Review of Modeling Approaches at the Freshwater and Saltwater interface in Coastal Aquifers

Abstract

Around 40% of the world’s population depends on coastal aquifers for freshwater supply but natural and anthropogenic drivers threaten groundwater availability. Of these drivers, saltwater intrusion (SWI) is one of the most critical and is increasingly affecting coastal areas worldwide. Interest in coastal aquifers has significantly increased, as demonstrated by the growing number of publications in which researchers describe various approaches to illuminate the importance of coastal aquifers, specifically with regard to SWI. The state of research and knowledge of the coastal SWI issue has been reviewed herein. The review includes a discussion of select geophysical and field methods and tools which can inform the numerical modeling of coastal aquifers. MODFLOW was identified as the most often used numerical modeling platform. Further, while many research sites, particularly in the United States, were identified where field studies and geophysical methods, mostly geoelectric ones, added important value to the numerical modeling of the SWI process in the coastal zone, in some regions of the world, data scarcity was identified as the main challenge. Overall, numerical modeling, combined with geophysical methods, is a valuable tool for studying SWI and managing coastal water resources.

1. Introduction

Coastal aquifers are a transition zone from freshwater to saltwater and can constitute an important source of drinking water for coastal communities worldwide. Coastal areas attract people and developments and typically have a high population density, which can increase demands on fresh groundwater resources [1]. Increasing coastal development, over-pumping, and climate change exacerbate the existent complexity of coastal aquifer management and protection and can result in profound and irreversible impacts on coastal communities’ drinking water supplies and other local uses, including septic systems and coastal lagoons [2].

Saltwater intrusion (SWI) is the encroachment of saline water into freshwater aquifers along coastal areas. SWI is driven by density gradients between seawater and freshwater. Also, hydraulic gradients between coastal groundwater levels and sea levels can further enable ocean water to intrude the freshwater aquifers, typically forming a wedge (toe) under the fresh groundwater [3,4] or cause up-coning in cases of over-pumping coastal aquifers and inundation due to hurricanes and storm surges (Figure 1). Drivers of SWI may vary in time and place. The process can be complicated, as they might combine and increase the magnitude and the extent of saltwater intruding landward. SWI drivers can also be related to climate change, such as changing precipitation patterns, warmer temperatures, and extreme weather events, as well as anthropogenic stressors including urbanization, land management, and over-pumping of groundwater [5]. Projected sea level rise (SLR) will further increase the landward pressure at the mixing interface, enabling saltwater to intrude further inland.

Fresh groundwater is mixed with saltwater through the processes of dispersion and molecular diffusion within a transition zone of dispersion. Longitudinal and transverse dispersivity play a significant role controlling the width of the mixing zone between fresh groundwater and seawater [6]. Geologic heterogeneity, hydraulic properties, stream discharge, and dynamic forces such as tidal fluctuation, wind-driven currents, groundwater recharge variation, and sea level changes control the temporal and spatial extent of the transition zone [7].

Once it has occurred, SWI under natural conditions is an irreversible process on a small time scale (centuries or millennia) [8]. Remediation options such as artificial recharge or desalination of the salinized water exist but are expensive.

The impact of climate change, SLR, and over-pumping on saltwater intrusion in coastal aquifers is of great concern worldwide; it varies spatially and temporally [9,10,11,12,13,14,15,16]. For instance, SWI was documented in many of the coastal aquifers of the United States, Mexico, and Canada [7,17,18,19,20,21,22]. Ferguson et al. [23] found that coastal watersheds with a low topographic gradient, which comprise less than 8% of the United States, will be significantly impacted by inundation due to SLR.

Figure 1. Conceptual models illustrate the impact of SLR (a) and groundwater extraction (b), including SWI and saltwater inundation. Adapted and modified after [23].

Figure 1. Conceptual models illustrate the impact of SLR (a) and groundwater extraction (b), including SWI and saltwater inundation. Adapted and modified after [23].

Coastal aquifers are susceptible to natural and anthropogenic changes (Figure 2), which affects their groundwater quality and quantity [24,25,26]. Natural drivers vary in time and scale, including SLR, storm surges, hurricanes, droughts, and vertical movement through the land, which impact fresh groundwater availability in coastal areas and can cause inundation and coastal flooding, shoreline erosion, SWI, and land degradation [5]. Anthropogenic drivers, including over-pumping, coastal development, land use, and mining, have detrimental effects on coastal aquifers and may introduce pollution to coastal ecosystems and facilitate habitat destruction [27]. Natural and anthropogenic drivers can work simultaneously and cause synergistic effects and exacerbate the problem, posing greater risks to the environment and the well-being of the inhabitants of coastal regions [28,29].

Various numerical and geophysical tools, including field and lab methods, have been developed and used to simulate and assess the impact of natural and anthropogenic factors on water resources and environmental systems in coastal areas. Numerical tools are powerful methods used to solve complex problems applying differential mathematical equations to describe a groundwater system’s physical processes and boundary conditions. The models consider the aquifer’s heterogeneity, anisotropy, and asymmetry, as well as its interaction with surface water features [30]. Numerical models require hydrogeologic information, hydrologic stresses, and long-term monitoring data, which poses a significant challenge in using numerical models since such data are often scarce and unavailable because it is costly, time consuming, and labor-intensive [31]. Different codes have been developed to simulate hydrogeological and hydrological processes, including the study of the fate and transport of contaminants [32,33,34], and to manage and optimize coastal aquifers [35,36,37,38,39,40,41,42,43]. Numerical modeling approaches are used broadly to simulate SWI based on variable-density flow using different codes, such as SUTRA [44], SEAWAT [45], FEFLOW [46], FEMWATER [47], and MOCDENS-3D [48]. In some instances, numerical models have been used in combination with geophysical approaches [49,50,51].

