HS8.2.6 | Integrating understanding across the land-ocean continuum: multidisciplinary approaches to studying saltwater intrusion and submarine groundwater discharge
Integrating understanding across the land-ocean continuum: multidisciplinary approaches to studying saltwater intrusion and submarine groundwater discharge
Convener: Albert Folch | Co-conveners: Audrey Sawyer, Valentí Rodellas Vila, Holly Michael, Ester ZancanaroECSECS, Marta CosmaECSECS, Iva AljinovićECSECS
Orals
| Tue, 25 Apr, 14:00–17:55 (CEST)
 
Room B, Wed, 26 Apr, 08:30–10:10 (CEST), 10:45–12:20 (CEST)
 
Room B
Posters on site
| Attendance Wed, 26 Apr, 16:15–18:00 (CEST)
 
Hall A
Posters virtual
| Attendance Wed, 26 Apr, 16:15–18:00 (CEST)
 
vHall HS
Orals |
Tue, 14:00
Wed, 16:15
Wed, 16:15
Coastal aquifers are transitional zones that play a vital role not only providing water resources for coastal societies, but also controlling the exchange of water and chemical constituents between land and ocean and thus influencing coastal marine ecosystems. Traditionally, they have been approached by two different scientific communities, one which focuses on the sustainability of water resources and is particularly interested in sea water intrusion (SWI), and another which focuses on fluxes of solutes supplied by groundwater to the coastal ocean, i.e. submarine groundwater discharge (SGD). As a result, the understanding of the bidirectional groundwater-seawater fluxes is often partial and/or limited. Nevertheless, recent technological, methodological and knowledge advances (e.g. new (hydro)geophysics and (micro)biological approaches, improved (bio)geochemical analytical capabilities, development of new sensors and modelling tools) have allowed scientists to monitor and approach these coastal systems in comprehensive and integrative manner as never before. This session aims to bring together multiple disciplines and perspectives on coastal aquifers. We solicit studies involving SWI, SGD, or both, in order to advance a broad conceptual framework of groundwater in the land-ocean continuum, quantify the dynamic biogeochemical processes and model mechanisms and factors driving freshwater-saltwater dynamics that occur across local to regional scales; from the vadose zone to aquifer systems and submarine groundwater discharge. A better understanding of SWI and SGD from hydrogeologic and oceanographic perspective can help improve management of coastal groundwater and ecosystems and assess its current and future global importance.

Orals: Tue, 25 Apr | Room B

Chairpersons: Albert Folch, Holly Michael
Coastal groundwater: Fresh groundwater offshore and SGD implications
14:00–14:05
14:05–14:25
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EGU23-10443
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HS8.2.6
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solicited
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On-site presentation
Jiu Jimmy Jiao, Chong Sheng, ShengChao Yu, and Xin Luo

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, 23–28 Apr 2023, EGU23-10443, https://doi.org/10.5194/egusphere-egu23-10443, 2023.

14:25–14:35
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EGU23-9399
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HS8.2.6
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On-site presentation
Wei-Li Hong, Aaron Micallef, Claudia Bertoni, Michela Giustiniani, Katrin Schwalenberg, Ariel Thomas, Elizabeth Quiroga-Jordan, and Hiba Wazaz

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, 23–28 Apr 2023, EGU23-9399, https://doi.org/10.5194/egusphere-egu23-9399, 2023.

14:35–14:45
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EGU23-4573
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HS8.2.6
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On-site presentation
Connor Cleary, David Dempsey, and Leanne Morgan

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, 23–28 Apr 2023, EGU23-4573, https://doi.org/10.5194/egusphere-egu23-4573, 2023.

14:45–14:55
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EGU23-15503
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HS8.2.6
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On-site presentation
Ariel Thomas, Shuangmin Duan, Zhihui Zou, and Aaron Micallef

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, 23–28 Apr 2023, EGU23-15503, https://doi.org/10.5194/egusphere-egu23-15503, 2023.

14:55–15:05
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EGU23-13371
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HS8.2.6
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On-site presentation
Aaron Alorda-Kleinglass, Isabel Ruiz-Mallen, Valentí Rodellas, Sergio Rossi, Genuario Belmonte, Marc Diego-Feliu, and Jordi Garcia-Orellana

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, 23–28 Apr 2023, EGU23-13371, https://doi.org/10.5194/egusphere-egu23-13371, 2023.

15:05–15:15
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EGU23-2163
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HS8.2.6
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On-site presentation
Yi Liu, Yurong Song, and Jiu Jimmy Jiao

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, 23–28 Apr 2023, EGU23-2163, https://doi.org/10.5194/egusphere-egu23-2163, 2023.

15:15–15:25
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EGU23-8510
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HS8.2.6
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On-site presentation
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Willard Moore, Samantha Joye, Ryan Sibert, Alan Shiller, Amy Moody, and Claudia Benitez-Nelson

Submarine groundwater discharge (SGD) is recognized to supply nutrient elements nitrogen and phosphorus to coastal waters. In some cases, these nutrients are essential for biological productivity; in other cases, the nutrients are in excess or change relative proportions such that they impact community structure and/or increase algal blooms. Often overlooked is the role of reducing substances in salty SGD such as H2S, NH4+, CH4, and DOM, which create a direct demand for dissolved oxygen (DO) and lower its concentration in estuarine and coastal waters. We call this SGD-Oxygen Demand or SGD-OD. These reduced substances primarily result from the oxidation of carbon in aquifers and aquicludes by seawater sulfate. Thus, coastal aquifers transitioning from freshwater to seawater due to seawater intrusion are most vulnerable. Saturated seawater DO concentrations are on the order of 200 µM; reducing DO in coastal waters to <150 µM induces biological stress on many organisms; reducing DO to <60 µM (hypoxic conditions) can be deadly. Studies have directly correlated DO depletion with increased SGD off the coast of South Carolina and Mississippi, USA, and in the Yangtze delta, China. These depletions initially affect near-bottom dwelling organisms and may be recognized by sudden fish kills. In this talk we will review a data base of reducing substances in coastal groundwaters and illustrate how the discharge of this water could impact estuaries and coastal waters. We will show additional examples where we hypothesize SGD-OD is occurring in hopes others will have the resources to investigate these areas.  

How to cite: Moore, W., Joye, S., Sibert, R., Shiller, A., Moody, A., and Benitez-Nelson, C.: The Direct Effect of Submarine Groundwater Discharge on the Concentration of Dissolved Oxygen in Estuarine and Coastal Waters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8510, https://doi.org/10.5194/egusphere-egu23-8510, 2023.

15:25–15:35
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EGU23-15067
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HS8.2.6
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On-site presentation
Korhan Özkan, Burak Kuyumcu, Serhat Ertuğrul, İdil Ilgaz Kaya, C. Serdar Bayari, S. Fatih Özmen, Kanchan Maiti, Koray Özhan, and Ekin Akoglu

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, 23–28 Apr 2023, EGU23-15067, https://doi.org/10.5194/egusphere-egu23-15067, 2023.

Coffee break
Chairpersons: Marta Cosma, Ester Zancanaro, Iva Aljinović
Coastal groundwater: Seawater intrusion (Tuesday, Time Block 2)
16:15–16:35
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EGU23-15232
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HS8.2.6
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solicited
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On-site presentation
Gualbert Oude Essink, Daniel Zamrsky, Jude King, Joost Delsman, Jarno Verkaik, and Marc Bierkens

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, 23–28 Apr 2023, EGU23-15232, https://doi.org/10.5194/egusphere-egu23-15232, 2023.

16:35–16:45
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EGU23-2859
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HS8.2.6
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ECS
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On-site presentation
Stephan L. Seibert, Janek Greskowiak, Leena Karrasch, Bernd Siebenhüner, Gualbert H.P. Oude Essink, Joeri van Engelen, and Gudrun Massmann

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 21st 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, 23–28 Apr 2023, EGU23-2859, https://doi.org/10.5194/egusphere-egu23-2859, 2023.

16:45–16:55
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EGU23-9457
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HS8.2.6
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ECS
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On-site presentation
Rachel Housego, Anner Paldor, Ryan Frederiks, Fengyan Shi, and Holly Michael

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, 23–28 Apr 2023, EGU23-9457, https://doi.org/10.5194/egusphere-egu23-9457, 2023.

16:55–17:05
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EGU23-9687
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HS8.2.6
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ECS
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On-site presentation
Ryan S. Frederiks, Anner Paldor, Lauren Donati, and Holly A. Michael

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, 23–28 Apr 2023, EGU23-9687, https://doi.org/10.5194/egusphere-egu23-9687, 2023.

17:05–17:15
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EGU23-17249
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HS8.2.6
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ECS
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On-site presentation
Ignacio Farias, Gualbert H.P. Oude Essink, Perry de Louw, and Marc F.P. Bierkens

Calculating groundwater salinity requires the computation of variable density groundwater flow coupled with salt transport in a 3-D gridded space. This type of groundwater modelling requires large amounts of hydrogeological information, computational power, and thorough hydrogeological knowledge of the area being studied. To overcome limitations in knowledge, data availability and computer run times, these models are simplified which may lead to a lack of accuracy in the results. In this setting, enlarging the cell size and/or simplifying surface-groundwater interactions (boundary conditions, BC) are common solutions to achieve feasible runtimes at the expense of precision.

