models.

References

Caterina, D., Beaujean, J., Tanguy, R., & Nguyen, F. (2013). A comparison study of different image appraisal tools for electrical resistivity tomography. Near Surface Geophysics, 11, 639-657. https://doi.org/10.3997/1873-0604.2013022

Oldenburg, D. W., & Li, Y. (1999). Estimating depth of investigation in dc resistivity and IP surveys. Geophysics, 64(2), 403-416. https://doi.org/10.1190/1.1444545

Paepen, M., Deleersnyder, W., De Latte, S., Walreavens, K., & Hermans, T. (2022). Effect of Groundwater Extraction and Artificial Recharge on the Geophysical Footprints of Fresh Submarine Groundwater Discharge in the Western Belgian Coastal Area. Water, 14(7), 1040. https://doi.org/10.3390/w14071040

Paepen, M., Hanssens, D., De Smedt, P., Walraevens, K., & Hermans, T. (2020). Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge (SGD) in the littoral zone. Hydrology and Earth System Sciences, 24, 3539-3555. https://doi.org/10.5194/hess-24-3539-2020

How to cite: Paepen, M., Deleersnyder, W., Walraevens, K., and Hermans, T.: The quantitative meaning of resistivity data in a coastal setting: a Belgian case study, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5524, https://doi.org/10.5194/egusphere-egu23-5524, 2023.

Submarine groundwater discharge (SGD) is significant to coastal water chemistry and ecology. Nonetheless, the majority of SGD flux to the ocean comprises circulated seawater. This study deals with seawater circulation in coastal aquifers on a global scale in order to assess solute fluxes through SGD into the ocean. While the circulated seawater does not affect the water budget, it has a much higher impact on the ocean solutes budget due to water-rock interactions. We present a global assessment of saline SGD mechanisms' role using numerical simulations and analytical calculations. The numerical model simulates three main circulation mechanisms in coastal aquifers: density-driven circulation (long-term), tidal-driven nearshore circulation, and tidal pumping (short-term), while we calculate the wave-driven benthic exchange flux analytically using the same settings of the numerical model. The model tests the typical range of geohydrological parameters such as hydraulic conductivity, hydraulic gradient, tidal amplitude, and more. Our results revealed that: (1) increasing hydraulic conductivity increases the density-driven and decreases the tidal-driven nearshore circulations; (2) increasing the hydraulic gradient (or freshwater recharge) has no significant effect on the density-driven circulation while it slightly decreases the short-term nearshore circulation; (3) tidal pumping fluxes are a relatively large fraction of the overall SGD flux (30%-60%). Together with global hydraulic parameter distributions, the model results enable assessing the global SGD component of seawater circulation. Preliminary results reveal that the total density-driven SGD is about 0.5-1% of the river fluxes to the oceans. Based on the enrichment of calcium in the long-term SGD component, our global assessment of the calcium flux through density-driven flow may reach the same calcium flux through rivers into the ocean. 

How to cite: Levy, Y. and Kiro, Y.: Towards global quantification of seawater circulation in coastal aquifers, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2263, https://doi.org/10.5194/egusphere-egu23-2263, 2023.

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.

How to cite: Rodriguez-Puig, J., Rodellas, V., Diego-Feliu, M., Alorda-Kleinglass, A., Alorda-Montiel, I., Manzano, M., Alcolea, A., Jiménez-Martínez, J., and Gilabert, J.: Assessing different methods to quantify Submarine Groundwater Discharge, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12823, https://doi.org/10.5194/egusphere-egu23-12823, 2023.

