1 Introduction
Seawater intrusion is often a major constraint to optimal use of fresh groundwater from coastal aquifers. Excessive groundwater abstraction to meet growing demands from an increasing coastal population and the expected rise in mean sea level from global warming will cause seawater to encroach farther inland and threaten the available groundwater supply. Seawater intrusion occurs in many coastal and deltaic areas around the world. When saltwater travels inland to production wells, underground water supplies become useless. In agriculture, this could cause soil salinity problems resulting to poor crop yields and the substitution of more salt-tolerant crops over indigenous crops [1].
The contaminated aquifer is sometimes abandoned resulting in the loss of a precious groundwater resource. Thus, present-day and future water supply engineers and managers face the challenges of optimal exploitation of fresh groundwater and the control of seawater intrusion [2]. There are different types of pollutants that can be found in groundwater, such as nitrate, heavy metals, and saltwater. Intrusion of saltwater is the most common contamination occurrence in coastal aquifers [3]. Intrusion of saltwater occurs when saltwater displaces freshwater in an aquifer. The phenomenon can occur in deep aquifers with the advance of saline waters of geologic origin, in shallow aquifers from surface waste discharge, and in coastal aquifers from the invasion of seawater [4].
New methods to control saltwater intrusion in coastal aquifers are presented and discussed in details; also the advantages and disadvantages of each method were highlighted. Finally, investigation and control of saltwater intrusion in Egypt, especially in the Nile Delta aquifer, are discussed. The possibility of applying new methods to control saltwater intrusion in Egypt is presented.
2 Saltwater Intrusion Control Methods
- 1.
Reduction of pumping rates
- 2.
Relocation of pumping wells
- 3.
Use of subsurface barriers
- 4.
Natural recharge
- 5.
Artificial recharge
- 6.
Abstraction of saline water
- 7.
Combination techniques
A number of numerical models had been developed and used to help in understanding the relevant process that causes saltwater intrusion in coastal aquifers and identify suitable methods of control. Extensive research has been carried out to investigate saltwater intrusion in coastal aquifers. However, only limited amount of research has been directed to study the control of saltwater intrusion. The following sections present the traditional methods that have been used to control saltwater intrusion in different locations. Also, the advantages and disadvantages of each one are discussed [6].
2.1 Reduction of Pumping Rates
- 1.
Increase in public awareness of the necessity of water to save water
- 2.
Reduction of losses from the water transportation and distribution systems
- 3.
Reduction of water requirement in irrigation by changing the crop pattern and using new methods for irrigation such as drip irrigation, canal lining, etc.
- 4.
Recycling of water for industrial uses, after appropriate treatment
- 5.
Reuse of treated wastewater in cooling and irrigation and recharge of groundwater
- 6.
Desalination of seawater
A number of models have been developed to control saltwater intrusion by reducing pumping rates from the aquifer or using optimization models to optimize the abstraction and control the intrusion of saline water. Zhou et al. [7] used a quasi-three-dimensional finite element model to simulate the spatial and temporal distribution of groundwater levels. The objective of the model was to maximize the total groundwater pumping from the confined aquifer and control saltwater intrusion by relocating the wells. Amaziane et al. [8] coupled the boundary element method and a genetic algorithm for the optimization of pumping rates to prevent saltwater intrusion. Qahman and Larabi [9] investigated the problem of extensive saltwater intrusion in Gaza aquifer, Palestine, using the SEWAT code. Different scenarios were considered to predict the extension of saltwater intrusion with different pumping rates over the time.
Advantages
Reduction of abstraction rates and use of other water resources help to increase the volume of freshwater which retards the intrusion of saltwater.
Disadvantages
Control of saltwater intrusion using this method has some limitations. The control of abstraction rates cannot be fully achieved especially from private stakeholders. Also the alternatives proposed for facing the shortage of water and the increased demands may be very costly, especially when freshwater is not available and requires transportation. Desalination of seawater as an alternative has some limitations; it is still expensive, is a source of pollution, and requires a large area of land. The treated wastewater is also costly. Furthermore, this can only be a temporary solution because it does not prevent saltwater intrusion but it attempts to reduce it; but with the population growth and increasing demands, the problem will not be controlled [6].
2.2 Relocation of Pumping Wells
Changing the location of pumping wells by moving the wells to more inland positions aims to raise the groundwater level and maintain the groundwater storage. This is because in the inland direction, the thickness of the freshwater lens increases and the risk of upconing of saltwater decreases accordingly. Hong et al. [10] developed an optimal pumping model to evaluate optimal groundwater withdrawal and the optimal location of pumping wells in steady-state condition while minimizing adverse effects such as water quality in the pumping well, drawdown, saltwater intrusion, and upconing. The study involved experimental verification of the optimal pumping model to develop sustainable water resources in the coastal areas. Ojeda et al. [11] modeled the saline interface of the Yucatan Peninsula aquifer using SUTRA assuming an equivalent porous medium with laminar flow and two-dimensional flow. The models showed that the interface position is very sensitive to head changes, and a simple groundwater abstraction scheme was presented which was based on the well depth and the distance from the coast. Sherif and Hamza [12] used two models to simulate the problem of saltwater intrusion in the Nile Delta aquifer in Egypt in the vertical and horizontal directions. The two models were 2D-FED and SUTRA. The 2D-FED model was employed to simulate the current condition and predict the effect of the seawater level rise in the Mediterranean Sea under the condition of global warming. SUTRA was used to define the best location of additional groundwater pumping wells from the Nile Delta aquifer and to assess the effect of various pumping scenarios on the intrusion process.
