Seawater intrusion (SWI) is a worldwide problem that has been exacerbated by rising sea levels, climate change (CC), and excessive over-pumping (EOP) of coastal fresh groundwater (GW) resources. Many of the world’s coastal areas are characterized by dense settlements, with a large part of the world’s population residing within 60 km from the coast [1
]. More than 3 million Moroccans live in Morocco’s coastal areas, and this figure is gradually increasing [2
]. Indeed, in 2015, more than half of the population lived on the coast, which experienced drought and agricultural development with an increasing proportion of the rural population due to rural flight [2
]. As a result, GW overexploitation soon became a widespread problem, with several coastal regions around the world (Morocco [3
], Tunisia [7
], Libya [10
], Italy [11
], and the Netherlands [1
]) experiencing substantial SWI in aquifers, resulting in significant degradation of GW quality and, consequently, quantity [12
To better understand the saltwater intrusion process, numerous studies have been undertaken, including laboratory-scale investigations and numerical and analytical modeling studies [17
]. A probabilistic numerical model was built to predict the extent of saltwater intrusion into a coastal phreatic aquifer by Felisa et al. [17
]. Fahs et al. [18
] used the semianalytical solution of the dispersive Henry problem based on the Fourier technique to simulate heterogeneous and anisotropic coastal aquifers, taking into account the effects of the aquifer hydraulic parameters, boundary conditions, pumping and recharge rates, and aquifer depth. The effects of aquifer bed slope and seaside slope on saltwater intrusion were investigated by Abdelhamid et al. [19
] to examine the movement of the dispersion zone under different settings of bed and seaside slopes, and a numerical model (SEAWAT) was applied to the well-known Henry problem.
Nowadays, numerical modeling technology is an important tool for GW analysis. CC impacts, computational GW flow, and transport modeling have been investigated in many regions around the world [20
The impact of CC and sea-level rise (SLR) on SWI has been studied using a variety of models. Abd-Elhamid et al. [30
] examined the individual and combined effects of predicted SLR and over-pumping on SWI using a coupled transient density-dependent finite element model to simulate SWI in the Gaza aquifer. Ranjbar [31
] analyzed the effects of a 1 m gradual and immediate SLR associated with pumping activities on the location of seawater wedge toe in a shallow coastal aquifer on the Caspian Sea’s southern shores. Essink and Schaars [32
] developed a model to simulate GW flow, heat and salinity distribution, and seepage and salt load flux into the surface water system. The model was used to analyze the impact of future events such as CC, SLR, and land submergence, as well as human activities, on the GW system’s qualitative and quantitative features. Tiruneh and Motz [33
] used SEAWAT to explore the influence of SLR on the freshwater–saltwater interface, taking pumping and recharging into account. Canning [34
] presented the climatic variability certainty and uncertainty for Washington’s maritime water. He also gave a history of SLR in Puget Sound as a result of CC. All of these studies have demonstrated the importance of numerical modeling methods in the monitoring and successful management of coastal aquifers. Furthermore, the use of numerical models in distinct case studies provides significant obstacles due to site-specific properties of individual aquifers. Moreover, these models are crucial for developing curative solutions to reduce SWI. In earlier research, various approaches have been recommended, including artificial recharge (Yang et al. [35
]; Hussain et al. [36
]); abstraction, desalination, and recharge (ADR) approach (Abd-Elhamid and Javadi [37
]); reduced pumping rates (Sherif et al. [38
]; Rejani et al. [39
]); relocation of the pumping wells (Datta et al. [40
]; Mantoglou and Papantoniou [41
]); subsurface dams (Chang et al. [42
]); and cutoff walls (Shen et al. [43
]; Abdoulhalik and Ahmed [44
The Rmel-Oulad Ogbane coastal aquifer (ROOCA), situated in Morocco’s northwest, is well-known for its contribution to agricultural, economic, and social growth. Furthermore, GW is the sole source of domestic use for the urban and rural populations. The decrease in average rainfall from 1211 mm in 1963 to 378 mm in 2016 is attributable to the effects of CC, which triggers intermittent droughts and reductions in recharge, which directly influence the GW level; the effects of CC also include the influence of SLR on the GW level and salinity intrusion. This condition is aggravated by ROOCA EOP values ranging from 14.6 hm3
in 1961/62 to 21.52 hm3
in 2015/16, which are used in rural, urban, and agricultural areas for industrial and drinking water supply [45
]. This has culminated in a significant decrease in GW levels, which could potentially result in an aquifer water balance deficit, in addition to a depletion in GW from the reservoir and a high standard SWI in the coastal plains, posing a severe challenge to potential water supplies. Figure 1
summarizes mechanisms that affect coastal aquifers such as ROOCA. As a result, careful management of GW potential in these aquifer systems is needed, as is consideration of the CC. It is possible to create it using a methodological approach that combines geographic information system (GIS), climate parameters, and the numerical GW flow model, which is suggested and extended to the case of ROOCA in Morocco. This enables one to comprehend the factors that control the activity of the coastal aquifer’s freshwater/saltwater transition region under multiple input conditions and CC scenarios, such as Representative Concentration Pathway (RCP) 4.5 and RCP 8.5, which are two different types of pathways in CC. These instruments would help municipal water supply agencies in their economic and water resource growth planning.
