Coastal aquifers are dynamic, shaped by natural and anthropogenic boundary conditions acting on very different time scales from seconds (waves) up to millennia (sea-level rise). In subterranean estuaries (STEs), inland aquifers connect with the sea. With terrestrial freshwater and circulating seawater, chemically different waters mix in STEs and are modified before they discharge into coastal waters. The cooperative interdisciplinary project DynaDeep studies the subsurface of high-energy beaches, which have so far hardly been investigated due to the difficulties associated with working under high-energy conditions. We propose that these systems are particularly dynamic environments, where frequent sediment relocation affects groundwater flow and transport up to depths of tens of meters below the ground surface. This may lead to strong spatiotemporal variability of geochemical conditions, presumably attracting a unique microbial community. The state-of-the-art concept of groundwater flow and transport in STEs with an upper saline plume overtopping a freshwater discharge tube is likely distorted under such conditions, with consequences for the biogeochemical functioning of these STEs. Within DynaDeep, a unique cross-shore research site was established on the northern beach of the barrier island Spiekeroog facing the North Sea. It consists of permanent infrastructure, such as a pole with measuring devices, multi-level groundwater wells and an electrode chain. This forms the base for autonomous measurements, regular repeated sampling and interdisciplinary field campaigns using, for example, direct push techniques supported by modelling and experimental work to understand and quantify the functioning of the biogeochemical reactor. Field results show that morpho- and hydrodynamics are clearly affected by waves, tides and stormfloods. Stormfloods are particularly relevant and can be traced into the subsurface. In the infiltration zone, the groundwater is rather young as shown by age dating and temperature tracing. Oxygen and nirtrate reach deep into the subsurface and a redox transition from oxic to anoxic conditions occurs at 12-15 m depth. In contrast, relatively old and anoxic water discharges near the low water line. Numerical modelling aids in process understanding and hypothesis development, and generic models show that moderate deviations in hydrogeological parameters severely change both salinity as well as hydrogeochemical patterns. The dynamic nature of high-energy STEs, depending on frequencies and amplitudes of change in environmental conditions, as well as the global relevance for high-energy STEs have yet to be explored.

How to cite: Massmann, G. and the DynaDeep Project Team: The dynamic deep subsurface of high-energy beaches , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16758, https://doi.org/10.5194/egusphere-egu24-16758, 2024.

Coastal aquifers build the transition zone of freshwater and saltwater. Hence, large salinity gradients are encountered in the subsurface below beaches and it is important to assess the salinity in a high resolution in order to understand coastal groundwater flow dynamics and consequently geochemical and microbial processes in subterranean estuaries. Within the project DynaDeep, we used both geophysical and hydrogeological methods to determine the bulk and fluid electrical conductivities (bulk/fluid EC) with the aim to convert the EC to salinity to monitor its temporal and spatial changes. This was done at a high-energy beach on the North Sea Island of Spiekeroog.

Numerous EC techniques have been used to acquire a unique dataset since 2022, covering a 2D transect from the dune base to the low water line. The site was subject to strong topographic changes over the seasons. Among the methods applied, we used electrical resistivity tomography (ERT) to get access to 2D distributions every six weeks. Additionally, continuous monitoring was carried out using a saltwater monitoring system (SAMOS) with a vertical electrode chain down to a depth of 20 meters located at the high water line. Direct push (DP) data at various locations as well as fluid EC values from water samples gathered via DP give access to high resolution information. In three multilevel wells (four levels each at 6, 12, 18, and 24 meter depth below ground) we logged the fluid EC and temperature and took water samples on a regular basis.

For an especially dense dataset between January and March 2023 we compared in detail the applied EC methods and found a general agreement in all of the gathered data after suitable calibration and temperature correction. We furthermore derived a formation factor model for the conversion to salinity.

Finally, we a combined inversion of the ERT data with the additional data aiming for fluid EC directly under the assumption of this temporally fixed formation factor model. In contrast to standard inversion techniques, this allowed for a naturally occurring smooth transition of salinities over the different geological units, which was critical when analyzing the spatial and temporal changes.

How to cite: Skibbe, N., Günther, T., Schwalfenberg, K., Meyer, R., Reckhardt, A., Greskowiak, J., Massmann, G., and Müller-Petke, M.: Investigating spatio-temporal salinity dynamics in coastal aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18971, https://doi.org/10.5194/egusphere-egu24-18971, 2024.

Submarine groundwater discharge (SGD) is a complex hydrological process influenced by multiple mechanisms with various spatiotemporal scales. Quantifying SGD is complex because it depends on various driving forces (e.g., waves, tides, hydraulic gradient) and on the underlying geological context that modulate the occurrence and magnitude of SGD. This poses challenges to the accurate quantification of SGD, especially in heterogeneous nearshore environments. However, there is still limited understanding on SGD spatiotemporal patters hindering the accurate assessment of SGD implications for the coastal ocean.
The aim of this study is to use amphibious electrical resistivity tomography (AERT), which combines terrestrial and marine instruments, to assess spatiotemporal patterns of FSGD (Fresh Submarine Groundwater Discharge) in the land-sea interface. This cost-effective method enables prospecting SGD dynamics at the land-sea transition zone based on the resistivity variations in the subsurface induced by salinity changes. The resistivity study was conducted regularly to assess hourly spatiotemporal variations in two aquifers near Barcelona, Spain, with different geological contexts, including detrital and karst formations. The resistivity data in both study sites have been validated with in situ pore water sampling for physicochemical parameters, with a good agreement between resistivity and subsurface salinity. The time-lapse results indicate that even in micro-tidal environments such as the Mediterranean Sea, SGD patterns are highly dynamic, with the karst environment exhibiting a more significant proportion of freshwater in the marine sediments and faster changes than the alluvial context. This methodology proves effective for the spatial and temporal assessment of FSGD. These findings offer a valuable approach for monitoring subsurface salinity changes in coastal aquifers and enhance our understanding of small-scale and short-term SGD variations, which is fundamental to deriving reliable SGD and nutrient flux estimates. 

How to cite: Tur Piedra, J., Diego-Feliu, M., Ledo, J., Queralt, P., Marcuello, A., Rodellas, V., and Folch, A.: Exploring temporal and spatial dynamics of fresh submarine groundwater discharge: amphibious electrical resistivity tomography in the land-ocean transition zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19567, https://doi.org/10.5194/egusphere-egu24-19567, 2024.

Groundwater acts as a critical link between onshore and offshore environments, connecting freshwater systems to saline oceans.  With 40% of the world's population residing along coastlines, understanding coastal groundwater reserves is paramount. One open question involves the vital role of submarine groundwater springs in global hydrology, and how the distribution and groundwater flux can be better constrained across the coastline to better predict both groundwater discharge into the ocean and saltwater inflow into coastal aquifers. Especially urban areas pose unique challenges where water demand is high and groundwater exploration problematic since geophysical remote sensing techniques often interfere with surface and subsurface constructions (e.g. cables, pipelines etc.), making innovative approaches for groundwater exploration crucial for sustainable groundwater management.

