Salinity Barriers to Manage Saltwater Intrusion in Coastal Zone Aquifers During Global Climate Change: A Review and New Perspective

Abstract

Climate change will have a significant impact on saltwater intrusion in coastal aquifers between now and 2150. Global sea levels are predicted to rise somewhere between 0.5 and 1.8 m. To mitigate sea level rise, coastal aquifers will require intensive management to avoid inland migration of seawater that could impact water supplies. In addition to reducing pumping of freshwater, the construction and operation of salinity barriers will be required in many locations. Eleven types of salinity barriers were investigated, including physical barriers (curtain wall and grout curtains), infiltration canals filled with freshwater paralleling the coastline, injection of freshwater (treated surface water or wastewater), pumping or abstraction barriers, mixed injection and abstraction barriers, combined abstraction, desalination, and recharge (ADR), ADR hybrid barriers using various water sources including desalinated water and treated wastewater, compressed air barriers, aquifer storage and recovery dual use systems, biofilm barriers, and clay swelling or dispersion barriers. Feasibility of the use of each salinity barrier type was evaluated within the context of the most recent projections of sea level changes. Key factors used in the evaluation included local hydrogeology, land surface slope, water use, the rate of sea level rise, technical feasibility (operational track record), and economics.

1. Introduction

A large portion of the Earth’s population lives and works in the coastal zone, with nearly 2 billion people within 50 km and nearly 1 billion people within 10 km of the shoreline [1]. Aquifers containing freshwater in the coastal zone are among the most productive on Earth, but these have been subject to contamination with saltwater for the past 125 years [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. Saline water intrusion has other adverse impacts. For example, the lessening of freshwater discharge into the sea within the nearshore zone can impact the marine biology of this critical area in various ways [50].

Historically, the most common cause of saline-water intrusion in the coastal zone is the over-pumping of freshwater aquifers [51]. Aquifer hydrogeology, the construction of canals, the pattern of pumping, and the land surface elevation variation across the coastal zone may all have considerable impacts on the rate of saltwater intrusion [52,53,54,55]. Extreme hurricane surges and tsunamis can also exacerbate inland movement of saltwater in low-lying coastal areas [56,57,58,59]. Past and future climate change also impacts the position of the freshwater/saline water interface by changing the duration of droughts and tidal flooding caused by both the number and intensity of tropical storms that impact the coast [60,61,62,63,64]. All aspects of the water budget that affect coastal aquifer inflows and outflows impact interface dynamics [41,65].

Rising sea levels caused by global climate change will impact coastal regions globally, in addition to more direct existing anthropogenic impacts [66,67,68,69,70,71,72,73,74]. Numerous modeling investigations have been conducted in various settings globally to assess the magnitude of impacts to the freshwater/seawater interface [37,65,66,68,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101]. Studies of the projected landward movement of the saltwater interface have used different assumptions (e.g., sharp verses dispersed interface) and various analytical methods [71]. However, researchers have consistently reached the conclusion that the landward migration of the interface is a significant problem. Aquifer hydrogeology and heterogeneity also influence the distance that sea level rise extends inland but are rather difficult to assess in many complex geologic settings [92,102,103,104,105]. A key issue is the projected rates of sea level rise and the remedies that may be used to mitigate some of the impacts, particularly to inland groundwater supplies over the next century.

Additional impacts of sea level rise will be permanent inundation of low-lying areas and increased frequency and severity of temporary inundation from storms and extreme (king) tides, which will occur atop a higher sea level baseline [60,63,91,106,107]. It is very difficult to assess how the episodic tidal or hurricane surge flooding of coastal areas with seawater impacts the position of the interface [88,106,108,109]. In a modeling study of east–central Florida, Xiao et al. [110] concluded that the initial surge caused infiltration of seawater that impacted the unconfined aquifer. The downgradient movement of freshwater and infiltrated rainfall would take about 8 years to dilute and flush the seawater from the aquifer. Therefore, a single hurricane surge event may produce a temporary but not permanent effect.

As the interface migrates landward, existing production wells and wellfields may become subject to water quality degradation caused by vertical upconing of saline water [74,111,112,113,114].

This paper is a review and evaluation of the potential use of salinity barriers to curtail or slow the rate of landward migration of the freshwater/seawater interface. Seawater barriers have been used successfully to manage the interface in coastal aquifers in some locations since the 1940s [115]. The type of barrier that can be successfully designed and constructed is greatly dependent on local hydrogeology and both the anthropogenic impacts currently affecting the coastal aquifer and future projected conditions. However, it must be clearly understood that if the current sea level rise projections caused by climate change are greater than current estimates, many salinity barriers could fail, or modifications would be required to adapt to a new set of conditions.

2. Climate Change and Saltwater Intrusion

2.1. Fundamental Aspects of Saltwater Intrusion

Saltwater intrusion into coastal zone aquifers is controlled by the water budget of an aquifer or aquifer system and also by the local hydrogeology, including aquifer heterogeneity, and anthropogenic impacts [43,105]. Groundwater modeling is commonly used to simulate various water budget scenarios to assist in controlling saltwater intrusion [105]. The impacts of sea level changes may be different between unconfined aquifers and confined aquifers. In deep, confined aquifers, the position of the saltwater/freshwater interfaces is controlled by inland recharge away from the coastal zone, hydraulic characteristics, and the pumping of wells between the recharge area and coastline [35]. Remedial measures that can be implemented (e.g., salinity barriers) will have differing designs and effectiveness depending on local hydrogeologic conditions.

