Submarine groundwater discharge (SGD) and offshore freshened groundwater (OFG) are two interdependent aspects of coastal groundwater systems that have important effects on water security, freshwater budgets, and sustainable water management. OFG, defined as low-salinity groundwater stored within sediments beneath the seabed, has been identified on many passive continental margins worldwide and has been recognized as a potential unconventional freshwater resource. Conversely, SGD constitutes a key pathway for the release of nutrients and contaminants from the seabed into the coastal water column, influencing coastal biogeochemical cycles and marine water quality.
Understanding the distribution and dynamics of OFG and SGD requires a multidisciplinary approach that integrates geophysical surveys, drilling and sedimentological characterization, hydrogeochemical analyses, numerical modeling, and emerging tools such as remote sensing and artificial intelligence. However, both OFG and SGD investigations face significant challenges related to limited accessibility, sparse observational data, and the need for integrated hydrological, geological, and marine datasets. This session invites contributions that address the detection, characterization, and modelling of offshore and coastal groundwater systems, including site-specific case studies, methodological developments, and integrated modelling frameworks that link terrestrial groundwater, offshore aquifers, and coastal seas. We welcome studies on past and present emplacement of OFG, future evolution under climate change and sea-level rise, and the role of SGD in transporting freshwater and solutes to the ocean. Contributions focusing on data integration, innovative monitoring approaches, and management perspectives are particularly encouraged, especially in vulnerable coastal regions where water scarcity, pollution, and anthropogenic pressures are critical concerns.
Sicily (Italy) is among the regions most vulnerable to drought, with conditions expected to worsen under climate change, underscoring the need for sustainable water management and the exploration of unconventional water resources. This study investigates the Ragusa Oligo-Miocene Formation, a karstified and fractured carbonate aquifer with medium to high porosity, which outcrops across the Hyblean Plateau in southern Sicily. The primary objective was to assess and quantify the potential presence of fresh groundwater in the deeper and offshore extension of this carbonate aquifer along the southeastern coast of Sicily, where it is sealed beneath more recent deposits. To reconstruct its subsurface structure, data from 90 deep oil and gas wells, both onshore and offshore, were analysed. Geophysical logs were examined using advanced petrophysical methods, while hydrogeological data from onshore wells were integrated to refine the understanding of the regional aquifer system. The results provide clear evidence of freshened groundwater within the Ragusa regional aquifer, extending deeper than previously known, onshore and continuing offshore up to 10 km from the coastline. A preliminary, conservative volumetric estimate suggests approximately 3 km3 of fresh groundwater preserved in the offshore region of the study area, at depths between 500 and 1200 m below sea level. This discovery demonstrates the untapped potential of unconventional groundwater in both the deep onshore and offshore areas of southeastern Sicily, offering an additional solution to water shortage problems, and has significant implications for other countries along Mediterranean coastlines.
How to cite: Chiacchieri, D., Lipparini, L., Quiroga Jordan, E., Bencini, R., and Micallef, A.: Coastal fresh groundwater extending deep offshore from southern Sicily (Italy): assessment of the Ragusa Aquifer via petrophysical and 3D hydrogeological modelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23046, https://doi.org/10.5194/egusphere-egu26-23046, 2026.