Over the last decades, however, computational power has grown exponentially, which, in combination with recent developments involving parallel computing for groundwater models and ever-increasing resolution and availability of datasets, allow for unexplored model resolutions. This immediately raises the question of what level of detail is required to estimate a sufficiently accurate groundwater salinity distribution for the relevant salinization processes in coastal zones for a given management or policy objective.

This research will explore how groundwater salinity distribution and salt fluxes are affected by varying grid sizes and the parameterization of the surface water boundary conditions. To achieve this, we start from the datasets of the Dutch national hydrological model (LHM fresh-salt). Models of a selected area are created with varying grid resolutions from 10 to 250-meter grid cell size. The surface water features are discretized at these same resolutions resulting in specific datasets with scaled conductance values for each resolution. The models are run in cluster environment using a parallelized version of SEAWAT developed by Deltares called iMOD-WQ. With the model results we aim to quantify the effect of grid size on the groundwater salinity distribution and salt loads to the surface and identify the right balance between required resolution and computational effort. Ultimately, we intend to contribute to the development of objective guidelines for model-enabled fresh groundwater management in coastal aquifers.

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, 23–28 Apr 2023, EGU23-17249, https://doi.org/10.5194/egusphere-egu23-17249, 2023.

17:15–17:25
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EGU23-12120
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HS8.2.6
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On-site presentation
Paolo Salandin, Enrica Belluco, Luigi Bottegal, Matteo Camporese, Elena Crestani, Giovanna Darvini, Pietro Giaretta, and Tommaso Trentin

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, 23–28 Apr 2023, EGU23-12120, https://doi.org/10.5194/egusphere-egu23-12120, 2023.

17:25–17:35
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EGU23-7607
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HS8.2.6
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ECS
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On-site presentation
Ilja America - van den Heuvel, Jude King, Huite Bootsma, Joost Delsman, Gualbert Oude Essink, and Ida de Groot - Wallast

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, 23–28 Apr 2023, EGU23-7607, https://doi.org/10.5194/egusphere-egu23-7607, 2023.

17:35–17:45
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EGU23-15557
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HS8.2.6
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ECS
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On-site presentation
Thijs Hendrikx, Gualbert Oude Essink, Marios Karaoulis, and Marc Bierkens

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, 23–28 Apr 2023, EGU23-15557, https://doi.org/10.5194/egusphere-egu23-15557, 2023.

17:45–17:55
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EGU23-13948
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HS8.2.6
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ECS
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Virtual presentation
Dhanya Narayanan and Eldho t i

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, 23–28 Apr 2023, EGU23-13948, https://doi.org/10.5194/egusphere-egu23-13948, 2023.

Orals: Wed, 26 Apr | Room B

Chairpersons: Audrey Sawyer, Valentí Rodellas Vila
Coastal groundwater: Dynamics, characterization and biogechemical processes (I)
08:30–08:50
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EGU23-8357
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HS8.2.6
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solicited
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On-site presentation
Alicia Wilson

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, 23–28 Apr 2023, EGU23-8357, https://doi.org/10.5194/egusphere-egu23-8357, 2023.

08:50–09:00
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EGU23-5524
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HS8.2.6
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ECS
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On-site presentation
Marieke Paepen, Wouter Deleersnyder, Kristine Walraevens, and Thomas Hermans

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, 23–28 Apr 2023, EGU23-5524, https://doi.org/10.5194/egusphere-egu23-5524, 2023.

09:00–09:10
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EGU23-7567
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HS8.2.6
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ECS
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On-site presentation
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Helena Klettke, Leonie Kandler, and Martin Brede

In recent years, the discharge of groundwater has increasingly been regarded as a notable influence on the terrestrial-marine relation in the coastal zone with respect to the flux of nutrients, carbon and metals into coastal waters. Assessing the net amount of discharging groundwater is a challenge in itself, but the transport and mixing within the water column is an important topic that has been unrevealed so far.

To investigate the transport and mixing processes and their dependence on wave scenario and bottom topography, we performed synchronized particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) experiments using a passive tracer in a wave channel. At the bottom of the channel, a permeable seabed model with defined roughness features, such as ripples and coarser sediment, is mounted and perfused with a fluorescent tracer fluid that resembles the discharging groundwater. Different oscillating flows with variable wave amplitude and period were investigated over each seabed model. The correlation of the measured concentration and velocity fields gives the turbulent transport quantities of the tracer fluid within the water column. Additionally, Prandtl mixing lengths can be determined from the coupled PIV and PLIF results.

Results show a great influence of both, the bottom topography and the wave scenario, as well as their strong coupling through wave-seabed-interaction. Furthermore, different types of bottom roughness showed a great variation in turbulent transport, indicating that the roughness features should be treated in a more complex fashion than being modelled as scalar quantities.

How to cite: Klettke, H., Kandler, L., and Brede, M.: Turbulent transport and mixing of discharged groundwater on structured surfaces at the coastal benthic seafloor, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7567, https://doi.org/10.5194/egusphere-egu23-7567, 2023.

09:10–09:20
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EGU23-2263
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HS8.2.6
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ECS
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On-site presentation
Yehuda Levy and Yael Kiro

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, 23–28 Apr 2023, EGU23-2263, https://doi.org/10.5194/egusphere-egu23-2263, 2023.

09:20–09:30
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EGU23-12823
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HS8.2.6
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ECS
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On-site presentation
Júlia Rodriguez-Puig, Valentí Rodellas, Marc Diego-Feliu, Aaron Alorda-Kleinglass, Irene Alorda-Montiel, Marisol Manzano, Andrés Alcolea, Joaquín Jiménez-Martínez, and Javier Gilabert

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, 23–28 Apr 2023, EGU23-12823, https://doi.org/10.5194/egusphere-egu23-12823, 2023.

09:30–09:40
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EGU23-15562
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HS8.2.6
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ECS
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On-site presentation
Rezwana Binte Delwar, Nele Grünenbaum, Janek Greskowiak, and Gudrun Massmann

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, 23–28 Apr 2023, EGU23-15562, https://doi.org/10.5194/egusphere-egu23-15562, 2023.

09:40–09:50
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EGU23-14176
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HS8.2.6
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On-site presentation
Jesús Carrera, Laura Martínez-Pérez, Linda Luquot, Tybaud Goyetche, María Pool, Andrea Palacios, Laura del Val, Philippe A. Pezard, Marc Diego-Feliu, Valenti Rodellas, Juanjo Ledo, and Albert Folch

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, 23–28 Apr 2023, EGU23-14176, https://doi.org/10.5194/egusphere-egu23-14176, 2023.

09:50–10:00
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EGU23-15408
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HS8.2.6
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ECS
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On-site presentation
Tom Rowan, Raymond Flynn, Adrian Butler, Matthew Jackson, Gerard Hamill, and Shane Donohue

Climate change associated sea level increases and projected growth in global water consumption of about 1% per year (WWAP, 2018) are expected to place further demands on already heavily utilized coastal groundwater supplies. Water stress is anticipated to become more critical over the next decades (Werner and Simmons, 2009). Society’s over-reliance on coastal freshwater abstraction had led to an increased threat of Saline Intrusion (SI). In spite of these challenges, no widely applicable methods of tracking saline fronts in the subsurface exist, even though this capability could prove critical to stopping over abstraction (pumping) before SI occurs; observational boreholes offer a limited warning, and resistivity imaging is often too expensive and logistically infeasible, (MacAllister et al. 2016). An alternative approach to detecting imminent SI is needed. The ongoing goal of this work is to develop a robust and low-cost method of tracking SI in the sub-surfaces. 

Naturally occurring voltages, known as Self Potential (SP), occur when pressure and concentration gradients in the subsurface cause ion separations (Jackson et al., 2012) SP can be used to track SI, so long as the signal source mechanism is understood. There are two key sources of SP widely encountered in hydrology, those induced by pressure, electro-kinetic potentials (VEK), and exclusion-diffusion potentials (VED), due to ion concentration gradients moving through the subsurface.  

SP signals are generated relative to static reference electrodes, offering a signal reading per electrode. However, these signals drift over time making interpretation and comparison challenging. We present findings and insights of an investigation using travelling SP electrodes, moving vertically inside boreholes or wells, to generate SP profiles. Results offer new insights into relationships between SP and SI when logged over time. Profiles taken over the last year at a variety of coastal and inland sites in the UK build upon results from a controlled pumping experiment in Northern Ireland, completed in 2020 and which attempted to interpret these patterns and signals through machine learning. Filtering out background noise sources, (such as electrical interferance, tides, Magneto Telluric effects etc.) has allowed signatures to be more confidently generated and related SI under contrasting hydrogeological regimes. This novel methodology and initial findings are presented and the scope for widely application of the method discussed.

 

References

Jackson, M. D., et al.   (2012). Measurements of spontaneous potential in chalk with application to aquifer characterisation in the southern UK quarterly. J. Eng. Geol. Hydrogeol.