Different geographic locations and approaches have been addressed in past review studies; Caroti et al. [52] demonstrated in a case study in the Pisan coastline, Italy, the importance of the use of high-accuracy geomatic datasets like GNSS and LiDAR in monitoring the encroachment of salt wedges and improving model calibration and prediction. Other review studies have addressed the saltwater intrusion into coastal aquifers on a regional scale, for example Africa, India, and the Mediterranean [26,53,54]. Meanwhile, in 1993 Bobba [55] investigated mathematical model simplification and approximation approaches in his review study and suggested some improvements to and considerations of the processes and boundary conditions necessary to better simulate more complex problems like saltwater intrusion. In addition, Sreekanth and Datta [43] investigated the different approaches that have been implemented to manage coastal aquifers and saltwater intrusion through the development of simulation-optimization models that require a large computational capacity which led to the application of surrogate modeling techniques to reduce such a burden. None of the previous studies reviewed coastal aquifers from a global perspective, nor did they describe in detail the different numerical algorithms implemented to simulate the groundwater systems.

In this work, we retrieved studies that addressed costal aquifers and the natural and anthropogenic drivers threatening and impacting the groundwater, as well as selected investigative approaches which have been used at the freshwater and saltwater interface using the Scopus Database at the global scale. The main objective was to summarize a spectrum of modeling approaches that have been used in studying coastal aquifers and the associated saltwater intrusion issue. Additional information about geophysical field methods for studying SWI has also been provided. This review concludes with the identification of current research gaps and recommendations for future studies. Its findings should be useful for those who work on protecting and managing coastal aquifers.

2. Methodology

A combination of keywords (

Table 1. Keyword combinations used to retrieve published work related to coastal aquifers. Time range indicates the earliest published work found.

Keywords	Time Range	Number of Documents
“Coastal aquifer”	1953–2023	4468
“Coastal aquifer” and “model”	1953–2023	2012
“Coastal aquifer” and “groundwater” and “model”	1970–2023	1679
“Coastal aquifer” and “groundwater” and “model” and “saltwater intrusion”	1973–2023	345

) and the Scopus search engine were used to identify published work on coastal aquifers worldwide between 1953 (the earliest published work) and November 2023 (retrieval date). Since 1953, there have been 4468 publications (Table 1) that relate to the “coastal aquifer” keyword and there are currently at least 300 references per year that meet the search criteria, currently (Figure 3). Most of these publications focused on “coastal aquifer” in the United States followed by India and China (Figure 4). Subsequently, the literature search was confined further by adding “model”, “groundwater”, and “saltwater intrusion” to “coastal aquifer”. This resulted in 345 publications ranging from 1973 to 2023, where 43 countries have multiple publications (Figure 5). Of these 345, the majority (n = 285) were research articles, followed by conference papers (n = 43), and book chapters (n = 13). Only 4 review papers were identified, and the earliest goes back to 1993 [55], while the latest [26] was published in 2021. As mentioned above in the Section 1, some of the review studies focused on one region or area, while other reviews addressed management and optimization approaches, in addition to mathematical models used to simulate coastal aquifers.

Figure 6 summarizes our review process. In addition to the keyword, type of publication, study classification (numerical, geophysical, laboratory, and combined approaches), and type of data analysis (algorithm, application, and limitation), we also documented the year of publication, number of citations, study site, objectives, and major findings to only highly cited documents due to the large number. Also, only publications free of charge were reviewed.

3. Results and Discussion

Researchers who studied coastal aquifers worldwide have applied numerical, geophysical, and experimental approaches to qualify and quantify the sustainability and the impact of environmental and anthropogenic drivers on fresh groundwater. In the following sections, we will first discuss research related to the natural and anthropogenic drivers, followed by a review of numerical, geophysical, and experimental approaches, including their applications and limitations.

3.1. Natural Drivers

Climate change affects precipitation spatially and temporarily, affecting recharge rates to the groundwater aquifers. Shallow coastal aquifers have a shorter time response to changes in either wet or drought conditions. Average surface temperature is expected to rise globally (IPCC, 2001) [56]. Global SLR refers to the increase in the average height of oceans on earth and is attributed to both glacial ice melt and thermal expansion due to global warming. The rise in local sea level might differ in magnitude from global SLR due to the variation in vertical land motion from one site to another; for example, the sea level has risen nearly 20 cm between 1880 and 1980 in the Netherlands [57] compared to 16 cm of global sea level rise between 1902 and 2015 [58]. Investigations and modeling studies have predicted that the consequences of SLR will adversely impact the coastal regions through SWI and inundation [54,57,59,60].

The change in sea level resulted in a change in heads between seawater and groundwater and increased the pressure seaward on the mixing zone, forcing it to move further inland [57]. Climate change is predicted to affect future recharge rates; in a study by Döll [61], simulating global recharge change under four different climate change scenarios predicted that groundwater recharge will change significantly by the 2050s. For instance, it is predicted that groundwater recharge will decrease by 30–70% in some semi-arid climatic zones, and that around 16–19% of the global population will be impacted by a more than 10% decrease in recharge. Meanwhile, it is predicted that 26–38% of the global population will experience a >10% increase in groundwater recharge. Storm surge frequency is anticipated to increase with climate change and SLR which are expected to exacerbate the impact of SLR on SWI. Once storm surges occur, nearshore areas will flood with seawater, soil salinization will occur, and the groundwater aquifer could become saline too. A comparison between storm surges and SLR induced SWI on a shallow coastal aquifer in east-central Florida, USA, showed that storm surge impact from a category 3 hurricane is three times greater than that of SLR in the short term. And it is equivalent to the impact of a 95-year SLR of 0.3 m by the year 2100 [62].