Advantages
Moving pumping wells further inland helps to decrease the occurrence of upconing of saltwater [6].
Disadvantages
Control of saltwater intrusion by this method is costly and may face some obstructions such as buildings or the size of the aquifer may not allow such movement. It is also a temporary solution and does not prevent the intrusion of saline water into aquifers.
2.3 Subsurface Barriers
Sketch of a subsurface barrier
Barsi [13] developed two methods for optimal design of subsurface barriers to control seawater intrusion through the development of implicit and explicit simulation-optimization models. The main objective was to find the optimal design of subsurface barrier to minimize the total construction cost through the selection of the width and location of the barrier. James et al. [14] developed a process for selectively plugging permeable strata with microbial biofilm. These biofilm barriers can aid the prevention of saltwater intrusion by reducing the subsurface hydraulic conductivity. The main advantage offered by the biofilm barrier technology is that biofilm barrier construction can be achieved without excavation and therefore will be economic. Harne et al. [15] presented a 2-D subsurface transport model of saltwater considering the soil to be homogenous and isotropic under the influence of constant seepage velocity. They used the finite difference method to solve the transport equation. The model examined the efficiency of subsurface barrier to control saltwater intrusion.
Advantages
Using subsurface barriers helps to reduce the intrusion of saline water.
Disadvantages
Control of saltwater intrusion using subsurface barriers could be costly in terms of construction, operation, maintenance, and monitoring. It is also not efficient for deep aquifers [6].
2.4 Natural Recharge
Sketch of the natural recharge process
This method could be efficient for unconfined aquifers, but it could take a long time to recharge the aquifer depending on its properties. Ru et al. [16] used a quasi-three-dimensional model to simulate the movement of the interface between seawater and freshwater and evaluate the effect of constructing a dam to protect groundwater resources. The function of the subsurface dam was to collect the rainfall water and recharge the aquifer to increase the groundwater storage and retard saltwater intrusion. Bajjali [17] applied geostatistical techniques of GPI, IDW with GIS, to examine the effect of recharge dam on groundwater quality. The infiltrated water below the dam increases the aquifer storage of freshwater and pushes the saline water toward the sea.
Advantages
This method helps to prevent the runoff to flow directly to the sea and uses it to increase the groundwater storage in the aquifer and prevent the intrusion of saline water.
Disadvantages
Natural recharge depends on the soil properties and requires high permeability soil. Depending on the soil permeability, the recharge process could take a long time. The cost of construction of dams and weirs and their maintenance is very high. This solution is unsuitable for confined and deep aquifers.
2.5 Artificial Recharge
Sketch of recharge and abstraction wells
A number of researchers used this method to control saltwater intrusion in coastal aquifers. Narayan et al. [18] used SUTRA to define the current and potential extent of saltwater intrusion in the Burdekin Delta aquifer under various pumping and recharge conditions. The results addressed the effects of seasonal variation in pumping rate and artificial and natural recharge rates on the dynamic of saltwater intrusion. Papadopoulou et al. [19] developed a 3-D finite element-finite difference groundwater flow simulation model. The extension of the saltwater front along the coastal zone was calculated only hydraulically because his model did not consider diffusion due to different densities of saltwater and freshwater. Artificial recharge was presented using different scenarios including different well locations and different injection rates. The source of water used for recharge was the effluent of the wastewater treatment plant of the industrial zone after tertiary treatment.
Mahesha [20] studied the effect of battery of injection wells on seawater intrusion in confined coastal aquifers. He used a quasi-three-dimensional areal finite element model considering a sharp interface. He studied various conditions by changing the well spacing and intensity and duration of freshwater injection. He concluded that spacing between wells, injection rate, and duration of injection control the repulsion of the saline wedge. Mahesha [21] presented steady-state solutions for the movement of the freshwater/seawater interface due to a series of injection wells in a confined aquifer using a sharp interface finite element model. The model was used to perform parametric studies on the effect of location of the series of injection wells, spacing of the wells, and freshwater injection rate on the seawater intrusion. It was found that reduction of seawater intrusion (of up to 60–90%) could be achieved through proper selection of injection rate and spacing between the wells. Also, he studied the effect of double series of injection wells and compared it with the single series. He found that a double series performs slightly better than single series. Also, it was found that the staggered system of wells in the series is slightly better than the straight well system for long spacing. Vandenbohede et al. [22] presented a simulation model to study sustainable water management using artificial recharge of freshwater in the dunes of the western Belgian coastal plain by two recharge ponds. The recharged water was produced from secondary-treated wastewater effluent by the combination of ultrafiltration and reverse osmosis.
Advantages
Artificial recharge helps to increase the groundwater storage in the aquifer and prevent the intrusion of saline water.