MacAllister, et al. (2016), Tidal influence on self-potential measurements, Journal Geophysical Research Solid Earth.

Werner, A.D., Simmons, C.T., (2009), Impact of sea‐level rise on seawater intrusion in coastal aquifers. Ground Water.

WWAP, (2018), The United Nations World Water Development Report 2018: Nature-Based Solutions for Water. Paris, UNESCO.

 

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, 23–28 Apr 2023, EGU23-15408, https://doi.org/10.5194/egusphere-egu23-15408, 2023.

10:00–10:10
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EGU23-13500
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HS8.2.6
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ECS
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On-site presentation
Marc Diego-Feliu, Valentí Rodellas, Aaron Alorda-Kleinglass, Júlia Rodriguez-Puig, Irene Alorda-Montiel, and Albert Folch

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, 23–28 Apr 2023, EGU23-13500, https://doi.org/10.5194/egusphere-egu23-13500, 2023.

Coastal groundwater: Dynamics, characterization and biogeochemical processes (II)
Coffee break
Chairpersons: Albert Folch, Marta Cosma, Holly Michael
10:45–10:55
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EGU23-15792
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HS8.2.6
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ECS
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On-site presentation
Maria Chiara Porru, Stefania Da Pelo, Claudio Arras, Francesca Lobina, Rosa Cidu, Francesca Podda, and Riccardo Biddau

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 δ18OH2O e δ2HH2O, δ11B, δ18OSO4, δ34SSO4, 87Sr/86Sr. Isotope analyses of δ18OH2O e δ2HH2O 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. δ18OH2O & δ2HH2O, δ11B, δ18OSO4 & δ34SSO4 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, 23–28 Apr 2023, EGU23-15792, https://doi.org/10.5194/egusphere-egu23-15792, 2023.

10:55–11:05
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EGU23-16742
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HS8.2.6
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ECS
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On-site presentation
Christian Roumelis, Fabian Willert, Maria Scaccia, Susan Welch, Rachel Gabor, Jesús Carrera, Albert Folch, Miquel Salgot, Alycia Insalaco, and Audrey Sawyer

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, 23–28 Apr 2023, EGU23-16742, https://doi.org/10.5194/egusphere-egu23-16742, 2023.

11:05–11:15
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EGU23-9177
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HS8.2.6
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ECS
|
On-site presentation
Erwin Don Racasa, Jakob Kienzler, Sate Ahmad, Cheryl Batistel, Simeon Choo, Miaorun Wang, Anna-Kathrina Jenner, Bernd Lennartz, and Manon Janssen

Most research on submarine groundwater discharge (SGD) focuses on sandy beaches. Less studies have investigated environments with low hydraulic conductivity (Ks) 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 Ks 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 Ks 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, 23–28 Apr 2023, EGU23-9177, https://doi.org/10.5194/egusphere-egu23-9177, 2023.

11:15–11:25
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EGU23-12009
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HS8.2.6
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ECS
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On-site presentation
Nele Grünenbaum, Janek Greskowiak, Rezwana Binte Delwar, and Gudrun Massmann

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, 23–28 Apr 2023, EGU23-12009, https://doi.org/10.5194/egusphere-egu23-12009, 2023.

11:25–11:35
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EGU23-5781
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HS8.2.6
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On-site presentation
Janek Greskowiak, Stephan Seibert, Vincent Post, and Gudrun Massmann

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, 23–28 Apr 2023, EGU23-5781, https://doi.org/10.5194/egusphere-egu23-5781, 2023.

11:35–11:45
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EGU23-6488
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HS8.2.6
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ECS
|
Virtual presentation
Juuso Ikonen, Nina Hendriksson, Samrit Luoma, Yann Lahaye, and Joonas Virtasalo

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, 23–28 Apr 2023, EGU23-6488, https://doi.org/10.5194/egusphere-egu23-6488, 2023.

11:45–11:55
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EGU23-7125
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HS8.2.6
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ECS
|
On-site presentation
Sophie ten Hietbrink, Ji-Hoon Kim, Arunima Sen, Aivo Lepland, Beata Szymczycha, Seyed Reza Saghravani, Jochen Knies, and Wei-Li Hong

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, 23–28 Apr 2023, EGU23-7125, https://doi.org/10.5194/egusphere-egu23-7125, 2023.

11:55–12:05
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EGU23-7303
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HS8.2.6
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On-site presentation
Yan Zheng, Peng Zhang, Xuejing Wang, Yinghui Wang, Hailong Li, and Junjian Wang

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, 23–28 Apr 2023, EGU23-7303, https://doi.org/10.5194/egusphere-egu23-7303, 2023.

12:05–12:15
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EGU23-1962
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HS8.2.6
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On-site presentation
Lotta Purkamo, Cátia M. Ehlert von Ahn, Tom Jilbert, Muhammad Muniruzzaman, Annette Kock, Herrman Bange, Anna Jenner, Michael E. Böttcher, and Joonas Virtasalo

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, 23–28 Apr 2023, EGU23-1962, https://doi.org/10.5194/egusphere-egu23-1962, 2023.

12:15–12:20

Posters on site: Wed, 26 Apr, 16:15–18:00 | Hall A

Chairpersons: Audrey Sawyer, Marta Cosma, Valentí Rodellas Vila
A.149
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EGU23-321
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HS8.2.6
John Richins, Kevin Befus, and Kirk Rodgers

Terrestrial groundwater discharges to coastal drainage networks as baseflow and to coastal waters as fresh submarine groundwater discharge. If groundwater discharge comprises a large portion of inflow or has a different composition than the receiving waters, these groundwater fluxes can strongly influence the water quality of coastal waters. However, the spatial distribution of such fluxes makes quantifying the net effect of groundwater discharge difficult with field data. Furthermore, future climate conditions may change how groundwater interacts with surface water through changes in precipitation, evapotranspiration, and sea level. Mobile Bay is at the nexus of anthropogenic-driven sea level rise and water quality issues, making it an ideal study location. Therefore, we are developing and calibrating MODFLOW-based groundwater flow models using scripted workflows within the Python programming language and the FloPy library to investigate three questions: 1) How much groundwater discharge occurs, 2) where does the groundwater discharge occur, and 3) how do environmental variations associated with climate change affect the location and volume of groundwater discharge to Mobile Bay. By answering these three questions, we will provide valuable knowledge regarding variations in the magnitude and location of coastal groundwater discharge due to possible environmental changes.

How to cite: Richins, J., Befus, K., and Rodgers, K.: Coastal Groundwater Discharge Simulations/Models for Mobile Bay, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-321, https://doi.org/10.5194/egusphere-egu23-321, 2023.

A.150
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EGU23-1372
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HS8.2.6
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ECS
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Lena Thissen, Janek Greskowiak, and Gudrun Massmann

Freshwater lenses (FWLs) are an important source of drinking water on many islands in the world. Thus, it is important to study their volumes. Many case studies have already been carried out on real world islands to approximate the FWL volume of the respective islands. Also, generic studies on FWLs exist that consider idealised island shapes such as circular or strip islands. However, to the authors’ knowledge there is no study so far that describes the general relationship between the island’s shape and the freshwater volume of its lens. Here we show that there is a relationship between these two quantities. In our approach, we characterized the shape of the islands using a circularity parameter; the volume was approximated using this shape and a simple numerical steady-state Ghyben-Herzberg approach. While we found a strong relationship between island shape and FWL volume, the relationship between the island shape and the depth of the FWL was less clear. The findings of this study can help to estimate the FWL volume on islands for which there are no case studies available.

How to cite: Thissen, L., Greskowiak, J., and Massmann, G.: Estimating Freshwater Lens Volume based on Island Circularity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1372, https://doi.org/10.5194/egusphere-egu23-1372, 2023.

A.151
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EGU23-1392
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HS8.2.6
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ECS
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Mareike Schloo, Laura Braeunig, Victoria Burke, Janek Greskowiak, and Gudrun Massmann

The salinization of groundwater due to saltwater intrusion in coastal regions requires efficient mitigation strategies, e.g., the infiltration of desalinated water via Managed Aquifer Recharge, such as ponded infiltration or injection wells, to ensure the supply of drinking water. It has been shown that the infiltration of desalinated water into an aquifer may result in a series of deteriorating chemical reactions. To counteract this problem, the goal of the cooperative project “innovatION” is to develop a new desalination membrane, which aims to reduce mainly monovalent ions only to achieve a more sustainable and efficient desalination technology.

To give an outlook on possible water-sediment interactions during the infiltration of monovalent desalinated water into aquifers, column experiments were realised using different dune sediments from the Island of Langeoog, North-West Germany. The experimental data suggest cation exchange and calcite dissolution as the main processes occurring (compare abstract Braeunig et al.). For a process-based quantitative description and analysis of all relevant processes and their interactions, PHREEQC models were created for the individual experiments. The models support the experimental data and the hypothesized reaction network, and even allowed for the identification of  reactions (e.g. cation exchange and calcite dissolution), as well as their impact on the overall system behaviour.