In unconfined aquifers, the flow pattern is controlled by the aquifer geology (e.g., siliciclastic verses carbonates) [116,117], land surface altitudes in the coastal zones [92], aquifer hydraulic properties (hydraulic conductivity, effective porosity, and dispersion) [104,118], hydraulic gradients that may or may not be anthropogenically affected [32,119,120], and density [107,121,122]. A conceptual diagram of the groundwater flow pattern at the saltwater/freshwater interface in a coastal aquifer is shown in Figure 1. The flow within coastal aquifers is somewhat cyclic, as has been described by Kohout [8]. The inland movement of seawater in unconfined aquifers can be exacerbated by the construction of drainage canals and other structures that can provide man-made conduits that allow unimpeded tidal water to move inland (Figure 2).

In confined or semi-confined aquifer systems, the freshwater/saltwater interface positions in each aquifer lie at positions controlled by unique sets of hydraulic characteristics and gradients within the individual aquifers [123,124,125,126] (Figure 3). In addition, thermal circulation can impact saline water movement, especially in thick carbonate aquifers containing sufficiently high vertical hydraulic conductivity [127].

2.2. Projected Sea Level Changes

Projections of future global sea levels based on tide gauges, satellite observations, and various climate models have evolved over time [128,129,130]. A compilation of projected sea level changes based on multiple future greenhouse gas and development scenarios was prepared by Fox-Kemper et al. [130] for the period of 2021 to 2150 (Figure 4). Note that the likely range of midpoint values lies between 0.7 and 0.8 m for the year 2100 and near 1.5 m in 2150 based on this compilation.

Some key observations that support these projections were made on real data collected from in 1901 to present. Fox-Kemper et al. [130] found that global mean sea level rose faster in the 20th century compared to any century in the last 3000 years, and the rate of rise increased in incremental time periods. The total rise was 0.2 m. They project that sea level will continue to rise through 2100 and perhaps beyond, depending on the continued contributions of greenhouse gas emissions. Contributions to sea level rise include thermal expansion of the oceans, melting of the Greenland and Antarctic ice sheets and glaciers, land-water storage, ocean dynamics, gravitational, rotational, and deformation effects, and the glacial isostatic adjustment and other drivers of vertical land motion (Fox-Kemper et al. [130]). While most scientists do not espouse extreme scenarios of sea level rise, it is important to place the current projections into perspective. The complete meltdown of the Greenland ice sheet represents approximately 7.4 m of global sea level rise [131]. The last time the Earth had a low amount of grounded ice was in the Eocene (53–49 Ma), when sea level was 70 to 76 m higher than present [132].

Based on the current most reliable projections of sea level changes, accompanied by other climate impacts, the use of saltwater intrusion barriers will have considerable applications in the next two centuries. If more extreme changes in sea level occur because of factors not fully incorporated into climate change models (e.g., unaccounted for feedback loops), then saltwater intrusion barriers would become irrelevant, as populations would have to move inland away from the uninhabitable coastal zone.

3. Salinity Barrier Types and Applications

A key issue in evaluating various potential solutions to saltwater intrusion is to first gain a thorough understanding of the subsurface conditions unique to the location via a full hydrogeologic characterization [133]. A variety of different types of salinity barriers are available that can be designed and constructed to partially or fully control the inland encroachment of saline water [134,135]. The selection of the type of barrier to design, construct, and operate is based on the local hydrogeologic conditions and the evaluation process involving simulations that consider present and future aquifer pumping [136,137,138,139,140,141,142].

3.1. Physical Vertical Salinity Barriers

Physical barriers, such as cut-off walls and grout curtains, can be used to limit the landward migration of the saltwater wedge [143,144,145,146,147,148,149,150,151,152,153,154,155,156]. This method has been applied primarily to unconfined aquifers with the barriers spanning the full aquifer thickness (Figure 5).

The methodology applied to construction of physical barriers varies depending on the geology and aquifer thickness [157,158]. Where an aquifer is less than 10 m in thickness, a trench can be dug and filled with a combination of bentonite and cement if the trench can be maintained without collapsing. Slurry walls can also be installed by drilling closely spaced wells and pumping emulsified bentonite into the boreholes under pressure to create a surface barrier at least 61 cm wide. Structural integrity can be added to the design by using cement mixed with bentonite [159].

Injected cement grout curtains have been used to reduce groundwater flow at the edges of dams, in deep mines, and surrounding tunnels [160,161,162]. Grouting to reduce groundwater flow has been used in mining to depths over 1000 m below surface, and so this method could also be used in deep confined aquifers to prevent saltwater intrusion. The alteration of aquifer hydraulic conductivity using cement grouting or any other method can inhibit saltwater intrusion in any type of coastal aquifer [163].

Another application of curtain walls, slurry walls, or grout curtains is to create some freshwater storage in areas where a river or stream occurs adjacent to groundwater containing seawater or brackish water [164] (Figure 6). These conditions commonly occur in delta areas and other near-coast environments.

While physical salinity barriers have some applications, their use along the shoreline has limitations based on the local tidal range, the frequency of surge flooding from tropical storms, and the local rate of sea level rise. In the event of tidal flooding that overtops a physical salinity barrier, a system could be equipped with a pump-out system located adjacent to the up-gradient side of the barrier (Figure 7)

A barrier wall can also be combined with freshwater injection to keep any bypassed saline water from intruding further inland [165]. There are various combinations of physical barriers that combine slurry walls with deeper grout curtains to adjust to local hydrogeologic conditions.