IODP3-NSF Expedition 501 drilled three sites along a 45 km-long, NNW-SSE transect on the New England continental shelf offshore Nantucket and Martha’s Vineyard to characterise an extensive offshore freshened groundwater (OFG) system. Site M0112 (41 m water depth; 338 m below seafloor [mbsf]) is nearest to shore, Site M0111 (42 m water depth; 393 mbsf) is the middle site, and Site M0113 (54 m water depth; 325 mbsf) is farthest offshore. Each site was investigated with drilling, coring, wireline logging (including formation conductivity and nuclear magnetic resonance), and groundwater pumping. Long-term observatories were also lowered in holes at Sites M0112 and M0113 to collect formation temperature and pressure data. The expedition was operationally successful, with 71% core recovery; 10,500 litres of water sampled through pump tests for post-cruise analyses; and the use of temporary PVC casing for wireline logging. Offshore (May-July 2025) and onshore (Bremen, Marum Onshore Operations, January- February 2026) analyses document an unconsolidated sedimentary package consisting predominantly of alternating layers of sand and mud; however, some coarser interbeds were observed at Site M0112. Beds were generally thicker in the shallow section and thinner at depth. Additional shore-based analyses will provide more detailed lithostratigraphic characterisation, sedimentary age, and stratigraphic ties between sites. Interstitial water and pumped groundwater from Sites M0111 and M0112 document a transition from seawater salinity to less than 10% of seawater salinity within the upper 125 mbsf, and salinity remains low until it increases at depths greater than 300 mbsf. At Site M0113 two freshened zones with salinity that is 50-60% of seawater salinity exist above 300 mbsf. Preliminary interpretations suggest that aquifers and confining units have similar salinity in the freshened zones. These freshening patterns are consistent with interpretations of marine-based electromagnetic surveys previously collected along the drilling transect. Compared to salinity, interstitial water alkalinity demonstrates more nuanced patterns, with similar values in sands layers but variable values in mud layers. In addition to shipboard analyses, numerous sediment, water, gas, and microbiological samples were collected for post-expedition research to understand freshwater emplacement mechanisms, timing of emplacement, and volumes of this extensive OFG system. This integrated research program not only elucidates the freshened system beneath the southern New England shelf, but also has implications for other OFGs worldwide.
How to cite: Le Ber, E., Dugan, B., Robinson, R. S., and Everest, J. D. and the IODP3-NSF Expedition 501 Scientists: IODP3-NSF Expedition 501: Offshore Freshened Groundwater in the New England Continental Shelf, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19272, https://doi.org/10.5194/egusphere-egu26-19272, 2026.
Alternating deposition of marine and terrestrial sediments commonly produces multi-layered aquifer-aquitard systems in coastal zones. Under the influence of vertical hydraulic gradients, hydraulic connections may develop between adjacent aquifers, leading to the formation of interlayer leakage. However, the extent to which the vertical leakage influences groundwater flow and freshwater-saltwater mixing processes in coastal aquifers remains poorly understood. Moreover, submarine groundwater discharge (SGD) and salinity dynamics vary across daily, monthly, and annual timescales in response to tidal, spring-neap and seasonal forcings. To date, no study has systematically compared the cross-timescale dynamics of salinity distribution and submarine groundwater discharge (SGD) in unconfined and semi-confined aquifers under leakage conditions. To address these knowledge gaps, this study combines laboratory experiments and numerical simulations to investigate the dynamic mechanisms of flow and transport in coastal aquifer-aquitard systems. The results demonstrate that upward leakage induces unstable freshwater fingering within the saltwater wedge of the unconfined aquifer, promoting the extension of the mixing zone from the wedge margin into its interior. Compared with steady-state conditions, tidal fluctuations reduce upward leakage from the semi-confined aquifer to the unconfined aquifer, thereby increasing horizontal freshwater discharge from the semi-confined aquifer to the sea and further alleviating seawater intrusion within it. When seasonal inland recharge is considered, both saltwater wedges and SGD in the unconfined and semi-confined aquifers exhibit pronounced periodic variations; however, leakage-affected saltwater wedges and internal saltwater circulation display irregular interannual variability. Relative to non-tidal conditions, tidal forcing reduces the amplitude of saltwater-wedge fluctuations in the unconfined aquifer driven by seasonal inland input, while amplifying the corresponding variability in the semi-confined aquifer. Furthermore, the combined effects of spring-neap tides and inland input variability result in dual monthly and quarterly fluctuations in SGD from both aquifers, whereas saltwater-wedge dynamics in the semi-confined aquifer respond to this coupling primarily at the quarterly timescale.