How to cite: Schloo, M., Braeunig, L., Burke, V., Greskowiak, J., and Massmann, G.: Hydrogeochemical modelling of water-sediment interactions during infiltration of monovalent-partial desalinated water into different dune sediments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1392, https://doi.org/10.5194/egusphere-egu23-1392, 2023.

A.152
|
EGU23-2160
|
HS8.2.6
|
ECS
Chong Sheng, Jiu Jimmy Jiao, Xin Luo, Jinchao Zuo, Lei Jia, and Jinhe Cao

The ~100 ka Milankovitch cycles during the Quaternary generated a significant sea level fluctuation with the lowest sea level of 120 m below the present level at the Last Glacial Maximum in the coastal areas. The large-river deltaic estuaries (LDEs), due to the proximity to sea environments, therefore, may archive the periodical transgression and regression information. During delta-front progradation, the sedimentation process is usually dominated by coarse-grained fluvial deposits, and the river networks extend further to the sea, whereas during marine transgressions, fine-grained marine sediments dominated by clay and silt are deposited. From a hydrogeological perspective, this geologic scenario leads to the formation of multi-layered aquifer-aquitard systems in current continental shelves. Therefore, we hypothesize that the offshore freshened groundwater (OFG) may be widely distributed in the LDEs and their adjacent continental shelves.

Pearl River is the second largest river in China in terms of water discharge, and the accompanying subaqueous paleo delta extends to the slope at the northern margin of the South China Sea with an offshore distance of 200 km. To address the key scientific issues raised in OFG of the LDEs and their adjacent shelves, we have studied the offshore hydrogeology, marine seismic profiles, and porewater hydrogeochemistry in the subaqueous paleo-delta and adjacent shelf of the Pearl River. A total of 31 offshore boreholes with high-resolution porewater geochemistry profiles have been obtained in this area. These boreholes have led to an identification of a large and unexpected OFG with a volume of ~523.3×109 m3, with the freshwater (salinity < 1 PSU) extending as far as 55 km offshore. The total OFG volume is twice of the annual discharge of the Pearl River. The distribution of the OFG is closely related to the morphology of the subaqueous paleo-delta of the Pearl River, where the buried paleochannel system is widely distributed. The values of δ2H and δ18O together with the chlorinity of the OFG in the Pearl River Estuary and adjacent shelf clearly reveal its meteoric origins. Besides, the systematic analysis of water quality indices including major ions, nutrients, heavy metals, and trace elements indicates that the OFG can be used as potable water with minor treatment or raw water source for effective desalination. Hotspots of OFG in the LDEs and their adjacent shelves, likely a global phenomenon, have a great potential for useful water resources for highly urbanized coastal areas suffering from water shortage.

How to cite: Sheng, C., Jiao, J. J., Luo, X., Zuo, J., Jia, L., and Cao, J.: Occurrence of offshore freshened groundwater in the Pearl River Estuary and adjacent continental shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2160, https://doi.org/10.5194/egusphere-egu23-2160, 2023.

A.153
|
EGU23-2505
|
HS8.2.6
|
ECS
Xiaogang Chen and Ling Li

Mangroves and saltmarshes are important coastal blue carbon ecosystems characterized by high soil carbon storage. Groundwater flow and associated carbon export in mangroves and saltmarshes is the frontier and challenging issue in the estimates of coastal blue carbon sinks. Large amounts of groundwater-derived sediment carbon outwelling remains in the ocean and may represent an important carbon sink, which are potentially significant yet poorly understood components of mangrove and saltmarsh carbon budgets. This study aims to how to quantify the groundwater flow and related carbon fluxes, analyze their driving mechanisms, and then reassess the carbon budget and carbon sink potential of mangroves and saltmarshes.

How to cite: Chen, X. and Li, L.: Groundwater Flow Mechanisms and Related Carbon Sink Potential in Coastal Blue Carbon Ecosystems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2505, https://doi.org/10.5194/egusphere-egu23-2505, 2023.

A.154
|
EGU23-2506
|
HS8.2.6
Husam Baalousha, Behshad Koohbor, Marwan Fahs, and Anis Younes

Coastal aquifers are very important as they provide water supply for more than one billion inhabitants around the world. Due to the increased stresses on the coastal aquifers and the anticipated sea level rise from climate change, the risk of seawater intrusion is high. Most of coastal aquifers around the Mediterranean and in the Middle East are comprised of fractured limestone. While numerous studies have been done to investigate flow in coastal aquifers, these studies consider classical flow in un-fractured aquifers. The seawater intrusion in fractured aquifers remains unexplored.

This study focuses on simulating seawater intrusion in fractured porous media using the dual porosity approach (DP). A new numerical model is developed for sweater intrusion with the DP approach, based on advanced finite element schemes. The newly developed model is applied to the common benchmark of Henry’s problem and verified by comparison against a semi-analytical solution and a standard finite element solution obtained with COMSOL Multiphysics. The newly developed model is used to perform a sensitivity analysis for understanding the effect of parameters on the model predictions. The results show that the salinity in the domain is mainly controlled by the mass exchange rate between fractures and porous matrix. The DP approach is compared to the discrete fracture (DF) approach, via an inverse approach procedure. Synthetic observation data are generated with the DF approach and then used to calibrate the DP model. Agreement between the predictions of the DP and DF approaches depends on the fracture density.

How to cite: Baalousha, H., Koohbor, B., Fahs, M., and Younes, A.: Comparing dual porosity approach and discrete fracture network for modelling seawater intrusion in fractured porous media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2506, https://doi.org/10.5194/egusphere-egu23-2506, 2023.

A.155
|
EGU23-3293
|
HS8.2.6
Ji Won Hwang, In Wook Yeo, and Dae Hyoung Lee

Islands rely on groundwater for their water resources. Groundwater overdraft lowers groundwater levels and causes seawater intrusion, which results in groundwater shortage and deterioration. Many remote small islands in the Yellow Sea of Korea suffer from water shortages. To supply 100 m3/d of groundwater to 197 island residents, a subsurface dam was constructed in Anma island (5.8 km2), Korea, from Sep. 2019 to Oct. 2020. This study assessed the effect of the subsurface dam on the groundwater sustainability of the small island. The Quaternary alluvium (0.48 km2), located in the central part of Anma island, is a main aquifer and extends to a thickness of about 40 m, which rests on the Cretaceous tuff. The water table lies approximately 5 m deep below ground and fluctuates within alluvial aquifers. The subsurface dam was installed through the alluvial layer to the top of the tuff to reduce groundwater discharge to sea and was built across the coastal alluvial aquifers. Aquifer tests found hydraulic conductivity for alluvium and tuff to be 3.910-6 and 2.8410-6 m/sec, respectively. The average water levels measured in Sep. 2019 from the five observation wells in alluvial aquifers increased in Sep. 2021 by 0.28 m (9%) after the dam construction, but the precipitation also increased by 18% in the same period. Therefore, the increase in water level could not account for enhanced groundwater storage due to the dam. Numerical simulations were carried out with annual precipitation of 1,100 mm and a recharge rate of 7.2% to evaluate the change in groundwater storage before and after the subsurface dam. The simulation results showed that the subsurface dam contributes to an increase of only 0.06 m in water level in alluvial aquifers. This represents an increase in groundwater storage of 5,760 m3 in the alluvial aquifer (considering the specific yield of 0.2), which amounts to 15% of the annual groundwater recharge of 38,016 m3 in alluvial aquifers. The additional increase in groundwater storage due to the subsurface dam could complement the scheduled groundwater development of 36,000 m3/y from the alluvial aquifers, particularly when the recharge declines due to drought. The simulation also indicated that regardless of the subsurface dam, seawater intrusion was found to be insignificant due to the small amount of groundwater pumping at 100 m3/d.

How to cite: Hwang, J. W., Yeo, I. W., and Lee, D. H.: Evaluating subsurface dams for the sustainable use of groundwater in remote small islands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3293, https://doi.org/10.5194/egusphere-egu23-3293, 2023.

A.156
|
EGU23-3641
|
HS8.2.6
|
ECS
Kyra H. Adams, Benjamin Hamlington, Cedric David, John Reager, Audrey Sawyer, Brett Buzzanga, and Jacob Fredericks

Coastal regions are home to more than 40% of the world’s population and often depend on fresh groundwater resources to sustain economic, residential, and recreational activities. However, coastal groundwater is under threat from saltwater intrusion (SWI), due in part to rising sea level and climate change. In this work we provide, for the first time, a global assessment of SWI and SWI vulnerability due to regional differences in sea level rise and predicted changes in recharge, leveraging NASA datasets of recharge, seawater density, and IPCC AR6 sea level rise. We show that climate-driven recharge changes drove 45% of watersheds to experience SWI, while SLR drove 92% of watersheds to experience SWI. By synthesizing various global datasets within an analytical framework, the work provides the first step towards evaluating the coastal impacts of saltwater intrusion in a changing climate. 

How to cite: Adams, K. H., Hamlington, B., David, C., Reager, J., Sawyer, A., Buzzanga, B., and Fredericks, J.: Estimating saltwater intrusion using remote-sensing datasets and analytical approaches, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3641, https://doi.org/10.5194/egusphere-egu23-3641, 2023.