3.2. Salinity Barriers Using Surface Water

Saline-water intrusion can be controlled by increasing recharge landward of the saline-water interface using canals, basins, surface spreading, and other means. One strategy to help prevent saltwater from migrating inland is to design and construct a freshwater canal that parallels the coast [150,166,167] (Figure 8). This strategy is greatly dependent on the hydrogeologic characteristics of the unconfined aquifer that requires protection. The increase in hydraulic head between the freshwater/saltwater interface and the landward area to be protected could inhibit the inland movement of the interface to some degree. However, in the case of highly stratified aquifers with production wells located on the landward side of the barrier, saltwater bypassing of the canal could occur. Therefore, the depth of penetration of the canal into the aquifer and the head maintained in the canal are important factors governing its effectiveness. Another issue is the potential for seawater to enter the canal during normal tides, spring tides, or hurricane surges. The construction of levees on each side of the canal would reduce the potential for saltwater movement into the canal and would allow the water level in the canal to be controlled at a higher level during peak water-use periods.

Another method of managing surface water in coastal areas to inhibit horizontal saltwater intrusion is the concept of designating a salinity line running parallel to the coast [168]. Where drainage canals discharge freshwater to tidal water, water control structures can be constructed to both block the uninhibited landward flow of tidal water and increase the head of freshwater upstream of the control structure, which can cause the interface to migrate seaward (Figure 9). Where freshwater canals are connected to tidal water and residents have boats, control structures can be designed to both maintain boater access (lock structure) to tidal water and to reduce the impacts of saltwater intrusion (Figure 10). Saline water intrusion in estuaries may be reduced by artificially constructed or enhanced dunes in channels [169].

3.3. Horizontal Salinity Barriers Using Injection Wells

The most common type of salinity barrier to limit the landward movement of the freshwater/seawater interface is injection using one or multiple wells located parallel to the shoreline [46,170] (Figure 11). The specific design of the pumping or abstraction barrier is based on the hydrogeology of the aquifer [65,150,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185]. Barriers of this type have been operating along the Californian coast for extensive periods of time, including the West Coast Basin Barrier (1951), the Dominguez Gap Barrier (1971), the Alamitos Barrier (1964), and the Talbert Seawater Intrusion Barrier (1976) [138,186].

The wells typically used to create a salinity barrier in a siliciclastic unconfined aquifer are constructed with the base of the screen near the bottom of the aquifer and with sufficient screening upwards to create a mound that prevents any bypassing (Figure 11). In carbonate rocks, the same general practice of design is used, but open-hole construction is commonly employed. Again, test drilling and hydraulic assessment (including groundwater modeling) are required to optimize the barrier design.

Horizontal salinity barriers inject either treated or partially treated freshwater. The permitting of salinity barrier wells is regulated by the Underground Injection Control rules of the U. S. Environmental Protection Agency and of states under primacy agreements, and the injected water is required to meet at least primary drinking water standards [187]. Highly treated domestic wastewater can be used to supply salinity barrier injection wells and is considered to be an excellent and cost-effective method of reusing water [150,180,188,189,190,191,192,193,194,195,196]. An example of a salinity barrier design using injected domestic wastewater is conceptually shown in Figure 12. An additional discussion on saltwater intrusion barriers in operation is contained in Section 4.2.

3.4. Horizontal Salinity Barriers Using Pumping Systems

The landward movement of the freshwater/seawater interface can be controlled by salinity barriers constructed using abstraction wells located parallel to the shoreline [46] (Figure 13). The specific design of the pumping or abstraction barrier is based on aquifer hydrogeology [197,198,199,200,201,202,203]. Depending on the design of the abstraction wells, the water withdrawn can range in quality from nearly seawater to brackish water. In most cases, the extracted water is discharged back to the sea, but could be used to supply a seawater desalination plant [204]. Some abstraction barriers are purposely designed to limit the extracted water to be brackish so that it can be used to supply a brackish-water reverse osmosis desalination plant that operates at a significantly lower cost compared to a seawater reverse osmosis desalination plant [199,205]. The co-use of an abstraction salinity barrier with desalination is discussed in Section 3.7.

While abstraction barriers can effectively limit the horizontal migration of the interface, a fundamental water management issue must be considered. A certain amount of freshwater is lost during the pumping process unless it is used in some beneficial manner (e.g., for a desalination supply). An economic assessment of the lost water versus the benefits of protecting inland water users from a decline in water quality would need to be made to justify the use of this method.

3.5. Concurrently Operated Multi-Barriers or Mixed Barriers

Saltwater intrusion barriers using both abstraction and injection wells have been proposed to provide a high degree of protection for interior freshwater aquifers [167,203,206,207,208,209,210] (Figure 14). This type of dual barrier has technical merit, but does create a need to obtain a suitable freshwater source for injection wells. In addition, there is some loss of freshwater when pumping the abstraction wells. The injected water may require treatment to prevent clogging of the wells and, in the United States, to meet potable water standards (USEPA [211]). Ebeling et al. [203] could not locate any currently operating mixed withdrawal/injection system in the world.

There are additional possible mixed barrier systems that could be designed and constructed using physical barriers, recharge ponds, and pumping wells [135,211]. However, local surface water hydrology and the hydrogeology of the unconfined aquifer system would require considerable investigation to justify the economics of these mixed barrier systems.