How to cite: Zhang, J. and Lu, C.: Dynamic Mechanisms of Flow and Transport in Coastal Aquifer-Aquitard Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7387, https://doi.org/10.5194/egusphere-egu26-7387, 2026.
Submarine groundwater discharge (SGD) significantly influences ocean chemistry, yet quantifying solute fluxes remains challenging due to the complex interplay of freshwater and saltwater within coastal aquifers operating at different temporal and spatial scales. The key lies in differentiating distinct saltwater flux components and characterizing their end-member compositions.
We developed a novel geochemical approach to isolate and quantify long-term density-driven seawater circulation in coastal aquifers. By compiling an extensive global dataset of coastal groundwater chemistry from onshore wells, we identified systematic deviations from conservative mixing models: enrichment in Ca and Sr and depletion in Na and K. These signatures reflect water-rock interactions occurring over multi-year timescales during mostly density-driven circulation, distinct from rapid tidal/wave-driven exchanges that show conservative mixing. Our novel approach quantifies the long-term SGD component by comparing major element enrichment and depletion in subterranean estuary samples (collected from seepage meters and piezometers) against an end-member composition derived from our global compilation of onshore well data.
To validate our methodology, we applied our mass balance approach to Indian River Bay, Delaware. Based on Ca and Sr enrichment (12 and 0.24 meq/L, respectively) and K depletion (5 meq/L), we calculated long-term circulation at 9±4% of total saline SGD. After correcting for wave-driven circulation, both fresh SGD and long-term circulation represent ~1% of total SGD, consistent with global estimates and extrapolating to 1.2-3.6×10³ km³/y globally. Sr-based flow field mapping further constrains circulation patterns within the coastal aquifer.
This study demonstrates that geochemical tracers can effectively partition SGD components across spatial scales, providing a framework for quantifying long-term seawater circulation impacts on coastal and ocean biogeochemistry.
How to cite: Kiro, Y., Duque, C., and Michael, H.: Quantifying Long-Term Seawater Circulation in Coastal Aquifers: A Novel Geochemical Approach Validated at Indian River Bay, Delaware, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16486, https://doi.org/10.5194/egusphere-egu26-16486, 2026.
Submarine groundwater discharge (SGD) can be a significant source of nutrients, carbon and other substances to coastal seas, with detrimental effects on the marine ecosystem, such as eutrophication and acidification. In Hanko, in the northern Baltic Sea off Finland, SGD occurs through several small depressions (pockmarks, <25 m wide, <2.5 m deep) on a sandy seafloor slope ca. 200 m from the shoreline at water depths of ca. 11 m. Sediment porewater profiles of Cl–, δ2H and δ18O sampled in September 2019 documented a wide range of discharge rates from the pockmarks – from consistent and relatively strong (0.31 cm/day) to moderate (0.02 cm/day) to cessated discharge. Reactive transport modeling showed that groundwater advection in consistent flow-dominated pockmarks forced the key biogeochemical processes and microbial activity (sulphate reduction, methane production) into a few centimetres thick zone below the sediment surface (Purkamo et al., 2022, Geochim. Cosmochim. Acta).
Here we present results from our extensive revisit to the Hanko pockmarks and onshore groundwater observation wells in June 2025. Pockmark sediment samples were collected for bulk geochemical and grain size analyses. Pockmark porewater, overlying water column and groundwater samples from nearby wells were collected for the analysis of a wide range of parameters such as δ2H, δ18O, δ13CDIC, major nutrients and ions. Water column and groundwater samples were also analysed for Ra and Rn activity, and groundwater samples were analysed for stable and radioactive noble gases. Preliminary results show significant temporal variability in discharge rates and biogeochemical conditions in the pockmarks.