3.2. Anthropogenic Drivers

Coastal development puts more pressure on groundwater, hastens the depletion of groundwater aquifers, and reduces the amount of groundwater available to discharge to streams and wetlands [7]. Ferguson and Gleeson [23] found that the vulnerability of coastal aquifers to SWI due to groundwater abstraction is more than it is for the anticipated SLR of 0.59 m by 2090–2099 in coastal watersheds with a greater hydraulic gradient (>0.001) and high population density. Over-pumping of freshwater is taking place to meet the increased demand from different sectors including domestic, industrial, and agriculture. Unsustainable extraction lowers water levels in the aquifers and leads saline water to encroach into freshwater aquifers, and reduced fresh groundwater storage, which might cause the abandonment of water supply wells due to salinization. Over-exploitation of groundwater and its consequences on coastal aquifers at different scales have been addressed in many coastal areas worldwide [11,41,53,54,63]. Vörösmarty et al. [60] investigated the impact of population growth and climate change on global water resources under different scenarios between 1985 and 2025 and anticipated that the impact due to population growth and development is a more determining factor than climate change. Another study showed that land use changes in watersheds impact sediment transport to the coastal zones and cause changes to the shape of the coastline, which is an important parameter in determining the position of the mixing interface in coastal aquifers [64]. SWI is of great concern to the inhabitants of coastal areas worldwide and seriously threatens the availability of fresh groundwater, human livelihood, and coastal ecosystem. These impacts, which affect around 20% of the world’s coastal aquifers, are expected to worsen in the future taking into consideration climate change and population growth [17]. In the following sections, we systematically present drivers of SWI and various approaches used to study, model, and monitor their impacts.

3.3. Saltwater Intrusion

Saltwater intrusion is a naturally occurring process in coastal freshwater aquifers due to the difference in densities of both the saltwater and freshwater, which allows ocean water to intrude into the freshwater aquifer and form a wedge (toe) under the fresh groundwater [57]. Drivers of SWI may vary in time and place. The process can be complicated, as they might combine and increase the magnitude and the extent of saltwater intruding landward. SWI drivers can also be related to climate change, such as changing precipitation patterns, warmer temperatures, and extreme weather events, as well as anthropogenic stressors including urbanization, land management, and over-pumping of groundwater [5].

SWI has been addressed in various environmental and geological settings; for example, Evans et al. [12] emphasized the importance of multi-scale flow consideration when studying SWI, since salinization of a deep confined aquifer has occurred by downward leakage from the upper aquifers instead of intrusion from the seaward end at the embayment scale rather than at the nearshore scale. Additionally, Karst aquifer systems have complex fracture networks, which impose a great challenge when studying SWI. Kresic and Panday [65] demonstrated that fractures might act as a drain that attracts water and causes the saltwater interface to intrude further inland and form a large diffusing zone. In a study about freshwater water resource anomalies within continental shelf aquifers in New England which was carried out by Person et al. [15], they concluded that the observed freshening of the coastal plain aquifers could be explained by the combined effect of meteoric recharge and subglacial recharge in addition to groundwater discharge along submarine canyons causing the freshwater lens to be ‘‘pushed’’ 90 km further offshore Long Island.

Impacts of SWI can be of tremendous magnitude, causing environmental degradations and detrimental social and economic effects within coastal communities [66]. SWI can also cause detrimental impacts on the available quantity and quality of fresh groundwater, increase corrosion to the existing infrastructure, and increase water treatment cost. Also, groundwater is vital for the sustainability of ecosystems, and there is a need for a better understanding of coastal aquifers providing fresh groundwater. The coastal lagoon’s ecosystem is a habitat for residents and migrating shorebirds as well as for fish and shellfish productivity and plays a vital role in filtering pollutants from discharging streams and providing a means of protection from flooding and erosion [67].

3.4. Approaches and Models

Commonly, three approaches are used to study SWI processes in coastal aquifers. Numerical modeling was the most common approach (around 80%) followed by geophysical methods, such as Electrical Resistivity Tomography (ERT), Vertical Electrical Sounding (VET), and Ground-Penetrating Radar (GPR). Relative to each other, geophysical methods have been applied to a lesser extent due to the complexity and intensive labor required, alongside the high cost of the instruments. Lastly, the lab-based approach is the least common due to the complexity and associated high cost. Other approaches include analytical, conceptual, and statistical. It is also common to combine and implement more than one approach.

3.5. Numerical Approaches

Table 2. Summary of selected numerical studies.

