Coastal Groundwater Research Articles

ABSTRACT Half a century after the only national-scale report on saltwater intrusion (SWI) in Lebanon, the evolution of this hazard is re-examined. SWI proxies from 4000+ field measurements and chemical samplings are collected from a network of 276 sites. To interpret this dataset, the coast is divided into seven large coastal areas which are then sub-divided into 14 smaller coastal hydrogeological zones (CHZs) reflecting uniform hydrogeological conditions and properties. Hydrochemical analyses characterized the groundwater types of the coast. Fresh–brackish to brackish–salt NaCl or CaCl water types attaining elevated salt content are present in several zones, and ongoing salinization to variable extents affects four CHZs. Porous-unconsolidated aquifers suffered the largest salinization spread compared to fractured karstic aquifers. Over-pumping is the main SWI driver in many zones. Comparing to older results, fast-ascending SWI impacts most coastal groundwater resources. The database from this study is shared to serve urgently needed continuous monitoring of SWI proxies and groundwater resources management.

Tracking the trace element pollution and associated health risk in urban coastal groundwater, eastern Saudi Arabia

Abstract Interactions between surface flows and groundwater in beaches can influence erosion and accretion, wave overtopping, groundwater levels and salinization, and transport of nutrients and pollutants. Laboratory experiments using transparent crushed quartz and optically matched mineral oil as proxies for sand and water allow the degree of saturation to be computed at pore‐scale (0.7 mm resolution) enabling detailed investigations of the wave runup driven infiltration into a beach in a wave flume for a range of slopes and flow boundary conditions. The evolution of the wetting front resulting from wave runup on an initially unsaturated beach is described in detail, including the formation of an infiltration wedge in the subsurface of the swash zone and the wave‐driven rise in fluid elevation inside the beach. The elevation of the runup for each event is found to be related closely to the saturation of the beach face, reaching an equilibrium state once the subsurface in the swash zone reaches capacity. The back wall boundary condition in the flume has a significant role in how subsurface flows increase saturation within the beach, especially with boundary head elevations greater than the initial phreatic surface. The results of these novel experimental observations are used to develop dimensionless relationships between the surface wave runup and the subsurface saturation rates. To improve monitoring and interpretation of future coastal groundwater studies, three distinct cross‐shore regimes are defined for assessing change in subsurface fluid elevation in the beach.

Abstract Removing carbon dioxide (CO2) from the atmosphere is required for mitigating climate change. Large-scale direct air capture combined with injecting CO2 into geological formations could retain carbon long-term, but demands a substantial amount of energy, pipeline infrastructure, and suitable sites for gaseous storage. Here, we study Earth system impacts of modular, sun-powered process chains, which combine direct air capture with (electro)chemical conversion of the captured CO2 into liquid or solid sink products and subsequent product storage (sDACCCS). Drawing on a novel explicit representation of CO2 removal in a state-of-the-art Earth system model, we find that these process chains can be renewably powered and have minimal implications for the climate and carbon cycle. However, to stabilize the planetary temperature two degrees above pre-industrial levels, CO2 capturing, conversion, and associated energy harvest demand up to 0.46% of the global land area in a high-efficiency scenario. This global land footprint increases to 2.82% when assuming present-day technology and pushing to the bounds of removal. Mitigating historical emission burdens within individual countries in this high-removal scenario requires converting an area equivalent to 40% of the European Union’s agricultural land. Scenarios assuming successful technological development could halve this environmental burden, but it is uncertain to what degree they could materialize. Therefore, ambitious decarbonization is vital to reduce the risk of land use conflicts if efficiencies remain lower than expected.

Abstract Groundwater is crucial to sustaining coastal freshwater needs. About 32 million people in the coastal USA rely on groundwater as their primary water source. With rapidly growing coastal communities and increasing demands for fresh groundwater, understanding controls of continental-scale coastal groundwater salinity is critical. To investigate what hydrogeological factors (e.g. topography, hydraulic conductivity) control coastal saline groundwater at continental scales, we have simulated variable-density groundwater flow across North America with the newly developed Global Gradient-based Groundwater Model with variable Densities (G3M-D). The simulation results suggest that under a steady climate and pre-development conditions (i.e. steady 30-year mean groundwater recharge, no withdrawals nor sea level rise) saline groundwater is present in 18.6% of North America’s coastal zone, defined as up to 100 km inland and up to 100 m above mean sea level. We find that the coastal zone is particularly vulnerable to containing saline groundwater at low hydraulic gradients (<10−4) and large hydraulic conductivities (>10−2 m d−1). To analyze model parameter sensitivities, i.e. which parameters control the resulting distribution of saline groundwater, we utilize the inherent spatial model variability. We find that hydraulic gradient, topographic gradient, hydraulic conductivity, and aquifer depth are important controls in different places. However, no factor controls coastal groundwater salinization alone, suggesting that parameter interactions are important. Using G3M-D based on G3M, a model that previous work found to be strongly controlled by topography, we find no controlling influence of recharge variability on the saline groundwater distribution in North America. Despite a likely overestimation of saline interface movement, the model required 492 000 years to reach a near-steady state, indicating that the saline groundwater distribution in North America has likely been evolving since before the end of the last ice age, approximately 20 000 years ago.