The authors would like to thank the European Commission and the Research Council of Finland, Swedish Research Council FORMAS, Polish Research Council NCBR, Estonian Research Council ETAG (Mobilitas 3.0 programme), Spanish Research Council AEI, and the Israeli Ministry of Energy and Infrastructure for funding in the frame of the collaborative international consortium SecuCoast financed under the 2023 Joint call of the European Partnership 101060874 — Water4All.
How to cite: Virtasalo, J., Hong, W.-L., Szymczycha, B., Suuroja, S., Folch, A., Reznik, I., Majamäki, R., Wasiljeff, J., Diego Feliu, M., Ram, R., Lanndér, R., Asmala, E., Purkamo, L., and Luoma, S.: Variable impact of submarine groundwater discharge on the patchiness of seafloor biogeochemical conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9290, https://doi.org/10.5194/egusphere-egu26-9290, 2026.
Submarine groundwater discharge (SGD) is an important transport pathway between land and ocean. The term SGD covers a wide range of processes, compositions and origins, including not only the direct discharge of fresh groundwater, but also diffuse and brackish fluxes through permeable sediments. The mixing zone between fresh groundwater and seawater in the coastal sediments forms a subterranean estuary (STE) where chemical elements undergo biogeochemical transformations before reaching the ocean. However, biogeochemical processes within STEs along tidally influences tropical coastlines, particularly under strong monsoonal rainfall, remain poorly constrained. This study evaluates these processes along tidally influenced section of the Odisha coast, India, which receives annual rainfall of about 1550 mm. Sampling was conducted during the pre-monsoon (May 2024) and post-monsoon (October 2024) seasons. Seawater, groundwater and sediment porewater (down to 125 cm) were collected along intertidal-zone transects parallel to the coastline. Samples were analyzed for the measurement of several parameters including nutrients, major and trace elements and carbon species. The surface seawater and pore waters along the shoreline showed a large difference in salinity values between the two seasons: during the pre-monsoon, salinities reached up to 36 PSU, while after the monsoon, the salinities decreased to a maximum of 30 PSU. During the post-monsoon season, more number and lower salinity spots were detected along the coastline, suggesting that SGD is an important phenomenon in the region causing the formation of an STE. Salinity values were positively correlated with the elevation of the beach, and a seepage line indicating the presence of diffuse SGD was found at about 2m above the sea level. The concentration of nutrients in the different systems suggests that STE plays a role in the transport of nutrients towards the sea. Further studies will continue to better understand the final subterranean element fluxes to the coastal waters of this interesting location.
How to cite: Ehlert von Ahn, C. M., Nayak, S. K., Gupta, N., Ramasamy, M., Raju, N. J., and Moosdorf, N.: Geochemical investigation of a subterranean estuary influenced by tides on the east coast of India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13190, https://doi.org/10.5194/egusphere-egu26-13190, 2026.
Coastal basaltic aquifers are inherently heterogeneous, making the identification of groundwater-seawater interactions and submarine groundwater discharge (SGD) zones challenging using conventional hydrogeological approaches. SGD represents a critical pathway for nutrient transport, contaminant dispersion, and freshwater flux into coastal environments. Limited subsurface exposure and complex fracture-controlled flow systems further increase uncertainty in delineating these zones along basaltic coastlines. To address these challenges, this study integrates Very Low Frequency Electromagnetic (VLF-EM), Transient Electromagnetic (TEM), and magnetic methods to characterize groundwater–seawater interactions along Western Coast Beach in the Raigad district, Maharashtra, India. The study area lies within the Deccan Traps, the largest basaltic lava province in India. VLF-EM and TEM surveys were employed to identify conductive structures associated with coastal aquifers and saline water intrusion, while ground magnetic data were analyzed to delineate structural controls and zones of reduced magnetic susceptibility. These integrated geophysical interpretation reveals preferential groundwater flow pathways connecting onshore aquifers to the coastal zone, indicating areas of active groundwater–seawater exchange. The study demonstrates that the combined use of electromagnetic and magnetic methods effectively reduces uncertainty in coastal groundwater investigations and provides a robust framework for identifying SGD zones, thereby supporting sustainable coastal groundwater management.