Authors	Year	Approach	Code	Location	Findings
[68]	2021	Numerical	SEAWAT	Spain	Fresh and saltwater do not exchange temperatures due to a barrier in the aquifer
[11]	2021	Numerical	SEAWAT	Morocco	Identified the impact of climate change, over-pumping, and SLR on coastal aquifers
[65]	2021	Numerical	MODFLOW-USG	-	Evaluated the use of MODFLOW-USG to simulate groundwater flow and transport in karst aquifers
[13]	1987	Numerical	FINITE ELEMENT	-	Improved 3D finite element modeling to alleviate computation burden for simulating saltwater intrusion
[69]	2017	Numerical	FEFLOW	Malaysia	Assessed the impact of high demand on groundwater on SWI in coastal aquifers
[70]	2013	Numerical and Experimental	SEAWAT	-	Investigated the mixing-zone profile in homogeneous and stratified aquifers
[71]	2007	Numerical	MODFLOW	RI, USA	Assessed groundwater average fluxes into salt ponds and coastal areas in southern Rhode Island, USA
[19]	2004	Numerical	SEAWAT	MA, USA	Investigated the impact of groundwater pumping and SLR on the lower Cape Cod aquifer system.
[16]	2010	Numerical	SEAWAT	VA, USA	Highlighted the capability of a 3D SWI model to accurately forecast future encroachment of saltwater at specific well sites.
[72]	2009	Numerical and Experimental	SUTRA	-	Successfully mapped and characterized the extent of the mixing between freshwater and saltwater in a laboratory-scale
[73]	2013	Numerical and Experimental	SEAWAT	-	Were the first to investigate contaminant transport and flow dynamics within the saltwater wedge
[74]	2019	Numerical and Experimental	MODFLOW SEAWAT	-	Investigated the effects of sea level fluctuation, groundwater pumping, and freshwater recharge on SWI
[75]	2016	Numerical	SUTRA	-	Assessed the impacts of SLR and groundwater abstraction on seawater intrusion
[76]	2015	Numerical	MODFLOW SEAWAT	-	Simulated saltwater sharp interface, which was found to be less complex and more computationally efficient.
[77]	2018	Numerical	MODFLOW SEAWAT	Iran	Carried out an integrated assessment of the long-term effect of climate change, SLR, and urbanization on coastal groundwater systems
[78]	2016	Numerical	SEAWAT	FL, USA	Assessed the impact of climate under various scenarios of SLR on groundwater in shallow coastal aquifers

summarizes selected studies that used numerical codes for the simulation of SWI in coastal aquifers. These numerical codes use either the finite difference method (FDM) or finite element method (FEM) to solve partial differential equations. Commonly used numerical modeling approaches for simulating coastal zone aquifers are (in no particular order) MODFLOW, SUTRA, SEAWAT, FEFLOW, FEMWATER, and MOCDENS-3D.

3.5.1. FEFLOW

FEFLOW (Finite Element subsurface Flow system) is a modeling program developed in 1987 and primarily used for simulating groundwater flow and mass and heat transfer in porous media, as well as calculating groundwater age [46]. It uses finite element approximation to solve for variable-saturation systems and variable density-dependent mass and heat transport, which provides more capabilities to simulate complex systems. However, it is difficult to master and is not in the public domain. Overall, FEFLOW is a powerful and versatile tool for modeling groundwater and transport processes in complex systems. It can handle variable-density and variable-saturation conditions, and it is suited for simulating SWI in coastal aquifers.

The 3D finite element modeling approach was improved by [13] to alleviate the computation burden for simulating SWI using coupled flow and transport equations under various aquifer settings and boundary conditions. However, they acknowledged that the code requires further improvement to simulate more complex three-dimensional problems. FEFLOW was used for various environmental problems, especially in coastal areas where SWI is a major concern. For example, the high demand for groundwater and increase in pumping under various scenarios in Kg. Salang, Tioman Island, Pahang, Malaysia, were found to exacerbate the problem of SWI and induce further landward encroachment of saltwater [69]. Similarly, Gad and Khalaf [79] found that groundwater levels will keep declining and saltwater will further encroach landward of the coastal aquifer in North Sinai, Egypt, under different pumping scenarios to satisfy agricultural needs, taking into consideration an increase in SLR because of climate change. Meanwhile, in another study conducted in southern Portugal [80], the long- and short-term impact of climate change, groundwater pumping, and adaptation measures on SWI was investigated under different scenarios. The authors found that by the year 2099, the freshwater–saltwater interface will encroach up to (3.5 km) inland. Meanwhile, the short-term impact showed some decrease of around (0.5 km). They proposed the use of the managed aquifer recharge adaptation measure to alleviate the impact of climate change and reduce recharge. However, the study did not consider the interaction between surface water and groundwater. Fu-lin Li et al. [81] studied the effectiveness of subsurface physical barriers in halting the freshwater–saltwater interface from migrating into the aquifer using a sandbox model and numerical simulation. They found that a barrier with a permeability of (K = 3.7 × 10⁻⁸ m/s) or lower is effective in preventing the encroaching of SWI. Their work provides important information about the mechanism of subsurface barriers. Yet, it needs to be applied to a real case and large-scale problems.

3.5.2. SUTRA

Saturated-Unsaturated Transport (SUTRA) is a public-domain computer program designed for groundwater modeling by the USGS [44,82]. SUTRA is also designed to simulate density-dependent flow and transport with either heat or a dissolved species for variable-saturation systems, which makes it suitable for 2D regional-scale SWI with dispersed or sharp transitions. SUTRA-MS is a modified version of SUTRA that simulates heat and multiple-species transport in a groundwater system [83]. It uses both finite element and finite difference approximation for solutions either in 2D or 3D simulations.

The SUTRA model has been mostly used to investigate the impact of projected SLR rates on coastal aquifers and submarine groundwater discharge (SGD). Evans et al. [12] conducted a field and modeling study in South Carolina, USA, to investigate the impact of SLR on groundwater flow and salinity and found that SGD from the salt marsh is three times as much as that from the beach and inner shelf, and the total fluxes of SGD decreased significantly with future predicted SLR increase. On the other hand, Hussain and Javadi [75] used SUTRA to assess the hypothetical impacts of rising sea levels as a climate change indicator and groundwater abstraction on seawater intrusion. They found that SLR will have a substantial impact on the future of coastal fresh groundwater and combined with over-pumping they will augment SWI into the aquifer. Additionally, SUTRA was used to investigate the effect of tidal force and amplitude on salinity distribution fluctuation in the upper saline plume (USP) and lower salt wedge (SW) in an unconfined shallow coastal aquifer [84] and demonstrated how strong the relationship is between tides and the amount of mixing of freshwater and saltwater in a sloping beach aquifer. Meanwhile, ref. [72] successfully mapped and characterized the extent of the mixing between freshwater and saltwater in a laboratory-scale SWI experiment using SUTRA and they alleged that this technique would provide a better understanding of the saltwater mixing patterns in coastal aquifers; however, this method is best suited for a situation where advection is the dominating factor in saltwater mixing and neglects the effect of molecular diffusion. In conclusion, SUTRA is a powerful tool for simulating groundwater flow and transport in variable-density and variable-saturation conditions. It can be used to tackle a wide range of groundwater problems including SWI and SGD in coastal aquifers. However, SUTRA comes with some challenges and limitations because it is considered less user friendly, more complex to create a three-dimensional model, and not suitable for complex geometry problems.

