Search

The quality of groundwater is deteriorating globally, leading to an increase in pressure on water resources, particularly in coastal regions. In the quest for mitigating water scarcity under changing climatic conditions, alternative water sources such as offshore freshened groundwater (OFG) have come into scientific focus. Estimates suggest that globally the freshwater volume within OFGs amounts to half a million cubic kilometres. This volume corresponds to more than one century’s worth of freshened water assuming present-day consumption rates. Despite the global significance of OFGs, our understanding of their spatial dimensions, volumes, and geological controls beneath the seafloor remains limited. Discoveries have largely been serendipitous, occurring during borehole drilling. Few studies have effectively integrated point-scale ground-truthing data from boreholes with regional measurements to accurately delineate the extent of OFGs. Furthermore, questions persist regarding the connectivity of OFGs to terrestrial aquifers. On this cruise, we investigated a newly-discovered OFG site within the Gulf of Corinth, Greece. We acquired electromagnetic and geochemical data to derive the spatial extent of the Gulf of Corinth OFG to understand if this low-salinity anomaly is due to present-day recharge through an onshore aquifer system or, alternatively, a remnant of past sea-level low stands.

The study of offshore freshened groundwater (OFG) is gaining importance due to population growth and environmental pressure on coastal water resources. Marine controlled source electromagnetic (CSEM) methods can effectively map the spatial extent of OFG systems using electrical resistivity as a proxy. Integrating these resistivity models with sub-surface properties, such as host-rock porosity, allows for estimates of pore-water salinity. However, evaluating the uncertainty in pore-water salinity using resistivity models obtained from deterministic inversion approaches presents challenges, as they provide only one best-fit model, with no associated estimate of uncertainty. To address this limitation, we employ trans-dimensional Markov-Chain Monte-Carlo inversion on marine time-domain CSEM data, acquired in the Canterbury Bight, New Zealand. We integrate resistivity posterior probability distributions with borehole and seismic reflection data to estimate pore-water salinity with corresponding uncertainty estimates. The results highlight a low-salinity groundwater body in the center of the survey area, hosted by consecutive silty- and fine-sand layers approximately 20–60 km from the coast. The posterior probability distribution of resistivity models indicates freshening of the OFG body toward the shoreline within a permeable, coarse-sand layer 40–150 m beneath the seafloor, suggesting an active connection between the OFG body and the terrestrial groundwater system. The approach demonstrates how Bayesian inversion constrains the uncertainties in resistivity models and subsequently in pore-water salinity estimates. Our findings highlight the potential of Bayesian inversion to enhance our understanding of OFG systems and provide uncertainty constraints for hydrogeological modeling, thereby contributing to sustainable water resource development. Key Points A Bayesian workflow is employed to evaluate uncertainty in pore-water salinity estimates Offshore groundwater in Cante

For millennia, humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectiv

Contains fulltext : 292784.pdf (Publisher’s version ) (Open Access)

Although marine controlled source electromagnetic (CSEM) methods are effective for investigating offshore freshened groundwater (OFG) systems, interpreting the spatial extent and salinity of OFG remains challenging. Integrating CSEM resistivity models with information on sub-surface properties, such as host-rock porosity, allows for estimates of pore-water salinity. However, deterministic inversion approaches pose challenges in quantitatively analyzing these estimates as they provide only one best-fit model with no associated estimate of model parameter uncertainty. To address this limitation, we employ a trans-dimensional Markov-Chain Monte-Carlo inversion on marine CSEM data, under the assumption of horizontal stratification, collected from the Canterbury Bight, New Zealand. We integrate the resulting posterior distributions of electrical resistivity with borehole and seismic reflection data to quantify pore-water salinity with uncertainty estimates. The results reveal a low-salinity groundwater body in the center of the survey area at varying depths, hosted by consecutive silty- and fine-sand layers approximately 20 to 60 km from the coast. These observations support the previous study’s results obtained through deterministic 2-D inversion and suggest freshening of the OFG body closer to the shore within a permeable, coarse-sand layer 40 to 150 m beneath the seafloor. This implies a potential active connection between the OFG body and the terrestrial groundwater system. We demonstrate how the Bayesian approach constrains the uncertainties in resistivity models and subsequently in pore-water salinity estimates. Our findings highlight the potential of Bayesian inversions in enhancing our understanding of OFG systems, providing crucial boundary conditions for hydrogeological modeling and sustainable water resource development.