How to cite: Maurya, S., Sarkar, K., John Ateng, A., and Singh, A.: Insights into Groundwater-Seawater Interaction using Magnetic and Electromagnetic Data along the Western Coast of India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10660, https://doi.org/10.5194/egusphere-egu26-10660, 2026.
Offshore freshened groundwater (OFG) is increasingly recognised as a potentially significant, yet still poorly constrained, freshwater resource stored on continental shelves. In the northern Adriatic Basin, OFG presence is supported by low salinity interstitial water (< 1 g/L) in a few hydrocarbon exploration-related drilling sites. Building on these observations, assessing whether OFG is widespread across the basin, and constraining its distribution, characteristics, emplacement mechanisms, water quality, and resource potential, requires a multifaceted investigation. Here, we synthesise the current state of knowledge for the Northern Adriatic Basin system, based on results obtained to date from integrated 3D modelling and regional monitoring datasets.
Onshore-offshore connectivity of high-permeability layers is supported by 3D geological geostatistical modelling. We built a 3D geological model of upper Plio–Quaternary sediments and simulated permeable facies distributions using Sequential Indicator Simulation, capturing depositional anisotropy consistent with mixed fluvial-coastal processes. The model supports laterally extensive, southward-dipping permeable units that extend offshore and remain connected across the coastline, providing a physical basis for OFG occurrence and storage. Active flow interaction is further supported by groundwater flow modelling. A transient groundwater flow model was developed and calibrated; simulated coastal exchange indicates that the Northern Adriatic Basin is hydraulically active but characterised by a very small offshore-directed freshwater flux, implying minimal present-day active recharge. Finally, regional onshore hydrochemical analysis suggests that OFG quality has not been substantially affected by anthropogenic inputs, although its potential for utilisation must be evaluated carefully. Hydrochemical results from coastal confined aquifers (multi-decadal monitoring combined with new sampling) delineate distinct groundwater families and age characteristics. Near the coast (and consistently offshore), the most chemically evolved end-member is characterised by strongly reducing conditions (elevated NH₄⁺ and redox-sensitive metals) and is consistent with emplacement during the Last Glacial Maximum (LGM) sea-level lowstand. Together, these findings support the presence of offshore permeable reservoirs saturated with freshwater that was likely emplaced during or before LGM, with minimal subsequent contribution, and currently under strongly reducing conditions, implying potential potability limitations.
How to cite: Corradin, C., Giustiniani, M., Camerlenghi, A., Zini, L., Bertoni, C., Thomas, A. T., Micallef, A., Zamrsky, D., Surian, B., and Barago, N.: Offshore freshened groundwater in the Northern Adriatic Basin: Insights from integrated 3D modelling and hydrochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13990, https://doi.org/10.5194/egusphere-egu26-13990, 2026.
Coastal regions worldwide face increasing water stress, making unconventional resources like Offshore Freshened Groundwater (OFG) critically important. However, characterizing these vast subterranean reservoirs is hindered by the scarcity of direct subsurface data, and current predictive methods are either too coarse for local assessment or qualitative in nature. This study introduces a novel quantitative methodology to predict OFG distribution using machine learning (ML) trained on a synthetic dataset derived from geologically realistic surrogate models. The workflow involves generating numerous surrogate models of continental shelves based on globally available geomorphological data. We then run numerical simulations of variable-density groundwater flow on these models, forced by glacial-interglacial sea-level cycles, to create a robust training dataset linking geological geometry to OFG system characteristics. This study details the parameterization of surrogate continental shelf models from 8 distinct global regions into numerical feature vectors suitable for ML. Initial results indicate that key geometric parameters, such as the offshore extent of the primary aquifer and the inland topographic gradient, are first-order controls on the volume and distribution of emplaced OFG. This proof-of-concept validates that the surrogate modelling framework can effectively capture the sensitivity of OFG systems to geological controls. Ultimately, this methodology highlights a potential pathway to overcoming the data-scarcity challenge, enabling the development of a predictive tool for rapid, quantitative assessment of OFG resources on continental margins worldwide.
