Coastal Groundwater Research Articles

The coastal zone presents complex hydrodynamic interactions among inland groundwater, reservoir water, and intruding seawater, with important implications for ecosystem functioning and water quality. However, the relative roles of hydraulic connectivity and seawater-driven salinity gradients in shaping microbial communities at the aquifer-reservoir interface remain unclear. Here, we integrated hydrochemical analyses with high-throughput 16S rRNA gene sequencing to investigate bacterial community composition, assembly processes, and co-occurrence network patterns across groundwater_in (entering the reservoir), groundwater_out (exiting the reservoir), and reservoir water in a coastal system. Our findings reveal that seawater intrusion exerts a stronger influence on groundwater_out, leading to distinct chemical profiles and salinity-driven environmental filtering, whereas hydraulic connectivity promotes greater microbial similarity between groundwater_in and reservoir water. Groundwater samples exhibited higher alpha and beta diversity compared to the reservoir, with dominant taxa such as Comamonadaceae, Flavobacteriaceae, and Rhodobacteraceae serving as indicators of seawater intrusion. Community assembly analyses showed that homogeneous selection predominated, especially under strong salinity gradients, while dispersal limitation and spatial distance also contributed in areas of reduced connectivity. Key chemical factors, including TDS, Na+, Cl-, Mg2+, and K+, strongly shaped groundwater communities. Additionally, groundwater bacterial networks were more complex and robust than those in reservoir water, suggesting enhanced resilience to salinity stress. Collectively, this study demonstrates that salinity gradients can override the effects of hydraulic connectivity in structuring bacterial communities and their networks at coastal interfaces. Our findings provide novel microbial insights relevant for understanding biogeochemical processes and support the use of microbial indicators for more sensitive monitoring and management of coastal groundwater resources.

Located on the northern coast of Java near Jakarta, Muara Gembong is confronted with significant environmental health threats due to land control issues, land conversion, and pollution from the Citarum River. These factors contribute to poor water quality in residential areas, making the water unsuitable for clean water use. The objective of this study is to investigate the vulnerability and factors influencing the health resilience of populations who rely on groundwater. The research utilizes qualitative and quantitative correlational methods, gathering data from 40 respondents in Pantai Mekar Village, Muara Gembong, West Java, through questionnaires. Three proposed hypotheses were processed using SPSS. The findings reveal that environmental health in Muara Gembong, particularly sanitation and clean water access, remains a significant problem, leading to increased vulnerability to infectious diseases. Key discoveries include a substantial relationship between knowledge of groundwater quality and community health resilience (0.000 < 0.05), a link between water sanitation attitude in coastal areas and health resilience (0.002 < 0.05), and a combined effect of groundwater quality knowledge and sanitation attitude on health resilience (0.001 < 0.05). Collectively, groundwater quality knowledge and sanitation behavior contribute 36.5% to the health resilience of coastal groundwater users.

Problem definition: Most of the world’s population relies on coastal aquifers for freshwater supplies. Groundwater is experiencing substantial overdrafts and facing ever-mounting freshwater demand. Existing groundwater management strategies are myopic and fail to coordinate production and the operation of protection approaches, including seawater intrusion barriers (SWIBs) and managed aquifer recharge (MAR). Motivated by the urgency of sustainable groundwater management, we investigate how to optimize the joint operations of groundwater production, protection (by injecting fresh water through SWIBs), and replenishment (via MAR). Methodology/results: We model a central planner’s decision on groundwater production, freshwater injection quantities, and artificial replenishment using stochastic dynamic models and identify that the optimal groundwater management policies follow a threshold-type structure. We find that SWIBs and MAR are strategic complements, except in cases with very high groundwater levels, when they turn into strategic substitutes. When the penalty for low groundwater levels decreases, the planner should use SWIBs more aggressively if groundwater levels are low and more conservatively if they are high. A similar pattern holds when natural recharge becomes more abundant, assuming that the natural recharge quantity has no impact on the purchasing cost of imported water. Moreover, we calibrate our model using real data sets in Orange County, California and find that the joint operations of SWIBs and MAR expand groundwater operational flexibility. In contrast, employing SWIBs alone comes at the expense of a lower groundwater level. Managerial implications: Our analysis offers strategic guidance on when to use SWIBs and MAR as complements or substitutes based on groundwater levels. It highlights the value of their joint operation in stabilizing groundwater, especially amid worsening droughts. Funding: This work was partially supported by the National Natural Science Foundation of China [Grants 72242106, 72242107, and 72188101]. Supplemental Material: The online appendix is available at https://doi.org/10.1287/msom.2022.0441 .