Offshore freshened groundwater (OFG) and submarine groundwater discharge (SGD) are important components of coastal hydrologic systems. A lack of understanding of offshore groundwater systems and their interactions with onshore systems along the majority of global coastlines still exists due to a general paucity of field data. Recently, controlled-source electromagnetic (CSEM) techniques have emerged as a promising noninvasive method for identifying and characterizing OFG and SGD. Unfortunately, only a few systems are available in academic and research institutions worldwide, and applications are limited to specific regions. These systems are often limited by relatively high deployment costs, slow data acquisition rates, logistical complexity, and lack of modification options. A relatively inexpensive and user-friendly CSEM system is needed to overcome these limitations. We present the initial theoretical and practical developments of SWAN — a low-cost, modular, surface-towed hybrid time-frequency domain CSEM system capable of detecting OFG and SGD to water depths of 100 m. A field test of the system was carried out in the central Adriatic Sea at water depths between several tens to approximately 160 m to illustrate its capabilities. Through its ability to facilitate continuous measurements in both the time and frequency domain, the system has demonstrated its effectiveness in acquiring high-quality data while operating at towing speeds ranging from 2.5 to 3 kn. The resulting data coverage enables the system to detect variations in subsurface resistivity to depths of approximately 150–200 m below seafloor. With its modular, user-friendly design, SWAN provides an accessible, cost-efficient means to investigate the hydrogeology of shallow offshore environments.

Various electromagnetic (EM) techniques have been developed for exploring natural resources. The novel frequency-domain semi-airborne controlled source electromagnetic (semi-AEM) method takes advantages of both ground and airborne techniques. It combines ground-based high-power electrical dipole sources with large scale and spatially densely covered magnetic fields measured via airborne receivers. The method can survey the subsurface down to approximately 1000 m and is particularly sensitive towards conductive bodies (e.g. mineralized bodies) in a more resistive host environment. However, the signal-to-noise ratio of semi-AEM is lower than that of ground-based methods such as long-offset transient electromagnetics (LOTEM), mainly due to the limited stacking time and motion induced noise. As a result, the semi-AEM often has reduced depth of investigation in comparison to LOTEM. One solution to overcome these flaws is to analyse and interpret semi-AEM data together with information from other EM methods using a joint inversion. Since our study shows that LOTEM and semi-AEM data have complementary subsurface resolution capabilities, we present a 2-D joint inversion algorithm to simultaneously interpret frequency-domain semi-AEM data and transient electric fields using extended dipole sources. The algorithm has been applied to the field data acquired in a former mining area in eastern Thuringia, Germany. The 2-D joint inversion combines the complementary information and provides a meaningful 2-D resistivity model. Nevertheless, obvious discrepancies appear between the individual and joint inversion results. Consequent synthetic modelling studies illustrate that the discrepancies occur because of i) differences in lateral and depth resolution between the semi-AEM and LOTEM data caused by different measuring configurations, ii) different measured EM components, and iii) differences in the error weighting of the individual datasets. Additionally, our synthetic study sugges

For millennia humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspective

The Tristan da Cunha islands are located 450 km east of the Mid-Atlantic Ridge and 60 km north of the Tristan da Cunha transform fault, and fracture zone system in the Atlantic Ocean. The islands are products of intraplate hotspot volcanism. However, several controversies are associated with theories for the origin of the Tristan da Cunha hotspot. Previous magnetotelluric data 3-D inversion results based on finite-difference algorithm couldn’t image plume-like vertical structure, unlike many geological and geochemical studies (Baba et al., 2017). In this study, we revisit the electrical structure of the region by inverting combined vertical magnetic transfer functions and the MT responses from seafloor stations around the islands and those newly obtained on the Tristan da Cunha and Nightingale islands. The topographic effect is one of the keys to obtaining a more reliable subsurface structure, and therefore, we use two finite-element-based inversion methods of Usui (2021) and Grayver (2015). For inversion analysis, we compared the topographic responses from these methods and a finite-difference-based forward modeling method (Baba & Seama, 2002; Baba & Chave, 2005) used in the previous study (Baba et al., 2017). The responses for sites with minimum topography effects are almost identical from all the three methods except for some sites where difference is observed in shorter periods. This could be due to the difference in the mesh resolution around the sites. The responses for sites with significant topography effects show a reasonable match for the finite-element methods in longer periods. In contrast a significant difference is observed in responses from all three methods for short periods. The comparison of modeled responses proves the applicability of Usui’s inversion scheme to MT data from this region. In the workshop, the inversion results from Usui’s inversion method will be presented.