3.5.3. MODFLOW

MODFLOW is a public-domain groundwater model originally developed and regularly updated by the United States Geological Survey (USGS) [33]. It is currently the most widely used groundwater simulation program around the world due to proficient documentation, worked examples, and ease of use. Its modular structure allows for specific processes and packages to be added as needed. MODFLOW uses finite difference approximation and allows users to simulate various processes and stresses, such as rivers, recharge, pumping, and, most important in the context of saltwater intrusion, variable-density flow. Previous MODFLOW 2000 and 2005 versions can be coupled with the Saltwater Intrusion Package (SWI2), developed by the USGS, to simulate SWI using finite difference approximation [85,86]. Unlike other common modeling software, SWI2 simulates the saltwater–freshwater interface for regional coastal aquifers where diffusion and dispersion do not play a significant role. Despite the benefit of this package to the user in reducing model run-time, making it more computationally efficient for creating models, it does have some limitations: SWI2 only accounts for the saturated zone. In addition, it does not account for diffusion and dispersion, resulting in a sharp representation of the saltwater–freshwater interface. Likewise, resistance to vertical flow is neglected, and it cannot accurately simulate the inversion of seawater overlying freshwater that can sometimes occur at inland salt ponds. The current version of MODFLOW 6 was released in 2016 [87,88], and uses a new integrated package called the Buoyancy package instead of SWI2. This package comprehensively simulates variable-density flow in saturated zones where physical processes like diffusion and dispersion are significant. Researchers have been using MODFLOW to investigate a variety of groundwater problems. For instance, Masterson et al. [71] assessed groundwater long-term average fluxes into salt ponds and coastal areas in southern Rhode Island.

The saltwater sharp-interface approach based on the Ghyben–Herzberg relation is less complex and computationally more efficient. Nevertheless, caution is warranted when implementing it under future management and climate change scenarios [76], as it would be affected by variation in aquifer properties and different management and climate scenarios. Additionally, SWI2 efficiently reduced the computation burden and saved running time when coupled with a stochastic inverse model to simulate the freshwater–seawater interface in a heterogeneous multi-aquifer system and a three-dimensional variable-density groundwater flow. This method partially overcomes the limitation associated with sharp-interface approximation [89]. Overall, MODFLOW is a powerful and widely used groundwater model that can simulate various hydrological processes and stresses including density-driven flow when coupled with the appropriate packages.

3.5.4. SEAWAT

SEAWAT is a public-domain computer program designed for the simulation of variable-density groundwater flow, multi-species solute, and heat transport in saturated zones using a finite difference approximation developed by the USGS [90]. The fundamental application of SEAWAT is for simulating SWI in coastal aquifers, brine migration, thermal plume transport, and groundwater age [91]. Unlike SWI2, it accounts for dispersion and diffusion, an aspect of the mixing zone between freshwater and saltwater [92].

SEAWAT has been widely used to simulate the impact of climate change, over-pumping, and sea level rise (SLR) on coastal aquifers and the development of SWI [9,10,11,16,77,78,93,94,95,96]. For example, El Hamidi et al. [11] used SEAWAT to simulate the impact of climate change, over-pumping, and SLR on coastal areas, and showed that saltwater will encroach into the groundwater aquifer more than 5 km inland with a salinity up to 25 g/L. In another study, the impact of storm surges from hurricanes on SWI was found to be more significant in the short term than SLR [62]. SEAWAT was also used to assess and evaluate remediation and management strategies to alleviate SWI problems [9,95]. Other studies have used SEAWAT for other purposes; Blanco-Coronas et al. [68] investigated the impact of geothermal activities in coastal aquifers and the mixing of fresh and saltwater. Badaruddin et al. [6] experimentally investigated the effect of longitudinal and transverse dispersivity on the width of the mixing zone. Kalakan and Motz [97] assessed the impact of vertical anisotropy on SWI and seawater recirculation in coastal aquifers and found that SWI is significantly greater than recirculation when coupling variable-density flow with flow and transport rather than using constant-density flow. SEAWAT is a widely applicable and reliable tool for studying the dynamics and interactions of freshwater and saltwater in coastal aquifers, which can simulate complex scenarios and evaluate remediation and management options to mitigate SWI problems. However, it only can be implemented in the saturated zone.

3.5.5. Limitations and Assumptions of Numerical Modeling Approaches

Complex natural systems, especially coastal groundwater systems, are difficult to accurately simulate and predict [98]. Modeling tools are used to conceptualize the dominant hydrological and hydrogeological processes and simplify the complex interactions taking place in such systems [99]. Numerical models, in particular, are powerful tools used to solve complex problems, but at the same time, they require detailed hydrogeologic information, hydrologic stresses, and long-term monitoring data, which poses a significant challenge in using numerical models since such data are often scarce due to financial, temporal, and labor burdens [31].