ABSTRACTIn coastal lowland and plains, where dense populations and extensive agriculture thrive, fresh water is a necessity. Management of coastal groundwater requires quantitative models of groundwater flow and transport to reveal salinity distributions and submarine groundwater discharge (SGD). Historically, computational burdens in modelling coastal groundwater necessitated the use of 2‐D models along cross‐sections. However, a variety of different features add fully three‐dimensional complexity to near‐shore groundwater flow and complex patterns of saline‐fresh groundwater mixing. Shoreline geometry is one such feature which has not been systematically assessed. We modelled mixing processes between freshwater and saltwater in three dimensions using the coupled surface/subsurface flow and transport code HydroGeoSphere. Various concave and convex coastlines were constructed with geomorphological features characteristic of those in China. Our simulation suggested that the lateral groundwater flow can cause significant velocity perpendicular to the plane of the cross‐sections, accounting for from 2% to 12% of longitudinal velocity. The lateral velocity component of groundwater increases with shoreline curvature, causing nonlinear responses of lateral saltwater circulation. The mixing zone and SGD change as a function of coastline curvature, which depends mainly on the magnitudes of convergent/divergent flow. In addition, shoreline curvature increases the mean travel time of fresh SGD, while a limited impact on the travel time of saline SGD is found. Our results highlight the nonnegligible influence of coastline geometry on lateral groundwater flow, freshwater‐saltwater mixing, and SGD characteristics in commonly concave and convex shorelines. This study has important implications for management of groundwater resources, comprehension of biogeochemical processes along the coastal lowlands and plains.

Relative assessment of constrained multiple objective optimization techniques for optimal pumping strategy design to mitigate saline water intrusion in coastal groundwater systems

The muddy coastal region of North Shandong, China, including Laizhou Bay and the Huanghe River (Yellow River) Delta, is a complex depositional environment where land and sea intersect, creating diverse water types and intricate coastal groundwater formation and evolution. This study focuses on the shallow Holocene aquifer (SHA) and the deep Pleistocene aquifer (DPA) groundwater, using hydrogeochemical, isotope analysis and numerical simulation methods to infer the source of water and salt and hydrogeological chemical processes. The results reveal that the groundwater is a mix of seawater, freshwater, and brine, with significant differences in hydrochemical types and isotopic signatures between the SHA and DPA aquifers. The SHA groundwater is dominated by low salinity (TDS ≈ 8 g/L), with the freshwater dominated by Cl-Na and Cl-Na·Mg hydrochemical types. In contrast, the DPA groundwater is characterised primarily by high salinity (TDS ≈ 72 g/L) and the Cl-Na type. δ18O-δ2H deviates from the precipitation line and is close to the seawater evaporation line, indicating stronger seawater intrusion and salt accumulation processes. Interestingly, δ18O and δ2H stable isotopes' relative abundance in the DPA brine samples from the Huanghe River Delta (at a burial depth of ~260 m) and Huanghe River water samples bear a resemblance, suggesting a strong correlation between the river water and the subsurface brine water source in the EPA. The Hydrochemical Facies Evolution Diagram (HFE–Diagram) analysis shows 63.77% of SHA samples underwent desalination, while 79.31% of DPA samples experienced seawater intrusion, this was restricted by structural constraints and rock salt dissolution. This study provides new insights into the hydrogeochemical evolution of coastal aquifers.