Disadvantages
The artificial recharge has been discussed in many papers as a solution to control saltwater intrusion. However, some of these models ignored the source of freshwater especially in the areas that suffer from scarcity of water. On the other hand, the cost of freshwater in cases where it is not available in these areas and transported from other places has not been taken into consideration. Using desalinated seawater is costly and could be a source of pollution. The treated wastewater was used in many areas for recharge to control saltwater intrusion. This technique is often costly and ineffective in the areas where excessive groundwater pumping occurs [23], [18]. Treated wastewater affects the aquifer properties and requires at least 1 year before abstraction from the aquifer. At present there is a growing opposition against artificial recharge by infiltration at the land surface, because it occupies a large area.
2.6 Abstraction of Saline Water
This method of control aims to reduce the volume of saltwater by extracting brackish water from the aquifer and returning to the sea. Figure 3 shows a sketch of an abstraction well. Johnson and Spery [24] presented different methods to control saltwater intrusion in different states in the USA. In California they extracted saline water and desalinated it using RO treatment process. The treated water was blended with untreated groundwater to produce water suitable for domestic delivery. In Los Angeles, injection wells were used to protect the coastal aquifers from saltwater intrusion. Potable and highly treated wastewater was injected into the wells. Maimone and Fitzgerald [25] applied three-dimensional groundwater flow models, dual-phase sharp interface intrusion model, radial upconing model, and single-phase contaminant transport model to develop a coastal aquifer management plan. Two techniques were used: the development of new well locations further inland and use of RO treatment for desalinating brackish water and using it for domestic purposes. Sherif and Kacimov [26] suggested pumping brackish water, encountered between the freshwater and saline water bodies, to reduce the extension of seawater intrusion. They used SUTRA to examine different pumping scenarios in the vertical view, and the equiconcentration lines and velocity vectors were identified for the different cases. They concluded that seawater intrusion problems could be controlled through proper pumping of saline groundwater from the coastal zone.
Kacimov et al. [27] investigated seawater intrusion in coastal unconfined aquifers of Oman, experimentally, analytically, and numerically. Water table elevation, capillary fringe, moisture distribution, and EC were observed and measured in a pilot site Al-Hail, Oman. Laboratory measurements of conductivity and capillary rise were conducted in repacked columns in the laboratory. Evaporation from a shallow horizontal water table to a dry soil surface was modeled by HYDRUS2D. An analytical Dupuit-Forchheimer model was developed for the planar part of the catchment with explicit expressions for the water table, sharp interface, and stored volume of freshwater. SUTRA code is used to study a variable density flow in a leaky aquifer with line sinks modeling freshwater withdrawal and evaporation. Both analytical and numerical models proved that pumping of saline groundwater from coastal aquifers would mitigate the migration of seawater into the aquifer and would contribute to the enhancement of the groundwater quality that is consistent with the findings of Sherif and Hamza [12].
Advantages
Abstraction of saline water decreases the volume of saline water in the aquifer and protects pumping wells from upconing.
Disadvantages
The main problem in abstraction of brackish water is the disposal of the saline water. Many researchers have attempted to solve this problem, but it is still a subject of current research. Brackish water is suitable only for certain types of crops, and using it in cooling may cause corrosion to the systems. The disposal of brackish water into the sea could affect the marine life in these areas, fishing, and tourism activities. Another problem is that increasing abstraction of saline water could, in some cases, increase the intrusion of saltwater [6].
2.7 Combination Techniques
Combination between two or more of the above methods can help to eliminate the disadvantages of these methods and give better control of saltwater intrusion. Zhou et al. [7] and Hong et al. [10] used a combination of reduction in pumping rates and relocation of pumping wells to control saltwater intrusion. Maimone and Fitzgerald [25] used the development of new well locations further inland and use of RO treatment for desalinating brackish water and using it for domestic consumption to reduce the abstraction from the aquifer. Narayan et al. [18], Paniconi et al. [28], and Barrocu et al. [29] used reduction of pumping rates and recharge of freshwater to the aquifer. Johnson et al. [30] used a combination of three techniques: subsurface barriers, reduction of abstraction rates, and recharge of freshwater. Fitzgerald et al. [31] used change of well locations, reduction of pumping rates, and recharge of freshwater.
The combination of freshwater’s injection and extraction of saline water can reduce the volume of saltwater and increase the volume of freshwater. Only few models have been developed for this technique. Mahesha [32] studied the control of seawater intrusion by a series of abstraction wells for saline water alone and also in combination with freshwater injection wells in a confined aquifer using a vertically integrated two-dimensional sharp interface model under steady-state conditions. It was found that the combination of injection wells with extraction wells produced excellent results to the individual cases for larger well spacing and small rates of injection. Rastogi et al. [33] developed a two-dimensional steady-state numerical model to study seawater intrusion problem involved. The model was used to investigate the efficiency of seawater control measures involving two scenarios: (1) freshwater recharge and (2) combination of freshwater recharge and saltwater discharge wells. The study found that depth, location, and head of recharge wells are important parameters that can control saltwater intrusion. The model confirmed that combined recharge and discharge system is more effective in controlling seawater intrusion.