Controlling seawater intrusion (SWI) into freshwater aquifers is crucial for preserving water quality in coastal groundwater management. This research evaluates the performance of three machine learning (ML) models: eXtreme Gradient Boosting (BO-XGB), Light Gradient Boosting Machine (BO-LGB), and Categorical Gradient Boosting (BO-CGB) in predicting the SWI wedge length. A database of 345 numerical simulations was compiled from previous research, and Bayesian Optimization (BO) with fivefold cross-validation was used to fine-tune the models. The inputs included abstraction well distance (Xa), abstraction well depth (Ya), recharge well distance (Xr), recharge well depth (Yr), abstraction rate (Qa), artificial recharge rate (Qr), and SWI wedge length (L). Results show that BO-CGB consistently achieved the best performance, with high R2 values (0.996 in training and 0.969 in testing) and low RMSE values (0.439 m in training and 1.327 m in testing). SHapley Additive exPlanations (SHAP) analysis highlighted that Qa and Qr had the most significant impact on SWI wedge length predictions, followed by Xa and Ya. Partial Dependence Plot (PDP) analysis revealed a strong negative correlation between flow variables Qa and Qr and wedge length, while Xr displayed a more complex, non-linear pattern. BO-CGB emerged as the most reliable model for predicting SWI wedge length. To facilitate practical application, an interactive Graphical User Interface (GUI) was developed, enabling users to input variables and receive instant predictions, enhancing the practical usability of the ML models in managing SWI in coastal aquifers.

Abstract The spatial variability of the degree of salinity of soils and groundwater is a major issue concerning the sustainable management of water resources. The problem is stressed in coastal plain areas where saline bodies of different origin coexist. We present an extensive geoelectrical resistivity dataset collected in the coastal region of Ambon city. The application of ERT (electrical resistivity tomography) method is to identify lithology below the earth’s surface and determine the dispersion pattern of seawater intrusion in the research area. The area of ERT survey is located approximately 12 m from the coastline using three measurement lines of Wenner Schlumberger electrode configuration. The length of line 1 is 300 m in the south north direction, and the length of line 2 and 3 is 100 m each in the east-west direction. The electrode on line 1 is 10 m and the electrode spacing on line 2 and 3 is 5 m. The 2D resistivity modeling results show a low value resistivity anomaly zone which is interpreted as a water saturated region. The measurement results of electric surveys cannot be interpreted uniquely. Misinterpretation of the analysis results should be minimized by studying the restriction and limitation of the analysis method among other factors. It is recommended that results of other investigations be used to support this evaluation. The resistivity interpretation results are synchronized with the TDS (total dissolved solid) distribution value, allowing for the potential for seawater intrusion into Passo coastal groundwater. The dispersion pattern of subsurface seawater intrusion zones in the study area is sparse. This has resulted in the aquifer layer being interpreted as having been polluted by seawater intrusion with a resistivity ranging from 1.0 - 4.5 Ωm at depths up to 5.0 m with the direction of flow movement from East to West whose distribution is concentrated in the West.

Optimizing coastal groundwater quality predictions: A novel data mining framework with cross-validation, bootstrapping, and entropy analysis.

Abstract Widespread anthropogenic activities pollute groundwater that eventually seeps out to the coastal ocean. Here, we resolve nutrient transformations and fluxes in 11 sandy subterranean estuaries (STEs) with contrasting nutrient sources and development trajectories. Coastal groundwater nitrogen pollution stems from sewage discharge and land use change. Anthropogenically derived groundwater nutrient fluxes with high N/P ratios (∼170) accounted for 22%–61% of riverine inputs into China's coastal waters, providing an additional source of nutrients that can fuel coastal eutrophication and algal blooms. Sandy STEs remarkably attenuated ∼84% of nitrogen pollution, minimizing the impact of submarine groundwater discharge (SGD) on coastal water quality. Hence, STEs deliver an overlooked ecosystem service that is particularly important in highly polluted coastal aquifers. Protecting STEs and recognizing the integrated nature of groundwater and seawater is thus important in coastal water quality management initiatives.