In this study, we aim to address the complex dynamics of coastal karstic groundwater systems in urban regions, where meteoric waters discharge into the ocean through coastal and submarine freshwater springs, while concurrently facing the risk of saltwater intrusions. Our investigations in the bay of Antalya (Turkey) aim to provide a comprehensive understanding of the land-ocean transition zone in the karstic groundwater systems and provide new tools for future groundwater monitoring in coastal regions.

We employ advanced hydroacoustic and resistivity methods, combining onshore and offshore electrical resistivity tomography with electromagnetic measurements to bridge the gap between onshore and offshore domains. This integration of geophysical datasets enables us to (1) delineate karstic groundwater flow pathways from land to ocean, (2) identify coastal and submarine freshwater springs, and (3) assess the risk of saltwater intrusions along the coastline.

The study showcases the potential of offshore geoelectric measurements as a tool for groundwater investigations in urbanized coastal regions. The proposed approach will facilitate exploration efforts for groundwater in urbanised karstic areas, but much more importantly will facilitate monitoring strategies to avoid intrusions of saltwater into freshwater aquifers. Our findings contribute valuable insights for water management strategies in Antalya, with implications for safeguarding todays and future freshwater resources.  

How to cite: Hoffmann, J., Erkul, E., Yolcubal, I., Haroon, A., Yogeshwar, P., Fischer, S., Sen, E., Rabbel, W., Sener, A., Schneider von Deimling, J., Tezkan, B., Peksen, E., Micallef, A., Gasimov, E., Kaplanvural, I., Gross, F., Sander, L., and Baris, S.: Assessing freshwater plumes, offshore freshened groundwater and the risk of salt intrusions in urbanised karstic groundwater systems using combined resistivity methods , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19992, https://doi.org/10.5194/egusphere-egu24-19992, 2024.

Due to their saturated conditions resulting from frequent inundations, coastal floodplains are crucial in sequestering atmospheric carbon and regulating nutrient cycling. Tidal inundations can mobilize dissolved oxygen (DO), a critical driver of biochemical function, towards the subsurface, stimulating microbial activity when DO-rich surface water mixes with anoxic groundwater (i.e., hydrodynamics of surface/groundwater exchange driven by periodic flooding). While our knowledge of these systems has improved over the years, we need a greater understanding of the spatiotemporal variability of essential biochemical drivers within tidal floodplains. Mainly, measuring DO at sufficient temporal resolution in such rapidly changing environments to resolve the relative influence of each factor is challenging, limiting our ability to predict how chronic sea-level rise will impact coastal floodplain functionality and spotlighting the urgency to fill this knowledge gap.

We used a state-of-the-art optical DO probe (Opti O₂) to continuously measure subsurface DO concentrations at 5 min resolution over ~4 years at Beaver Creek, a freshwater creek in Washington, USA. Following the removal of a barrier in 2014, the site floods during tidal events with water from Gray’s Harbor, located at the northwestern Pacific coast. This data set is the first measurement in a coastal environment with the requisite temporal resolution to obtain in-situ, subsurface oxygen consumption time series (see attached image). With our data, we investigate the evolution of processes following a controlled sea level rise experiment. Co-located instruments monitoring surface water and groundwater levels, salinity, and meteorological parameters (including rainfall, air temperature, barometric pressure, solar flux density) let us parse the critical drivers of coastal floodplain DO dynamics. To understand how drivers of subsurface biogeochemical processes fluctuate across tidal cycles, we used wavelet analyses to explain the interactions between DO and water levels. We observed multiple oxygenation events (34 clusters of hot moments from June 2019 to date) followed by subsequent returns to anoxia. We used information theory to explore DO’s relationship with the hydro-meteorological data. This work highlights the importance of multi-year, high-frequency in-situ measurements, such as DO, to elucidate the non-linear coupling of climate, hydrology, and biochemistry in coastal floodplains.

How to cite: Ghosh, R., Grande, E., and McIntire, C.: Oxic hot moments in a coastal floodplain highlight the bidirectional flow of surface water-groundwater exchanges at the terrestrial-marine interface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14357, https://doi.org/10.5194/egusphere-egu24-14357, 2024.

Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying chemical constituents such as nutrients, pollutants and carbon. Mixing between saline seawater and fresh terrestrial groundwater in the subsurface is complicated by catchment morphology, variable density flow and very dynamic boundary conditions across temporal scales (e.g. tides, storms, yearly variations in terrestrial groundwater levels). Thus, tracing water and nutrients fluxes through the subterranean estuary is not trivial, especially when attempting to quantify temporal dynamics on time scales from days to weeks. In this work we use long-term (months) temperature profile measurements and numerical heat modelling to investigate the dynamics of water fluxes through the beach sediments into the Königshafen Bay, Sylt Island, North Germany. Temperature measurements were complemented by stable isotope (δ¹⁸O, δ²H)  and pore water chemical measurements to infer the origin of water discharging into the bay. The results showed that the temporal fluxes vary considerable depending on season, location and catchment characteristics. The freshwater flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge. Moreover, it appears that either the isotope signature of the islands fresh groundwater is variable or there are at least two end-members contribute to the freshwater signature. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. Overall temperature measurements and heat modelling combined with pore water chemistry show potential to understand the dynamics in water and element exchange through the subterranean estuary and thus help to understand local water and material fluxes and transformations at the land-ocean interface.

How to cite: Gilfedder, B., Böttcher, M. E., von Ahn, C. M. E., and Frei, S.: Quantifying dynamic water fluxes and origin at the land-sea interface from days to weeks using temperature and stable isotopes: An example from Königshafen, Sylt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5707, https://doi.org/10.5194/egusphere-egu24-5707, 2024.

Abstract 

Amidst increasing human activities and complex interactions between the ocean and land, coastal zones have emerged as dynamic yet vulnerable regions within global ecosystems. Understanding the interplay between groundwater and the ocean is crucial for studying submarine groundwater discharge (SGD) and seawater intrusion (SWI), thereby protecting coastal environments and water resources. This study aims to propose a coupling strategy between groundwater flow models and physical oceanographic models to accurately simulate the interactions between coastal groundwater and the ocean.

In this research a three-dimensional hydrodynamic model based on TELEMAC-3D was first constructed to simulate marine conditions under varying salinities and temperatures. Subsequently, a groundwater model using MODFLOW6 was developed. An efficient coupling strategy was introduced through the integration of Python, facilitating precise integration of the hydrodynamic and groundwater models. Validation results of the coupled model against published experimental results demonstrated high accuracy, closely replicating laboratory outcomes within acceptable error margins.

By combining three-dimensional hydrodynamics with groundwater modeling, this study not only represents an innovative attempt at coupling but also provides a robust tool for understanding the complex mechanisms of interaction between the ocean and land.

How to cite: Jin, J., Espino, M., Fernández, D., and Folch, A.: Integrating groundwater flow models with physical oceanographic models in coastal regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11418, https://doi.org/10.5194/egusphere-egu24-11418, 2024.