Tanapol et al. [34] studied the effectiveness of controlling methods in unconfined aquifers by using scaled-down physical models. The investigated methods included freshwater injection, saltwater extraction, and subsurface barrier. They concluded that freshwater injection is more favorable than the saltwater extraction and subsurface barrier method. A freshwater injection rate of about 10% of the usage rate can effectively push the interface toward the shoreline and maintain the water quality in the vicinity of pumping wells. Acostaa and Donadoa [35] conducted a laboratory scale simulation of seawater hydraulic barriers in confined coastal aquifers considering the effects of stratification. The best hydraulic barrier performance was observed in the extreme point of wedge, where 17.8% of intrusion reduction, in homogeneous media, and 78.9% in stratified media were observed. The highest reduction of seawater intrusion was achieved with the highest injection rate. Stratification affected the performance of the hydraulic barrier. Smaller injection rates were necessary to reduce seawater intrusion in stratified medium.
3 New Methodologies to Control Saltwater Intrusion
Coupling of flow and solute transport models with optimization techniques to address important groundwater quality management problems has started early 1980s. The motivation for the development of simulation-optimization (S/O) approach for the control of saltwater intrusion is the enormous cost of groundwater remediation. The S/O approach has been shown to be capable of reducing the remediation costs of contaminated land in several real-world applications [36]. The S/O approach can be used efficiently in groundwater remediation system design and in other groundwater quality management problems including saltwater intrusion control. The S/O approach is interesting because it can both account for the complex behavior of a groundwater flow system and identify the best management strategy to achieve a given set of prescribed constraints [37].
3.1 Simulation-Optimization Models
Simulation models have been applied for the management of groundwater resources seeking for optimal management strategy by trial-and-error. This has proved to be time-consuming and laborious. In addition, the results obtained may not be optimal. The main reason for this is the inability of this approach to consider important physical and operational restrictions. To accommodate these restrictions, coupling of the simulation model with a management model is the generally adopted procedure. For groundwater systems, combined simulation and management models may adequately predict the behavior of the system and provide the best solution for problems such as planning of long-term water supply or preventing seawater intrusion. A relatively small number of studies have concentrated on the control of saltwater intrusion linking simulation with optimization models to effectively determine the optimal solution for control of seawater intrusion [37].
3.1.1 Simulation Models
Modeling of fluid flow and solute transport for a site involves development of a mathematical model of the site system being studied and to use this model to predict the value of hydraulic heads and salt concentrations at points of interest at different times. Numerical simulation of seawater intrusion, assuming that mixing occurs at the transition zone between seawater and freshwater, involves the solution of the partial differential equations representing the conservation of mass for the variable density fluid (flow equations) and for the dissolved solute (transport equation). The numerical solution of coupled fluid flow and solute transport is based on solving the governing equations with the boundary and initial conditions by the iterative solution scheme. Recently, a number of simulation codes were developed and capable of simulating saltwater intrusion such as SEAWAT and SUTRA [37].
3.1.2 Optimization Models
The optimization problem can be solved through trial-and-error adjustment or through a formal optimization technique. Numerical simulation models can be used to examine a limited number of design options by trial-and-error. Due to the simplicity of the trial-and-error method, it is widely used. However, testing and checking hundreds to thousands of trial solutions is tedious and cannot guarantee that the optimal solution has been identified. On the other hand, the optimization technique can be used to search for the optimal solution in a wide search space of design variables and, equally important, to prove whether a particular management scenario or remedial alternative is feasible in terms of meeting the management objective and satisfying all the constraints [37].
Optimization model can be defined in terms of an objective function and a set of constraints. Groundwater management problem includes two sets of variables: decision variables and state variables. Decision variables are the variables that are used to define different alternative decisions, such as pumping or injection rate of a well, well location, and well depth. Decision variables can be managed in the calculation process to identify the optimal management policy or strategy. State variables are the variables that describe the flow and transport conditions of an aquifer, such as head and salt concentrations, which are the dependent variables in flow and transport equations. In a coupled S/O model, simulation components update the state variables, and optimization components determine optimal values of decision variables [36]. Optimization models are widely used in groundwater planning and management. In recent years, a number of simulation models have been combined with optimization techniques to address groundwater management problems. The complex behavior of the groundwater system and identification of the best management strategy under consideration of the management objectives and constraints can be achieved easily by the combined simulation and optimization modeling processes. In the last two decades, genetic algorithms (GAs) have received increased attention from academic and industrial communities for dealing with a wide range of optimization problems.
3.2 Genetic Algorithm (GA)
GA is an optimization technique based on the process of biological evolution. GA was first introduced by Holland [38] and followed by Goldberg [39], Davis [40], Michalewicz [41], Mckinney and Lin [42], and Haupt and Haupt [43] among others. GAs are a family of combinatorial methods that search for solutions of complex problems using an analogy between optimization and natural selection. GA mimics biological evolution based on the Darwinist theory of survival of the fittest, where the strongest offspring in a generation are more likely to survive and reproduce. The GA method starts with randomly generating an initial set of solutions, called the initial population. Each member of this population represents a possible solution of the problem, encoded as a chromosome. A chromosome is a string of symbols, usually a binary bit string. The population of chromosomes evolves through a cycle that involves selection, crossover, and mutation. Each cycle is referred to as a generation. After many generations, the population will contain chromosomes that represent near optimal solutions to the problem. GA was applied to a wide variety of problems in engineering including groundwater management, water resources, and seawater intrusion. An overview to the application of GA in these fields is presented in the following sections [36].