A.157
|
EGU23-3786
|
HS8.2.6
Mohammed Benaafi, Sani Abba, and Isam Aljundi

Seawater intrusion is considered the main threat to fresh groundwater resources in coastal regions, especially in arid areas such as Saudi Arabia. For decades, the coastal community in the eastern province of Saudi Arabia has depended on groundwater for domestic and agricultural uses. The current study objective is to assess the degree of seawater intrusion and its impact on the shallow groundwater in the eastern coastal region of Saudi Arabia. We integrated three techniques, including hydrogeology, hydrochemistry, and electrical resistivity. A hydrogeological and electrical resistivity field survey was conducted, coupled with in situ measurement of the shallow groundwater's hydrochemical properties (pH, ORP, DO, EC, temperature, and turbidity). This was followed by collecting 24 water samples for the laboratory analysis of hydrochemical properties (major cation and anion). Geospatial maps of salinity (electrical conductivity (EC)) and chloride were established to trace their concentrations from the sea landward. The hydrochemical spatial maps were correlated with the geoelectrical measurements to delineate the lateral and vertical extent of seawater intrusion in the studied shallow coastal aquifer. The hydrochemical results show that chloride and sodium are the dominant ions, indicating seawater intrusion is the source of elevated salinity. The salinity (EC) and chloride concentrations of the tested GW range from 6452 μS/cm to 44420 μS/cm, and 1041 mg/L to 8523 mg/L, respectively. The study results indicate that the shallow groundwater has been contaminated with seawater up to around 4 km landward, with a maximum depth of 30 m. The main conclusion of the current study is that decades of overexploitation of groundwater aquifers in the studied region have resulted in salinity contamination of shallow groundwater by seawater intrusion to levels of saline and brackish water. The result of this study may help the decision-maker implement a proper management measure to safeguard groundwater resources for coastal communities and develop a plan to remediate the shallow groundwater aquifer through desalination and reinjection for the environment and ecosystem benefits.

How to cite: Benaafi, M., Abba, S., and Aljundi, I.: Impact assessment of seawater intrusion on shallow coastal groundwater in eastern Saudi Arabia using a multidisciplinary approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3786, https://doi.org/10.5194/egusphere-egu23-3786, 2023.

A.158
|
EGU23-4624
|
HS8.2.6
|
ECS
Shengchao Yu, Xiaolang Zhang, Jiu Jimmy Jiao, and Hailong Li

Tide and wave are both significant driven forces of submarine groundwater discharge (SGD). The hydrodynamic process of tidal SGD is well studied while wave induced part is still challenging to quantify. In this paper, an analytical study was performed to investigate wave pumping-driven SGD in subsea sediments. The loading effect of seawater weight overlying the seabed has been considered in the groundwater flow equation. Two dimensional simulations governed by this analytical solution were applied to examine water head fluctuations within permeable sediments. The analytical solution was validated by simulated water head fluctuations, which revealed the influence of poro-elastic property of sediment on the hydrodynamic process within the permeable seabed. The analytical and simulated results may provide guidance for assessing the wave-induced SGD and shed light on modeling the biogeochemistry process in wave dominated porous seabed environment.

How to cite: Yu, S., Zhang, X., Jiao, J. J., and Li, H.: Loading effect on wave pumping driven seawater-groundwater circulation in submarine aquifer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4624, https://doi.org/10.5194/egusphere-egu23-4624, 2023.

A.159
|
EGU23-5121
|
HS8.2.6
Jan Scholten, Jan Schroeder, Feng-Hsin Hsu, and Volker Liebetrau

The radium isotopes (224Ra, t½ = 3.7 d;223Ra, t½ =11.5 d; 228Ra, (t½ = 5.7 y); 226Ra, t½ =1600 y) are well established tracers for the detection and quantification of submarine groundwater discharge (SGD). In the Eckernförde Bay (Western Baltic Sea) SGD associated with pock marks have been observed at many locations on the seabed (~ 20-25m water depths). In order to investigate the spatial and temporal SGD variability we repeatedly measured between 2016 and 2020 the radium (224Ra, 223Ra, 228Ra, 226Ra) distribution in the water column of the Eckernförde Bay and the adjacent Kiel Bay. In general, the water-column radium profiles are characterized by relatively low radium concentrations in the upper water column (~ <15m water depth, salinity ~ 12-18) and significantly higher ones in deep waters (~ 20-25m water depth; salinity ~ 21-25). High radium occurs also in areas far off the coast where pock marks have previously not been reported. Monthly/bi-monthly measurements at the time-series station Boknis Eck (Eckernförde Bay) revealed that this high radium occurs only between spring and autumn, a period, where bottom waters have low or negligible oxygen content. This observation may indicate that processes other than SGD may contribute to seasonal changes in deep water radium. In the presentation possible other radium sources like e.g., diagenetic radium supply from anoxic sediments, sediment bioturbation and resuspension, advection of deep waters from the North Sea, are discussed in order to understand to what extent the radium distribution in the western Baltic Sea can still be interpreted as a tracer of SGD.

How to cite: Scholten, J., Schroeder, J., Hsu, F.-H., and Liebetrau, V.: Widespread occurrence of high radium concentrations in bottom waters of the Eckernförde and Kiel bays (Western Baltic Sea): Are these related to submarine groundwater discharge?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5121, https://doi.org/10.5194/egusphere-egu23-5121, 2023.

A.160
|
EGU23-6141
|
HS8.2.6
|
ECS
Rena Meyer, Janek Greskowiak, and Gudrun Massmann

Beach aquifers are the contact zone between the terrestrial groundwater and the ocean. Where the fresh groundwater mixes with the recirculating saltwater biogeochemical reactions take place and change the composition of the water. These biogeochemical reactions and resulting element net fluxes are linked to residence times and dispersive mixing processes that depend on dynamic density-driven groundwater flow and transport processes. In the present study we investigate the effect of dynamic hydro- and morphological boundary conditions and aquifer parameters on the groundwater flow- and transport processes in the deep subsurface of a high energy beach along a 2D transect perpendicular to the shore by means of a density-driven flow and transport model (SEAWAT). The ‘classical’ concept of a beach aquifer in the intertidal zone describes the stable establishment of three separated, layered water bodies. The fresh water discharge tube (‘FDT’) separates the wave and tide induced upper saline plume (‘USP’) from the saltwater wedge (‘SW’). Recent research challenges this concept of stable position of water bodies under high energy beach conditions which are characterized by high wave and tidal amplitudes. Our modelling results show that dynamic beach morphology has an important effect on the spatio-temporal flow and transport patterns as well as on the salt distribution and residence times in the deep beach aquifer and results in the formation of several time variable FDTs and USPs. Moreover, our research shows the sensitivities of the freshwater-saltwater interface in the subsurface to hydraulic conductivity, anisotropy, dispersion and boundary conditions like storm surges and fresh water inflow. The result is a complex, temporally and spatially variable picture of the ‘classical’ stable USP with dynamic interfaces. Hence, our results elucidate the physical conditions in the deep subsurface that are relevant for the spatio-temporal distribution of chemical reactions that are linked to the mixing interfaces between the water bodies. As a consequence the biochemical reactions might be enhanced with effects on the element net fluxes to the sea. 

How to cite: Meyer, R., Greskowiak, J., and Massmann, G.: Effects of variable beach morphology, storm surges and aquifer parameters on the salinity distribution in the deep subsurface of a high energy beach – a generic modelling approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6141, https://doi.org/10.5194/egusphere-egu23-6141, 2023.

A.161
|
EGU23-7483
|
HS8.2.6
|
ECS
Xue Meng, Yangxiao Zhou, Jinguo Wang, Tibor Stigter, Fatima Mussa, Dinis Juizo, and Yun Yang

Groundwater is an important source for water supply in Maputo City, Mozambique. A groundwater monitoring network has been established for systematically monitoring of groundwater level and salinity changes. Automatic data loggers were installed to register groundwater levels and EC values every hour. Three observation wells were installed in the coast to detect seawater intrusion. The measured groundwater levels show clearly tidal effects. In this study, time series analysis methods were used to identify dominant periodic changes in groundwater levels (GWLs), effects of tide, and estimation of aquifer diffusivity using tidal effect parameters. Autocorrelation and cross-correlation analysis were used to estimate the periodic components and lag time between the tide and GWLs, respectively. Spectral analysis was used to ascertain the dominant periodic components in the tide and GWLs by means of estimating amplitude spectrum and power spectrum density. Cross-spectral analysis was used to determine the lag time between the tide and GWLs by means of estimating cross-power spectrum density. Furthermore, wavelet analysis was used to investigate changes of periodic components over the measured period. The estimated amplitudes and lag times were used to estimate aquifer diffusivity. The results identified dominant periodic component with a 12hour period both in the tide and GWLs. However, groundwater level is lag behind the tide with 2-4 hours depending on the distance of the observation wells to the costal line. The wavelet analysis results show no changes of dominant periodic components over the time. Therefore, the estimated amplitude and lag time were used to estimate aquifer diffusivity. The estimated parameter values are 2.72595E-05 h/m2, 6.97843E-05 h/m2, and 6.14906E-05 h/m2 from observation wells. These correspond values of transmissivity as 440 m2/d, 172 m2/d, and 195 m2/d, respectively. The estimated transmissivity values are useful for constructing saltwater intrusion models.