Submarine groundwater discharge is an important pathway of various nutrients and solutes from the land to the ocean. The groundwater discharging across the seabed interface is exposed to the highly dynamic conditions of the coastal ocean. The flow-topography interaction in the benthic boundary layer defines these dynamics and affects how the discharged groundwater is transported and mixed within the water column. Additionally, the flow-topography interaction can also drive convection in the seabed that can enhance fluxes across the seabed interface. 
To investigate these effects, laboratory experiments are conducted, where waves are generated over gravelly sand beds. In these types of beds, gravel protrudes from the seabed. The protruding part is altered to change its size and effective slope, to quantify their respective impact on the flow conditions. Additionally, water with a fluorescent dye seeps through the seabed, resembling groundwater discharge. A coupled PIV-LIF approach (Particle Image Velocimetry, Laser Induced Fluorescence) is used to measure the velocity field and the concentration of discharged water in the water column simultaneously. Both quantities are then correlated, which gives the turbulent Reynolds flux. This approach grants insights on the distribution, transport, and mixing of discharged water within the water column. 
Both, the seabed geometry and the wave scenario, significantly influence the turbulent flux. The results show different processes, such as wave pumping and separated vortex pumping, which drive convection in the seabed and alter the discharge rates. This leads to different concentrations of discharged water being measured in the water column, depending on the boundary conditions. While the outcome of these processes can be visualized and quantified in the water column from experimental data, complimentary surface-subsurface modeling holds the potential of additionally resolving the flow field within the seabed and expanding the investigated two-dimensional region to a three-dimensional domain. 

How to cite: Klettke, H., Kandler, L., Grundmann, S., and Brede, M.: Flow-topography interaction over gravelly sand beds and the implications for transport of submarine groundwater discharge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5043, https://doi.org/10.5194/egusphere-egu24-5043, 2024.

Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two opposite components of hydrological cycle that occur across the land-sea continuum and their understanding are imperative for development and management of coastal groundwater resources. This study has attempted to identify the SGD and SWI sites along the water stressed coastal plains of Odisha through a three tier investigation system and quantify the SGD and associated chemical fluxes (nutrients and metals) though seepage metric measurements. A total 340 samples (85 each i.e., 30 porewater, 30 seawater and 25 groundwater in two post-monsoons and two pre-monsoons) were collected and their in-situ physicochemical parameters were measured along ~145 km of the coastline. Considering high groundwater EC values (> 3000 μS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre-monsoons and < 25 ppt in post-monsoons), four probable SGD zones were identified. The high positive hydraulic gradient (> 10 m) near SGD site and negative gradient (< 0 m) near SWI site along with anomalous SST (due to cold/warm groundwater input) at SGD locations validated the identified locations. Lee type seepage meters were installed at SGD identified sites during the low tide period and the measured fluxes ranged from 0 to 4247.973 m³ m^-2 year^-1 in post-monsoon and 0 to 1470.46 m³ m^-2 year^-1 in pre-monsoon. The metal fluxes were found in the order of B > Sr > Li > Ba > Al > As > Fe > Ni > Cu > Co > Mo > Be > Mn with highest flux of B (2962.86 mmol. m^-2 year^-1) at Puri beach which was 7x of Sr-flux and 30x of Li-flux respectively. Among the measured nutrients, DSi fluxes were higher than NO₃^- and PO₄^3- and the highest flux of DSi (849.86 mmol. m^-2 year^-1) was observed at Puri beach. These nutrients and metal fluxes may lead to an increased chance of eutrophication/algal blooms at the SGD-identified locations and influence the sensitive coastal-marine ecosystems of Odisha; therefore, further investigations focusing on biogeochemical species transformation are needed along the fresh-saline interface.

Keywords: Submarine Groundwater Discharge, Seawater Intrusion, Nutrient flux, Metal flux, Seepage meter, Odisha Coast.

How to cite: Nayak, S. K. and Nandimandalam, J. R.: Evaluation of Submarine Groundwater Discharge and Chemical Fluxes along the Coastal Plains of Odisha, India., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11535, https://doi.org/10.5194/egusphere-egu24-11535, 2024.

Large-scale coastal groundwater models (LCGMs) covering areas of several tens of thousands of km² can provide valuable insights into (supra)regional coastal groundwater system dynamics over time. This is crucial in understanding the current and future state of transboundary groundwater resources as well as identifying potential hotspots for fresh groundwater shortages in coastal regions worldwide. Recent developments in code parallelization, namely the SEAWAT based iMOD-WQ code (Verkaik et al. 2021) and high performance computing open new possibilities in building LCGMs using open source tools like Python and available global datasets as input into the LCGMs. These LCGMs, despite simplifications and uncertainties related to hydrogeological data availability, yet provide first order approximations of groundwater conditions in data scarce large-scale regions.. In this research, we describe current and future developments of our tool and demonstrate potential opportunities in using these LCGMs as basis for further developments in (supra)regional water management in data-scarce regions.

The recent development of parallel open-source iMOD-WQ signalled an important breakthrough in variable-density groundwater flow and salt transport modelling with regular (and irregular) grids options and geological data ensemble modules. Whereas in before only serial simulations using the regular SEAWAT code were possible, now these simulations can be split into multiple (practically at least tens of) partitions and executed in parallel, leading to significant reduction in computation time. This opens possibilities in terms of both the physical and temporal size of the LCGMs. The LCGMs developed in this research cover several tens of thousands km² and simulate groundwater salinity dynamics over a full glacial-interglacial cycle (viz. approx. 125 kA). The HydroBASINS global-watershed-boundaries dataset is used to delineate the boundary of the LCGM in the inland part of the model domain. Our LCGMs also cover the offshore continental shelves; those are manually outlined and added to the selected HydroBASINS. The top elevation is derived from a global DEM dataset (GEBCO) while the bottom elevation is estimated by the bottom of the unconsolidated sediment formations as well as the sedimentary rock formations (limited to siliclastic lithology). Using this approach can lead to considerable uncertainties and therefore, whenever local hydrogeological input data is available (e.g. bore logs, groundwater salinity, extractions), we use tools like GEMPY to improve the hydrogeological model in our LCGM building tool. We believe that building a LCGM using global datasets is a necessary first step to provides valuable information for (supra)regional coastal groundwater management in data-scarce regions.

Verkaik, J., J. Van Engelen, S. Huizer, M.F.P. Bierkens, H.X. Lin, and G.H.P. Oude Essink. 2021. “Distributed Memory Parallel Computing of Three-Dimensional Variable-Density Groundwater Flow and Salt Transport.” Advances in Water Resources 154 (August): 103976. https://doi.org/10.1016/j.advwatres.2021.103976.

How to cite: Oude Essink, G., Zamrsky, D., King, J., Achmad Syaehuddin, W., and Bierkens, M.: Building large-scale 3D coastal groundwater models with iMOD-WQ and global datasets, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10924, https://doi.org/10.5194/egusphere-egu24-10924, 2024.