In a coastal aquifer under tidal influence, seawater recirculates beneath the intertidal zone along the classical saltwater wedge (SW). The recirculating seawater on top of the saltwater wedge is called upper saline plume (USP). In between the USP and SW, a freshwater discharge tube (FDT) prevails. Both fresh and saline water components leave the aquifer and flow into the sea as submarine groundwater discharge (SGD). Due the density-gradient across the saline/fresh interface, the USP is prone to instability, resulting in the fingering flow. Whether the USP becomes instable or not depends on several factors, for instance, hydrological (tidal amplitudes, storm surges, precipitation, freshwater flux), morphological (beach slope, aquifer depth), and physical-chemical (temperature, pressure, and dissolved solids of the fluid) boundary conditions as well as aquifer physical properties (porosity, permeability). Unstable USP, which tends to sink to the bottom of the aquifer generating salt fingers, has been described in the context of numerical studies and physical experiments. Yet, flow and transport patterns and the effects of the boundary conditions and parameters described above are not well understood. USP alters the travel path and time of terrestrial nutrients, metals, and contaminants from the coastal aquifers to the marine environment, necessitating a thorough investigation to reveal its critical role in the hydrological cycle. 

For the present study, laboratory sand tank experiments have been carried out to evaluate the effect of homogeneous/heterogeneous conditions, beach slope, fresh groundwater influx and tidal amplitude on USP instability. The results have been used to delineate the conditions that either promote or suppress fingering flow in a tide affected aquifer.  The results define beach slope as the foremost parameter, along with tidal frequencies and beach morphology for the instability of USP. The tank experiments also support the general idea that the presence of low permeable layers disrupts USP formation. Regardless of the aquifer medium, the 3D effect of the salt fingers was observed during the experiments. Furthermore, the laboratory results are found to be consistent with results from previously undertaken generic simulations for field scale conditions. It appears that both laboratory and field scale behavior can be predicted in previously developed non-dimensional stability diagram that separates unstable from stable conditions. 

How to cite: Delwar, R. B., Grünenbaum, N., Greskowiak, J., and Massmann, G.: Fingering Flow in the Subterranean Estuary under Tidal Influence, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15562, https://doi.org/10.5194/egusphere-egu23-15562, 2023.

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.

How to cite: Carrera, J., Martínez-Pérez, L., Luquot, L., Goyetche, T., Pool, M., Palacios, A., del Val, L., Pezard, P. A., Diego-Feliu, M., Rodellas, V., Ledo, J., and Folch, A.: Seawater Intrusion and Submarine Groundwater Discharge studies at the Argentona site in Spain, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14176, https://doi.org/10.5194/egusphere-egu23-14176, 2023.

How to cite: Rowan, T., Flynn, R., Butler, A., Jackson, M., Hamill, G., and Donohue, S.: Automated vertical Self Potential gradient logging and analysis for the tracking of Saline Intrusion, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15408, https://doi.org/10.5194/egusphere-egu23-15408, 2023.

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.

How to cite: Diego-Feliu, M., Rodellas, V., Alorda-Kleinglass, A., Rodriguez-Puig, J., Alorda-Montiel, I., and Folch, A.: How are radiotracers shaping the research in submarine groundwater discharge?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13500, https://doi.org/10.5194/egusphere-egu23-13500, 2023.

Many coastal areas around the world, especially low-lying delta areas, have a high density population and host important economic activities. In such context groundwater abstraction for public water, irrigation and private water supply can lead to over-exploitation and seawater intrusion phenomena. Saltwater intrusion is a critical socio-economic and environmental issue in the coastal plain of Muravera, south-eastern Sardinia (Italy). Since the early fifties the natural hydrodynamic equilibrium between groundwater, surface-water and seawater has been deeply modified by human interventions mainly related to the development of agriculture and tourism activities. The aim of this work is to deepen the knowledge about groundwater recharge areas, salinization mechanisms and water chemistry evolution through a combined hydrogeological and multi-isotopic approach. In this frame, a monthly piezometric and electrical conductivity monitoring survey was carried out for one year, integrated with chemical and isotope analyses of δ¹⁸O_H2O e δ²H_H2O, δ¹¹B, δ¹⁸O_SO4, δ³⁴S_SO4, ⁸⁷Sr/⁸⁶Sr. Isotope analyses of δ¹⁸O_H2O e δ²H_H2O from two precipitation samples are also performed to provide a reference for local meteoric composition.