The study aimed to identify and monitor potential submarine groundwater discharge (SGD) zones by integrating thermal infrared (TIR) remote sensing with groundwater dynamics, particularly groundwater level and electrical conductivity along the Southeastern Arabian Sea (SeAS) coastline. Sea surface temperature anomalies were detected in pre- and post-monsoon seasons using TIR imagery to identify SGD sites along a ~ 600-km stretch of SeAS. Coastal rivers and streams were delineated to identify sea surface temperature anomalies from surface runoff. Groundwater level and electrical conductivity of coastal aquifers were analyzed from 2017 to 2022 to characterize potential SGD zones. The results show varying cold-water anomalies linked to SGD of colder groundwater into the ocean, with notably higher anomalies post-monsoon due to increased groundwater recharge from monsoonal rainfall. Some anomalies are linked to direct surface water discharge from coastal rivers/streams. Groundwater level analysis shows that about 240km of coastline shows groundwater levels reaching up to 50m above mean sea level (msl) seasonally, leading to significant SGD during pre- and post-monsoon periods. Some areas with low groundwater levels in both seasons experience seawater intrusion, putting about 250km of coastline at risk of SGD extinction due to groundwater levels near msl. The electrical conductivity of groundwater shows that 86.4% of the coastline has fresh SGD, while 12.9% and 0.6% have brackish and saline SGD zones, respectively. Overall, the study reveals spatiotemporal variation in SGD along the SeAS, with findings useful for coastal groundwater management and ecosystem protection.

In recent times, groundwater (GW) contamination has been rising at an alarming rate in the freshwater-scarce dry and coastal areas of the world in terms of arsenic (As) and fluoride (F−). This study focused on the vulnerability assessment of ‘As and F−’ contamination of GW in the Gangetic delta region by evaluating GW quality and potential health issues. After analyzing the health risk, it is noticed that about 65% area of the Ganges delta come over the moderate to excessive health risk zone, which is detected in the northeast, east, and central portion of this region due to high population pressure on GW consumption. The findings of this study will be fruitful to the local, national, and international environmentalists, water scientists, and supervision experts to mitigate such harmful condition sustainably and give safeguard to the local people by supplying fresh drinking water by undertaking some preventive measures.

The development of Padang City in the coastal area is potentially faced with seawater intrusion. This study was conducted to determine the characteristics of shallow groundwater on the coast of the city using stable oxygen -18 (18O) and hydrogen -2 (2H) or deuterium isotopes, which are abundant in the nature. The isotopic analysis revealed variations in groundwater isotopic composition, indicating the possibility of diverse recharge sources. Although this study was primarily focused on salinity identification, the isotopic data provided preliminary insights into the influence of local meteoric recharge. Water samples were taken at several locations 5 m below the ground surface, and one sample of seawater was collected at an elevation of 0 m, which comes directly from sea water. On the average, they were located 270 m from the sea. The spectrometry of these water samples produced isotope ratios expressed per thousand or mil, which were then plotted on a graph illustrating the relative abundances of oxygen (18O) and deuterium (2H). Analyses of the stable 18O and 2H isotopes found two water samples close to the local meteoric water line (LMWL) and one sample interacting or mixing with seawater. The mixing effect is likely the product of evaporation and interaction between water and oxide minerals that compose the aquifer lithology, i.e. loose (sand) deposits. Based on the electrical conductivity, these samples had brackish water.