High-resolution three-dimensional variable-density groundwater flow and coupled salt transport models (3D-VD-FT models) are useful instruments to support coastal groundwater management strategies and to project 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 temporal and spatial data and knowledge of groundwater salinity processes in the subsurface. Current understanding in saltwater intrusion research is mainly based on theoretical, experimental and numerical studies, with intensive monitoring field studies being uncommon. In this paper, we describe a methodology that combines a high-resolution 3D-VD-FT model with an intensively monitored pilot in a coastal area in the Netherlands to improve our understanding of fresh-saline groundwater dynamics in response to multi-level groundwater extractions. We assess the applicability of 3D-VD-FT models to reproduce observed groundwater salinity changes in response to extractions. Moreover, we evaluate multiple extraction regimes of fresh and brackish groundwater. Subsequently, critical pumping rates are determined to secure fresh groundwater supply and the preventive effect of brackish groundwater extractions on saltwater intrusion is evaluated. Preliminary results show improvement of predictions on the scale of individual wells compared to previous studies conducted on similar scales. The 3D-VD-FT model captures the observed salinity trends such as downconing fresh groundwater and upconing saline groundwater, both of which occurred in response to withdrawals. However, the model’s absolute accuracy of downconing and upconing groundwater salinity still requires improvement.

How to cite: Hendrikx, T., Oude Essink, G., and Bierkens, M.: Detailed monitoring and simulation of groundwater salinity in response to extractions in coastal aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12630, https://doi.org/10.5194/egusphere-egu24-12630, 2024.

Soil temperature regulates biogeochemical processes and is a key environmental factor affecting salt marsh ecosystems. Previous studies on soil temperature and heat transport in intertidal marshes predominantly focused on short-term changes, leaving seasonal variations still unclear. This study conducted a yearlong field and modeling investigation to examine the temporal and spatial variations of soil temperature in a creek-marsh section under estuarine and meteorological influences. The porewater flow and heat transport processes were simulated using SUTRA-MS. The response of soil temperature to air and tidal water temperature conditions will be discussed here in detail. We will also discuss the impacts of tide-induced porewater circulations on soil temperature response. Finally, there will be a discussion on quantifying the thermal effects of tidal water and air on hourly and depth-averaged shallow soil temperatures in the creek-marsh system.

How to cite: Yu, X., Xu, X., Zhan, L., Cheng, H., and Xin, P.: Seasonal temperature patterns and variability in salt marshes: Field study and numerical simulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5152, https://doi.org/10.5194/egusphere-egu24-5152, 2024.

Seawater intrusion threatens the freshwater resources and the ecosystem in coastal areas. Spatial heterogeneity of hydraulic conductivity is a common feature of coastal aquifers and can significantly influence the location of the seawater-freshwater interface. Conventional approaches primarily rely on numerically solving the advection-dispersion models (e.g. SEAWAT) to determine the interface in three-dimensional heterogeneous aquifers. However, these approaches are mostly computationally intensive. This study developed a semi-analytical approach for determining the location of the sharp seawater-freshwater interface, considering the steady-state seawater intrusion in three-dimensional heterogeneous confined aquifers. The approach decouples the hydraulic conductivity as a spatial variable from the conventional potential definition and defines a new potential expression. A two-dimensional partial differential equation is employed to describe the three-dimensional heterogeneous aquifer system by calculating the transmissivity above the sharp interface. Finally, the potential distribution can be obtained by solving the governing equation using finite-difference method, allowing for determining of the morphology of the sharp interface. 2D cross-sectional and 3D confined coastal aquifers with Gaussian random hydraulic conductivity fields were generated to validate our approach. The seawater-freshwater interfaces were estimated using both the semi-analytical approach and SEAWAT. Comparison analysis showed that the developed semi-analytical approach can accurately simulate the interfaces by correcting the dispersion effect. This study provides a cost-effective means for estimating the seawater-freshwater interface location in three-dimensional heterogeneous confined aquifers. It is useful for future seawater intrusion study of three-dimensional systems and for managing subsurface freshwater resources efficiently.

Key Words: seawater intrusion, semi-analytical, three-dimensional, stochastic heterogeneity

How to cite: Liu, Y., Fan, B., Zhang, J., and Lu, C.: A semi-analytical method for delineating steady-state seawater-freshwater interface in three-dimensional stochastic heterogeneous confined coastal aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2276, https://doi.org/10.5194/egusphere-egu24-2276, 2024.

In coastal areas, saltwater intrusion causes a depletion of the resource by reducing potable and irrigation freshwater supplies and causing severe deterioration of groundwater quality.This trend is observed in Pwales Valley – Maltese Island, where the water resource management plays a crucial role for the environmental sustainability of the area, given the importance of intensive agricultural activity along this valley. In order to tackle such phenomenon, actions or adaptation measures against climate change are strongly required.  For example, Managed Aquifer Recharge (MAR) is an increasingly important water management strategy to maintain, enhance and secure stressed groundwater systems and to protect and improve water quality.For accurately plan a MAR facility, it is crucial to define a hydrogeological model of the studied area, with the use of traditional hydrogeological measurements and innovative unconventional techniques. In recent years, Electromagnetic Induction (EMI) measurements, based on subsurface electrical conductivity data, have been increasingly used for investigating the saltwater intrusion dynamics due to their high sensitivity to the salinity.In the study area of Pwales Valley, a MAR scheme is being planned and, for this aim, a hydrogeological model has been developed through an EMI survey.More than 20,000 apparent electrical conductivity (ECa) data were collected to generate a quasi 3D high-resolution model of electrical conductivity of the Pwales Valley. The results highlighted the spatial extension of the tongue-shape salt water intrusion from east to west along the valley, as well as some geological-hydrogeological peculiarities such as the thickness of the salt wedge and the irregular top surface of the bottom impermeable layer, otherwise undetectable with other techniques. This approach confirms to be a useful tool for an effective hydrogeological characterization, essential for planning mitigation and tackle climate changes actions or adaptation measures, such as a MAR plant.

How to cite: De Carlo, L., Turturro, A. C., Caputo, M. C., Sapiano, M., Mamo, J. A., Balzan, O., Galea, L., and Schembri, M.: Mapping saltwater intrusion via Electromagnetic Induction (EMI) for planning a Managed Aquifer Recharge (MAR) facility in Maltese Island, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20298, https://doi.org/10.5194/egusphere-egu24-20298, 2024.

Coastal aquifers, a vital drinking water (and agricultural) resource for over a billion people, are facing increasing risks of seawater contamination due to the dual challenges of population growth and climate-induced sea-level rise. With water stress expected to intensify and water use consumption growing annually, there is an urgent need for innovative methods to track saline intrusion in these aquifers.  Current monitoring techniques like observational boreholes are limited in their warning capabilities, while resistivity imaging, despite its effectiveness, is prohibitively expensive and logistically challenging, (MacAllister et al. 2016).