Results from hydrochemistry analysis show the occurrence of seawater-freshwater mixing, extending up to 4 km inland. δ¹⁸O_H2O & δ²H_H2O, δ¹¹B, δ¹⁸O_SO4 & δ³⁴S_SO4 isotopes analysis confirms the mixing processes and indicates the meteoric origin of recharge waters for both shallow and semi-confined aquifers. Moreover, a clear correlation between precipitation and seawater H₂O isotopic composition is observed. Strontium isotopes ratio has allowed the identification of four main groundwater flow paths, including lateral recharge from bedrock, surface water infiltration from the Flumendosa river and Rio Flumini Uri, and the occurrence of young mixing processes between fresh and sea waters. Outcomes from the combined investigation approach are crucial in the implementation of an integrated and sustainable management system which aims, on the one hand, at slowing the process of saltwater intrusion, and on the other hand to meet socio-economic needs for local communities’ development.

How to cite: Porru, M. C., Da Pelo, S., Arras, C., Lobina, F., Cidu, R., Podda, F., and Biddau, R.: A coupled hydrogeological and multi-isotopic approach to investigate saltwater intrusion in a coastal groundwater system (Sardinia, Italy), EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15792, https://doi.org/10.5194/egusphere-egu23-15792, 2023.

Water tables in coastal aquifers respond to a variety of hydrologic forcings, including precipitation, coastal flooding, and tides. The water table response to these forcings has the potential to impact water quality by affecting the fate and transport of nitrogen, particularly in coastal environments where nitrogen can accumulate in soils and water. To investigate the urban and agricultural reactions involving N that occur near the water table, a meter-long column containing reconstructed coastal soil and aquifer layers from a Mediterranean site was made. We continuously monitored in-situ redox potential, soil moisture, and water pressure and collected frequent pore water samples for analysis of dissolved organic carbon (DOC) and dissolved inorganic nitrogen species over 16 days while imposing water table fluctuations by injecting local groundwater rich in nitrate-N (~15 mg/L). In-situ redox potential in shallow soils (40 cm depth) ranged from -600-600 mV, which is indicative of alternating conditions favorable for aerobic and anaerobic respiration. Redox potential increased upon saturation and declined again as soils drained, with more subtle changes occurring during the first wetting and drying cycle and greater changes occurring during repeated cycles. Pore water analysis shows mobilization of DOC and ammonium-N in shallow soils and removal of nitrate-N in sandy aquifer layers. More specifically, DOC, nitrate-N, ammonium-N, and nitrite-N were greatest in the organic soils and decreased down the column into the sandy aquifer layers. Toward the end of the experiment, the column was inundated with seawater collected from the Mediterranean to simulate a flooding event, causing an increase in all N-species concentrations below 10 cm as seawater transported the nitrogen and DOC contaminants to depth. In contrast, when the column was flooded from the bottom, nitrate-N concentrations decreased as the soils became saturated, oxygen was depleted, and denitrification occurred. Overall, we see how water table dynamics impact the fate and transport of nitrogen in groundwater as soils are repeatedly saturated from above and below.

How to cite: Roumelis, C., Willert, F., Scaccia, M., Welch, S., Gabor, R., Carrera, J., Folch, A., Salgot, M., Insalaco, A., and Sawyer, A.: Water table dynamics in coastal aquifer sediments alter nitrogen fate: Observations from soil column experiments, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16742, https://doi.org/10.5194/egusphere-egu23-16742, 2023.