How to cite: Meng, X., Zhou, Y., Wang, J., Stigter, T., Mussa, F., Juizo, D., and Yang, Y.: Tidal effects on groundwater levels in Maputo, Mozambique, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7483, https://doi.org/10.5194/egusphere-egu23-7483, 2023.

A.162
|
EGU23-10144
|
HS8.2.6
Abrahan Mora and Jürgen Mahlknecht

This study shows the dynamics of several trace elements in groundwater of a coastal sedimentary aquifer of western Mexico affected by seawater intrusion. The results of the chemical analyses performed in groundwater indicated that the elements Ba, Sr, Li, Fe, Co, V, Se, and Re are mobilized from the aquifer sediments to the aqueous phase during seawater intrusion, whereas Rb is removed from solution during salinization. Overall, the mobilization/removal of these elements may be due to processes such as cation exchange, the increase of the ionic strength due to increasing salinity and the weathering of biogenic carbonates in an oxidizing environment. Other elements such as Mo, Ni, Cr, Ta, and W displayed low or no mobilization during the seawater intrusion. This may indicate that dissolution of biogenic carbonates is not their main sources and that these elements are not affected by the ionic strength caused by groundwater salinization. Finally, other elements such as As, U, Ge, Sb, Cu, and Mn displayed an undefined tendency, showing concentrations above and around the theoretical seawater mixing line. These elements may be likely affected by other processes rather than seawater intrusion, which may explain their undefined tendency during groundwater salinization. In general, geochemical processes such as carbonate/sulfate complexation (in the case of U), organic matter complexation (in the case of Cu), metalloid co-occurrence (As, Ge, and Sb), and redox processes (in the case of As and Mn) may play a key role in determining the concentration levels of these elements in this coastal groundwater system affected by seawater-freshwater mixing.

How to cite: Mora, A. and Mahlknecht, J.: Dynamics of trace elements during seawater intrusion in a sedimentary aquifer of western Mexico, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10144, https://doi.org/10.5194/egusphere-egu23-10144, 2023.

A.163
|
EGU23-10384
|
HS8.2.6
Il Hwan Kim and Sunwoo Chang

Water use in island is mostly dependent on groundwater. Saltwater intrusion has occurred in aquifers in island, and available freshwater is decreasing. Sea level rise due to the climate crisis is increasing the range of saltwater intrusion in coastal aquifers. Saltwater intrusion is driven by complex mechanisms in coastal aquifers, including sea level rise, decrease of fresh submarine groundwater discharge (FSGD) and pumping from coastal aquifers, and geologic properties of coastal aquifers. FSGD from coastal aquifers, coupled with sea level rise, has a significant impact on saltwater intrusion and can reduce the amount of available water resources. Previous FSGD studies have focused on local areas where a large amount of discharge is observed. The existence of FSGD was diagnosed or estimated through various approaches such as field observation, isotope tracking, and water balance analysis. In this study, quantitative analysis of FSGD was performed using the freshwater-saltwater interface estimation formula of coastal aquifers. The location of the freshwater-saltwater interface was calculated using the Ghyben-Herzberg (G-H) equation, and the freshwater above the interface was estimated by FSGD. The geographic information system (GIS) was used to estimate FSGD using observation data over a large area. It was used to interpolate the observation data in a large area in grid units, and the Inverse Distance Weight method was used as the interpolation method. The interpolated data was used to input data for estimating FSGD. The study area was Jeju Island, the largest island in Korea. Quantitative estimation of FSGD can be used as a scientific basis for establishing water resource management plans in island.

Acknowledgement

Research for this paper was carried out under the KICT Research Program (project no.20220275-001, Development of coastal groundwater management solution) funded by the Ministry of Science and ICT.

 

How to cite: Kim, I. H. and Chang, S.: Quantitative estimation of fresh submarine groundwater discharge in Jeju island using geographic information system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10384, https://doi.org/10.5194/egusphere-egu23-10384, 2023.

A.164
|
EGU23-10642
|
HS8.2.6
Xiayang Yu, Pei Xin, Zhaoyang Luo, and Li Pu

Freshwater injection is a practical and efficient solution to mitigate seawater intrusion in overexploited coastal aquifers. Previous studies predominantly considered isothermal conditions and overlooked temperature contrasts. How thermal effects of freshwater injection regulate flow processes and salinity distributions is poorly understood. This study investigated the dynamic characteristics of salinity distributions and seawater recirculation in coastal aquifers subjected to freshwater injection and tides. The processes were simulated using SUTRA-MS (a model simulating porewater flow coupled with salt and heat transport). The transience of upper saline plume and saltwater wedge responding to injected freshwater will be discussed here in detail. We will also discuss the thermal plume-induced changes in salinity distributions and water effluxes. Finally, the overshoot of total water efflux in response to the thermal impacts of freshwater injection will be discussed.

How to cite: Yu, X., Xin, P., Luo, Z., and Pu, L.: Effects of freshwater injection on tidally influenced coastal unconfined aquifers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10642, https://doi.org/10.5194/egusphere-egu23-10642, 2023.

A.165
|
EGU23-11444
|
HS8.2.6
Gualbert Oude Essink, Daniel Zamrsky, Jude King, Joost Delsman, Jarno Verkaik, and Marc Bierkens

Accessible and reliable freshwater sources are essential for human communities and freshwater ecosystems worldwide. In coastal regions, groundwater is the main freshwater sources for drinking water thanks to its high quality, easy accessibility and relatively constant supply. Both anthropogenic (e.g. poor water management and rising population numbers) and natural (e.g. climate change related sea-level rise and storm surges) create additional pressure on coastal freshwater resources. This pressure can lead to declines in fresh groundwater availability caused by salinization and over-exploitation, especially in densely populated areas with intensive agricultural production that already have a high freshwater demand. Groundwater salinization can have severe negative impacts on environmental, economic and human health conditions in these areas. Understanding of current and future threats to fresh groundwater availability by salinization allows coastal communities to better adapt to these risks. Groundwater salinity models are typically applied to study groundwater salinization in local and regional settings and thus provide information for water management bodies to improve their mitigation and adaptation measures. The added value of a global 3D coastal groundwater salinity map would be that it provides important insights into the most threatened regions worldwide, while also identifying coastal regions with similar groundwater salinization risks and similar suitable mitigation and adaptation measures to tackle them. The global 3D groundwater salinity map can also be used as a starting point to evaluate future groundwater salinity developments under multiple climate change and socio-economic scenarios. Hitherto, there were several key obstacles preventing us from building a global 3D groundwater salinity map; the most important ones being the lack of standardized global hydrogeochemical, geological and geophysical datasets and inadequate computational resources and numerical codes. Recent developments in code parallelization (e.g. iMOD-WQ and in due time MODFLOW6) and access to high performance computing allows us to simulate global 3D groundwater salinity by splitting the world into smaller regional scale 3D groundwater salinity models and simulating these in parallel. Moreover, the advancement in available global datasets and creation of a unified global hydrogeological database and schematization allow us to better estimate regional subsurface conditions. Here, we demonstrate the process of building the global 3D groundwater salinity map and show its potential applications. Ultimately, identifying the most threatened regions in near future can lead to better water management strategies to limit the negative impacts of groundwater salinization on fresh groundwater resources, and/or to come up with strategies to explore additional new ones.

How to cite: Oude Essink, G., Zamrsky, D., King, J., Delsman, J., Verkaik, J., and Bierkens, M.: 3D groundwater salinity mapping of the global coastal zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11444, https://doi.org/10.5194/egusphere-egu23-11444, 2023.

A.166
|
EGU23-11976
|
HS8.2.6
Ivan Lovrinovic, Iva Aljinovic, and Veljko Srzic

The river Neretva coastal aquifer system is the largest agricultural area along Croatia's Adriatic Sea coast. With the construction of an embankment, pumping stations, and melioration channels, a once-marshy area has been transformed into an area with favourable agricultural conditions.  Due to its proximity to the sea, numerous anthropogenic impacts in the region, and climate change, groundwater and surface water in the study area are presently significantly influenced by the salinization process. 
Multiple times per year, groundwater and surface water samples are collected to fully comprehend saltwater processes and the origin of saltwater in the study area. Within each sample, the concentration of major ions (K+, Na+, Ca+, Mg2+, Cl-, HCO3- and SO42-) is determined under laboratory conditions.
The analysis of major ions indicates that seawater intrusion is the primary source of salinization in both unconfined and confined aquifers. In contrast to unconfined aquifer, confined aquifer is minimally affected by precipitation and surface water regimes. In addition, samples of groundwater from unconfined aquifer are categorized into three groups based on their degree of contamination with seawater. The samples from the Diga area that are closest to the Adriatic Sea are most significantly influenced by the seawater. The groundwater quality in the Jasenska area varied significantly between dry and rain periods, whereas groundwater samples from the area of Vidrice revealed the lowest level of seawater contamination.   Neretva river surface water samples reveal the presence of a salt wedge, while river Mala Neretva samples indicate that river Mala Neretva is the primary source of freshwater in the study area during dry season.