Models adopt a set of assumptions to simulate and solve a specific problem, which in turn imposes some limitations on the solution and its applications. Among the common assumptions and limitations is the requirement for the spatial and temporal discretization of the model domain in terms of grid size and stress period, which depends mainly on the scale and the complexity of the problem, data availability, and computational resources [30]. Additionally, a large set of reliable and accurate input data, such as hydraulic properties of the layers, boundary conditions of the system, sinks and sources, initial conditions, water levels, and geology and subsurface stratigraphy, is needed. Generally, such data are often not completely available or accessible, scarce, and often associated with large uncertainty affecting the reliability of the models. In addition, it is more challenging with transient SWI models [100].

Furthermore, models fail to capture the exact heterogeneity and anisotropy inherent in the aquifers, which vary based on material composition and vary from one location to another, controlling groundwater and solutes’ flow path and travel time. In addition, they may not be able to comprehend and accurately represent the interaction between the different hydrological processes, varying layers, and interaction between surface water and groundwater [31]. Also, models are built on governing equations to solve for groundwater and solute transport, which in some situations may not comprise all the relevant physical, chemical, and biological processes that take place in the groundwater system, or it is too complex to incorporate all these processes [101,102]. Another aspect limiting the use of numerical models is their availability to the public and how well documented they are. Some of the modeling software are associated with license purchase and maintenance expenses; usually, they have enhanced graphical representation and more user-friendly data input. On the other hand, open-source models are available for public use, with options for developers to add more functionalities to them; usually, they require more training and data manipulation, but they are well documented with real problems.

3.6. Geophysical Approach

Despite the increase in accessibility and computational power that has come with modeling software in recent years, another practice in obtaining coastal aquifer characterization consists of geophysical methods. Geophysical techniques have been advancing since the 1980s due to increasing convenience, transportability, and lower cost [103]. Ground-based geophysical field methods offer validation in data collection and calibration success in modeling; however, the optimal combination of geophysical techniques will vary depending on location and research purpose.

Table 3. Summary of geophysical studies.

Contributor	Year	Approach	Location	Findings
[104]	2013	Review	-	Review of saltwater intrusion literature as an issue and current methods to research. Offers direction for future research.
[105]	2013	Geophysical	University of Technology Camp/Iraq—Baghdad	Compared Wenner, dipole–dipole, and Wenner–Schlumberger arrays to characterize subsurface structures in Baghdad. Found Wenner–Schlumberger to have greatest success.
[106]	2016	Geophysical	Thermaikos Gulf, Greece	ERT was helpful in filling gaps in geochemical analysis of a coastal aquifer, finding an affected area with high Cl− concentrations and offering valuable insights for studying similar rarities in other regions.
[107]	2019	Geophysical	Lower Bari Doab, Punjab, Pakistan	ERT + VES may reduce the need for boreholes to obtain data on the interface between the fresh and saltwater.
[108]	2004	Geophysical	Baeksu-eup, Youngkwang-gun, Korea	Coupling electrical sounding, drilling, and well logging proved valuable for quantifying seawater intrusion and enhancing overall use of geophysical data.
[109]	2013	Geophysical	Borkum Island, Germany	Use of GPR to collect high-resolution information about the groundwater table, sediment stratigraphy, and hydraulic properties complemented existing data from monitoring wells used in simulations.
[110]	2022	Geophysical	Neyveli Basin, Tamil Nadu, India	Used VES to identify potential areas of groundwater based on transverse resistance, longitudinal conductance, and longitudinal resistivity.
[111]	2018	Geophysical	Nagapattinam and Karaikal, South India	VES employed to isolate layers of saline intrusion in coastal aquifers.
[112]	2006	Geophysical	Bells Creek, Brisbane Australia	Use of GPR to construct the architecture of an aquifer.
[113]	2002	Geophysical	Frederick, Maryland	Compared and contrasted three different electrode arrays to characterize karst terrane. Dipole–dipole method worked best to keep costs low and accuracy high.

lists select methods used in geophysical studies of coastal zone processes.

3.6.1. Wells and Point Source Data

For over a century, point source measurements from observation wells, water sampling, and borehole drilling have been reasonably accessible ways to validate and correct characterization of groundwater and aquifers. Hwang et al. [108] demonstrated this method by utilizing 12 drilled wells in Baeksu-eup, Youngkwang-gun, Korea, to collect hydrogeological, geochemical, and geophysical data to map the spatial distribution of seawater intrusion in a coastal aquifer. They then established a relationship between groundwater resistivity and the corresponding NaCl concentration. The methodology proved valuable for quantifying SWI and strengthening the use of geophysical data in aquifer characterization. Nevertheless, borehole and well monitoring are challenging [103]. Given the complexity and dynamics of coastal aquifers, monitoring and managing their vulnerability and production potential demands more than just well data.

3.6.2. Electrical Resistivity Tomography

Electrical Resistivity Tomography (ERT) is a minimally invasive geophysical tool that has gained popularity in recent years. ERT operates by sending an electrical current into the ground via a set of electrodes while measuring the potential difference using another set of electrodes. These values allow for the calculation of the subsurface’s material resistance. Material resistance can be calculated using Ohm’s law (V = IR), as seen in Figure 7. Material resistance is inversely proportional to conductivity and can be converted to a measure of salinity [49]. Not only is ERT utilized in SWI studies, but it can also provide insight into the distribution of different lithological materials in the subsurface, map potential flow paths, and assist in determining the heterogeneity of a coastal aquifer.