GA has been applied to a number of groundwater management problems. A number of researchers incorporated groundwater simulation models with GA to solve groundwater management problems such as maximizing extraction from an aquifer, minimizing the cost of water supply, and minimizing the cost of aquifer remediation and pump-and-treat. Among the researchers who applied GA to groundwater management problems are Mckinn and Lin [44], Rogers and Dowla [45], El Harrouni et al. [46], and Aly and Peralta [47]. Recently, GA has been used to identify cost-effective solutions for pump-and-treat groundwater remediation design. In the field of water resource management, GA has been applied to a variety of problems such as calibration of rainfall-runoff models and pipe network systems and operation of reservoir systems. A number of researches applied GA for calibration of rainfall-runoff models (e.g., [48–50]). A large number of applications in pipe network optimization using GA were presented by Murphy et al. [51] and Farmani et al. [52, 53].
On the other hand, development and application of optimization techniques in association with saltwater intrusion considering density-dependent flow are relatively recent and very few. Das and Datta [54, 55] presented a multi-objective management model. The objectives were to maximize pumping from the freshwater zone and minimize pumping from the saline zone in order to control saltwater intrusion. The nonlinear finite difference of the density-dependent miscible flow and salt transport model for seawater intrusion in coastal aquifers was embedded within constraints of the management model. Das and Datta [56] developed a nonlinear optimization method for minimizing total sustainable yield from specific location of the aquifer while satisfying salinity constraints. Gurdu et al. [57] developed a management model to maximize the total pumping rate from wells considering saltwater intrusion into the aquifer. They used SUTRA code for groundwater simulation and the general algebraic modeling system code to execute the optimization model.
Gordon et al. [58] developed a model for optimal management of a regional aquifer under salinization. The source of salinization was from irrigation water percolating over some part of the aquifer, influx of saline water from faults in the aquifer bottom, and inflow from laterally adjacent saline water bodies. The objectives of management were to maximize the total amount of water pumped for use and to minimize the total amount of salt extracted with the water. The simulation model used a finite element formulation for the flow and a streamline upwind Petrov-Galerkin formulation for the transport. The model computed the gradient of the state variables (heads and concentrations) with respect to the decision variables (pumping rates at wells). The gradients were then used in a Bundle-Trust non-smooth optimization procedure to achieve an improved solution. Rao and Rao [59] presented S/O approach to control saltwater intrusion through a series of extraction wells. They used SEAWAT code to simulate saltwater intrusion and the simulated annealing algorithm for solving the optimization problem. Two objective functions were considered: first to maximize groundwater pumpage for beneficial purposes and second to minimize the pumping in extraction wells which were used to extract saline water and dispose to the sea.
Javadi et al. [60] presented the development and application of a simulation-optimization model to control seawater intrusion in coastal aquifers using different management scenarios: abstraction of brackish water, recharge of freshwater, and combination of abstraction and recharge. The model was based on the integration of a genetic algorithm optimization technique and a coupled transient density-dependent finite element model. The developed model was applied to analyze the control of seawater intrusion in a hypothetical confined coastal aquifer. The efficiencies of the three management scenarios are examined and compared. The results showed that combination of abstraction and recharge wells is significantly better than using abstraction wells or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. The results from the study would be useful in designing the system of abstraction/recharge wells to control seawater intrusion in coastal aquifers and can be applied in areas where there is a risk of seawater intrusion.
3.3 Examples of New Methods to Control Saltwater Intrusion
The new methodologies used to control saltwater intrusion in different locations of the world are present in this section.
3.3.1 Abstraction of Saline Water and Recharge Using Surface Ponds
Schematic sketch for abstraction of saline water and recharge using surface ponds
3.3.2 Abstraction, Desalination, and Recharge (ADR)
- 1.
Abstraction of brackish water to reduce the saline water volume in the aquifer and reduce the intrusion of saltwater.
- 2.
Desalination of abstracted brackish water using RO treatment process to produce freshwater from brackish water for recharge. This step is very important to produce freshwater in the areas where freshwater is scarce. The advantages of RO as a desalination method are discussed in details in the next section.
- 3.
Recharge of treated water to increase the fresh groundwater volume to prevent the intrusion of saltwater.
Schematic sketch for abstraction, desalination, and recharge (ADR)
The combination of abstraction and recharge techniques is considered one of the most efficient methods to control saltwater intrusion. It increases the volume of fresh groundwater and decreases the volume of saltwater. This method is capable of retarding saltwater intrusion. It also has lower energy consumption, lower cost, and lower environmental impact. ADR can be used to increase the water resources in coastal regions by increasing the abstraction of saline water and desalination. The excess reclaimed water can be used directly for different purposes or injected into the aquifer to increase the groundwater storage [6].