3.6. Abstraction, Desalination, and Recharge (ADR)

Abstraction wells are used to feed a reverse osmosis desalination system that is used for recharge [203,212,213,214]. The original abstraction–desalination–injection (ADR) concept was described by Abd-Elhamid and Javadi [213] and is shown in Figure 15.

An assumption made by Abd-Elhamid and Javadi [213] is that the cost of brackish water desalination was low enough to make the ADR concept economically viable based on research published by Jaber and Ahmed [215]. However, as the total dissolved solids (TDSs) of the abstracted water is rather uncertain, and if the value rises above the 10,000–12,000 mg/L range, the use of brackish water membranes becomes unviable, and a seawater membrane would be required [216]. Pumping of water from at or near the interface will likely produce a variable TDS concentration flow that affects the design and operation of a brackish water reverse osmosis (BWRO) system, causing operational challenges and creating the necessity to make expensive modifications to the plant to allow future operation. In addition, no cost was allocated to the disposal of the concentrate for the process. Therefore, ADR may not be cost-effective by itself.

A minor modification of the ADR concept by adding additional development of upgradient production wells could create a more economic system [203,214]. Even if the water quality from the abstraction well causes the use of a more expensive desalination process, the additional protected freshwater could balance the economics. The example shown in Figure 16 shows the concept applied to an unconfined aquifer, which may cause some additional treatment challenges, such as pretreatment to remove dissolved iron and dissolved organic matter. However, the concept applied to confined aquifers, which tend to contain lower concentrations of dissolved iron and organic matter, could be more economical. The concept could be applied to any aquifer type but requires a thorough economic analysis after obtaining real data on the local hydrogeology and water chemistry of the target aquifer or aquifer system.

3.7. Evaluation of Hybrid Barrier/Water Treatment Types to Control Saltwater Intrusion

A modification of the ADR concept was suggested by Koussis et al. [217] (Figure 17). In this case, brackish water pumped from the abstraction well would be treated using either BWRO or modified SWRO with a TDS concentration significantly below normal seawater concentrations and used for water supply. The injection wells would use highly treated wastewater to maintain a dual barrier against saltwater intrusion. The major advantage of this concept is that any minor concentrations of undesirable chemicals found in the wastewater would be treated at the desalination plant and would never become problematic. A case could be made during the Underground Injection Control permitting that additional treatment processes for chemicals such as per- and polyfluoroalkyl (PFAS) compounds should not be required.

A management model was developed by Javadi et al. [143] to assess various types of salinity barrier types, including recharge with desalinated water, recharge with treated wastewater, abstraction of brackish water and return to the sea, desalination of abstracted brackish water and recharge with excess desalinated water (ADR), desalination of abstracted brackish water and recharge with treated wastewater, and the abstraction of brackish water and disposal of the of the abstracted water to the sea combined with treated wastewater recharge. They concluded that the most cost-effective and environmentally friendly method was the desalination of abstracted brackish water to be used for freshwater supply and recharge with treated wastewater. This method produced potable water and trapped and treated any trace contaminants in the wastewater.

3.8. Pumped Air Barriers

Injection of compressed air is an alternative to using water in some form (freshwater or treated wastewater) to create a hydraulic barrier to inhibit saltwater intrusion [218,219,220]. The first laboratory experiment to demonstrate that compressed air inhibits the passage of saltwater in porous media was performed by Dror et al. [221]. Later, Sun and Semprich [218] used a numerical model to demonstrate that saltwater can be displaced from a confined aquifer, thereby creating a barrier (Figure 18). Use of an air barrier was not demonstrated for use in an unconfined aquifer because the air could escape through the top of the aquifer. Zang and Li [220] investigated the use of an air barrier to inhibit saltwater intrusion in an unconfined aquifer using a numerical model. They concluded that injection of compressed air in an unconfined aquifer was less effective compared to a confined aquifer. However, in heterogeneous unconfined aquifers containing semi-confined units above the base of the aquifer, compressed air injection did move the seawater/freshwater interface seaward. Based on this very interesting research, the use of compressed air in what could be termed a semi-unconfined aquifer may be feasible. However, very detailed hydrogeologic investigations would be required to assess where injection of compressed air would be effective.

3.9. Management of Saltwater Intrusion Using Aquifer Storage and Recovery

Aquifer storage and recovery (ASR) can be used to manage seasonal variations in the supply of freshwater and simultaneously to inhibit saltwater intrusion [222,223,224] (Figure 19). Where an ASR system is located in a coastal zone, it can be operated to control the position of a freshwater/seawater interface by injecting more water than is recovered to stabilize the water budget of any aquifer meriting protection [222].

Misut and Voss [222] modelled the use of ASR to ascertain impacts on the saltwater/freshwater interface in the Lloyd Aquifer underlying New York City. They demonstrated that by injecting more water than recovered for many cycles, it caused the interface to stabilize or move seaward. The Lloyd Aquifer is a confined aquifer that was historically over-pumped, resulting in the landward migration of seawater.

Hussain et al. [223] modeled a wadi aquifer located in the Fujairah emirate in the United Arab Emirates. They assessed several injection and recovery scenarios and found one that allowed a certain quantity of injected water to be recovered while still maintaining the position of the freshwater/seawater interface. This demonstrated that an ASR system can be used conjunctively to control the interface and to meet seasonal water supply needs. Careful monitoring of these conjunctive systems is necessary to operate them at a high level of efficiency.