Several studies utilized the ERT method to study the freshwater–saltwater interface. Kazakis et al. [106] conducted fifteen ERT surveys perpendicular to the coastline of eastern Thermaikos Gulf, Greece, and Hasan et al. [107] conducted eleven surveys in the semi-arid region of Lower Bari Doab, Punjab, Pakistan; both showed success in using ERT to monitor SWI, and in delineating the fresh–saltwater interface in coastal aquifers. These studies were paired with another geophysical method: drilling boreholes to determine lithological constraints in the subsurface. ERT surveys are typically combined with a form of ground truthing such as boreholes or point source well monitoring to correct potential misinterpretation of resistivity values, such as clay mimicking a saltwater layer or bedrock mimicking freshwater [103].

Goebel et al. [115] deployed an ERT survey over 40 km of the Monterey Bay coast in central California. This region has previously experienced and is currently threatened by SWI into coastal aquifers. The study area lacked well data, making the ERT surveys a necessity to track the inland extent of SWI. This research contained one of the longest continuous observation transects of its time and advanced the plausibility of ERT as a primary monitoring tool. Satriani et al. [116] conducted the ERT survey in the Basilicata region of southern Italy to map SWI. A similar study conducted by Greggio et al. [117], in Ravenna, Italy, narrowed in on tracking the dynamics of the freshwater lens and carried out a vulnerability analysis due to SLR.

Selecting the appropriate electrode array is dependent on environmental factors while using the ERT. Zhou et al. [113] applied dipole–dipole and Wenner–Schlumberger arrays to a karst setting in Maryland, with the dipole–dipole array being the most effective while also keeping costs low. In Baghdad, Alwan [105] also utilized all three methods, finding Wenner–Schlumberger to be best suited in the silty clay soil of the study area.

3.6.3. Vertical Electrical Sounding

Vertical Electrical Sounding (VES) is a similar technique to ERT in that it is used to investigate subsurface structures and characteristics and functions on the same principles (Figure 8). While ERT is imaging based, VES is a 1D point-based method for depth profiling used to estimate D-Z parameters including transverse resistance, longitudinal conductance, and longitudinal resistivity to investigate the fresh–saline interface [107].

Hasan et al. [107] proved there to be a strong correlation between ERT and VES results. Gopinath et al. [111] conducted 32 VES measurements in Nagapattinam and Karaikal, South India, and found five different layers of subsurface materials with varying resistivity values. Higher resistivity values indicated aquifers free from intrusion, and lower resistivity values indicated contamination via SWI. Vijayaprabhu et al. [110] demonstrated VES’s ability to map potential groundwater-bearing zones derived from varying subsurface electrical properties. Based on the D-Z parameters, fresh, moderate fresh, and saline aquifers were discerned. These types of ground-based details can contribute to making local- and regional-specific models more precise.

3.6.4. Ground Penetrating Radar

Ground-Penetrating Radar (GPR) is a non-invasive geophysical method used to examine and map the extent of the freshwater–saltwater interface. Historically, GPR was primarily employed in the construction industry to map subsurface objects or lithology but has become a rapidly growing tool in hydrogeology. GPR works by sending a high-frequency electromagnetic wave into the ground and measuring the reflected wave (Figure 9). Reflections occur in the subsurface due to the varying electrical properties of the sediments [118]. GPR can collect high-resolution information about the groundwater table, sediment stratigraphy, and hydraulic properties of coastal aquifers, which can complement data from monitoring wells, especially when paired with existing boreholes [109].

Since GPR offers a lower cost method of mapping the lithology of the subsurface compared to drilling new wells, it can also be used to improve on and calibrate numerical models. Ezzy et al. [112] used GPR to construct the composition of an aquifer within an entire coastal plain in preparation for improving conceptual groundwater models. Due to the portability of the equipment, over 50 km (around 31 mi) of profiles were able to be collected, providing a vast research area, which is a rarity compared to other geophysical methods.

3.6.5. Limitations and Assumptions of Geophysical Approaches

Limitations of geophysical methods as a collective include labor constraints, time, specialized equipment, and training. Point source well monitoring is only as effective as the number of observation wells in the scope of the research area, which severely limits their capacity to offer supportive evidence on their own. The limitation of ERT revolves around its inability to differentiate between lithological layers and water characteristics, as it captures only resistivity values. A particularly clay-like environment may yield low resistivity values that could be mistaken for saltwater regions without further geologic analysis [104]. Due to this, it is often coupled with seismometer or well log data to determine subsurface makeup. ERT is also an approximation between apparent resistivity values and true resistivity values, which means it is not an exact representation of environmental conditions.

With visual interpretation of GPR and VES, data quality can correspond with operator skill and may yield varying results [111]. Nearly all definitive studies mix a combination of geophysical methods with ground truthing or modeling methods, proving that there is no solo technique that will give straight forward data regarding coastal aquifer characterization. Depending on the goal of each study, a different combination of tools may be best. The resolution of resulting images may also become a nuisance in study sites with significant noise.

Resistivity methods often do not capture characteristics of groundwater, such as flow or recharge, which are integral factors in understanding a coastal aquifer system [119]. Repeated measurements can assist in closing these knowledge gaps but require more time and money. In addition to the quality of resulting data and reliability, geophysical tools may come with a host of operator setbacks of which time and money come at the forefront. Often, specialized training is required for researchers to utilize these techniques, which can become a financial burden for firms and institutions [103]. The time and resources, including the operation and maintenance cost required to operate some of these surveys, include labor expenses and equipment transportation costs. Despite the drawbacks, geophysical methods remain a popular, successful approach to characterizing the coastal subsurface as it pertains to aquifers.

3.7. Combined Approach (Numerical and Geophysical)

Given the laborious characteristics and location-derived constraints of geophysical methods, some researchers have opted to combine them with numerical methods in hopes of a more validating and often simpler product. These two techniques, in conjunction, offer validation of field data and the opportunity to simulate extended or predicted results of a designated research area.

Table 4. Summary of combined geophysical and numerical studies.