How to cite: Lovrinovic, I., Aljinovic, I., and Srzic, V.: Evaluation of surface and groundwater quality and identification of saltwater sources by hydrogeochemical analysis in river Neretva coastal aquifer system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11976, https://doi.org/10.5194/egusphere-egu23-11976, 2023.

A.167
|
EGU23-12125
|
HS8.2.6
|
ECS
Marta Cosma, Ester Zancanaro, Iva Aljinović, Francesco Morari, Veljko Srzić, Pietro Teatini, Luigi Tosi, Alessandro Bergamasco, Anna Botto, Matteo Camporese, Chiara Cavallina, Cristina Da Lio, Sandra Donnici, Ivan Lovrinović, Ivan Racetin, Luca Zaggia, Claudia Zoccarato, and Paolo Salandin

Saltwater intrusion in coastal aquifers is a global problem recently worsened by anthropogenic activities (e.g., aquifer overexploitation, hydraulic reclamation and drainage of low-lying areas) and climate change effects (e.g., severe droughts, sea level rise) that contribute to reduce groundwater natural recharge, water quality, and agricultural production. Many low-lying coastal plains facing the Adriatic Sea are strongly affected by saltwater intrusion with serious consequences on agricultural activities and tourism that may become dramatic in a relatively short time due to climate change. In this framework, this work aims to identify monitoring strategies to characterize the process of saltwater intrusion under the effects of climate change and recommend appropriate countermeasures in two Adriatic low-lying coastal plain: south of the Venice Lagoon (north-eastern Italy), and at the Neretva River mouth (south-eastern Croatia).

Geomorphologic, stratigraphic, hydrogeologic, and agricultural data were collected to characterize the aquifer system at both sites and assess the effects of seawater intrusion on agricultural productivity. Saltwater intrusion was monitored and analysed through monitoring systems that provide qualitative and quantitative information on the processes influencing groundwater and surface water dynamics within the two coastal systems. Moreover, laboratory physical models were developed to serve as benchmarks for the numerical models used to simulate the field results. Numerical modelling reliably implements boundary and initial conditions defined in-situ on both sites, simulates existing states, specifies different scenarios, and predicts salinization dynamic changes caused by climate changes.

The results of the research activities include the development of specific tools for the management of agriculture-related activities and freshwater resources in coastal areas including vulnerability assessment, mitigation plans, and countermeasures against salt contamination. These results were obtained by integrating the findings gained on both sites, considering differences and peculiarities of the specific areas that are representative of many low-lying plains located on both sides of the Adriatic coast.

This study has been funded by the contribution from the EU cofinancing and the Interreg Italy–Croatia Cross Border Collaboration (CBC) Programme 2014–2020 (Priority Axes: Safety and Resilience) through the European Regional Development Fund as a part of the projects MoST  (AID: 10047742) and SeCure (AID: 10419304).

How to cite: Cosma, M., Zancanaro, E., Aljinović, I., Morari, F., Srzić, V., Teatini, P., Tosi, L., Bergamasco, A., Botto, A., Camporese, M., Cavallina, C., Da Lio, C., Donnici, S., Lovrinović, I., Racetin, I., Zaggia, L., Zoccarato, C., and Salandin, P.: Approaches and methodologies to monitor and mitigate saltwater intrusion in the Adriatic coastal plains, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12125, https://doi.org/10.5194/egusphere-egu23-12125, 2023.

A.168
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EGU23-16042
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HS8.2.6
|
ECS
Jose Tur-Piedra, Juanjo Ledo, Pilar Queralt, Alex Marcuello, Marc Diego-Feliu, Aaron Alorda-Kleinglass, Valentí Rodellas, and Albert Folch

In recent decades, there has been a growing interest in accurately characterizing the process of fresh groundwater discharge into the coastal ocean since it represents a significant pathway for solute from land to sea. Although SGD research has focused on quantifying water and solute fluxes, little is known about the physical forces, mechanisms, and distribution of SGD.

Investigating interaction processes between the fresh and saltwater under the sea bottom represents a logistical and technical challenge. Traditionally, electrical geophysical methods have been solely used in the terrestrial part, and during the last decades, the method techniques have been introduced to marine environments. However, these methods are limited by their characteristics in obtaining subsurface resistivity data in the first few meters of the coast. Especially in the land-sea transition zone of microtidal environments, where many of the most critical processes take place, this data is traditionally not available. In this study, we developed a combination of methods to bridge the data gap between the terrestrial and marine realms. This study aims to characterize the FSGD in two aquifers from different geological contexts (alluvial and karstic) on the Mediterranean coast near Barcelona.

To study the transition zone of the aquifer with a good spatial resolution, the amphibious electrical tomography method has been chosen, combining a terrestrial and aquatic line fixed in contact with the marine sediment. Profiles perpendicular to the coastline has been made in a shallow water area to obtain electrical resistivity data of the seabed at a local scale. The configuration used is Dipole-Dipole and Wenner-Schlumberger, with an investigation depth of 10 m. Based on the results, designing a 3D model of electrical resistivity in marine sediments was possible. In parallel to getting electrical resistivity data, manual piezometers were used to bring porewater samples at different points.

We have been able to measure the presence of freshwater or brine, and we have identified differences in the geometry depending on the geological context, where the karst environment is the one that presents a more significant proportion of freshwater saturating the marine sediments. Therefore, amphibious electrical tomography is a non-invasive method that detects resistive zones of marine sediments associated with discharge processes and can be instrumental in characterizing the presence and distribution of SGD.

 

Acknowledgments

This work was partly funded by the Spanish Government (grant no. PID2019-110212RB-C22) and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency.

How to cite: Tur-Piedra, J., Ledo, J., Queralt, P., Marcuello, A., Diego-Feliu, M., Alorda-Kleinglass, A., Rodellas, V., and Folch, A.: Combining terrestrial and marine electrical resistivity methods to improve data acquisition in the land-sea transition zone., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16042, https://doi.org/10.5194/egusphere-egu23-16042, 2023.

A.169
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EGU23-17061
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HS8.2.6
|
ECS
Guillem Buxó-Escapa, Gustavo Cárdenas Castillero, Alejandro Adán, Michal Kuráž, and Albert Folch

Submarine groundwater discharge (SGD) has received increasing attention as it has been proven to be a fundamental hydrological process that supports many coastal biogeochemical cycles and, even more, social-ecological systems and their implications in saltwater intrusion dynamics. Fresh SGD (FSGD) is very dependent on climatology, whereby, in an analogous form as a river discharge, this event is tightly attached to the aquifer . This research aims to evaluate the impact of climate change on FSGD based on historical precipitation and groundwater level data and future climatic projections (precipitation and temperature).

The daily precipitation and potential evapotranspiration time series from the public source were recorded between January 1, 1996, and October 31, 2022, in Cabrils hydrometeorological station in Northeastern Spain, where the trend shows a brief decrease in the precipitation in the latest years. The Box-Ljung forecasting method with an autoregressive integrated moving average (ARIMA) model was used to predict the changes in precipitation for projected years. The ARIMA models, validated with 26 years of data (1996–2022), were used for predicting precipitation up to 2050. To estimate the effect on FSGD, piezometric data from the Argentona alluvial, close to the Cabrils station, was used. From the water table data, the hydraulic gradient can be defined and  FSGD calculated based on Darcy’s law.

Based on the precipitation series, recharge is calculated compared with groundwater historical levels of the aquifer in the coastal zone, estimating its effect on groundwater discharge. With the rise of extreme events due to global warming, we could face a change in FSGD dynamics in the years to come. With high precipitation events expected to be more frequent, FSGD may be more discontinuous and with higher peaks, with direct implications in saltwater intrusion dynamics and the coastal biogeochemical cycles.

Keywords: Submarine groundwater discharge, Aquifer recharge, Precipitation, Climate change, Extreme climate events.

How to cite: Buxó-Escapa, G., Cárdenas Castillero, G., Adán, A., Kuráž, M., and Folch, A.: Temporal variations of groundwater tables and implications forsubmarine groundwater discharge: a case study the Mediterranean Spanish coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17061, https://doi.org/10.5194/egusphere-egu23-17061, 2023.

A.170
|
EGU23-15750
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HS8.2.6
|
ECS
Bella Almillategui, Albert Folch Sancho, Valenti Rodellas Vila, Maarten Saaltink, Alejandro Adan, Jose Tur-Piedra, Clara Ruiz-González, Daniel Romano Gude, Marc Diego Feliu, and Jesús Carrera

Coastal aquifers are characterized by their unique land-sea interaction and the main difficulty in studying these systems is the complexity of their biogeochemical cycles. Important processes in the “mixing zone” of coastal systems are associated with the ecosystem’s diversity, and supply and exchange of chemical compounds. This coastal body known as a “subterranean estuary” (STE), is characterized by a free connection to the sea, creating an interface between freshwater and seawater. The distribution and composition of substances flowing from land to sea change because the submarine groundwater discharges (SGD) considerably dilute the seawater that has invaded the aquifer through the free connection to the sea (SWI, Sea Water Intrusion).