Maliva and Missimer [219] described operational ASR systems located in New Jersey that not only provides both peak season water supplies, but also controls salinity within a coastal aquifer. The Wildwood, New Jersey ASR system is a good example of this type of system [224,225,226,227,228].

3.10. Biofilm Barriers

Biofilms are known to significantly reduce flow in porous media [229,230,231,232]. Song et al. [229] documented the clogging of a sandstone aquifer during the operation of an ASR system. This issue is most often associated with ASR using treated wastewater or partially treated surface water and in both cases, the injected water contains bacteria and relatively high nutrient concentrations. The biofilm on the walls in the aquifer outside of the borehole is nearly impervious and renders the well dysfunctional, thereby requiring cleaning or redevelopment. Specific types of microbes have been used to experimentally study the formation of biofilms in porous media, including Gram-negative Pseudomonas aeruginosa or Pseudomonas putida and Gram-positive Bacillus subtilis [233,234,235,236,237].

It is well known that thin biofilms in the shallow subsurface can form virtually impervious barriers to the flow of water. This has been demonstrated on the walls of adsorption fields in septic tank systems, which is called a biomat [237]. The biomat or biofilm forms an impervious barrier surrounding the adsorption field, causing it to stop allowing percolation of the wastewater, thereby causing system failure. Based on the rapid growth of biofilms and their clogging of aquifer porous media, James et al. [230] suggested that the purposeful biofouling of an aquifer at the desired locations could create a salinity barrier.

The creation of a biofilm barrier in an aquifer requires a mix of the correct bacteria and nutrients to facilitate the creation of an active biofilm. This would likely be best accomplished in a pumping barrier using wastewater, which could cause the biofilm layer to form within the injection plume. Although some laboratory work has been completed on biofilm formation in groundwater systems, it has been limited mostly to biofilms forming on well screens in injection wells. This is considered to be experimental technology that has some potential for creation of a salinity barrier, but will require additional research.

Another experimental barrier type uses injected chemical concoctions to cause minerals to precipitate within porous media and create targeted subsurface barriers. [237]. Again, the development of this type of barrier requires additional research to assess if the barrier would remain intact over long periods to maintain its integrity.

3.11. Purposeful Activation of Expanding Clays to Inhibit Saltwater Intrusion

In certain siliciclastic aquifers, detrital or authigenic clays, when exposed to freshwater, tend to swell or disperse and permanently reduce aquifer hydraulic conductivity [238,239,240]. Smectites in particular tend to swell when exposed to freshwater [238]. Another process of hydraulic conductivity reduction caused by water/clay interactions is clay dispersion. Gray and Rex [241] have documented that clay dispersion causes a reduction in hydraulic conductivity in very permeable sandstones to less than 1% of the original values. Mays [242] reviewed the geochemical processes involved in clay dispersion.

In ASR systems located in aquifers subject to clay swelling or dispersion, the aquifer can be pretreated to prevent swelling or dispersion [243]. A detailed discussion of various pretreatment methods is contained in Maliva and Missimer [219]. However, if the process that was causing clay dispersion is purposely initiated, it theoretically could be used to create a salinity barrier to inhibit movement of the seawater/freshwater interface landward. This barrier type is limited to siliciclastic aquifers that contain the clay minerals that are subject to either swelling or dispersion, but could be very effective depending on the distribution of the clay within the most permeable sections of the aquifer being managed. However, it is well known that once clays have dispersed or swelled and have reduced aquifer hydraulic conductivity, the change is largely irreversible [244]. This is therefore another barrier type that merits future research to ascertain if it is viable for site-specific applications.

3.12. Vertical Salinity Barriers

Saline water intrusion into coastal aquifers is often caused by the upconing of higher TDS water into existing wells and well fields [54,74,112,113,114,245]. Some types of salinity barriers have been proposed to inhibit the upward movement of the saline water into wells in both unconfined and semiconfined aquifers. These barriers could be termed vertical salinity barriers. Farid [246] developed a well design that is proposed to allow for the capture of freshwater in a well while simultaneously inhibiting the upward movement of high salinity water (Figure 20). Some of the freshwater pumped from the well is re-pumped below the base of the well. This method can be used primarily in unconfined aquifers.

Another type of vertical salinity barrier was proposed for the City of Daytona Beach, Florida wellfield [193]. This wellfield taps the Upper Floridan Aquifer, which is recharged by both freshwater moving downward from the unconfined aquifer via a leaky confining unit and upward flow of underlying saline water through an underlying leaky confining unit. The vertical salinity barrier involves the construction of an injection well that penetrates the upper part of the lower semiconfined aquifer and recharges it with freshwater to inhibit the upward movement of the saline water through the confining unit (Figure 21).

Well system designs can be used that tend to limit saltwater intrusion, such as the use of horizontal wells and shallow collection trenches [247]. These systems have been used at locations where the freshwater lens has limited thickness, such as on carbonate islands.

4. Current Applications of Salinity Barriers (Examples)

4.1. Salalah, Oman Salinity Barrier

An injection barrier has been operating for more than 15 years to protect a coastal unconfined, siliciclastic aquifer [248,249,250]. About 40 injection wells are used to create the salinity barrier, using tertiary-treated domestic wastewater. The injection wells are located approximately parallel to the coast between 0.9 and 1.3 km inland from the sea. The wells are located about 300 m apart, are 40 m deep, and contain well screens to avoid collapse. Combined observation data and modeling results show that the barrier has been effective in pushing the freshwater/saltwater interface seaward, thereby meeting the objectives of the project.