Most research on submarine groundwater discharge (SGD) focuses on sandy beaches. Less studies have investigated environments with low hydraulic conductivity (K_s) such as coastal peatlands, which are abundant along the southern Baltic Sea coast. Coastal peatlands, which have been drained for agricultural purposes, hold high quantities of carbon, nitrogen, and other compounds that could possibly be released to the sea upon rewetting of these sites. In this study, we simulated groundwater flow from a coastal rewetted fen with a peat layer extending out into the sea to understand the short– and long–term dynamics of SGD, quantify SGD water and matter fluxes, and assess the impact of a storm surge on SGD and seawater intrusion. Five-year (2016 – 2021) daily 2D numerical simulations of groundwater flow were based primarily on monitored groundwater and seawater level data and field-gathered soil hydraulic parameters. Hydraulic conductivities of geological layers were optimized against measured water levels. Manual seepage meter measurements were conducted and water samples were collected. The modeled seepage rates fitted the measured ones well. Our results reveal that SGD and seawater intrusion are highly dynamic and vary spatially and temporally. Two dominant submarine discharge areas were observed: 1) near the beach (up to ~30 m from shore) where mean seepage rates based on nodal water velocities reach up to 12.4 cm d^-1 with waters originating from the dune dike and recirculated seawater; 2) seeps from the aquifer at about 60 m distance from the coast with discharge rates of 1.1 cm d^-1 on average. Mean seepage rates from the discharge areas are comparable to other wetland and sandy environments. The low K_s of the peat layer limits water exchange between the peatland and the Baltic Sea to these regions. The groundwater-seawater interface below the dune moves between the beach and the central dune on an hourly to weekly basis. However, the extent of the interface changes at a seasonal scale. Higher SGD fluxes occur in spring and summer while seawater intrusion increases during fall and winter, as a consequence of the seasonal variations of the peatland’s water level and the resulting hydraulic gradient. During storm surges, higher seawater intrusion fluxes are expected, while low seawater would lead to higher SGD fluxes. The mean daily net flux which represents land-derived SGD from the peatland is 0.15 m² d^-1 (range: -6.12 m² d^-1 to 1.63 m² d^-1), with the highest intrusion occurring during the 2019 storm surge and the highest SGD occurring two days after the surge event. Our mean daily net flux compares well with previous studies but total SGD, which includes recirculated seawater, is likely underestimated. Nearshore carbon and nitrogen SGD concentrations are higher than ambient seawater concentrations demonstrating the potential impact of SGD on local biogeochemistry. Our findings show that SGD is an important coastal process even from low-lying and low K_s coastal peatlands. We emphasize the importance of conducting more interconnection studies between peatland hydrogeology and geochemistry disciplines to better understand SGD processes in these environments.

How to cite: Racasa, E. D., Kienzler, J., Ahmad, S., Batistel, C., Choo, S., Wang, M., Jenner, A.-K., Lennartz, B., and Janssen, M.: Seasonal dynamics of submarine groundwater discharge from a rewetted coastal peatland, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9177, https://doi.org/10.5194/egusphere-egu23-9177, 2023.

In coastal aquifers, terrestrial freshwater and seawater mix and form typical compartments in the subsurface (subterranean estuary, STE), a zone of intense biogeochemical reactions. Flow and transport processes are mainly driven by density differences and- if present- tides. Typically, an upper saline plume is encountered overtopping a freshwater discharge tube and a saltwater wedge. Contrary to the general view of the hydraulic conditions in the STE, latest studies proclaim that the subsurface salinity distribution is less stable and more dynamic than previously thought, especially under the influence of strong morphodynamics. Also, the occurrence of the phenomena of fingering flow has been observed in modelling studies. In this study, physical tank experiments were conducted to compare unstable and stable flow conditions in the STE with respect to the formation of iron oxides that may form in the transition zone between the oxygen-free, iron(ii)-containing, terrestrial freshwater and the oxygen-rich seawater. The results illustrate how biogeochemical processes in the STE are linked to the hydrodynamics as salt fingering flow strongly influenced the location and extent of ferric iron oxidation and the precipitation of Fe(III)hydroxides.

How to cite: Grünenbaum, N., Greskowiak, J., Binte Delwar, R., and Massmann, G.: Visualizing reaction zones in tidal subterranean estuaries using physical tank experiments, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12009, https://doi.org/10.5194/egusphere-egu23-12009, 2023.

