Development of suitability map for managed aquifer recharge: Case study, West Delta, Egypt

The increased population in Egypt coupled with the growing industrial and agricultural developments has increased the pressure on available water resources that are almost reaching available water supply. However, the Nile water, which accounts for 93.7% of Egypt's Water Budget, has not increased and alternative resources for additional supplies are still very limited (Abdelhafez et al. 2020). Furthermore, the unprecedented impacts of climate change (CC) are expected to encompass greater concerns throughout the country, with severe environmental, social, and economic implications that may arise due to variations in the Nile River Basin (NRB) flows intensified by the promising development plans for upstream countries (Gado & El-Agha 2021).

According to El Agroudy et al. (2014), 3–5 million acres of Egypt's cultivated land would be lost due to the water storage in front of the Renaissance Dam which is expected to reduce incoming water to Lake Nasser by roughly 25–33 billion m³ annually.

Egypt's yearly water consumption exceeds 80.25 billion m³, while the annual water budget for Egypt is estimated at around 60 billion m³. This results in an annual shortfall of approximately 20 billion m³ (MWRI 2021). Respectively, the per capita share in fresh renewable water has decreased tremendously in the last three decades to less than the international critical limit. The country is thus moving from the water stress situation to water scarcity (Negm 2019). Furthermore, the recent studies concerning the future impacts of CC on the NRB suggest that the variations in precipitation patterns along with the increase in evaporation rates are among the main factors that are expected to affect the water budget in Egypt (Gado et al. 2019).

Despite the fact that the Nile River flow is predicted to rise throughout the period (2010–2039), the results also show that the flow will later decrease due to increased evaporation and the decrease in precipitation rates (Beyene et al. 2010). Additionally, a predicted variation in the average Nile flow is estimated by 2025, from −9 to +12%, which will cause a fluctuated annual flow ranging between 50 and 65 Billion Cubic Meter (BCM) compared to the present share of 55.5 BCM that is based on the 1959 Nile Waters Agreement (Conway and Hulme).

In Egypt, although rainfall only contributes to about 1.3 BCM annually, studies reveal decreasing trends in precipitation predicated across the country coupled with increased trends in temperature that are expected to mirror an estimated increase in evapotranspiration rates (Gado et al. 2019). This in return shall cause major impacts on groundwater recharge, evaporation from water surfaces (like Lake Nasser), environmental requirements (e.g., maintaining the water quality of canals and lakes), and most importantly irrigation water requirements (IWRs) which are expected to increase for winter crops from 6.1 to 7.3% in 2050 and from 4.9 to 5.8% for summer crops due to CC (Mahmoud & El-Bably 2019).

The Nile Delta accounts for two-thirds of Egypt's agricultural land; however, less than two-thirds of its area is under cultivation. The aquifer in the Nile Delta named the Nile Aquifer is considered a vast leaky aquifer bounded by an upper semi-permeable layer and a lower impermeable rocky layer.

Infiltration from excess irrigation water is considered the main recharging method for the aquifer considering the very limited rainfall amounts that infiltrate through the upper clay layer (Leaven 1991).

This, in return, has resulted in a subsequent decrease in groundwater levels that led to a severe consequence on the dynamic balance between fresh and saline water bodies in the Nile Delta aquifer. Moreover, over-abstraction resulted in the deterioration of the available quality and quantity of groundwater resources. Hence, an extensive seawater body has intruded the aquifer forming the major constraint against its exploitation (Mabrouk et al. 2013).

Using innovative subsurface storage methods similar to managed aquifer recharge (MAR) proved to have several advantages including improvements in the quality of groundwater aquifers, rehabilitation of depleted groundwater levels and prevention of saltwater intrusion in coastal zones (Stefan & Ansems 2017).

MAR systems generally are composed of several components that influence the suitability of their implementation in specific locations. To date, most MAR implementation guidelines stress the need for proper planning to ensure the efficiency, profitability and sustainability of the systems. Hence, the selection of suitable recharge sites is considered a crucial step that influences the selection of appropriate MAR techniques and governs its implementation and success.