Self-Potential (SP), naturally occurring voltages arising from ion separation in the subsurface, is a promising geophysical technique to identify and manage saline intrusion, provided its source mechanisms are well understood. There are two key sources of SP in hydrology. Electro-kinetic potentials (V_EK), due to fluid flow induced by pressure gradients, and exclusion-diffusion potentials (V_ED), due to ion concentration gradients in the subsurface. The balance of these effects depends on a variety of variables including the physical and chemical properties of geological material in which the saline-fresh water interface is located. Spatial and temporal changes in these potentials provide  insight into the location and behaviour of the saline-fresh interface.

This study introduces a novel SP profiling approach that employs both fixed and movable electrodes within a borehole, greatly enhancing the data acquired from SP measurements. Observations across various UK locations have revealed SP profiles exceeding 50mV. Coastal aquifers including those of Chalk, Gravel and Sand have been investigated. This talk presents not only gathered results but also details insights in the practical assessment of these gradients. Notably, these SP gradients are dynamic, with changes seemingly connected to the movements and proximity of the saline interface. The findings are corroborated by laboratory experiments and numerical models, showing that the dynamics of SP gradients can serve as an early indicator of saline intrusion in coastal aquifers.

Bibliography

MacAllister, DJ., Jackson, M. D., Butler, A. P., and Vinogradov, J. (2016), Tidal influence on self‐potential measurements, J. Geophys. Res. Solid Earth, 121, 8432– 8452

How to cite: Rowan, T., Butler, A., Flynn, R., Hamill, G., Donohue, S., and Jackson, M.: Logging of self-potential gradients to track saline intrusion in chalk, gravel and sand aquifers around the United Kingdom, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20333, https://doi.org/10.5194/egusphere-egu24-20333, 2024.

The ability of mixed physical barrier (MPB) as a seawater intrusion countermeasure was explored in heterogeneous coastal aquifer settings. The performance of MPB was examined in a synthetic aquifer containing a low permeability interlayer sandwiched between two layers of high permeability (case HLH). The performance of the MPB was compared to that of a single cut-off wall for various hydraulic gradients in a laboratory setting, whereby performance was measured in terms of measuring the percentage of reduction of the intrusion length. Also, numerical simulations were conducted using SEAWAT to validate and further examine the effects of various layering patterns on MPB performance. In total, five additional heterogeneous scenarios were simulated, including a scenario where a low permeability layer was set at the top of the aquifer (case LH), at the lower part of the aquifer (case HL), at the top and bottom part of the aquifer (case LHL), and two cases with monotonically increasing/ decreasing permeability from top to bottom. The sensitivity of the percentage reduction to the MPB design and hydrogeological parameters was examined thereafter. Experimental results demonstrate that the MPB could perform better than the single cutoff wall, with up to 55 % more reduction of the intrusion length. Also, the numerical results showed that the MPB remained effective regardless of the stratification patterns adopted, whereby it achieved at least around 70% SWI length reduction. The results also showed that the effectiveness of MPB was very sensitive to the thickness of the middle layer as well as the permeability ratio. While increasing the thickness of the interlayer induced a negative impact on the MPB performance, a reduction in the permeability of the interlayer induced better reduction. The findings of this study provide insight into the main parameters affecting the performance of the MPB system in realistic layered heterogeneous coastal aquifer scenarios and further evidence of its reliability as a practical countermeasure for SWI.

How to cite: Abdoulhalik, A., Ahmed, A., and Abd-Elaty, I.: The Protective Effect of Mixed Physical Barrier in Heterogeneous Aquifers: Experimental and Numerical Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9972, https://doi.org/10.5194/egusphere-egu24-9972, 2024.

Increased groundwater extraction, coupled with potential declines in recharge rates, is anticipated to negatively impact the sustainability of groundwater resources (Mehdizadeh, 2019). Coastal aquifers are particularly vulnerable, as these changes can result in a significant risk of saltwater intrusion (SI). While the fundamental mechanisms of SI are well-understood, tracking the encroachment of saline water into coastal aquifers and its risk to water extraction sources remains a complicated and expensive task. Studies have shown that self-potential (SP) could serve as an effective tool for remotely monitoring the movement of saline-freshwater interfaces due to SI (Graham, 2018).

Self-potential voltages, which originate from subsurface pressure and concentration gradients, occur when these gradients lead to ion separation. This separation results in an electrical potential and a subsequent electron flow to preserve electrical neutrality. These potentials, usually in the millivolt range, can be observed and recorded in the field using electrodes. SP primarily consists of two types: electro-kinetic potentials (VEK), arising from differing flow velocities, and exclusion-diffusion potentials (VED), stemming from ion concentration gradients with varying mobilities. A previous study recorded tidal signatures in SP within a Chalk borehole located less than 2 kilometres from the English Channel (MacAllister et al., 2016). More recent research has detected similar signatures in a sand aquifer on the north coast of Northern Ireland and in a gravel aquifer on the south coast of England.

In each instance, the tidal signature most prominently reflected the M2 (Principal lunar-semidiurnal) component, although in some cases other, less prominent, elements were also identified. Analysis of water level, electrical conductivity and temperature data suggests that these signatures were not due to electrokinetic potentials from tidally induced flows in and around the borehole. Rather, a more likely explanation is that nearby saline-freshwater interfaces, inducing exclusion-diffusion potentials, are responsible. Whilst further investigation is necessary to quantify, model, and fully comprehend these signals, the detection of tidal signatures in an increasing and diverse number of aquifers suggests that self-potential might be a viable technique for monitoring and providing an early warning of saline intrusion.

Bibliography
Graham, M. T., MacAllister, DJ., Vinogradov, J., Jackson, M. D., and A. P., Butler, (2018). Self‐potential as a predictor of seawater intrusion in coastal groundwater boreholes. Water Resources Research, 54, 6055– 6071.
MacAllister, DJ., Jackson, M. D., Butler, A. P., and Vinogradov, J. (2016), Tidal influence on self‐potential measurements, J. Geophys. Res. Solid Earth, 121, 8432– 8452.
Mehdizadeh, S., Badaruddin, S. and S. Khatibi, (2019). Abstraction, desalination and recharge method to control seawater intrusion into unconfined coastal aquifers. Global Journal of Environmental Science and Management, 5, 107-118.

How to cite: Butler, A., Rowan, T., Hamill, G., Flynn, R., Donohue, S., and Jackson, M.: Detection of Tidal Signatures in Self-Potential Monitoring of UK Aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11169, https://doi.org/10.5194/egusphere-egu24-11169, 2024.