Reliable data are the base of all scientific analyses, interpretations and conclusions. Evaluating data in a smart way speeds up the process of interpretation and conclusion and highlights where, when and how additionally acquired data in the field will support knowledge gain. An extended SMART monitoring concept is introduced which includes SMART sensors, DataFlows, MetaData and Sampling approaches and tools. In the course of the Digital Earth project, the meaning of SMART monitoring has significantly evolved. It stands for a combination of hard- and software tools enhancing the traditional monitoring approach where a SMART monitoring DataFlow is processed and analyzed sequentially on the way from the sensor to a repository into an integrated analysis approach. The measured values itself, its metadata, and the status of the sensor, and additional auxiliary data can be made available in real time and analyzed to enhance the sensor output concerning accuracy and precision. Although several parts of the four tools are known, technically feasible and sometimes applied in Earth science studies, there is a large discrepancy between knowledge and our derived ambitions and what is feasible and commonly done in the reality and in the field.

Offshore groundwater systems have been suggested as alternative sources of potable water in islands and coastal regions. In this study, we integrate offshore controlled-source electromagnetic (CSEM) with borehole data to identify an offshore groundwater system in the Canterbury Bight, New Zealand. CSEM data were acquired with a seafloor-towed system along four profiles and 2-D inversion was carried out using MARE2DEM to derive resistivity models along each profile. Moreover, a trans-dimensional Bayesian inversion was conducted to assess the distribution of plausible resistivity-depth models. The study area was previously investigated during IODP Expedition 317 in which a pore-fluid salinity anomaly (24 psu at 40 mbsf) was recorded in borehole U1353. A comparison between the CSEM resistivity model and the resistivity-depth profile converted from pore-water salinity within the borehole shows a strong correlation between CSEM and borehole data at the closest waypoint to site U1353. We show through a Markov-Chain Monte Carlo approach that our estimates of seafloor resistivity agree with the measured borehole data. The computed resistivity distributions at the borehole provide significant evidence that the CSEM inversion models can be used to extrapolate groundwater inferences from the borehole onto a basin scale providing improved geophysical imaging capabilities for offshore freshened groundwater.

Scientists are using a variety of geochemical, geophysical, and numerical methods to study offshore freshened groundwater and better understand its role in the global water cycle.

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Micallef, A., Person, M., Berndt, C., Bertoni, C., Cohen, D., Dugan, B., Evans, R., Haroon, A., Hensen, C., Jegen, M., Key, K., Kooi, H., Liebetrau, V., Lofi, J., Mailloux, B. J., Martin-Nagle, R., Michael, H. A., Mueller, T., Schmidt, M., Schwalenberg, K., Trembath-Reichert, E., Weymer, B., Zhang, Y., & Thomas, A. T. Offshore freshened groundwater in continental margins. Reviews of Geophysics, 59(1), (2021): e2020RG000706, https://doi.org/10.1029/2020RG000706., First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography-driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three-dimensional hydrological models. We show that such advances, combined with site-specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub-seafloor geomicrobiology., This study has received funding from the European Research Council (ERC), under the European Union's Horizon 2020 research and innovation program (grant agreement No. 677898 (MARCAN) to A. M.) and the U.S. National Science Foundation (NSF FRES 1925974 to M. P.; NSF OCE 0824368 to B. D.; and NSF EAR 1151733 to H. A. M.). T. M., B. W. and Y. Z. were funded by the SMART project through the Helmholtz European Partnering Initiative (Project ID Number PIE-0004) involving GEOMAR and the University of Malta.

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 126(10),(2021): e2021JB022228, https://doi.org/10.1029/2021JB022228., Seafloor massive sulfide deposits form in remote environments, and the assessment of deposit size and composition through drilling is technically challenging and expensive. To aid the evaluation of the resource potential of seafloor massive sulfide deposits, three-dimensional inverse modeling of geophysical potential field data (magnetic and gravity) collected near the seafloor can be carried out to further enhance geologic models interpolated from sparse drilling. Here, we present inverse modeling results of magnetic and gravity data collected from the active mound at the Trans-Atlantic Geotraverse hydrothermal vent field, located at 26°08′N on the Mid-Atlantic Ridge, using autonomous underwater vehicle and submersible surveying. Both minimum-structure and surface geometry inverse modeling methods were utilized. Through deposit-scale magnetic modeling, the outer extent of a chloritized alteration zone within the basalt host rock below the mound was resolved, providing an indication of the angle of the rising hydrothermal fluid and the depth and volume of seawater/hydrothermal mixing zone. The thickness of the massive sulfide mound was determined by modeling the gravity data, enabling the tonnage of the mound to be estimated at 2.17 ± 0.44 Mt through this geophysics-based, noninvasive approach., The authors would like to thank the captain, crew, and scientific team from the 2016 R/V Meteor M127 and 1994 R/V Yokosuka MODE'94 cruises for all their work collecting the data modeled in this study. C. Galley is funded through an NSERC Discovery Grant and Memorial University's School of Graduate Studies Grant., 2022-03-29