Ocean warming and ice melting, according to the Intergovernmental Panel on Climate Change (IPCC), could raise global sea levels by up to ~60 cm by 2100 [46
The effects of climate-induced SLR will become more apparent as the intensity of the phenomenon increases, especially in low-elevation coastal areas (Figure 2
). The majority of African countries tend to be in grave danger due to low levels of development along with projections of fast population increases in coastal areas. Under these conditions, the SLR at the aquifer’s seaside boundary, which is an additional pressure head, would be imposed. Water table gradient and/or the piezometric head would decrease, resulting in further interference. Many hydraulic, geometric, and transport parameters influence the SWI operation. Each aquifer has its set of circumstances, and the sharp interface approach cannot be applied to all of them, because of the transient conditions which often reign in the exploitation of aquifers.
For this purpose, the study’s goals are to (i) describe and characterize the case study (e.g., geography and climate) (Section 2.1
); (ii) define the geological and hydrogeological setting of the study area (Section 2.2
and Section 2.3
); (iii) understand the hydrodynamic functioning of this hydrogeological system by quantifying the components of the GW mass balance (Section 2.4
); (iv) design a conceptual model of the ROOCA (model set-up, initial and boundary conditions, model calibration and validation, time step of the simulations, the distribution of recharge from all sources and pumping, etc.); (v) design a computational model for simulating saltwater intrusion in the ROOCA using SEAWAT code; and (vi) determine the expected future forcing on the ROOCA aquifer due to population increase, water demand (over-pumping), CC (rainfall reduction), and SLR extrapolations to assess the water balance terms and SWI volumes under CC scenarios (Section 3
GW is the important source of freshwater in the Rmel-Oulad Ogbane coastal plain in the low Loukkos basin in Morocco, where 98% of the domestic water, as well as the entire industrial water supply, is dependent on GW. Therefore, it is imperative to protect GW from SWI in this area. Hence, there is a need for tools that can guide and assist the manager in decision-making regarding the use, management, and planning of water resources. Currently, these decision support elements are provided by efficient technical tools such as GIS, geostatistical analysis, and conceptual and mathematical models, as developed in this research.
Using data from boreholes and hydrogeological investigations, we modeled the aquifer reservoir using a 3D GSIS. Then, a geostatistical model was produced from physicochemical, piezometric, and hydrodynamic parameters. These outputs were used to update the water balance in 2013/2014 and to develop a good conceptual model of the aquifer. Finally, to predict the current extent of SWI and provide useful information for the protection of GW resources, a three-dimensional numerical model of density-dependent GW flow and miscible salt transport of the subsurface aquifer was developed to assess the current extent of SWI in the study area with the aim of determining the impacts of CC due to increasing of temperatures, decreasing precipitation, and SLR during the 21st century. The developed model incorporated regional geologic, geographic, and hydrogeological features. The model input parameters were determined from analysis of well logs, well driller’s reports, and pumping tests. All these inputs were used to simulate three-dimensional variable-density GW flow under steady and transient states.