How to cite: Thomas, A., Zamrsky, D., Micallef, A., and D'Amico, S.: Predicting Offshore Freshened Groundwater via Machine Learning and Surrogate Modelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14257, https://doi.org/10.5194/egusphere-egu26-14257, 2026.
Offshore freshened groundwater (OFG) represents a significant but underexplored global water resource. Although OFG systems are strongly influenced by paleo sea-level fluctuations, the specific hydrological controls governing present-day salinity distributions remain difficult to constrain due to limited data availability and uncertainties in model parameterization.
We present our latest results from two OFG systems: (1) offshore New Jersey, USA, and (2) Canterbury Bight, New Zealand. Both studies integrate available data such as high-resolution seismic profiles, borehole constraints, geochemical and isotopic data, and paleo-hydrogeological modeling. Offshore New Jersey, OFG was primarily emplaced during sea-level lowstands over the past ~100 kyr, when large portions of the continental shelf were exposed to the atmosphere. Subsequent marine transgression led to partial salinization through diffusive and density-driven mixing with seawater. However, the duration of interglacial submergence has been insufficient to fully salinize the OFG, allowing relic freshwater from pre–Last Glacial Maximum and earlier interglacials to persist. In Canterbury Bight, simulations indicate that modern onshore recharge contributes only a limited fraction of the OFG. The majority of seaward OFG has a mean groundwater age of less than ~40 kyr, suggesting a dominant origin from local meteoric recharge during late-Pleistocene sea-level lowstands. In addition to diffusion and compaction-driven flow, topographically driven lateral flow across the continental shelf played a key role in OFG emplacement. OFG volumes and paleo-submarine groundwater discharge along the continental shelf have varied periodically with glacial–interglacial sea-level changes. Similar to New Jersey, the current OFG system in Canterbury Bight is at non-steady state and becomes gradually salinized by overlying seawater.
Overall, this study sheds light on the effect of changing paleo-hydrological conditions in shaping continental-shelf groundwater systems and provides a framework for assessing the occurrence, evolution, and vulnerability of OFG along passive continental margins.
How to cite: Hensen, C., Sheng, C., Müller, T. H., Thomas, A. T., Micallef, A., and Schmidt, M.: Offshore freshened groundwater (OFG) systems – results and implications from two continental margins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7547, https://doi.org/10.5194/egusphere-egu26-7547, 2026.
With extreme climatic events and increasing populations, water stress in regions of Europe is becoming critical. Offshore Freshened Groundwater (OFG) is increasingly being identified within continental margin sedimentary sequences worldwide, and has potential to be used as an industrial, agricultural, or domestic/potable resource, especially as a mitigation to drought during extreme climatic events. As part of an international effort under the Horizon Europe Water4All project RESCUE (RESources in Coastal groundwater Under hydroclimatic Extremes), we have used extensive subsurface petrophysical and geophysical datasets, alongside machine learning approaches, to generate detailed static reservoir models for a region of the Southern North Sea. Neutron, density and sonic porosities from well log data are used to train the spatially aware EMBER machine learning algorithm against acoustic impedance data, which is derived from 3D seismic reflection and well data. We demonstrate a strong predictive capacity of the trained algorithm to predict porosity from acoustic impedance for the interpreted formations by blind well testing. Permeability is also derived from well logs using the Timur and Holmes-Buckle relationships, before also training EMBER for permeability prediction. Our results provide a detailed, strongly data based, and fully spatially constrained determination of the porosity and permeability distribution for an area of the Southern North Sea, which can be used for dynamic modelling of OFG emplacement during sea level lowstands associated with the last glacial maximum.