The efficiency of submarine groundwater discharge (SGD) in transporting solutes into coastal environments during glacial periods remains poorly understood. Moreover, the absence of observational constraints on offshore groundwater emplacement times hinders our understanding of glacial-driven SGD timescales and subsequent solute fluxes. This knowledge gap presents challenges in predicting the impact of ice sheet collapse on critical solute discharge into peripheral oceans. An SGD site with methane seepage offshore northern Norway that experienced drastic changes due to Fennoscandian ice sheet dynamics offers insights into glacial-interglacial transitions and their consequences for offshore groundwater circulation. Radiocarbon (¹⁴C) contents of the dissolved inorganic carbon along with chlorinity contents of the upward-advected fluids reveal that the groundwater transit times of the seawater component coincide with the retreat of the Fennoscandian ice sheet from the continental shelf. This suggests that seawater intrusion replaced offshore freshening, flushing the freshened aquifer with seawater. Decelerating groundwater discharge velocities and aquifer salinization as a consequence of glacial unloading allowed the precipitation of authigenic carbonates, sequestering discharged methane. Reduced groundwater advection velocities facilitated the migration of the sulfate-methane transition zone into the marine sediments, while the aquifer salinization likely increased Ca²⁺ concentrations, promoting carbonate precipitation. Our geochemical evidence conclusively shows that the decreased hydraulic head gradients, coupled with aquifer salinization, mitigated the escape of methane from the subsurface.

How to cite: ten Hietbrink, S., Patton, H., Szymczycha, B., Sen, A., Lepland, A., Knies, J., Kim, J.-H., Chen, N.-C., and Hong, W.-L.: Seawater intrusion and methane sequestration followed the retreat of the Fennoscandian ice sheet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1485, https://doi.org/10.5194/egusphere-egu24-1485, 2024.

Biogeochemical cycling of organic matter across the land-ocean continuum (LOC) is important in driving energy and material exchange between the fresh water and saline water ecosystems, including the direction and magnitude of the uptake of carbon dioxide. The “reactivity” of dissolved organic matter (DOM) can range from highly bioavailable (labile) to hardly bioavailable (recalcitrant). Originated in soil biogeochemistry, “priming” refers to when something (nutrients such as inorganic N and P) is added to soil, it affects the rate of decomposition of the soil organic matter (SOM). This concept has recently been introduced to OM cycling across the LOC, for example, the “recalcitrant” DOM can become more bioavailable in coastal waters affected by high anthropogenic inputs of inorganic N and P. This study examines a set of surface fresh water, groundwater and coastal water samples in Pearl River Delta region to understand how biological information (eDNA, usually known with a short degradation time) may be interpreted in the context of still unclear mechanisms of how fast dissolved organic matter degrades in the environment, which we also characterize at molecular level. Metagenomics analysis of eDNA samples collected from Dapeng Bay, Shenzhen has revealed a high range of diverse antibiotic resistance genes (ARG), as well as over 100 genera of eukaryotes. In this highly anthropogenically affected Bay with elevated inorganic N and P inputs by a local stream, ARGs and human pathogens are abundant in the influent of a waste water treatment plant, though a large number of them were efficiently removed with the combination of a wastewater treatment plant and an engineered wetland. However, some ARGs and human pathogens persisted in fresh water downstream of the WWTP, in sea water collected at the beach and in the bay. It is worth noting that the groundwater collected at the beach exhibited the lowest abundance of ARGs and human pathogens. However, minor traces of sulfonamide (a group of semi persistent antibiotics) resistance genes were detected. This study hopes to shed light on whether the highly labile eDNA can be used as a biomonitoring tool or not across the LOC. This will likely depend on the understanding of how the sources and reactivity of DOM affect eDNA degradation. 

How to cite: Zheng, Y., Palomo, A., Ma, Y., Hopwood, M., Wang, J., Zhang, P., Li, H., Feng, L., Zheng, Y., and Zhang, C.: Chemical and biological information in dissolved organic matter across the land-ocean continuum , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14500, https://doi.org/10.5194/egusphere-egu24-14500, 2024.

Flooded caves within carbonate coastlines serve as important conduits for carbon transport and transformation prior to groundwater expulsion into the sea. To investigate the sources, magnitude and biogeochemical reactions regulating carbon sources within an unconfined coastal aquifer, we analyzed the concentrations and δ¹³C values of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) at four sites along a flow path, a 6-km shoreline-perpendicular transect of flooded coastal caves in the northeastern Yucatan Peninsula, Mexico. The study revealed significantly higher DOC concentrations after regional rainfall compared to a mid-summer drought, suggesting precipitation as a key driver of the downward DOC flux from the surface. The decomposition of organic matter in the saturated soils of mangroves and tropical forest is the source of high DOC (on average, 678 µM; δ¹³C-DOC = −28‰) in the shallow fresh groundwater. The regional seaward tendency of DOC in the upper aquifer to become more ¹³C-enriched is primarily driven by increased mixing near the coast with saline groundwater, which is lower in concentration (70 µM, on average) and more ¹³C-depleted (δ¹³C-DOC = −26‰) than the seawater DOC (158 µM; −19‰). Evidence for net DOC consumption, along with positive and negative changes in δ¹³C-DOC values, is consistent with microbe-mediated transformation of organic matter primarily occurring in the upper aquifer’s low salinity waters. Diminishing DOC, coinciding with 4- to 5-fold increase in DIC concentrations while δ¹³C-DIC becomes more positive, implies that organic matter diagenesis also enhances carbonate dissolution. These landscape-level observations reveal hydrologic and biogeochemical factors that regulate the internal functioning of a coastal aquifer ecosystem and influence the quality and quantity of carbon exported to the coastal sea, where the Mesoamerican Barrier Reef resides.

How to cite: Brankovits, D., Pohlman, J., Martínez García, A., and Alvarez, F.: The origins and fate of dissolved organic carbon in a density-stratified carbonate aquifer on a tropical coastal landscape, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20144, https://doi.org/10.5194/egusphere-egu24-20144, 2024.

Long-term warming of the continental shelf of the Canadian Beaufort Sea has caused decomposition of submarine permafrost. Ancient dissolved carbon preserved in submarine permafrost could be transported and released into seawater by submarine groundwater discharge derived from thawing permafrost. However, the rate and scale of such a carbon emission is currently unclear. To fill this knowledge gap, we investigate the δ¹³C, Δ¹⁴C, and composition of sediment pore fluid from samples retrieved from a shelf edge site, where rapid seafloor depressions as a result of permafrost thawing have been observed. Downcore decrease of water isotopic signatures indicate widespread meteoric freshwater seepage from the region. The Δ¹⁴C values of dissolved inorganic carbon (DIC) in pore fluids indicate an ancient source of DIC (up to 7.7 cal kyr BP). The carbon isotopic mass balance calculation with Δ¹⁴C and δ¹³C of DIC suggest an input of ancient DIC with little radiocarbon, which is not from in-situ dissolution of carbonate. In other words, mixing of DIC from in-situ degradation of local organic carbon and overlying seawater DIC cannot explain the observed Δ¹⁴C values of DIC. Based on these results from porewater profiles, we suggest a lateral discharge of low-chlorinity fluid carrying such an ancient DIC to shallow sediments as a result of the decadal degradation of submarine permafrost.

How to cite: Chen, N.-C., Kim, J.-H., Hong, J.-K., and Hong, W.-L.: Release of ancient dissolved carbon by thawing submarine permafrost in the Canadian Beaufort Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5199, https://doi.org/10.5194/egusphere-egu24-5199, 2024.