Previously conducted hydrological investigations and testing scenarios in Egypt for MAR implementation began in 1994 (El Arabi 2012). The main objective of these studies was to assess the feasibility of artificial recharge projects (El-Fakharany, Zeinab 2013) and delineate recharge areas (Ajjur et al. 2021). Most of these efforts focused on hydrological assessments and field investigations of MAR technologies at the local level. As a result, the establishment of a methodological framework addressing both the linkages between a particular recharge technique and suitability mapping was often overlooked.

The development of suitability mapping to evaluate several MAR sites in the Nile Delta can be used as a reference tool to prioritize the selection of various MAR operational sites. By using a minimum number of criteria that are easily derived from existing studies, it can facilitate defining possible integrated solutions that consider the combined effects of CC and development challenges using a holistic perspective. It also helps set future research needs that could lead to sustainable solutions.

The purpose of this paper is to develop a customized approach for improving the selection of MAR implementation sites in West Delta by capturing the interlinkages between a specified MAR technique (spreading methods) and MAR suitability mapping processes using the pairwise comparison matrix (PCM). To optimize the environmental advantages of the chosen recharge technique. The developed approach was created with a range of constraining and factorial considerations in mind, including diverse hydrological, hydrogeological, and socio-economic aspects, using remote sensing data and GIS-based tools.

To start the development process of the suitability mapping in this study, several measures were taken into account to optimize the selection of the study area and establish the development approach based on the best scoring and analytical procedures, the following sections describe the applied measure in detail.

The selection of the study area in West Delta was due to several conditions. One crucial requirement was the presence of existing operational MAR sites in the study area, ideally using similar recharge sources (spreading methods). These sites were essential in examining the reliability of the developed suitability map. In Western Delta fringes, several experiments on the artificial recharge of groundwater aquifers were conducted. These experiments examined several recharge techniques including injection wells and spreading methods, which demonstrated promising results with average infiltration rates that ranged between 0.1 and 0.4 m/day depending on the location of the basin and the mode of operation using Nile flood water and treated wastewater (El Arabi 2012). Finally, the location was also selected considering the projected high needs for water demand due to possible expansion in agricultural development in the area and particularly the newly developed West Delta Agricultural project.

Suitability mapping is considered a crucial element in the planning phase of the MAR system (Dillon 2005). The use of suitability maps is considered an initial step toward conducting extensive field applications to verify the implementation parameters; hence, it is considered a highly effective tool that can be used to inform strategic MAR site planning (Sallwey et al. 2018).

As emphasized by several studies such as Pedrero et al. (2011) and Saraf & Choudhary (1997), remote sensing and GIS have been widely used for the groundwater exploration and identification of artificial recharge sites using reclaimed wastewater. On the other side, the application of an operational research approach, such as a multi-criteria decision analysis (MCDA), is considered an important milestone in the development of regionally mapping reservoir storage suitability. The results obtained from MCDA can help decision-makers in strategic planning if combined with extensive aquifer characterization feasibilities and sensitivity analysis to verify the validity of the proposed suitability sites (Rahman et al. 2012). The criteria that should be selected to evaluate the MAR suitability mapping should be chosen to be complete, non-redundant, and minimal (Malczewski 1999).

Following the completion of the data collection stage, a constrained method was used to screen restricted areas using constrain mapping techniques.

Once completed, various operational considerations had to be evaluated. Therefore, the second step in the development of the suitability map was factor mapping which encountered the compilation of various layers that represent crucial factors. These factors were categorized as follows:

• (1)

  Hydrological parameters are mainly associated with the aquifer characteristics, properties and parameters that influence the infiltration of water from the surface according to the selected MAR technique.

• (2)

  Operational and socio-economic parameters define the suitability of the land for establishing MAR projects taking into consideration the socio-economic demands while minimizing the operational costs associated with the implementation.

Each factor under the outlined categories was classified into appropriate classes in a GIS environment using natural grouping inherent in the data (De Smith et al. 2015). The factors were then standardized using a common scale of 1–5 (where 1 = unsuitable, 2 = low suitability and 5 = highly suitable). This was later followed by the integration of the MAR-classified factors using the analytical hierarchical process (AHP). The AHP process mainly aims to enable the creation of a framework for the decision-making process by providing a mechanism for assigning proper weights to different criteria (Yalew et al. 2016).