Coastal aquifers are critical freshwater resources that face increasing threats from contamination and saltwater intrusion. Traditional approaches for characterizing these aquifers are challenged by complex dynamics, high-dimensional parameter spaces, and significant computational demands. This study presents an innovative method that combines an Auto-Regressive Convolutional Neural Network (AR-CNN) surrogate model with the Iterative Local Updating Ensemble Smoother (ILUES) for the joint inversion of contamination source parameters and hydraulic conductivity fields. The AR-CNN surrogate model, trained on synthetic data generated by the SEAWAT model, effectively approximates the complex input–output relationships of coastal aquifer systems, substantially reducing computational burden. The ILUES framework utilizes observational data to iteratively update model parameters. A case study involving a heterogeneous coastal aquifer with multipoint pollution sources demonstrates the efficacy of the proposed method. The results indicate that AR-CNN-ILUES successfully estimates pollution source strengths and characterizes the hydraulic conductivity field, although some limitations are observed in areas with sparse monitoring points and complex geological structures. Compared to the traditional SEAWAT-ILUES framework, the AR-CNN-ILUES approach reduces the total inversion time from approximately 70.4 h to 16.2 h, improving computational efficiency by about 77%. These findings highlight the potential of the AR-CNN-ILUES framework as a promising tool for efficient and accurate characterization of coastal aquifers. By enhancing computational efficiency without significantly compromising accuracy, this method offers a viable solution for the sustainable management and protection of coastal groundwater resources.

ABSTRACTCoastal groundwater is a vital resource for coastal communities around the globe, and submarine groundwater discharge (SGD) delivers nutrients to coastal marine ecosystems. Climatic changes and anthropogenic actions alter coastal hydrology, causing seawater intrusion (SWI) globally. However, the selection of SWI and SGD study sites may be highly biased, limiting our process knowledge. Here, we analyse hydroenvironmental characteristics of coastal basins studied in 1298 publications on SGD and SWI to understand these potential biases. We find that studies are biased towards basins with gross domestic product per capita below (SWI) and above (SGD) the median of all global coastal basins. Urban coastal basins are strongly overrepresented compared to rural coastal basins, limiting our progress in understanding undisturbed natural processes. Despite the connection between anthropogenic activity and coastal groundwater issues, and the consequential overrepresentation of urban basins in coastal groundwater studies, perceptual (or conceptual) models of coastal groundwater rarely include anthropogenic influences aside from pumping (e.g., subsidence, land use change). Taking a holistic view on coastal groundwater flows, we have developed an editable perceptual model illustrating the current understanding, including both natural and anthropogenic drivers. As SGD and SWI in new areas of the globe are studied, we advocate for researchers to utilise and further edit this perceptual model to openly communicate our process understanding and study assumptions.

Multiple environmental tracers combined with a constrained Bayesian isotope mixing model to elucidate nitrate and sulfate contamination in a coastal groundwater system.

Aquifer storage and recovery have gained attention as a solution that utilizes submarine groundwater discharge (SGD) as a surrogate water resource to alleviate water scarcity and fill the demand gap. Characterizing SGD is crucial for using coastal groundwater and improving understanding of the interaction between continental water and seawater. This study employs fiber-optical distributed temperature sensing (FODTS) and the heat tracer to quantify the groundwater flux in a coastal aquifer in northern Taiwan. The fluxes in different sections along the borehole were estimated from the temperature response caused by the active heating tests and campier groundwater flux under different tidal conditions, providing information on potential water resources for water resource planning and management. According to the active heating tests, the material of the sections with high-temperature response mainly consists of a gravel–sand mixture. Based on the estimations of groundwater fluxes along the well, the sections with low sensitivity of temperature response have low hydraulic conductivity and low groundwater flux. The estimated thermal parameters at the site are consistent with those obtained from the borehole samples in the laboratory tests. The groundwater fluxes in different sections are calculated based on the temperature response observed from the FODTS. The groundwater fluxes along the well vary between 0.02 and 1.77 m/day. There are considerable differences between the estimated fluxes during the tidal cycle in a heterogeneous coastal aquifer, indicating the high uncertainty of estimated SGD along coastlines.

Integrated impacts of mariculture on nitrogen cycling processes in the coastal groundwater of Beihai, southern China

Groundwater flow and salinity dynamics in swash Zones: Combined effects of Evaporation, Waves, and geologic heterogeneity

Abstract Karst subterranean estuaries within globally ubiquitous carbonate aquifers are coastal groundwater ecosystems that provide an essential water resource for human populations. To understand the drivers of salinization within a coastal aquifer in the Yucatan Peninsula (Mexico), we employed hydroacoustics in flooded caves to observe how oceanic and atmospheric events facilitate mixing between the meteoric lens (fresh‐brackish groundwater) and the saline groundwater on tidal and episodic timescales. Precipitation during Tropical Storm Carlotta increased the flow and salinity of the meteoric lens without evidence for vertical mixing across the halocline. We postulate that vertical migration of haloclines in the conduit relative to those within the rock matrix during precipitation creates lateral density gradients that drive mixing, and ultimately creates a brackish layer within the meteoric lens. These results provide a mechanistic explanation for vertical and lateral exchange in a coastal carbonate aquifer, which has implications for groundwater response to future climatic change.