Contributor	Year	Approach	Code	Location	Findings
[49]	2017	Numerical, Geophysical	SEAWAT	Dutch Coast, the Sand Engine area, Netherlands	Time-lapse ERT proved successful in monitoring rapid changes in coastal environments, while coupling with a model showed accuracy in predicting storm surge and tidal outcomes.
[50]	2007	Numerical, Geophysical	Princeton Transport Code (PTC)	Heraklion in Crete, Greece	Successfully simulated groundwater flow using a 3D finite element–finite difference model established from the Princeton Transport Code (PTC) and produced the hydraulic head distribution over the entire interest area, compared to ERT surveys.
[51]	2019	Numerical, Geophysical, Geochemical	SEAWAT	Coast of Wadden Sea, Danish–German border	AEM and SEAWAT simulated saltwater intrusion were found to be very similar in results, proving the success of using each to validate.

lists some of these studies.

Koukadaki et al. [50] developed a method using a numerical simulation based on hydraulic conductivity that was derived from an Electrical Resistivity Tomography survey. The study area lacked borehole data or observation wells, commonly used to validate geophysical methods, so numerical modeling was applied to establish reliability. They successfully simulated groundwater flow using a three-dimensional finite element–finite difference model established from the Princeton Transport Code (PTC) and produced hydraulic head distribution over the entire area of interest. The maximum extent of SWI into the aquifer could also be simulated based on the geologic characterization from the ERT surveys, proving an accomplishment in joining geophysical and numerical methodology, as well as predicting the characterization of salinity without requiring verified geophysical data points.

The complexity and laborious nature required to run ERT surveys commonly limits their frequency to no more than a few per day. Due to the incremental changes in coastal aquifers, as they pertain to tidal cycling and precipitation variation, it is not often that ERT is used to demonstrate day-to-day changes. In a study by Huizer et al. [49], ERT was utilized to monitor changes over time using time-lapse surveys spanning two consecutive months. They demonstrated that time-lapse ERT can provide extensive temporal and spatial changes of SWI and the impact of short-term processes. Their research compared the geophysical and numerical modeling software SEAWAT version 4. As discussed earlier, SEAWAT’s strength is simulating variable-density groundwater flow and solute transport [11]. The simulated fresh–salt groundwater distribution was closely comparable with the observed patterns in the time-lapse ERT images, proving that the simulation was reasonably accurate in predicting groundwater flux. Again, borehole drilling was combined with another method to build the groundwater flow model.

At the Danish–German border near the Wadden Sea, geophysical, geochemical, and numerical density-dependent groundwater flow and transport models were utilized to understand the history of a saltwater-affected groundwater system and its potential response to past and future changes [51]. Airborne Electromagnetic (AEM) data were obtained from a geophysical database and geochemical data were obtained from another national database describing major ion samples. The numerical model created using SEAWAT was then validated with the AEM data for similarity. This research, along with Koukadaki et al. [50], proves the successes of simulations validated by geophysical data.

3.8. Experimental and Lab-Based Approach

The lab-based experimental approach simulates a real-world problem in a controlled environment. In the field of groundwater flow modeling, researchers have used this approach to test new methods and investigate the impact of one or multiple factors on fresh groundwater in coastal aquifers under a variety of hydrogeological settings [70,120,121,122]. Lab experiments are often accompanied by numerical model development for assessing the ability of the model to simulate the experimental conditions [70,72,73,123]. For instance, Guo et al. [74,120,121] successfully simulated different natural and anthropogenic factors like SLR, groundwater exploitation, tidal fluctuations, and fresh groundwater recharge in the lab to investigate and help understand their impact on SWI. In another lab experiment, Guo et al. [123] studied and examined contaminant transport and dynamics associated with SWI. Meanwhile, Chang and Clement [73] were the first to investigate contaminants transported within the saltwater wedge. Zhang et al. [122] experimentally investigated contaminant plume transport and behavior based on their densities in coastal aquifers with an existing saltwater interface. Additionally, Abarca and Clement [72] experimentally demonstrated the effectiveness of their method in characterizing the extent of the mixing zone of a saltwater wedge in a steady-state system.

4. Conclusions

This work has critically and systematically reviewed published literature which has addressed different approaches used to study and assess coastal aquifers in the past. The most common approaches were found to be the numerical approach, the geophysical approach, the lab-based experimental approach, and a combination of the former. Most of the coastal aquifer studies have addressed the SWI problem using variable-density groundwater flow models like SEAWAT, SUTRA, and FEFLOW. Meanwhile, the most common geophysical methods used were ERT, VES, and GPR. The limitations of these approaches vary based on the study’s purpose, site location and scale, accessibility, and data and resource availability. Approaches or combinations thereof are best tailored to site conditions, data availability, available expertise, equipment, and funding support.

The study of the coastal aquifers has been growing since the early 1980s, particularly in the continental United States [103]. But still there are insufficient data available in many parts of the world where few or no studies have been carried out. Also, we recognize that there are other field methods and technical approaches to studying SWI which are not covered herein. For instance, refs. [124,125] used hydrochemical and hydrogeochemical facies analysis to study the evolution of groundwater processes in saltwater intrusion zones. Consequently, further studies are needed to fill in local spatial gaps and to test combinations of other tool sets. Furthermore, increased data sharing and accessibility efforts would further improve current constraints on using numerical models.

Modeling of SWI is an important task which will become even more important because of climate change, sea level rise, and more extreme storm events in coastal areas, including managing the continuous increase in coastal population. Modeling, combined with other methods and tools, is therefore a valuable approach for managing coastal water resources, aiding in coastal infrastructure design, and ensuring the resiliency of coastal zones.