High-energy beaches mark a highly variable land-ocean transition where matter fluxes are modulated by dynamic subsurface biogeochemical reactions. At the beach face, seawater infiltration into the saline recirculation cell of the intertidal beach aquifer creates a high input of electron acceptors and organic matter. Microorganisms rapidly degrade fresh organic matter in the upper sandy beach layer under advective flow conditions. Filtration of particulate organic matter and constant supply of oxygen (O₂) in the shallow sand body result in much of this turnover taking place under predominantly oxic conditions. In temperate regions, this filter effect combined with seasonal seawater inputs results in a strong seasonality of reaction rates as well a seasonally heterogeneous distribution of rates. Additionally, subsurface transport dynamics of seawater containing biogeochemical reactants highly depends on the physical forcings such as tides, waves and the beach morphology, adding complexity to the system. We assume that the variable O₂-consuming degradation processes in the upper layer in combination with dynamic physical forcing regimes lead to a fluctuating oxycline in the beach aquifer. Therefore, the aim of our study was to investigate the impact of seasonally variable oxygen demand under different physical forcing regimes on redox zonation in the beach subsurface. We used O₂ consumption rates from the beach face at Spiekeroog Beach (Germany), measured down to 1m depth and over a year-long sampling campaign within the project DynaDeep, to develop a numerical reactive transport model at field scale. The results from the field data showed a strong seasonal depth dependency of O₂ consumption rates. Lowest rates were found in winter and increased substantially in summer, with the strongest increase in rates in the upper decimeters.Modelling case studies for a summer and a winter situation were carried out to simulate both, quasi-stationary and dynamic conditions. Model results show that the oxic zone is significantly larger in winter than in summer, aligning with the general O₂ distribution measured in the field. We found that in summer, dynamic tidal conditions lead to greater variations in O₂ concentrations than in winter. In addition, the model shows that during tidal inundation, O₂ can overcome the high consumption rates in the upper decimeters in summer, thereby increasing the oxic zone. Finally, the model will be used to explore the impact of additional physical forcings in order to better constrain the oxycline as a variable redox boundary for subsequent anoxic processes.

How to cite: Auer, F., Greskowiak, J., Reckhardt, A., and Holtappels, M.: Oxycline Variabilities in Intertidal Beach Aquifers Under Seasonally Variable Oxygen Consumption and Physical Forcing Regimes , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2212, https://doi.org/10.5194/egusphere-egu24-2212, 2024.

Intertidal permeable high energy beach systems represent complex biogeochemical reactors which attract increasing scientific attention. In these environments morphology variations lead to complex and dynamic groundwater flow paths, saltwater-freshwater mixing zones, and changing biogeochemical conditions. The aim of our study was to assess the spatio-temporal dynamics in the hydrobiogeochemistry of the continuum between a deep subterranean estuary (STE) and the surface of a high-energy beach on Spiekeroog Island (Germany). Several permanent wells distributed along a cross-shore transect (supratidal to intertidal zone) allowed for regular groundwater sampling down to 24 m below ground surface (mbgs). Additional direct push sampling helped to obtain a high resolution cross-sectional view on the deep STE groundwater biogeochemistry. We found salinities below 10 near the dunes increasing to a salinity of about 30 towards the intertidal zone. Tide- and wave induced seawater circulation reached down to more than 24 mbgs. Oxygen and NO₃^- penetrated 12-15 mbgs deep, at least in the supratidal to upper intertidal area. Below and towards the low water line, conditions were Fe-(hydr)oxide-reducing and accumulating Fe sulfides indicated active microbial net sulfate reduction. At few sites, the concurrent presence of dissolved NO₃^- and Fe indicated overlapping redox zones. Deep old freshwater from Spiekeroog’s fresh groundwater lens mixed with the saline groundwater in the lower intertidal zone and added nutrients, especially Si, but lowered dissolved Mn and Fe concentrations. Accordingly, these parameters followed the temporally varying location of freshwater discharge at this site. Except for this most seaward well site, biogeochemical conditions were found to be relatively stable in the zone below 12 mbgs of the STE and more variable above. Temporal changes related to seasonally varying input and processing of organic material seemed to be restricted to the top few mbgs around the high-water line, where tide-induced infiltration regularly adds young seawater to the beach system. In the next step, the results will be analyzed by reactive-transport modeling to allow for a further general understanding and extrapolation of flow and reaction dynamics in the deep subsurface below high-energy beaches.

How to cite: Reckhardt, A., Roberts, M., Böttcher, M. E., Meyer, R., Wurl, O., Pahnke, K., and Massmann, G.: Spatio-temporal dynamics of groundwater biogeochemistry in the deep subsurface of high-energy beaches, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9937, https://doi.org/10.5194/egusphere-egu24-9937, 2024.

Dissolved species of terrestrial and marine origins are transformed in Subterranean Estuaries (STEs) before they flow into the coastal oceans. The occurring biogeochemical reactions are highly complex, demanding the application of numerical reactive transport modeling (RTM) approaches to achieve a deeper process understanding. The objective of this study was to quantify the impact of organic matter degradation and secondary mineral reactions on the fate of dissolved species in a generic sandy STE. A comprehensive RTM approach was developed for this purpose, investigating the effects of ion activities, pH, pe, redox reactions, mineral equilibria (goethite, siderite, iron sulfide, hydroxyapatite and vivianite) as well as surface complexation. We found that the STE biogeochemistry was very sensitive to the assumed reaction network. For example, dissolved inorganic carbon and pH were mainly controlled by calcite and siderite dynamics. Dissolved Fe²⁺ and HS^- were precipitated as goethite, siderite and/or iron sulfides, respectively. PO₄^3- concentrations were strongly controlled by the formation of P-bearing minerals, e.g., vivianite and hydroxyapatite, as well as surface complexation. Our work helps to establish the relative importance of some of the major biogeochemical processes in the STE. In a next step, field data from a high-energy STE site on Spiekeroog (‘DynaDeep observatory’) will be used to explore which processes take place in real-world STEs.

How to cite: Seibert, S. L., Massmann, G., Meyer, R., Post, V. E. A., and Greskowiak, J.: Reactive transport modeling to study the impact of mineral reactions and surface complexation on the transport of dissolved species in a subterranean estuary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12252, https://doi.org/10.5194/egusphere-egu24-12252, 2024.