Due to the scarcity of some observed water quality data and continuously monitored head data, more field measurements, such as vertical salinity profile and trace element studies for dispersivity, need to be performed in order to improve the reliability of the model. For this performed calibration, a total of 22 observed hydraulic head values were used, resulting in good agreements between the observed and calculated hydraulic heads. When more data become available in the future, additional calibration would be needed. Moreover, an optimization model for rational management of the aquifer must be developed.
Climate projections used in this assessment comprise a combination of regional climate modeling projection data, generated from RICCAR, and a set of local observation datasets for precipitation and temperature for the study area. Projected temperature and precipitation were obtained from CNRM-CM5, EC-EARTH, and GFDL-ESM2M RCMs under RCP 4.5 and RCP 8.5 scenarios. These projections show a decrease in precipitation and an increase in temperature for both scenarios. As a result, CC would almost certainly have a detrimental effect on SLR, reducing the availability of new GW resources. The increase in sea level would affect coastal aquifers, shifting the saltwater interface farther inland. Indeed, the model took into account the predicted SLR and used it to adjust the boundary conditions.
The variation in recharge was determined by taking into account the variations in return from irrigation and climatic parameters (precipitation and temperature). The numerical simulations were conducted for a period of approximately 76 years and dealt with SWI relating to GW abstraction, climatic parameters, and SLR. The simulation results under RCP 4.5 show that the maximum extent (about 5.2 km) of SWI would increase in 2040 in the northwestern sector of the study area. The water quality would be most affected in the ONEE pumping area, which is directly adjacent to the seashore. As a result, the GW abstraction associated with CC is the primary driver of SWI in the study area. Furthermore, the reduction in recharge and the rise in sea level caused by CC exacerbate saltwater intrusion into the aquifers, reducing the fresh GW resources.
The primary impact of this SWI in the ROOCA would be unnecessary over-pumping that would deplete renewable water resources. However, this situation can be improved by the use of surface water for irrigation (provided from the neighboring dam reservoir), a desalination plant project for DWS, and artificial recharge of the aquifer. GW recharge with recycled water would be also an effective and feasible way to address the rapid GW depletion and saltwater intrusion in the ROOCA. Recycled water is a sustainable and reliable source of local water that should be viewed as a valuable resource. Indeed, GW recharge is an excellent utilization of recycled water as it provides natural storage (which allows for drought mitigation or withdrawal when demand for water increases) and soil treatment (with surface spreading), and it can be used to directly prevent SWI (with a direct injection barrier, for instance). Thus, the city of Larache, which is located directly at the northern limit of the study site, has a good potential for wastewater to be produced for the artificial recharge of the coastal fringe of the aquifer, which is distinguished by sand dunes, very favorable to infiltration and natural purification.
This would greatly increase the GW production in the coastal sectors of the aquifer and would protect freshwater from SWI. Such long-term results and findings will help the local decision-makers and all relevant stakeholders to better plan, manage, and improve the fresh GW resources for the ROOCA.
Morocco has more than 3500 km of coastline (two maritime facades: Atlantic, 2,934 km; Mediterranean, 512 km) with several thousand hectares of coastal plains, where irrigated agriculture is well developed, such as in this case study, in addition to more industries and the most important cities in the country (Casablanca, Rabat, Tangier, Agadir, etc.). These coastal plains contain coastal aquifers that are overexploited and threatened by SWI. Some large cities are already supplied with surface water, GW, and desalination water (Agadir, Casablanca, Al Hoceima). Therefore, this study will serve as a pilot study in order to implement the established methodology. It is also recommended to complement this study with an in-depth study on the choice of artificial recharge sites and a study to optimize the management of conventional and unconventional water resources in order to minimize the energy costs associated with desalination and wastewater treatment, especially during wet and dry periods caused by CC.
It is also recommended to extend this established methodology to study further similar coastal aquifers abroad in terms of climate conditions, such as in the Mediterranean region, as this will help the decision-makers in water resources planning and development and securing sustainable GW management of the coastal aquifers. This methodology can be completed by an impact modeling study based on the application of artificial recharge (from available sources) in well-chosen sites in order to improve the production of water resources and limit the entry of seawater into freshwater of the coastal aquifer.