Constrain mapping starts by defining specific criteria that serve as restrictive factors that are believed to prevent the implementation of MAR projects under certain conditions. The constraint criteria are usually described by a Boolean statement of suitability where the criterion is described by a binary system (1/0) (Eastman 1999). In this case study, the following constraint criteria were identified and applied as a mask layer considering the following parameters.

The ideal recharge method for treated wastewater is by using infiltration basins or spreading methods. The slope is considered one of the main determining factors in the selection of the artificial recharge technique and the area. The ideal slope for developing the recharge basins or percolation sites is between 0 and 5% as higher slopes increase the construction costs, runoff and soil erosion (Ahmadi et al. 2017).

The slopes constrain map was calculated based on a newly developed digital elevation model (DEM) that was generated for the study area based on the Shuttle Radar Topography Mission (SRTM) with a resolution of 1-arc second with a cell size of 30 m × 30 m. The classification of the slope ideal degree was developed based on the SOTER model in Table 1, which was used to evaluate the slope suitability for spreading methods.

The soil of the aquifer in the vadose zone advances the sewerage effluent treatment by removing most of the biological loads and by reducing the concentrations of chemicals. Hence, suitable sites for MAR should have a minimum of 5 m thickness of the unsaturated zone (EPA 2006). Another factor to determine the suitability of the aquifer depth was to prevent excessive rises of groundwater table due to the recharge and allow a sufficient vadose zone for final purification. In this layer, monitoring wells were used to generate the desired depth to the groundwater layer to generate a constrain map that excludes locations where the depth to groundwater was recorded to be less than 5 m. It also excluded any locations with rocky soils and water bodies.

The second stage of the development of the suitability mapping for MAR using reclaimed wastewater involved factor mapping and determining the associated scoring system that is needed to determine the weighted score of each parameter. The factor mapping process, in general, involved two main categories of criterion in Figure 5: hydrological properties and operational and socio-economic considerations. The MCDA method necessitates the transformation of the evaluation criteria to a unified scale that can be compared. In this case, a local form of a value scale was developed to take into consideration the various parameters. The factor criteria were standardized using an index ranging from 1 (minimum suitability) to 5 (maximum suitability), as presented in Table 2.

Estimating the recharge rate or the hydraulic load of the recharge system is dependent on the hydrological properties of the aquifer, which mainly involves data on the aquifer type, soil texture and hydraulic conductivity.

Generally, in the West Delta Area, the Nile aquifer system is considered the main aquifer in the Nile Delta. The Nile Delta aquifer is a semi-confined aquifer due to the upper clay layer. Also, it is considered a shallow aquifer that is recharged mainly by the infiltration of excess irrigation water or from the irrigation network. It is composed of a thick sand and gravel layer covered by a clay cap of varying depth up to 50 meters. It provides about 85% of total groundwater abstractions in Egypt. About 6.1 BCM/year is annually extracted from the aquifer (MWRI-Egypt 2013). Accordingly, an aquifer-type map was generated based on the state of the hydrogeological entity in the literature. In this thematic layer, a scale of standardization was assigned to the quaternary and late tertiary layers of the Nile aquifer system that occupies the West Delta flood plains and desert fringes.

For the soil texture, the large size particles of soil were considered most suitable for the recharge process as it facilitates surface infiltration, noting that the rate of infiltration increases with the particle size (Bagheri Bodaghabadi et al. 2015). Accordingly, the rank ‘unsuitable’ was given to soils with no information on topsoil as well as for very fine soil textures.

For the type of hydraulic entity, the standardization of this particular criterion was directly linked to the productivity level of the hydrological layer (aquifer) based on the calculated hydraulic conductivity calculation. Table 3 summarizes the standardization index that was assigned for each hydrological criterion in this study.

The operational assessment was undertaken considering the integration of two main criteria. The first was related to the distance to the wastewater treatment plants (WWTPs) (Table 4), which followed the guidelines of the U.S. Environmental Protection Agency concerning the acceptable transport length that should not exceed 8 km; otherwise, the transport costs for the recharged water become very expensive and can affect the overall costs of operations for the MAR site to become inefficient (EPA 2006).