Subterranean estuaries (STE) below beaches are biogeochemical reactors that modify the composition of fresh meteoric groundwater and recirculating seawater before they enter the ocean via submarine groundwater discharge (SGD), which can affect coastal ecosystems. Thereby, prevailing redox conditions have a major impact on the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within the STE. Due to the transient nature of the flow and transport within STEs as well as the variable hydrogeochemical boundary conditions, redox zoning in the STE is likely highly dynamic. Elucidating the factors that affect redox zoning and its dynamics is essential for the interpretation and understanding of hydrogeochemical data and the prediction of coastal solute fluxes. In the present study we investigated the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox behavior, i.e., redox zoning in the STE in a generic modelling approach. A 2D cross-shore density-dependent flow and reactive transport model was set up for this purpose, mimicking a beach aquifer exposed to high-energy conditions due to high tides, waves and storm floods. The results of this study show that redox dynamics can occur well down to a depth of 20 m. Morphodynamics were shown to be the most important factor for redox zoning in the STE. For cases where morphodynamics are less pronounced, e.g., at low-energy sites, storm floods and the seasonal temperature changes may be dominating. Seasonal changes in meteoric groundwater recharge rates seem to be least relevant for the redox dynamics in STEs. The results of the present study increase the understanding of STEs as biogeochemical reactors.

How to cite: Greskowiak, J., Seibert, S., Post, V., and Massmann, G.: The effect of dynamic hydro(geo)logical boundary conditions on redox-zoning in high-energy subterranean estuaries, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5781, https://doi.org/10.5194/egusphere-egu23-5781, 2023.

The impact of submarine groundwater discharge (SGD) on coastal sea biogeochemistry and water quality has been demonstrated in many recent studies. However, the isotopic behavior of terrestrially-derived solutes in the groundwater-seawater mixing zone of coastal aquifers (the subterranean estuary, STE) has been less studied, although solutes such as Li, S and Sr are commonly used as tracers of weathering and biogeochemical processes taking place in aquifers and in coastal sea sediments.

This study investigated the behavior of ⁸⁷Sr/⁸⁶Sr, δ⁷Li and δ³⁴S in the STE and three seafloor pockmarks with different degrees of groundwater influence, as constrained based on δ²H and δ¹⁸O, at the Hanko SGD site in Finland, in the northern Baltic Sea. These data were supplemented by groundwater and seawater measurements. ⁸⁷Sr/⁸⁶Sr showed non-conservative behavior with values elevated up to 0.0167 units above that expected for the conservative mixing in the STE and in the most groundwater-dominated pockmark (up to 100% groundwater), but the deviation was masked by much stronger seawater contributions in the other pockmarks. δ⁷Li values were shifted down to −1.75‰ below that expected for conservative mixing in the STE and in groundwater-influenced pockmark porewaters, whereas δ⁷Li was elevated up to 1.53‰ in the porewater of organic-rich mud in a pockmark where groundwater influence had ceased. δ³⁴S deviated between −16.78‰ and 10.51‰ from the conservative mixing in the STE and porewaters of groundwater-influenced pockmarks, while δ³⁴S was elevated up to 16.85‰ in the porewater of the pockmark with no groundwater influence.

In the Hanko STE, the isotopic fractionation of Sr and Li was explained by chemical weathering of silicate minerals and clay minerals, respectively, whereas δ³⁴S was fractionated by complex interactions of microbial sulfate reduction and sulfide reoxidation. In the pockmark porewater with no groundwater influence, δ⁷Li and δ³⁴S isotopes were enriched in the heavier isotopes as a consequence of early-diagenetic mineral formation in the organic-rich muds. The measured ⁸⁷Sr/⁸⁶Sr and δ⁷Li were higher than the previously estimated isotopic compositions of their groundwater-derived fluxes to the oceans, and partly higher than the global riverine values. The heterogeneity in the seafloor biogeochemical environment, caused by the focusing of SGD in pockmarks, resulted in strongly variable δ³⁴S of groundwater-derived S flux to the coastal ocean at a spatial scale of a few hundreds of meters.