Abstract Saltwater intrusion (SWI) is a critical, persistent and demanding environmental hazard affecting coastal habitats and groundwater sustainability worldwide. The present study uses an integrated geophysical approach to delineate the salinity‐affected zones in the coastal aquifers of the Contai–Mandarmani region in West Bengal, India. Vertical electrical sounding (VES), electrical resistivity tomography (ERT), induced polarization (IP) and self‐potential (SP) measurements were employed to carry out near‐surface geophysical studies at several sites. The flow path of 3D saltwater infiltration was analysed by collecting quasi‐3D ERT data. The 2D ERT results identify the zones with resistivity values below 2 Ω m and IP values below 7 ms as the layers affected by SWI. These findings exhibit a strong correlation with the results acquired from VES and quasi‐3D survey investigations. The resistivity results show that the shallow aquifers at a depth of less than 40 m below the Earth's surface are primarily affected by the SWI. In situ water samples were collected from the study area to obtain the geochemical parameters such as electrical conductivity (EC), total dissolved solids (TDS) and pH levels. High TDS (>700 ppm) and EC (>1500 µS/cm) readings from various locations within the study area confirm the groundwater salinity issue and validate our geophysical findings. The present study found that the major cause responsible for increased salinity levels in the inland regions is anthropogenic activities, including aquaculture, paddy fields and salt pans. Moreover, the frequent cyclones contribute to the rising salinity levels in the areas near to seacoast. This integrated study provides a valuable framework for monitoring salinity intrusion in the study area. It offers critical geophysical insights to inform policy and guide the planning of remedial measures for the sustainable use of coastal groundwater resources.

Impact of REE Mining on Coastal Groundwater: Numerical Modelling and Remediation Potential of Clay-Amended Laterites

Coastal aquifers in Terengganu, Malaysia, face increasing challenges in groundwater quality and availability, necessitating a comprehensive assessment of their vulnerability. This study investigated groundwater vulnerability and susceptibility in the coastal region of Terengganu, Malaysia, where coastal aquifers face threats to groundwater quality and availability. A comprehensive groundwater vulnerability assessment was conducted using the TRUST Index. This index-based approach considers the lithology, river proximity, well usage, distance to the seashore, and well type. Field investigations were undertaken to obtain real-time measurements of well behavior. This included conducting constant-rate pumping tests on four private wells to gauge hydraulic conductivity. Consequently, flow rates were meticulously monitored throughout these tests, and water level measurements and physicochemical assessments were conducted over a 120-minute duration. Following this, the data was analyzed utilizing AQTESOLV software to determine the hydraulic conductivity, transmissivity, and storativity of the aquifer. The data from MW4, MW16, and MW20 collectively indicate favorable hydraulic characteristics, suggesting water movement within the aquifer, ranging from 4.02 m3/day to 11.39 m3/day. In contrast, MW7 displays an unexpectedly high discharge rate of 19.77 m3/day, suggesting a highly permeable and efficient water-transmitting unconfined aquifer with limited water storage capacity. The vulnerability assessment classified the wells as Low, Moderate, and High vulnerability. Wells MW1, MW6, MW7, and MW20 were categorized as low vulnerability, indicating relatively secure groundwater quality and availability. Wells MW2, MW3, MW8, MW9, MW12, MW13, and MW14 were classified as moderately vulnerable, suggesting a moderate level of potential risk. Meanwhile, wells MW4, MW5, MW10, MW11, MW15, MW16, MW17, MW18, and MW19 were labeled as highly vulnerable, signifying a higher susceptibility to threats. The correlation matrix revealed insightful connections between hydrological and water quality parameters. The distance from the seashore is inversely correlated with salinity and specific conductance, signifying a reduced seawater water impact farther inland. Note that wells near rivers exhibit higher salinity, likely due to potential saltwater intrusion, emphasizing the importance of understanding these relationships in coastal aquifer systems. This study comprehensively assesses coastal groundwater vulnerability, behavior, and water quality. Its unique contributions lie in the meticulous hydraulic characterization and identification of unconventional well behavior. These findings emphasize the importance of considering temporal variations, local influences, and tailored management strategies for sustainable coastal groundwater resource utilization.