Abd-Elhamid and Javadi [62] presented a cost-effective method to control seawater intrusion in coastal aquifers. The methodology (ADR) includes abstraction of saline water and recharge to the aquifer after desalination. A coupled transient density-dependent finite element model was developed by for simulation of fluid flow and solute transport and used to simulate seawater intrusion. The simulation model was integrated with an optimization model to examine three scenarios to control seawater intrusion including abstraction, recharge, and a combination system, ADR. A comparison between the combined system (ADR) and the individual abstraction or recharge system was made in terms of total cost and total salt concentration in the aquifer and the amount of repulsion of seawater achieved. The results showed “that the proposed ADR system performs significantly better than using abstraction or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. ADR is considered an effective tool to control seawater intrusion and can be applied in areas where there is a risk of seawater intrusion” [62].
3.3.3 Treatment, Recharge, Abstraction, and Desalination (TRAD)
Schematic sketch for TRAD method
The main benefits of the (TRAD) methodology are to return the mixing zone (the interface between fresh groundwater and saline water) into the original status and to reach the dynamic balance between fresh- and saline groundwater through two processes of (A) abstraction of brackish groundwater to reduce the volume of saline water and (R) recharge of the aquifer by treated wastewater to increase the volume of fresh groundwater. The abstraction and recharge processes help to push saline water toward the sea. The desalination of brackish water (D) aims to produce freshwater from the brackish water and use it for different purposes such as domestic, irrigation, and industry. Desalination of seawater has a lot of problems such as high cost, high pollution (mainly carbon emission), large area of land that is required for plants, and disposal of the brine. “Desalinating brackish water is an efficient alternative to seawater desalination, because the salinity of brackish water is less than one-third of that of seawater. Therefore, brackish water can be desalinated at a significantly lower cost than sea water” [63]. Several methods can be used for desalination, but RO has been selected as a method for desalting brackish water, because it has many advantages; it requires simpler equipment, low energy, and low cost. The treatment of waste water (T) aims to overcome the scarcity of water in these areas and also to prevent the disposal of wastewater into seas and rivers. Overall, the methodology has lower cost and lower environmental impacts. It can be used to increase the water resources by increasing the abstraction of brackish water and desalination. The excess treated wastewater may be used directly for different purposes. The abstraction rates of the main aquifer can be increased to provide more freshwater and prevent loss of water that flows to the sea.
Javadia et al. [64] presented the results of an investigation on the efficiencies of different management scenarios for controlling saltwater intrusion using a simulation-optimization approach. A new methodology was proposed to control SWI in coastal aquifers. The proposed method was based on a combination of abstraction of saline water near shoreline, desalination of the abstracted water for domestic consumption, and recharge of the aquifer by deep injection of the treated wastewater to ensure the sustainability of the aquifer. The efficiency of the proposed method was investigated in terms of water quality and capital and maintenance costs in comparison with other scenarios of groundwater management. A multi-objective genetic algorithm-based evolutionary optimization model was integrated with the numerical simulation model to search for optimal solution of each scenario of SWI control. The main objective was to minimize both the total cost of management process and the total salinity in aquifer. The results indicated that the TRAD method is efficient in controlling SWI as it offers the least cost and least salinity in the aquifer.
4 Control of Saltwater Intrusion in Egypt
“Saltwater intrusion in a coastal aquifer is a highly complex and nonlinear process. The management of coastal aquifers requires careful planning of withdrawal strategies for control and remediation of saltwater intrusion.” A number of researches have been carried out to simulate saltwater intrusion in the Nile Delta aquifer. However, a limited number of studies were conducted to control saltwater intrusion into the aquifer and protect the groundwater storage. Sherif and Singh [65] examined different scenarios for an additional pumping of 2.3 billion m3/year from the Nile Delta aquifer. It was concluded that additional pumping practices should be located in the middle Delta to decelerate and minimize the intrusion of saline water. Gaamea [66] examined the use of scavenger wells to control saltwater intrusion in the Nile Delta aquifer. He used SUTRA code to generalize and simplify the use of scavenger wells as individual wells or in the shape of the field. Nasar [67] presented the environmental impact assessment of abstracting brackish water for desalination and recharging the brine water through deep wells to control the seawater intrusion into the Nile Delta aquifer. Designed charts for abstraction and recharge wells including discharge and recharge rates and locations and depths of pumping and injection wells were presented.