These processes were studied in an area 100 m long inland from the coastline and 30 m wide, in the alluvial aquifer of Argentona, Mataró, northeast of Barcelona, Catalonia (Spain). This experimental site was established in 2015 and has been equipped for intensive monitoring of coastal aquifer processes such as SWI and SGD. Currently, it is monitored with 25 piezometers (2 meters screened) consisting of 5 nests with 4 piezometers each (at 10m, 20m, 15m, and 25m intervals) and 4 individual piezometers. In this system described as a “multi-aquifer and reactive system”, we are characterizing the physicochemical and hydrogeochemical conditions associated with biogeochemical processes at different depths and seasonal variations. For this presentation, we will show new results of hydrogeochemistry, nitrogen isotopes, and microbiological parameters associated with different periods of monitoring that characterize the dynamics in the subterranean estuary. 

Acknowledgments: This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22), the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency, and the SENACYT – IFARHU – BID Scholarship.

How to cite: Almillategui, B., Folch Sancho, A., Rodellas Vila, V., Saaltink, M., Adan, A., Tur-Piedra, J., Ruiz-González, C., Romano Gude, D., Diego Feliu, M., and Carrera, J.: Detailed characterization of biogeochemical processes in coastal aquifers: variations along the subterranean estuary-mixing zone at the MEDISTRAES experimental site, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15750, https://doi.org/10.5194/egusphere-egu23-15750, 2023.

A.171
|
EGU23-17530
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HS8.2.6
Carlos René Green Ruiz, Valentí Rodellas, Júlia Rodriguez-Puig, and Juan Santos-Echeandía

Mar Menor, on the northern Mediterranean coast of Spain, is a coastal lagoon with a surface of 135 km2, surrounded by agriculture fields and several towns, that support an important touristic activity. In addition, mining has been an historical activity on the region. In order to elucidate the input of metals supplied by Submarine Groundwater Discharge (SGD) to the lagoon and the spatial and temporal distribution of potentially toxic elements in this coastal ecosystem, the concentrations of dissolved metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As and Cd) were measured in surficial water courses (“ramblas”), groundwaters (including piezometers and boreholes) and lagoon waters. Samples were collected in two sampling campaigns (July and November, 2021), representing contrasting climatic seasons. These results were combined with the estimation of SGD fluxes to the coastal lagoon, allowing evaluating the fluxes of the studied pollutants.  Occurrence of these pollutants in water from the lagoon has been previously registered, but none of those works has studied at the same time, and under a holistic point of view, all the possible aquatic pathways of dissolved metals, including the role of SGD as a conveyor of these potentially toxic elements.

Keywords: Metal release, Coastal pollution, Land-Sea interaction, Submarine Groundwater Discharge.

How to cite: Green Ruiz, C. R., Rodellas, V., Rodriguez-Puig, J., and Santos-Echeandía, J.: Potentially toxic element input into Mar Menor coastal lagoon (Spain) through submarine groundwater discharge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17530, https://doi.org/10.5194/egusphere-egu23-17530, 2023.

Posters virtual: Wed, 26 Apr, 16:15–18:00 | vHall HS

Chairpersons: Valentí Rodellas Vila, Iva Aljinović, Albert Folch
vHS.23
|
EGU23-1849
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HS8.2.6
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ECS
|
Soumya Kanta Nayak and N Janardhana Raju

Abstract: Seawater Intrusion (SWI) and Submarine Groundwater Discharge (SGD) are two opposite natural processes which plays significant role in sustainable management of coastal groundwater resources. Thus site specific investigation is necessary to comprehensively evaluate the timing and magnitude of SWI and SGD along water stressed coastal regions. The present study attempts to locate the potential SWI and SGD zones along the central Odisha coast which is experiencing water stress due to overexploitation, groundwater salinization as well as fresh groundwater loss through natural discharge. In this study groundwater level fluctuations along the coastal tract (below or above to mean sea level) and sea surface temperature anomalies (thermal contrast in sea water through LANDSAT -8 TIR imagery) were used as holistic approach to draw inferences about probable SWI and SGD sites before our field investigations. Further during post monsoon 2021 a total number of 93 water samples (34 pore water, 34 sea water and 25 groundwater) were collected along ∼145 km stretch coastline of Odisha state. The in-situ physicochemical parameters of pore water and sea water (pH, EC, TDS, salinity and temperature) were measured at every 1km gap along the coastline except the inaccessible sites using HANNA made portable multi-parameter water quality kit. The physicochemical anomalies observed in water samples were used as evidences for our initial holistic approach to identify SWI and SGD zones. The EC of groundwater sample varied from 99 to 6440 μS/cm with Mean±SD of 1238.6±1668.5, the EC of porewater varied from 8.25 to 44.47 mS/cm with Mean±SD of 39.0±7.4 and the salinity of porewater varied from 4.59 to 28.89 ppt with Mean±SD of 24.97±4.95. Groundwater samples with EC > 3000 μS/cm were considered as potential SWI zones and porewater samples with salinity < 25 ppt and EC < 35 mS/cm were considered as suspected SGD zones. A total number of 3 SWI zones and 6 SGD zones were identified in present work and this preliminary study will act as a baseline for detailed investigation of groundwater-seawater interaction process along the coast.

Keywords: Groundwater level fluctuation, Sea surface temperature, Pore water, Seawater Intrusion, Submarine Groundwater Discharge

How to cite: Nayak, S. K. and Raju, N. J.: A multi approach study of Groundwater level fluctuation, Sea surface temperature anomaly and Physicochemical parameters to assess Seawater Intrusion and Submarine Groundwater Discharge along Odisha coast, India., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1849, https://doi.org/10.5194/egusphere-egu23-1849, 2023.

vHS.24
|
EGU23-5604
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HS8.2.6
Olga Nitcheva, Albena Vatralova, and Donka Shopova

Coping  with water deficit in coastal areas is a challenge when groundwater extraction must be relied upon. The problem is exacerbated when favourable conditions appear for activation of marine intrusion. Such could be significant pressure on groundwater bodies from water abstraction, which causes a change in the water level and, accordingly, a change in the direction of the groundwater flow. The present paper presents hydrological and water resource management analysis  of factors for activation of marine intrusion on the Bulgarian Black Sea coast using data from  monitoring observations of water quantity and quality  and hydrological model simulations. Anthropogenic impacts are analyzed using information about the actual structure and dynamics of the water supply.

The results of the research show that in the Bulgarian Black Sea coastal zone the rainfalls in the past decade are below the country average. Hence marine intrusion processes are observed in the central and northern part of the region due to excess water intake from groundwater for domestic water supply. This imposes the necessity of searching alternative water sources from other territories and specific methods for water treatment.

Key words: water deficit, groundwater water abstraction, marine intrusion, Bulgarian Black Sea coast

How to cite: Nitcheva, O., Vatralova, A., and Shopova, D.: Natural and anthropogenic factors for activation of marine intrusion on the Bulgarian Black Sea coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5604, https://doi.org/10.5194/egusphere-egu23-5604, 2023.

vHS.25
|
EGU23-9655
|
HS8.2.6
Michela Giustiniani, Martina Busetti, Michela Dal Cin, Erika Barison, Aurélie Cimolino, Giuseppe Brancatelli, and Luca Baradello

Increasing demand for freshwater requires the identification of additional and less-conventional water resources. Amongst these, offshore freshened groundwater is considered a new opportunity to face increasing water demand and has been studied in different parts of the world. Here we focus on the north-eastern Adriatic Sea, where offshore aquifers could be present as a continuation of onshore aquifers. Geophysical data, especially offshore seismic data, as well as onshore and offshore well data were integrated and interpreted to characterize the hydrogeological setting via the interpretation of seismo-stratigraphic sequences. Two areas located in the proximity of the Tagliamento and Isonzo deltas were studied. Well and seismic data suggest that Quaternary sediments, extending from onshore to offshore areas, represent the most promising from an offshore freshwater resources point of view. Firstly, onshore well data confirm the presence of freshwater aquifer systems in proximity to the coastline, supporting the hypothesis of their continuation offshore. Secondly, during the glacial periods, a drop in sea level (about -120 m with respect to today during the Last Glacial Maximum ), provided the total emergence of the northern Adriatic Sea, that represented a fluvial plain, allowing the storage of freshwater. Moreover, a lower sea level position could lead to a higher groundwater gradients towards offshore areas. On the contrary, during the interglacial ones, the sea level was some meters higher than the present one (about +8 m during the last transgression in the Middle/Late Pleistocene), with mainly starved conditions. During the deglaciation phases, the fluvial drainage fed by melting glaciers produced the deposition of sediment above the plain. Below these sediments, the several kilometres thick pre-Quaternary carbonate and terrigenous sequences seem to host mainly salty waters.

How to cite: Giustiniani, M., Busetti, M., Dal Cin, M., Barison, E., Cimolino, A., Brancatelli, G., and Baradello, L.: Potential Water Resources in the North-Eastern Adriatic Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9655, https://doi.org/10.5194/egusphere-egu23-9655, 2023.