The 79th volume of the Bulletin of the Geological Society of Malaysia comprises eight original research articles that touch on techniques and approaches in geological sciences, some with potential for wider application. The lead paper by Misman et al. (2025) introduces drone technology for investigating coral reefs in Pangkor Island, Perak. The use of images from drone technology has immense potential to be advanced, for example in the study of fast and slow onset hazards. Zulkipli et al. (2025) deployed the portable Raman Spectrometry for the first time to analyse mineralogical properties of ancient pottery in Kuantan, Pahang. In combination with the conventional X-Ray Diffraction, the technique provided new insights on the manufacturing process, firing conditions and provenance of the artifacts. A novel approach for assessing flood risks associated with post-earthquake settings has been proposed for Kota Belud, Sabah by Sharir & Roslee (2025). The method draws on hydrodynamic modelling to identify areas at risk of floods and levels of vulnerability, to delineate communities that require enhanced resilience building. A combination of approaches was used to characterise subsurface karstic conditions by Nguyen et al. (2025) in Hanoi, Vietnam. The approach is useful to develop long-term strategies for mitigation and early warning of land subsidence in the densely populated areas. Ostadi et al. (2025) used the TRUST Index to determine the vulnerability status of coastal groundwater wells in Terengganu. Further investigation is required to establish the hydraulic characteristics of this coastal aquifer system and assess risks due to saltwater intrusion. The next batch of papers are on structural geology and sedimentology. Choong et al. (2025) draw on the K-Ar Age dating method to investigate deformation events in the Bentong-Raub Suture Zone in Peninsular Malaysia. Sedimentological analysis of outcrops in Miri, Sarawak and Brunei Darussalam was used to establish the relationship between lithofacies and depositional settings by Rahman et al. (2025). In the last paper of this volume, Pirot et al. (2025) uses microscopic analysis to characterise reservoir properties from surface outcrop samples in Iraq, which are comparable to results from scanning electron microscopy and core plug analysis. The following sections provide highlights from the deployment of these techniques and approaches.

Coastal beach is alternately covered by and exposed to external driving forces such as waves and tides, leading to significant response in the groundwater dynamics in coastal aquifer, which is closely related to various nearshore issues, e.g., pollutant transport and coastal erosion. A number of recent studies has been conducted to investigate the wave- induced groundwater dynamics (Yang et al., 2022; Zheng et al., 2023). However, the effect of wave periods on the coastal groundwater response is still unclear. In this study, a series of regular wave laboratory experiments were conducted to reveal the spatiotemporal features of the groundwater dynamics including the total water head and the seepage flow, paying particular attention to the wave period effect.

Coastal shallow groundwater is susceptible to adverse sea-level rise (SLR) impacts. Existing research primarily focuses on SLR-induced salinization of coastal aquifers. There is limited understanding of the magnitudes and rates of water table rise in response to SLR, which could lead to groundwater flooding and associated infrastructure challenges. This study used a variable-density groundwater flow model to quantify the transient movement of the water table in response to various SLR scenarios and rates, considering a range of aquifer parameters for both fixed-head and fixed-flux inland boundary conditions. The SLR scenario based on realistic and progressive SLR projections resulted in a smaller water table rise than the instantaneous or gradual SLR scenarios at 100 years, despite a final identical SLR. Rates of water table rise were always less than SLR, decreased with distance from the coastline, and were proportional to SLR. The magnitude and rate of water table rise in response to SLR were largest for fixed-flux conditions. It also took longer for the rate of water table rise to equilibrate after the commencement of SLR for fixed-flux conditions than for fixed-head conditions. As such, fixed-flux conditions represent a greater hazard for water table rise, and the maximum impact may not be experienced for decades. This delayed response poses challenges to planners and managers of coastal groundwater systems. Introducing a drain reduced water table rise more on the inland side of the drain than on the coastal side. Subsurface infrastructure may limit SLR impacts, but further effects need to be carefully considered.