Original publication: Ikonen, J., Hendriksson, N., Luoma, S., Lahaye, Y. and Virtasalo, J. J.: Behavior of Li, S and Sr isotopes in the subterranean estuary and seafloor pockmarks of the Hanko submarine groundwater discharge site in Finland, northern Baltic Sea, Applied Geochemistry, 147, 105471, https://doi.org/10.1016/j.apgeochem.2022.105471, 2022.

How to cite: Ikonen, J., Hendriksson, N., Luoma, S., Lahaye, Y., and Virtasalo, J.: Behavior of Li, S and Sr isotopes in the subterranean estuary and seafloor pockmarks of the Hanko submarine groundwater discharge site in Finland, northern Baltic Sea, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6488, https://doi.org/10.5194/egusphere-egu23-6488, 2023.

Freshwater in sediment pore fluids and methane seepage from the seafloor have often been observed concurrently in Arctic regions where submarine groundwater discharge (SGD) occurs, with advective flows potentially reintroducing ancient carbon into the modern ocean. It is hypothesized that hydraulic loading by ice sheets enhances submarine groundwater discharge, and subsequently methane transport. In the presence of microbial communities, large amounts of methane can be consumed by the anaerobic oxidation of methane (AOM), inducing precipitation of authigenic carbonates that inherit unique carbon isotopic signatures from methane. At an SGD site offshore Lofoten Islands, northern Norway (ca. 800 meters water depth), in the vicinity of the maximum extent of the Fennoscandian Ice Sheet, we observed a downcore decreasing chlorinity profile and a linear relation between δ¹⁸O and δ²H of the porewater, known as the local meteoric water line. This demonstrates a meteoric water contribution to the porewater. We also found methane-derived authigenic carbonates (MDACs) with depleted δ¹³C values (< -30 ‰ VPDB), suggesting that microbial (or thermogenic) methane was incorporated during MDAC precipitation. Moreover, δ¹⁸O values (> 2.5 ‰ VPDB) of MDACs indicate precipitation in the presence of ¹⁸O-enriched water, possibly a result of past hydrate dissociation. To assess the carbon cycle and timing of the methane seepage at the Lofoten SGD site, we investigated the radiocarbon contents of Total Organic and Inorganic Carbon in sediments (TOC, TIC), as well as Dissolved Inorganic Carbon (DIC) in porewater from multiple sediment horizons. The radiocarbon contents of DIC have the lowest values among the three carbon pools, in the order of 10-30 percent modern carbon. Their radiocarbon ages (~ 17,000 years BP) are in the order of the Last Glacial Maximum. Consequently, the DIC must have been closed off from the atmosphere due to long groundwater retention times. Alternatively, a methane source low in radiocarbon could have contributed to the DIC pool through AOM. The TIC pool showed radiocarbon content half of that of the TOC in the same sediment horizons, which can be explained by carbonate precipitation from the radiocarbon-depleted DIC pool. To further constrain in situ carbon cycling and advection velocities, a reaction-transport model using mass balance calculations of ¹²C, ¹³C and ¹⁴C has been applied. Radiocarbon content profiles of the DIC indeed imply the advection of old groundwater into the marine sediment porous media.

How to cite: ten Hietbrink, S., Kim, J.-H., Sen, A., Lepland, A., Szymczycha, B., Saghravani, S. R., Knies, J., and Hong, W.-L.: Submarine groundwater discharge and methane seepage driven by Fennoscandian Ice Sheet dynamics offshore northern Norway, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7125, https://doi.org/10.5194/egusphere-egu23-7125, 2023.