TDS distribution in the END aquifer for base case (a) areal and (b) vertical distribution
Results of different scenarios to investigate and control SWI in the Eastern Nile Delta aquifer
Case | Scenario | Scenario description | Intrusion length (km) | |
|---|---|---|---|---|
Equiconcentration line 35 | Equiconcentration line 1 | |||
Base case | Current situation | 75.75 | 90.25 | |
Different scenarios of investigating SWI due to climate change | 1 | SLR 100 cm | 77.00 | 90.75 |
2 | Increasing abstraction rate 100% | 79.60 | 91.50 | |
3 | Decreasing recharge 100% | 85.25 | 95.40 | |
4 | Combination of 1, 2, and 3 | 99.40 | 110.25 | |
SLR 100 cm | ||||
Increasing abstraction rate 100% | ||||
Decreasing recharge 100% | ||||
Different scenarios of controlling SWI | 1 | Decreasing abstraction 50% | 69.95 | 88.40 |
2 | Increasing recharge 50% | 70.75 | 88.00 | |
3 | Abstracting brackish water 50% | 61.00 | 87.80 | |
4 | Combination of 1, 2, and 3 | 59.80 | 85.90 | |
Decreasing abstraction 50% | ||||
Increasing recharge 50% | ||||
Abstracting brackish water 50% | ||||
Vertical distribution of TDS in the END aquifer due to different scenarios of SWI control (a) Scenario 1, (b) Scenario 2, (c) Scenario 3, and (d) Scenario 4
4.1 Application of New Methods to Control Saltwater Intrusion in Egypt
Groundwater is an important source for drinking, agriculture, and industry in Egypt. But pollution leads to lose large quantity of water and increase the gap between available and demand. Saltwater intrusion in the Nile Delta aquifer threatens billions of cubic meters of freshwater that are available for different purposes. According to the literature a limited number of studies have been carried out to control saltwater intrusion in the Nile Delta aquifer. The control of saltwater intrusion is very important to protect the available water resources and increase the national income through using this water in the development in agriculture and industry. The key to control saltwater intrusion is to maintain a proper balance between water being pumped from the aquifer and water recharged to the aquifer. Not all the solutions are economically feasible because they are considered long-term solutions, and the time to reach the state of dynamic equilibrium between freshwater and brackish water may takes tens of years [69].
- 1.
Optimization of abstraction rates
- 2.
Pump and treat (P&T)
- 3.
Recharge and abstraction (R&A)
These three techniques have been applied in different locations of the world (e.g., Gaza, Florida, USA, India). These three methods are capable of preventing the intrusion of saltwater into the Nile Delta aquifer, but the cost of applying these techniques may be high and may have some environmental and social impacts. The use of linked simulation-optimization models to assess these techniques should be considered. S/O models can help to optimize the abstraction from the aquifer and minimize the intrusion of saltwater. Pump and treat (P&T) can help to remediate the aquifer and provide a new source of freshwater. Recharge and abstraction (R&A) technique can help to prevent the intrusion of saltwater by keeping a proper balance between water being pumped from the aquifer and water recharged to the aquifer considering social, economical, and environmental issues.
4.1.1 Optimization of Abstraction Rates
Increasing pumping rate is considered the main cause of saltwater intrusion along the coasts because the natural balance between freshwater and saltwater in coastal aquifers is disturbed by abstraction which lowers groundwater levels, reduces the amount of fresh groundwater flowing to the sea, and ultimately causes saltwater to intrude coastal aquifer. The traditional method of reducing abstraction rates to control the intrusion of saline water was applied in different areas using simulation models (e.g., Sherif and Singh [65] , Scholze et al. [70], and Zhou et al. [7]).
But linked simulation-optimization models were recently applied to maximize the abstraction and control the intrusion of saline water. Bhattacharjya and Datta [71] used an artificial neural network as a simulator for flow and solute transport in coastal aquifer to predict salt concentrations of the pumped water. A linked simulation-optimization model was formulated to link the trained ANN with a GA-based optimization model to solve saltwater management problems. The objective of the management model was to maximize the permissible optimal abstraction of groundwater from the coastal aquifer for beneficial use while maintaining salt concentration in the pumped water under specific permissible limits. Qahman and Larabi [9] investigated the problem of extensive saltwater intrusion in Gaza aquifer, Palestine, using the SEWAT code linked with an optimization model. Different scenarios were considered to predict the extension of saltwater intrusion with different pumping rates over the time. Dokou et al. [72] presented a simulation-optimization model to manage saltwater intrusion in two unconfined coastal aquifers in Crete, Greece. The optimization model seeks to maximize groundwater withdrawal rates while maintaining the saltwater intrusion. A combination of a groundwater flow model (MODFLOW) with simple algorithm (GWM) was employed. The results showed that under the current pumping strategies, the saltwater intrusion front will continue to move inland, posing a serious threat to the groundwater quality of these regions. In both case studies, significant reductions in pumping are required to retract the saltwater intrusion front closer to the shoreline.
For Egypt, this technique should be considered with the other methods of controlling saltwater intrusion because using any control method with increasing abstraction rate is ineffective. Application of optimization techniques linked to simulation models to assess the efficiency of this method in controlling saltwater intrusion can help to avoid the limitations of the traditional method of reduction of abstraction rates and use of other water resources. The optimal management of abstraction from the Nile Delta aquifer has become necessary to protect the aquifer from depletion.
4.1.2 Pump and Treat (P&T)
Optimization models are widely used in groundwater remediation. Recently, a number of researchers incorporated groundwater simulation models with GA to minimize the cost of aquifer remediation and pump-and-treat. Among the researchers who applied GA to groundwater management problems are Rogers and Dowla [45] and Aly and Peralta [47]. Mckinn and Lin [44] applied a GA to groundwater resource management and pump-and-treat system design. Johnson and Rogers [73] used GA and neural network to select the optimal well locations and pumping rates in a remediation design problem. Zheng and Wang [74] applied GA to pump-and-treat system design optimization under field conditions. Recently, GA has been used to identify cost-effective solutions for pump-and-treat groundwater remediation design. Some researchers have applied GA to solve groundwater pollution remediation problems (e.g., [75–77]).