Seafloor massive sulfide deposits form in remote environments, and the assessment of deposit size and composition through drilling is technically challenging and expensive. To aid the evaluation of the resource potential of seafloor massive sulfide deposits, three-dimensional inverse modelling of geophysical potential field data (magnetic and gravity) collected near the seafloor can be carried out to further enhance geologic models interpolated from sparse drilling. Here, we present inverse modelling results of magnetic and gravity data collected from the active mound at the Trans-Atlantic Geotraverse hydrothermal vent field, located at 26o08'N on the Mid-Atlantic Ridge, using autonomous underwater vehicle (AUV) and submersible surveying. Both minimum-structure and surface geometry inverse modelling methods were utilized. Through deposit-scale magnetic modelling, the outer extent of a chloritized alteration zone within the basalt host rock below the mound was resolved, providing an indication of the angle of the rising hydrothermal fluid and the depth and volume of seawater/hydrothermal mixing zone. The thickness of the massive sulfide mound was determined by modelling the gravity data, enabling the tonnage of the mound to be estimated at 2.17 +/- 0.44 Mt through this geophysics-based, non-invasive approach.

Groundwater aquifers are important for the freshwater supply all over the world, especially in dry regions. It occurs that the water bearing rock matrix extends offshore below the sea floor, and as a consequence fresh water is embedded within good conducting seawater-saturated sediments. Whether an aquifer is open to seawater intrusion or closed, depends on the local hydrogeological setting. Prior studies showed that for high demand on fresh water and high exploitation rates, it is necessary to understand the interaction between sea and fresh water to prevent deterioration of the aquifer. Several geophysical surveys have been carried out in the past to investigate a coastal aquifer in the Palmahim region of Israel, which serves as one of the main reservoirs of the national water supply system. Time domain marine LOTEM and DED data have thereby been recorded on similarprofiles. Both results show a resistor at an assumed aquifer depth of roughly 100 m with a comparable offshore extension of 3.5 km from the coastline. However, until now the LOTEM and DED data have only been interpreted separately using 1D inversion and extensive 2D forward modeling. This study presents the first joint 1D and 2D inverse modeling studies of marine LOTEM and DED techniques using synthetic and field data. The synthetic results indicate large benefits of using a joint inversion of both appliedsurvey techniques. However, a joint inversion of the field data is still in progress. Yet, a 2D inversion of the DED data is promising and indicates a brackish transition zone at the expected lateral aquifer boundary. This finding seems consistent with prior studies and suggests an open aquifer scenario with seawater intrusion. This outcome is essential for the groundwater management and supply in Israel.

Offshore groundwater systems have been suggested as unconventionalsources of portable water in islands and densely populated coastal regions, where terrestrial groundwater resources are over-extracted or contaminated. In this study, we evaluate how well pore-water salinity can be efficiently estimated from controlled-source electromagnetic (CSEM) resistivity models. Time-domain CSEM methods are an effective tool to explore offshore groundwater bodies since the electrical resistivity of the seafloor is primarily determined by the characteristics of the pore-water within the sub-surface sediments. We integrate offshore TD-CSEM data with borehole data and multichannel seismic reflection to identify an offshore groundwater system in the Canterbury Bight. The Canterbury margin is located off the eastern coast of the South Island of New Zealand and was previously investigated during IODP Expedition 317 in which a pore-fluid salinity anomaly was recorded in borehole U1353. By focusing on the low-salinity zone, we carry out synthetic modelling, implement Bayesian inversion to derive probability density functions for resistivity as a function of depth, and finally transform it to distribution of probability density for pore-fluid salinity using Archie’s equation. Subsequently, we use a trans-dimensional Bayesian inversion to interpret measured CSEM data, and consequently use the derived probability density functions to estimate uncertainty of pore-water salinity. Having done the same procedure for different waypoints in the Canterbury Bight, we extrapolate salinity values on a basin scale and estimate the probability density distribution for pore-water salinity as a function of depth. We show that using the Bayesian sampling algorithm provides us with a more precise estimation of hydrogeological model parameters with their uncertainties by generating an ensemble of models instead of inferring only one model using deterministic inversion approaches.