How to cite: Phethean, J. J. J., Li, Z., Bertoni, C., and Corradin, C.: Rapid and large-scale reservoir modelling for offshore freshened groundwater applications: The North Sea story, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18084, https://doi.org/10.5194/egusphere-egu26-18084, 2026.
Submarine groundwater discharge (SGD) plays a crucial role in material transport and environmental variability in coastal regions. However, its spatial distribution is highly heterogeneous, making direct observation difficult and regional identification using remote sensing a persistent challenge. Since SGD often causes localized cooling in coastal waters, variations in sea surface temperature (SST) can serve as an initial indicator of potential discharge zones.
This study evaluates the feasibility of using SST data alone as an initial indicator of SGD along the coast of Taiwan. MODIS 8-day composite SST data are used to construct a long-term seasonal baseline. Temperature anomalies relative to this baseline are then analyzed to identify the spatiotemporal distribution patterns of coastal anomaly events.
The spatial characteristics of these anomaly events are examined across different seasons and compared with potential SGD zones reported in previous studies. This analysis explores whether SST, without integrating additional oceanic parameters, can provide reliable preliminary information for identifying SGD discharge zones and serve as a foundation for future multi-parameter integrated studies.
How to cite: Hou, T.-T. and Lin, Y.-C.: Using Sea Surface Temperature as an Initial Indicator for Identifying Potential Submarine Groundwater Discharge Zones along the Coast of Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3719, https://doi.org/10.5194/egusphere-egu26-3719, 2026.
Groundwater is an important, though highly system-dependent, regulator of cryosphere mobility and stability. However, current understanding of these processes is constrained by a scarcity of direct observations due to logistical challenges. Much of the evidence for groundwater–cryosphere coupling therefore derives from numerical modeling and conceptual frameworks. The occurence of offshore groundwater systems along present/past glaciated continental shelves & slopes provides a unique opportunity to constrain the boundary conditions governing the coupling between groundwater and cryospheric processes. This is because the recharge of these offshore groundwater bodies, located several tens to hundreds of meters below seafloor, requires steep hydraulic gradients allowing for robust attribution of flow drivers to changes in cryospheric conditions. In addition, offshore groundwater systems are generally located far from their fluid sources, and thus may respond the first when fluid recharge—for example, ice-sheet basal melt—weakens.
Six high-latitude offshore groundwater sites were investigated for sediment and fluid geochemistry: three sites proximal to past glaciation (Lofoten–Vesterålen from the Norwegian Sea, Fifång Bay close to the Stockholm archipelago, and the Gulf of Finland) and three others in the vincinity of modern glaciers/ice caps, submarine permafrost, or mud volcanoes (Tempelfjorden and Hornsund fjords in Svalbard, Victoria and Petermann fjords in northwest Greenland, and Beaufort Sea shelf and slope). Radiocarbon dating of the offshore groundwater suggest recharge events from early Holocene to pre-Holocene. The mixing of other radiocarbon sources in the sediments, such as carbon derived from degradation of particulate organic matter and dissolution of carbonates, complicates the interpretation of the groundwater signal. By comparing radiocarbon results from overlying seawater, organic matter, carbonate, and adjacent meteoric fluid sources (rivers and glacial ice) at these six locations, we discuss the limitations and potential for constraining the residence time of cryosphere-associated offshore groundwater.
How to cite: Hong, W.-L., ten Hietbrink, S., Chen, N.-C., Kim, J.-H., Pullyottum Kavil, S., Szymczycha, B., Lepland, A., Knies, J., Sen, A., Joonas, V., Suuroja, S., Liira, M., kirchner, N., and Jakobsson, M.: Fluid sources and water–rock interactions of cryosphere-associated offshore fossil groundwater in high-latitude regions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14724, https://doi.org/10.5194/egusphere-egu26-14724, 2026.