Abstract The interactions between the atmosphere, ocean, and beach in the swash zone are dynamic, influencing water flux and solute exchange across the land‐sea interface. This study employs groundwater simulations to examine the combined effects of waves and evaporation on subsurface flow and salinity dynamics in a shallow beach environment. Our simulations reveal that wave motion generates a saline plume beneath the swash zone, where evaporation induces hypersalinity near the sand surface. This leads to the formation of a hypersaline plume beneath the swash zone during periods of wave recession, which extends vertically downward to a maximum depth of 30 cm, driven by the resulting vertical density gradients. This hypersaline plume moves approximately 2 m landward to the top of the swash zone and down the beachface due to wave‐induced seawater infiltration and is subsequently diluted by the surrounding saline groundwater. Furthermore, swash motion increases near‐surface moisture, leading to an elevated evaporation rate, with dynamic fluctuations in both moisture and evaporation rate due to high‐frequency surface inundation caused by individual waves. Notably, the highest evaporation rates on the swash zone surface do not always correspond to the greatest elevations of salt concentration within the swash zone. This is because optimal moisture is also required—neither too low to impede evaporation nor too high to dilute accumulated salt near the surface. These insights are crucial for enhancing our understanding of coastal groundwater flow, biogeochemical conditions, and the subsequent nutrient cycling and contaminant transport in coastal zones.

A global analysis of nearshore and submarine springs: spatial distribution, controlling factors, and probability of presence

Denmark's complete reliance on groundwater for water supply presents a unique case study in management of natural resources, urban planning, and water resilience in the face of climate change. This paper examines the groundwater management strategies in Denmark in general, focusing on Denmark's four largest cities—Copenhagen, Aarhus, Odense, and Aalborg— each facing distinct challenges due to their demographic, geographical, hydrogeological, and economic characteristics. Through analysis of these cities' approaches to groundwater management, this research contributes to the global discourse on sustainable urban water supply systems. As coastal groundwater cities (CGC), these urban areas must navigate the complexities of sustaining growing populations, mitigating climate change impacts, and coastal processes while ensuring the long-term viability of their groundwater resources. Copenhagen and Aalborg, built atop semi-confined fractured and locally karstic carbonate rocks, highlights the specific challenges associated with karstic groundwater systems, while, Aarhus, and Odense built on glaciofluvial aquifers faces different issues. The different groundwater challenges in these cities underscores the importance of integrating urban development with water resource management and environmental sustainability, offering valuable insights and lessons learned for other regions facing similar challenges. This study, thus not only sheds light on Denmark's groundwater management practices, but also emphasizes the need for innovative solutions to ensure the resilience of urban water supply systems in a changing climate and increasing pressures of emerging organic contaminants and elevated concentrations of geogenic elements induced by water abstraction and fluctuating water tables. Advanced Danish monitoring and modelling tools applied to support decision-making and innovation within the water sector are continuously developed and improved to support resilient and sustainable management of the available water resources.

This paper presents a new approach to the spatiotemporal design of groundwater quality monitoring networks for coastal aquifers. A fusion model combines the outputs of several developed simulation models to make estimates more accurate. A modified GALDIT method is used to incorporate the aquifer vulnerability to saltwater intrusion. The value of information (VOI) theory is applied to determine sufficient monitoring wells. The groundwater quality monitoring network is designed by employing a robust decision-making (RDM) approach under different management strategies and economic considerations. This approach incorporates the deep uncertainties of some critical variables, including water level and total dissolved solids (TDS) concentration at the coastline and pumping flow rates of agricultural wells. The new methodology is implemented in the coastal Qom-Kahak aquifer, Iran. The results illustrate that the combination model has significantly improved evaluation criteria compared to individual prediction models. The fusion model results indicate that thirty monitoring wells would be ideal. The RDM-based analyses in the Qom-Kahak aquifer showed that an optimal network with 30 monitoring wells outperforms the current network regarding various criteria, such as VOI and variance of estimation error. The new well configuration also demonstrates a suitable spatial distribution. Given that the current sampling frequencies are unsuitable for areas with varying vulnerabilities, we recommend sampling every 3months in areas with moderate vulnerabilities and once every three seasons in areas with low vulnerabilities, based on the information transfer index. Finally, a management strategy in which the pumping rate should be less than 60% of the current rate is suggested to prevent saltwater intrusion into the aquifer.