Interest has grown in the land-ocean continuum as an energy and material exchange hotspot between the fresh water and saline water ecosystems, including its role in global carbon cycle. Despite progress, carbon cycle especially the biogeochemistry of dissolved organic carbon (DOC) in subsurface environment across the land-ocean continuum is inadequately illustrated. Using fluorescence spectroscopy and ultra-high-resolution mass spectroscopy, this study investigated the molecular characteristics of a broad array of dissolved organic matter (DOM) molecules in shallow groundwater from a coastal aquifer of Guangdong Province. A total of 21 groundwater samples were obtained from 5 multilevel monitoring wells (W1-W5) installed to depth 1 m – 13 m along a transect located 0 m to 65 m from the coast. The infiltration rate ranged from 0.79 to 23.51 m/d, possibly reflecting a less permeable layer between ~ 9 m to ~ 12 m depth. Above this layer consisted of clay lenses, salinity (3.93‰ - 32.43‰) decreases with depth, coinciding with a linear drop of ORP value from a high of +101 mV to a low of -131.90 mV at 8 m depth. The progressively more reducing condition with depth is likely fueled by DOM released from the clay, supported by simultaneous increases of DOC (0.46–2.36 mg/L), DIC (24.62–46.71 mg/L) and DIN (0.03–2.37 mg/L) concentrations and the fluorescence index (FI) with depth. Further, except for two samples (W3-13 m and W5-8 m) with low degradation index (IDEG) of 0.47 and 0.32, of the 11190 molecular formulae of DOM identified by ultra-high-resolution mass spectroscopy molecular formulae with high relative abundance (average ~86.0%) were present in the other 19 samples (90%), indicating the existence of a core pool of DOM compounds with similar molecular compositions despite a strong redox gradient and evidence for microbial processing based on fluorescence spectroscopy data. This core pool of DOM compounds displayed high IDEG (0.82±0.06), high %lignin-like DOM (85.9±2.6), and high abundances of carboxylic-rich alicyclic molecules (%CRAM: 69.5±3.4) that are generally considered to be refractory. Therefore, consumption of labile DOM is reasoned to have taken place, resulting in the prevalence of stable DOM in saline groundwaters of coastal aquifers. 

Key words: Coastal aquifers; Groundwater; Dissolved organic matter; 3D-EEMs; FT-ICR MS 

How to cite: Zheng, Y., Zhang, P., Wang, X., Wang, Y., Li, H., and Wang, J.: Molecular characterization of dissolved organic matter in groundwater of a coastal aquifer: microbial processing of sediment sourced organics , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7303, https://doi.org/10.5194/egusphere-egu23-7303, 2023.

Biogeochemical processes and microbial community structure were investigated in sediment cores from three pockmarks in Hanko, Finland, in the northern Baltic Sea, and compared to groundwater and seawater measurements. Three studied pockmarks varied with the rate of submarine groundwater discharge (SGD). Based on e.g., chloride and DIC concentrations from sediment porewaters, pockmark D had the strongest groundwater influence, while in pockmark E SGD had ceased and therefore this pockmark resembled typical Baltic Sea water and sediment. The pockmark B was the intermediate representative of SGD. The inactive pockmark E had orders of magnitude higher methane concentrations compared to the active pockmarks, but interestingly, this did not reflect on the copy numbers of methanogenesis marker gene (mcrA) results, as pockmark B had equal methanogenesis gene pool as the pockmark E. Sulfate reducer numbers measured with dsrB marker gene was highest in pockmark E sample but also many orders of magnitude higher in other pockmark sediments compared to seawater and groundwater, where the sulfate reducer numbers were only negligible. Reactive transport modeling (RTM) established that the porewater systems in pockmarks D and B were dominated by groundwater advection pushing reactants for biogeochemical reaction into a narrow zone at sediment surface. The advection reduced the organic matter accumulation which results in absence of sulfate-methane transition zone in these pockmarks and concentrates the microbial activity to these habitats. Microbial community structure revealed with phylogenetic marker gene amplicon sequencing reflects the groundwater in active pockmarks, as notable populations of ammonia-oxidizing archaea and nitrifying bacteria in pockmarks are mainly originating from groundwater. RTM also estimated low rates of sulfate consumption and low rates of methane, ammonium and DIC in the active pockmarks.

How to cite: Purkamo, L., von Ahn, C. M. E., Jilbert, T., Muniruzzaman, M., Kock, A., Bange, H., Jenner, A., Böttcher, M. E., and Virtasalo, J.: Groundwater flow impacts on microbial communities and biogeochemistry in seafloor pockmarks, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1962, https://doi.org/10.5194/egusphere-egu23-1962, 2023.