Various electromagnetic (EM) measuring techniques were developed to fulfil the requirements in diverse earth or resources explorations, such as the long-offset transient electromagnetic (LOTEM) and the semi-airborne EM methods. The novel semi-airborne frequency-domain electromagnetic system takes advantages of both ground and airborne techniques by combining ground-based high power sources with large scale and spatially dense covered data. However, its signal-to-noise ratio is still smaller in comparison with the ground-based method like LOTEM due to the limited stacking time. From the perspective of inversion, the data of different EM methods have distinct resolutions towards the subsurface resistivity structures and therefore they can provide complementary earth information. However, these distinct resolutions could also lead to different inversion results if each dataset is inverted individually, which may introduce confusions to the following interpretations. To reduce the ambiguities and parameter uncertainties, joint inversion algorithms are developed to couple spatially dense sampled semi-airborne data and horizontal electric fields (Ex) measured using LOTEM. Nevertheless, the 1D joint inversion faces convergence problems due to 2D effects in the field data. The synthetic modelling suggests that the 2D effects in different datasets lead to distinct artificial structures in the 1D inversion, which makes the 1D joint inversion unfeasible. Therefore, a 2D joint inversion algorithm was further developed for the frequency-domain semi-airborne EM data and the LOTEM transient electric fields. With its application, the newly developed 2D joint inversion of the semi-airborne and LOTEM Ex field data acquired in eastern Thuringia,Germany, converged successfully and a 2D conductivity model could be derived for the survey area. In the consequent 2D synthetic modelling studies, it is demonstrated that part of the discrepancies between the individual inversion result of eac

Many current research questions in Earth Sciences are related to understanding the complex geological processes that dictatethe location of resource formation. Multivariate and multi-disciplinary measurements across disparate spatial scales are common and generate databases that cross traditional geoscientific domains such as geophysics, geochemistry and geology. To cope with this mass of information, integrative data assessment approaches are essential for optimal information extraction from the available data, enabling more accurate prognoses of where to find previously undiscovered natural resources. This is especially true in the deep ocean, where data collection is inherently more difficult than on land. Here, we present an example from predicting the spatial distribution of seafloor massive sulfides (SMS) at the Trans-Atlantic Geotraverse (TAG) hydrothermal field using various sources of marine geophysical data, including high-resolution bathymetry and magnetics collected with an autonomous underwater vehicle (AUV), as well as conductivity data derived from Controlled-Source Electromagnetic (CSEM) measurements. Electrical conductivities are considered a direct SMS indicator as these exhibit a substantial conductivity variation to the surrounding host rock. Due to the size of the survey area, a spatial sampling of conductivity on a dense grid is not economical and would require many years of data acquisition. As a result, robust extrapolation of sparsely sampled conductivity data onto a regional scale seems efficient for predicting additional occurrences of SMS by integrating the acquired marine geophysical data into a Data Science framework. We show that this framework may improve current SMS predictions through associated prediction uncertainty, thereby detecting observational gaps and informing further sparse sampling of ground-truthing data (e.g. sampling or visual observations).

The seventh edition of the »World Ocean Review« focuses on the effects of climate change on the physics of the ocean and on its biotic communities; the consequences of fishing, shipping, resource extraction, energy production, and marine pollution; and the questions of how active substances from the ocean can be used and how the ocean can be managed in the future in such a way that both its protection and the participation of as many people as possible in its services and goods are ensured.

Carbonate lithologies host considerable quantities of the Earth's freshwater resources and partially supply a significant amount of the global population with drinkable water. Although they comprise substantial amounts of the coastlines, it is not known if these carbonate lithologies can sustain freshened groundwater offshore, and if this can help meet future water demands in coastal regions. To date, predicting volumes of freshened groundwater within marine carbonates has been challenging. Here, we integrate controlled source electromagnetic profiles with seismic reflection and core log data to derive a lithological model for the eastern carbonate margin of the Maltese Islands, one of the most water-starved countries in the world. Electrical resistivity models are used to guide lithological inference where seismic data provide limited information due to the superimposed seafloor multiple. We show that resistivity values within the Upper Coralline and Globigerina Limestone formations exceed the measured resistivity of seawater-saturated core log samples by at least a factor of four. This could be indicative of offshore freshened groundwater that occupies the pore space of the low permeability limestone along the eastern Maltese shelf. To validate this observation without further ground-truthing data, we use extensive forward modelling to show that a similar resistivity footprint can be achieved by localized interbedded low- porosity or highly cemented units. However, the spatial extent of such units across the entire eastern Maltese margin is geologically improbable. This points to the occurrence of offshore freshened groundwater that was likely emplaced during the last sea-level lowstand