Assessing the effects of ENSO-induced climate variability on shallow coastal groundwater reserves of north Patagonia, Argentina

Abstract Groundwater management involves a complex decision‐making process, often with the need to balance the trade‐off between meeting society's demand for water and environmental protection. Therefore effective management of groundwater resources often involves some form of multi‐objective optimization (MOO). Many existing software tools offer simulation model‐enabled optimization, including evolutionary algorithms, for solving MOO problems. However, such analyses involve a huge amount of numerical process‐based model runs, which require significant computational effort, depending on the nonlinearity and dimensionality of the problem, in order to seek the optimal trade‐off function known as the Pareto front. Surrogate modeling, through techniques such as Gaussian Process Regression (GPR), is an emerging approach to significantly reduce the number of these model evaluations thereby speeding up the optimization process. Yet, surrogate model predictive uncertainty remains a profound challenge for MOO, as it could mislead surrogate‐assisted optimization, which may result in either little computational savings from excessive retraining, or lead to suboptimal and/or infeasible solutions. In this work, we present probabilistic Pareto dominance criteria that considers the uncertainty of GPR emulation during MOO, producing a “cloudy” Pareto front which provides an efficient decision space sampling mechanism for retraining the GPR. We then developed a novel acquisition strategy to manage the solution repository from this cloud and generate an ensemble of infill points for retraining. We demonstrate the capabilities of the algorithm through benchmark test functions and a typical density‐dependent coastal groundwater management problem.

Mechanisms controlling spatial variability of geogenic ammonium in coastal aquifers: Insights from Holocene sedimentary evolution.

ABSTRACTPrevious studies on solute transport in coastal aquifers have mostly focused on the transport characteristics of instantaneous point sources. In actual situations, due to various causes of pollution, pollutants usually do not enter the aquifer instantaneously but continue to discharge into the groundwater at a certain speed. Due to different pollution conditions, pollutants usually enter the aquifer in various forms such as point sources and line sources. It is of great significance to study solute transport characteristics with different release modes in coastal aquifers for environmental impact assessment of coastal groundwater. This paper explored a laboratory‐controlled experiment to research the transport characteristics of solute with different release modes in coastal unconfined aquifers under tidal action. This paper presented that the solute plumes of each release mode affected by tidal fluctuations all follow the transport characteristics of a large step to the sea and a small step to the land. The rising tide suppressed the original change trend of the concentration at each point. The spreading range of the solute plume in each release mode had periodic fluctuation. With the increase of the hydraulic gradient during the falling tide, the spreading range of the solute plume increased, whereas with the difference of the transport velocity of the leading edge and trailing edge of the solute plume during the rising tide, the spreading range decreased slightly. The variation characteristics of the spreading range of the continuous source mainly depend on the continuously released rate of the tracer. When the continuous release rate of the tracer and the discharge to the sea reach a dynamic equilibrium, the solute transport velocity of the continuous source will eventually oscillate around a relatively stable velocity value, and the solute concentration value of each point in the aquifer will eventually oscillate around a relatively stable concentration value, especially the concentration of each point in the continuous line source will be approximately equal to the initial concentration.

Coastal inundation caused by seawater overtopping/overflow due to sea level rise and extreme coastal disaster events poses a significant threat to coastal groundwater resources. Seawater intrusion into coastal aquifers accelerates salinization, disrupting the natural freshwater balance and limiting sustainable water supply for drinking and agriculture. This study uses numerical simulations based on a porous body model to investigate the vertical seawater intrusion process triggered by coastal inundation. The inundation height, distance, and hydraulic gradient effects on salinization and recovery dynamics in coastal aquifers were analyzed. The results indicate that longer inundation distances cause more extensive salinization, whereas higher inundation heights accelerate vertical intrusion. Additionally, lower hydraulic gradients lead to prolonged retention of saline water, delaying recovery. In contrast, higher hydraulic gradients facilitate rapid discharge of intruded seawater, accelerating salinization recovery. The recovery process follows a logarithmic trend, initially rapid but slowing. These findings emphasize the importance of understanding the interplay between coastal inundation conditions and groundwater flow dynamics to effectively manage and protect coastal freshwater resources.