Subterranean estuaries (STEs) beneath sandy beaches are biogeochemical reactors that can modify the chemical composition of fresh groundwater and recirculating seawater. Compared to surface estuaries, the mechanisms underlying the transformation of dissolved organic matter (DOM) in the STEs remain challenging to disentangle, particularly in high-energy beaches where DOM supplied from marine and terrestrial sources is exposed to alternating redox conditions, salinity gradients, and dynamic flows. Our study is aimed at elucidating the spatio-temporal patterns of DOM sources and sinks in high-energy beach STEs from a case study site on a barrier island in the German North Sea. We present a geochemical analysis of freshwater lens groundwater (FWL), seawater (SW), and beach STE groundwater samples (STEGW) collected over different seasons. STEGW samples were collected from multilevel wells with sampling depths of 6m, 12m, 18m, and 24m close to the dune base (ML1), near the high-water line (ML2), and the low-water line (ML3), and the FWLGW was collected from wells located at the northwestern part of the island. All samples were analyzed for their dissolved organic carbon (DOC) concentrations, and humic-like/terrestrial components of DOM (FDOM) were determined via fluorescence spectroscopy. DOM samples were desalted through solid-phase extraction and molecularly characterized via ultra-high-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. We found a decrease of FDOM along the salinity gradient in the land-sea continuum (FWL > STEGW > SW). DOC concentrations were highest in the FWL and SW but relatively lower in STEGW samples, suggesting a role of this STE as net organic carbon sink. The DOM composition of the groundwater samples from all sampling stations was highly diverse, with a total of up to 10,000 detected molecular formulas. The numbers of detected molecular formulas in FWL samples were twofold higher than those in SW samples, with intermediate values observed in the STEGW. There was a general decline in terrestrial DOM signature in the land/sea continuum, suggesting the loss of terrestrial DOM from land to the sea. Our results indicate that STEs under high-energy beaches are powerful sinks for organic carbon from both marine and terrestrial sources.

How to cite: Amoako, K., Abarike, G., Waska, H., Niggemann, J., and Dittmar, T.: The sandy beach subterranean estuary as a potential organic carbon sink, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9178, https://doi.org/10.5194/egusphere-egu24-9178, 2024.

Eutrophication is a process driven by enrichment of water by nutrients, especially compounds of nitrogen and phosphorus, leading to: increased primary production, changes in the balance of organisms, and water quality degradation. It occurs naturally; however, human activities have accelerated its rate and extent through both point-source discharges and non-point loadings of limiting nutrients into aquatic ecosystems. Since the beginning of the XXI^st century, concern regarding eutrophication led to the adoption of policies aimed at combating eutrophication, such as the Water Framework Directive and the Marine Strategy Framework Directive. To implement these policies and achieve their goals, the proper comprehension of aquatic ecosystems structure and dynamics and the exchanges occurring among them are essential; in other words, to provide an integrated vision of the hydrosphere is key.

Under the umbrella of the AquaINFRA project (https://aquainfra.eu/), we are developing a new set of hydrodynamical and biogeochemical simulations of our case study, the Catalan coast (NW Mediterranean), to assess the risks and hazards to its coastal ecosystems. A regional circulation model (BFMcoupler; see Galiana, S. et al. at Session OS2.1) that takes into account continental inputs from both point and non-point source discharges is used, including from riverine to submarine groundwater discharges (SGD).

This work aims to focus on the relevance of SGD and their seasonal pattern in the north of the Creus Cape, the northernmost part of the Catalan coast.

The coastline of this zone is characterized by Mediterranean dry fields, fragmented  forests, few small urban areas and small catchment zones of temporal streams. It shows a 591 mm mean annual precipitation and 14.8 ºC mean annual air temperature. Data, collected from 2011 to 2016 (following Directives requirements) in five sampling stations located at 1m from the coast, reveal lower mean salinity (36.24) and higher mean nitrate (8.63 µM) and silicate (8.10 µM) concentrations compared with zones of similar characteristics, such as the Montgrí coast located further south (three sampling stations; 37.61; 3.18 µM; 2.23 µM), which indicate the presence of SDG. However, their mean chlorophyll-a concentrations are similar, 1.06 and 1.11 µg/l, which implies that primary production is not enhanced by nutrients. The seasonal pattern of this zone is similar to that observed at open and surface Mediterranean waters, except for the decrease in salinity and the increase in nitrate during autumn and winter, typically rain and wet periods, respectively. Therefore and according to Garcia-Solsona, E., et al. (2010), SDG are significant in coastal waters of the Mediterranean Sea, where their influence is more prominent in coastal zones without the influences of urban areas or rivers.

Currently, these SDG data are being included in the hydrodynamical and biochemical model of the AquaINFRA project for a better understanding of the eutrophication process in coastal waters. Thereby, they will provide information for the European Directives and, thus, improve the integrated management of aquatic ecosystems under the ecosystem approach.

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(This project has received funding from the European Commission’s Horizon Europe Research and Innovation programme under grant agreement No 101094434)

How to cite: Flo, E., Galiana, S., Ballabrera, J., Berdalet, E., Otsu, K., Pesquer, L., and Garcia, X.: Submarine groundwater discharges in the Creus Cape (NW Mediterranean): new data for an hydrodynamical and biochemical sea model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20033, https://doi.org/10.5194/egusphere-egu24-20033, 2024.

Coastal aquifers encompass a variety of flow patterns, with circulating seawater being a prominent phenomenon driven by diverse mechanisms of varying spatial and temporal scales. Understanding the volume of seawater circulation in aquifers is of great interest due to water-rock interactions and consequent modification of seawater composition. While the influence of circulating seawater on ocean biogeochemistry and ecology is well recognized, quantifying its actual effect is challenging due to the involvement of multiple mechanisms.

In this study, we employed element and isotope ocean budgets and groundwater flow modeling to quantify fluxes through specific mechanisms, enabling us to determine solute fluxes from coastal aquifers into the sea. Our budgets included all known ocean sources and sinks, such as river fluxes, mid-ocean ridge hydrothermal circulation, basalt weathering, and diffusive fluxes. We used major element budgets and δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr budgets to constrain the long-term SGD flux. Our sensitivity tests included steady-state conditions and scenarios where the hydrothermal fluxes and rivers were over or underestimated, and precipitation of carbonates was over-estimated. We found the steady-state, underestimation of both hydrothermal and river fluxes, and overestimation of carbonate fluxes reasonable scenarios.

Our findings, using groundwater flow models sensitivity tests and geospatial databases, reveal that benthic wave-driven circulation contributes the largest volume of circulating seawater, while other mechanisms such as nearshore circulation, tidal pumping, and density-driven circulation are 2-3 orders of magnitude smaller. However, the short duration (minutes) of water-rock interaction under the wave-driven circulation limits its potential to modify seawater. Long-term density-dependent circulation emerges as the most significant mechanism influencing ocean chemistry, primarily due to its extended time scale of water-rock interaction. The prevailing water-rock interaction process in coastal aquifers is identified as ion exchange, wherein circulating seawater returning to the sea becomes enriched in calcium and depleted in potassium and sodium. The annual volume of seawater circulating through the long-term process is calculated to be approximately 1000-2000 km³/y, resulting in calcium and potassium fluxes of 17 ± 6 and -3.36 ± 1.6 Tmol/y, respectively. Notably, these fluxes are of the same magnitude as solute fluxes from rivers.

How to cite: Kiro, Y. and Levy, Y.: Using ocean chemical budgets and groundwater flow models to quantify SGD individual component fluxes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9016, https://doi.org/10.5194/egusphere-egu24-9016, 2024.