This project has been operating for an extended period, but documentation on the operation has not been shared with the scientific world. It is an example of a barrier that could yield design insights to others with a similar hydrogeology.

4.2. Talbert Seawater Intrusion Barrier

One of the largest and best-operated salinity barriers in the world is in Orange County, California, and is called the Talbert Seawater Intrusion Barrier [186,251,252]. This combined salinity barrier and water reuse project injects highly treated domestic wastewater, at potable water standards, into a complex confined aquifer system (Figure 22 and Figure 23). In 2014, the injection capacity of the barrier was 265,000 m³/d [252]. In the 2021 Orange County Water District Groundwater Replenishment System Annual Report, the average injection rate was 104,000 m³/d [253]. There were 36 injection wells operating to protect five aquifers, as shown in Figure 23. Many of the wells or well clusters contain multiple injection depths. Periodic inspection of the injection wells is required to ascertain their need for maintenance and redevelopment.

4.3. Proposed Innovative MAR Project to Prevent Future Saltwater Intrusion

The Anne Arundel County, Maryland (USA) Managed Aquifer Recharge (MAR) Project, known as Our wAAter Program, proposes injection of highly treated domestic wastewater into the Upper and Lower Patuxent Aquifers, which are the regional primary sources of drinking water [254,255]. This project was conceived to achieve three goals: the removal of nutrient loading to the Chesapeake Bay estuarine system, a reduction in the potential for saltwater intrusion by increasing the potentiometric surface to mitigate pumping impacts, and a reduction in the potential for land surface subsidence by mitigating the head differential across clay confining units. One of the drivers of the project was the recognition that rising sea levels could impact the operations of their utility in the future. This project is one of the first in the world to attempt mitigation of seawater intrusion in a coastal aquifer before it becomes problematic.

5. An Assessment of Salinity Barriers to Mitigate the Impacts of Sea Level Rise in Coastal Aquifers

An evaluation matrix was constructed that shows all the barrier types as applied to unconfined and confined aquifers in the coastal zone with six key evaluation factors that include technological maturity (mature and used versus theoretical), environmental risk, permitting issues, capital cost (CAPEX), operating cost (OPEX), and long-term positive impact. A numerical value from 0 to 10 was assigned to the barrier type for each of the six evaluated categories, and each barrier type was assigned a numerical rating for application to unconfined and confined aquifers (

Table 1. Technical and economic assessment of salinity barriers to mitigate sea level rise.

Barrier Type	Technology	Environment Issues	Regulatory Issues	Capital Cost	Operating Cost	Long-Term Effectiveness	Total Score
Physical
Grout curtain (unconfined)	10	10	7	7	10	0	44
Curtain wall (unconfined) with pumping	10	7	6	6	8	1	39
Curtain wall (confined)	0	0	0	0	0	0	0
Grout curtain (unconfined)	10	9	8	6	10	0	43
Grout curtain (unconfined) with pumping	10	7	6	5	8	1	37
Grout curtain (confined)	9	9	7	2	10	10	47
Surface Water (unconfined only)
Canal	10	8	8	6	3	0	35
Canal with levees	10	6	7	4	3	5	35
Blockage of canal/control structures	10	10	9	8	6	0	43
Injection
Injected freshwater (unconfined)	10	9	7	7	5	4	42
Injected freshwater (confined)	10	9	7	6	5	10	48
Injected treated wastewater (unconfined)	10	7	5	4	2	4	32
Injected treated wastewater (confined)	10	7	5	3	2	10	37
Pumping
Brackish to tide (unconfined)	10	8	8	8	6	2	42
Seawater to tide (unconfined)	10	8	8	8	6	2	42
Brackish to tide (confined)	10	6	5	8	6	9	44
Seawater to tide (confined)	10	7	7	8	6	9	47
Mixed
Injection and pumping (unconfined)	10	6	5	6	5	2	34
Injection and pumping (confined)	10	7	6	4	5	10	42
Abstraction, Desalination, Recharge (ADR)
Brackish water pumped, treated (BWRO), and injected (unconfined)	10	8	8	3	3	5	37
Brackish water pumped, treated (BWRO), and injected (confined)	10	8	8	3	3	10	42
Seawater pumped treated (SWRO) and injected (unconfined)	10	8	8	1	1	5	33
Seawater pumped treated (SWRO) and injected (confined)	10	9	8	1	1	10	39
Brackish water pumped, treated (BWRO), and injected with freshwater use (unconfined)	10	9	9	3	3	5	39
Brackish water pumped, treated (BWRO), and injected with freshwater use (confined)	10	9	9	3	3	10	44
Seawater pumped, treated (SWRO), and injected with freshwater use (unconfined)	10	9	9	3	3	5	39
Seawater pumped, treated (SWRO), and injected with freshwater use (confined)	10	9	9	3	3	10	44
ADR with treated wastewater for injection and BWRO use for supply (unconfined)	10	9	9	3	5	5	41
ADR with treated wastewater for injection and BWRO use for supply (confined)	10	9	9	3	5	10	46
ADR with treated wastewater for injection and SWRO use for supply (unconfined)	10	8	8	3	2	5	36
ADR with treated wastewater for injection and SWRO use for supply (confined)	10	8	8	3	5	10	44
Pumped compressed air
Unconfined/ semi-unconfined aquifers	3	9	9	8	7	0	33
Confined aquifers	8	9	9	8	8	10	52
Aquifer Storage and Recovery
Unconfined aquifers	8	9	7	8	6	4	40
Confined aquifers	10	10	8	8	6	9	51
Biofilm
Biofilm (unconfined)	1	8	3	6	9	1	28
Biofilm (confined)	1	9	5	8	7	10	40
Clay, swelling or dispersion
Clay, unconfined	1	10	9	6	9	0	35
Clay, confined	1	10	9	5	7	10	42