For Egypt, this technique can be applied for remediation of aquifers contaminated by saltwater intrusion and/or other types of contaminants. The polluted water can be pumped, treated, and used for different proposes. GA can be linked with simulation models to minimize the cost of aquifer remediation and pump-and-treat. This technique can provide a new source of freshwater from polluted water to face the increased demands.
4.1.3 Recharge and Abstraction (R&A)
Combination techniques can help to eliminate the limitations of using individual methods and give better control of saltwater intrusion. Combination of recharge of freshwater and extraction of saline water can reduce the volume of saltwater and increase the volume of freshwater which help to control saltwater intrusion. A number of models have been used to assess the control saltwater intrusion using R&A (e.g., [33–35, 78]). Recently, linked S/O models were applied to maximize the abstraction and minimize the recharge to control the intrusion of saline (e.g., [59, 60]).
- (a)
Abstraction of saline water and recharge using surface ponds
- (b)
Abstraction, desalination, and recharge (ADR)
- (c)
Treatment, recharge, abstraction, and desalination (TRAD)
The developed models to assess the efficiency of these methods to control SWI were discussed above and the possibility of application to Egypt is discussed below.
Application of Abstraction of Saline Water and Recharge Using Surface Ponds to Egypt
This method is based on a combination of abstraction of saline water near shoreline and recharge freshwater to the aquifer using surface ponds. This method was presented by Javadia et al. [61]. The study presented the integration between a simulation model SUTRA and a genetic algorithm optimization tool in order to investigate the efficacy of abstraction of saline water and recharge using surface ponds. The results indicated that the combined system is better than using abstraction alone in controlling seawater intrusion as it gave the least cost and least salinity in the aquifer. For Egypt, this technique can be used to control saltwater intrusion in locations where freshwater for recharge is available. The source of freshwater could be treated wastewater or excess desalinated brackish water (if any). Also, storm water can be collected and used for recharging the aquifer. The abstracted saline water could be desalinated and used for different purposes.
Application of Abstraction, Desalination, and Recharge (ADR) to Egypt
The methodology ADR (abstraction, desalination, and recharge) includes abstraction of saline water and recharge to the aquifer after desalination. Abd-Elhamid and Javadi [62] presented the integration between coupled transient density-dependent finite element model and an optimization model to examine the efficiency of ADR to control saltwater intrusion. A comparison between the combined system (ADR) and the individual abstraction or recharge system was made in terms of total cost and total salt concentration in the aquifer and the amount of repulsion of seawater achieved. The results showed that ADR system performs significantly better than using abstraction or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. For Egypt, this technique can be used to control saltwater intrusion in locations where the width of the transition zone is small to reduce the cost of transporting the desalinated brackish water and use for recharge.
Application of Treatment, Recharge, Abstraction, and Desalination (TRAD) to Egypt
These methods is based on a combination of abstraction of saline water near shoreline, desalination of the abstracted water for different purposes, and recharging the aquifer by deep injection of the treated wastewater. Javadia et al. [64] presented the integration between SUTRA code and a genetic algorithm optimization tool in order to investigate the efficacy of (TRAD) method to control saltwater intrusion. The results indicated that the TRAD method is efficient in controlling saltwater intrusion in coastal aquifer as it offers the least cost and least salinity in the aquifer. For Egypt, the application of this technique to control saltwater intrusion could be useful as the width of the transition zone reaches some kilometers. This technique does not depend on the distance between recharge wells and abstraction wells because the treated wastewater is used for recharge and the abstracted saline water is desalinated and used for different uses.
- 1.
The source of freshwater for recharges can be addressed using treated wastewater or collect the storm water in a drainage system and inject to the aquifer will retard the intrusion of saline water and protect the coastal area from flash flood such as in the case of Alexandria city in 2015.
- 2.
The disposal of abstracted saline water can be addressed using desalination and use the desalinated water for different purposes or for recharge. The desalination problems including energy and brine disposal can be addressed using renewable energy such as solar energy with new techniques of desalination such as reverse osmoses (RO) or forward osmosis (FA). Brine can be used to produce salt or used for certain types of crops or fish.
- 1.
Population growth
- 2.
Climate change and sea level rise
- 3.
New reclamation projects
- 4.
Reducing recharge due to new structures on the Nile River and associated increase in abstraction rates from the aquifer.
5 Summary, Conclusion, and Recommendation
This chapter gave a background of different control methods that can be applied in different locations where saltwater intrusion is found. Excessive groundwater abstraction to meet growing demands due to increasing population and the expected rise in mean sea level due to global warming will increase seawater and threaten the available groundwater supply. The methods used to control saltwater intrusion in coastal aquifers were presented in this chapter. Also, the new methods that could be applied to control saltwater intrusion in coastal aquifers in Egypt, especially in the Nile Delta aquifer, were presented. Saltwater intrusion in costal aquifer must be controlled to protect freshwater. It is recommended to secure the needed financial support to apply the suitable technique to control saltwater intrusion in the coastal aquifer (Nile Delta aquifer) to protect groundwater in these aquifers. Also, monitoring of the performance of implemented techniques is very important.