). While the numerical assignments are somewhat subjective, they are based on sound reasoning (see Section 5.1, Section 5.2, Section 5.3, Section 5.4, Section 5.5 and Section 5.6). Many of the evaluation factors are related. Note that if it was determined that a barrier type had a zero long-term positive impact based on the projected rise in sea level, then it was not considered to be viable.

Climate change impacts not only include sea level rise but other impacts that affect the performance of various types of barriers. Increased frequency of storm surges of seawater based on tropical storm impacts is another key factor considered. Climate change includes ocean warming that has a high potential to increase the average number of annual tropical storms and possibly the intensity [256,257,258,259,260]. In addition, the duration of drought conditions can impact potential sources of freshwater that are needed to operate many types of salinity barriers [261,262].

5.1. Technology

A key evaluation factor is the technological maturity of each type of salinity barrier in terms of design and use. Many of the barrier types have been used for decades, such as recharge canals, injection wells, and pumping wells, so their effectiveness is well known. Combining mature barrier types with water treatment options still has a high level of proven functionality with a variety of improvements. New salinity barrier methods are under development, and some have been tested in laboratories, or only modeling studies have been conducted. While some of these new concepts may have viable applications, they have not been designed, constructed, and operated, so they do not have a high degree of technological development. Therefore, the score for technology is based on the knowledge obtained from operating these systems, as well as knowledge and improvements made based on operation. New ideas have a lower score based on the degree of conceptual development.

Rising sea levels can have an adverse impact on the long-term operation of many types of salinity barriers. As sea levels rise, beach erosion increases, which could destroy any facility located on or near the beach. Injection or withdrawal wells and their surface components, such as pumps and electrical transmission lines, are subject to damage or forced relocation. Higher sea levels can also increase the impacts of storms (e.g., tropical storm surge) and cause the destruction of surface infrastructure.

5.2. Environmental Issues

Salinity barriers can have varying degrees of environmental impacts on both land surface features, water availability, and water quality. For example, if a barrier type cuts off access to the beach or shoreline, coastal zone ecosystems could be impacted, and public access could be a problem. Any salinity barrier system that requires the injection of freshwater must have a source of supply for the freshwater, which could impact coastal wetlands or water supplies for existing users. Salinity barriers that withdraw water and discharge it to tidal waters may have impacts on the marine ecosystem (freshening during critical periods), or the loss of freshwater could be considered counter to water conservation. The use of treated municipal wastewater for injection could create nutrient discharges to tidal water that could create harmful algal blooms or add undesirable contaminants of emerging concern to the groundwater system. Salinity barrier types that have the lowest potential impact on the environment will have the highest evaluation scores.

5.3. Regulatory Issues

There is a tendency for salinity barrier types with a lesser degree of environmental impact to have the least problematic regulatory permitting issues. Salinity barrier wells require permits based on the federal Underground Injection Control regulations in the United States. This can create a degree of uncertainty in the permitting process. For example, if the source of water used for creation of an injection barrier is wastewater, then water treatment must meet all primary drinking water quality standards and, in some jurisdictions, most secondary standards. This additional treatment becomes an economic factor involving both the additional capital costs of the treatment system and the operating costs of water treatment and residuals disposal.

5.4. Capital Costs

The capital costs for salinity barriers include the design and construction costs for all primary and secondary components that allow the system to operate. Salinity barrier systems that contain water treatment components tend to have the highest capital costs, and the more complex the treatment, the higher the capital costs. Water treatment facilities, such as advanced wastewater treatment facilities, tend to use membrane water treatment processes that create residuals that require disposal, which add both capital and operating costs.

5.5. Operating Costs

The operating costs are related to the complexity of the barrier system and increase greatly as water treatment becomes a greater component. Systems that are designed with additional water use opportunities tend to mitigate the operating costs by obtaining greater opportunities to receive net additional water use for the operation instead of solely water losses. Each water treatment process added to the operation of a salinity barrier creates a significant increase in the overall operating cost of the system.

5.6. Long-Term Effectiveness

The evaluation of the long-term effectiveness of salinity barriers is based on the projected increase in global sea levels, as shown in Figure 4, which shows an increase of 0.5 to 1.8 m by 2150. Therefore, systems that can continue to operate effectively under the projected sea level changes receive the highest scores. Note that there is considerable uncertainty in the projected sea level increase based on climate change, and various local areas can have higher or lower rates of change based on local geological conditions.

5.7. A Discussion on the Results Reported in the Evaluation Matrix

Physical barriers, recharge canals (without levees), and new technology physical barriers could provide positive benefits for the most probable projected changes in sea level through 2100. However, for rises in sea level at or exceeding the upper range of projections and for later times, they would become inadequate.

Physical barriers constructed in confined aquifers generally have high feasibility scores. The compressed air barrier, which is still experimental, has the highest score for application to confined aquifers but is not feasible for use in unconfined aquifers where air could be lost out of the top of the aquifer.

The most commonly used barrier type, injection, has high scores for use in confined aquifers using both freshwater and/or highly treated wastewater. For unconfined aquifers, the scores are lower based on possible bypassing of the barrier over time or by storm surges—the slope of the coastal aquifer system is a major consideration. A major cost item with the use of treated wastewater is the necessity to meet drinking water standards and to limit nutrient discharge into coastal tidal waters. Generally, depending on the slope of the unconfined aquifer at the coast, all injection barriers have a reasonable degree of feasibility based on the most probable sea level projections.

The abstraction and mixed barrier type shows higher average feasibility scores compared to injection, but in all cases, some freshwater is recovered and discharged to tides. In the mixed barrier, some freshwater is required to charge the injection part of the barrier, so water treatment costs will impact the total score (reduce).

The ADR options contain some intriguing possibilities, especially the ones that reuse highly treated wastewater, use the BWRO water for public supply, and allow additional freshwater wells to be operated inland from the barrier. The scores for the options range from 33 to 46. In all cases, the barriers have significant positive impacts on management of the interface during rises in seawater to the end of 2150. Again, the type of barrier chosen must be linked to the local hydrogeologic conditions, the land surface altitude, susceptibility to tropical storm impacts, and the financial resources of the coastal communities using the barrier.

The co-use of aquifer storage and recovery in coastal confined aquifer systems has a very high feasibility score (51), particularly due to the dual benefit received. Also, it is a mature technology and has long-term potential for successful use. ASR usefulness to manage interface migration in unconfined aquifers is based on the local hydrologic and physical conditions at a given site.

Three experimental technologies were evaluated, including compressed air injection, creation of a biofilm barrier, and creation of a swelling and/or clay dispersion barrier. All are intriguing to various degrees. The compressed air barrier may be quite useful in confined aquifers, based on modeling, and appears to be quite feasible and thus merits field testing. The other two technologies will require further laboratory testing, modeling, and field testing before they can be considered as future barrier options.

6. Limits on Use of Salinity Barriers

In certain cases, extreme aquifer heterogeneity and very high hydraulic conductivities can prevent the effective operation of salinity barriers, such as karstic conditions where open conduits penetrate the interface and discharge freshwater on the continental shelf [263,264,265,266,267]. In some cases, the only method to control saltwater intrusion and to restore the coastal aquifer is to curtail pumping using an optimization method [135]. Low yield wells in low-permeability aquifers are commonly used in some coastal zone locations. When these wells initiate seawater intrusion, there is a very limited potential to use a salinity barrier to protect them.

The coastal plain slope in many global regions presents challenges to development of salinity barriers based on inland movement of seawater during tide changes. In these areas, the application of a salinity barrier for the management of the freshwater/saltwater interface may not be feasible for the middle range of future sea level changes.

Another limiting issue in the design, construction, and management of a salinity barrier is economics [268]. One effective method of assessing economic feasibility is to cross-check the costs with that of seawater desalination, which may be the most expensive alternative water supply for inhabitants of the coastal zone. Ghaffour et al. [205] provide some data on the CAPEX and OPEX from seawater desalination facilities. However, there is a major scaling effect for very large seawater desalination plants recently constructed in the Middle East, where the OPEX cost has reduced to less than USD 0.70/m³.

7. Conclusions

Climate change will have a negative impact on coastal aquifer water quality in the future. Not only will the rise in global sea levels exacerbate saltwater intrusion, but increased numbers of tropical storms may cause increased surge impacts, and a greater occurrence of long-term droughts may reduce recharge in many coastal areas. Therefore, water management in the coastal zone will require multi-faceted plans to mitigate the movement of the freshwater/seawater interface landward and to prevent impacts to freshwater supplies. Mitigation measures include a reduction in the use of freshwater from over-stressed aquifers.

The design, construction, and operation of salinity barriers will be needed to provide protection for coastal aquifer water supplies. Eleven types of salinity barriers were evaluated within the context of the projected global sea levels changes made by the International Panel on Climate Change and others, which conclude that global sea level will rise between 0.5 and 1.8 m by 2150. An evaluation matrix was created that included six factors applied to each barrier type (including numerous design variations) for unconfined and confined aquifers. The factors considered were technology, environmental risk, permitting difficulty, capital cost, operating cost, and long-term positive impacts of managing the saltwater/freshwater interface.

Many barrier types, including physical barriers, infiltration canals, and the application of injection and pumping barriers, may not be useful in the management of unconfined aquifers in the future because they will be inundated with seawater with minimal long-term positive benefits. The type of barrier that can be effectively operated greatly depends on the local hydrogeologic conditions in the aquifer and the slope of the coastal plain from the shoreline landward. Salinity barriers constructed in confined aquifers produce a higher degree of long-term benefit. A series of hybrid barriers generally classified as abstraction–desalination–recharge, with different design variations, appear to have the most positive long-term benefits. A new salinity barrier concept, the injection of compressed air into a confined aquifer, shows a high degree of promise for future use. Two theoretical barriers, the creation of a subsurface biofilm and the purposeful activation of clay expansion or dispersion, show some promise but will require greater research.

Laboratory, field, and modeling research needs to continue to evaluate existing and new barrier types to broaden the use of these technologies. In addition, greater documentation is needed on the operation of existing salinity barriers to allow engineers and scientists access to lessons learned in order to make design improvements in the future.