Weights were assigned to each criterion in relation to their relative importance according to the set of criteria. The AHP method that was used to assign the weights followed the ‘PCM’ developed by Satty in 1980 (Chen 2006). The determination of the PCM weighted factors was completed through INOWAS (Sallwey et al. 2019), a newly developed modeling platform. A INOWAS modeling platform is a Linux open-source empirical, analytical and numerical web-based modeling tool developed in 2019 to facilitate the development and optimization of MAR applications. The full code of the platform and tools can be accessed through the following link: https://inowas.com/about/.

Each criterion was compared to others in a grade from 0 to 8 reflected as follows: 0: equally important | 3: slightly more important | 5: much more important | 7: far more important | 8: extremely more important. The hydrological parameters were considered ‘slightly more important’ than the socio-economic, considering that they are crucial in the storing process from a technical point of view. Each score was later normalized and converted into relative weights as indicated in the matrix as shown in Table 5.

The map identified potential zones for the recharge of reclaimed water using spreading method techniques. According to the results, the studied area included four classifications namely ranging from low suitability to high suitability.

The application of the analysis revealed that higher potential areas for MAR implementation projects using spreading methods are located in the northeast part of West Delta, notably along the Nile River valley's left bank, and spread significantly across the valley to the south. This is influenced by the effect of the constrain mapping technique and particularly the slop criterion that revealed the presence of flatter terrain in the north and center of the study area, allowing a more suitable implementation of infiltration basins in comparison to the south. Moreover, although the soil texture indicated high suitability in the west and east parts, the integration of the WWTP index in conjunction with the aquifer types resulted in a change in the findings by shifting the MAR suitability index toward the north.

By examining the percentage of each suitability scale and linking it to hydrological and future socio-economic parameters, it is possible to specify which element may be changed to increase the likelihood of successful MAR projects and also to understand current limitations and inherent trade-offs. For example, despite the presence of viable treatment plants in the north that facilitates securing the recharged reclaimed water at low cost, low suitable areas were mainly located closer to the north Mediterranean coasts and the southern part, reflecting the slope constraints for steep terrains and the presence of Wadi deposits and volcanic formations. Areas of low suitability were also located around the west-north parts where the hydrological criterion seems to hinder the implementation of the MAR projects due to the low productivity of the hydrogeologic layer located in this particular area and considering the influence of the WWTP buffer that also increased the cost of operations.

In light of the above, the applied methodology proved to provide an adequate solution procedure to deal with the complexity of MAR suitability at a low cost in comparison to traditional field investigation that can be costly and time-consuming and may result in negative outcomes if initial suitability assessments were not conducted and were only based on hydrogeological considerations.

Furthermore, the obtained results demonstrated the method's applicability in assisting water practitioners in making better-informed decisions and modifying current plans for MAR implementation projects using spreading methods to better strategies groundwater resource management in the country by maintaining the aquifer's sustainability and improving groundwater quality in the West Delta area.

The validation of results in this study was accomplished through two processes. The first involved the validation of used soil-type datasets through field sampling in various locations throughout the study area, followed by the second validation process, which involved validating the suitability mapping ranking results against MAR testing results obtained from existing MAR sites within the study area, namely El-Bustan and Abu-Rawash.

Except for a small number of samples that did not exceed 10% across the whole study area, the field sampling results proved to be consistent with the soil type dataset used in the development of the suitability map, which indicates a high level of confidence in the obtained map.

Although the applied methodology proved to be a viable option for obtaining a good understanding of the regional conditions in West Delta and was verified according to local information, it should not be utilized to reflect site-specific conditions. Hence, it can be utilized as a useful tool for determining the best locations for future ‘site-specific’ field investigations. Furthermore, additional local-specific socio-economic assessments and considerations such as land use and crop types could be taken into account to precisely calculate future crop intensity and future crop water requirements on the local level in high-suitability sites (Kaini et al. 2020).

The feasibility of adopting MAR-suitable sites using reclaimed water was investigated in this paper in Egypt's West Delta area using multi-criteria decision analysis combined with the GIS and remote sensing techniques. While prior research in Egypt concentrated on the selection of MAR techniques or suitability areas, this article focused on the creation of a methodology that incorporates both variables, focusing on MAR spreading method techniques to benefit from the soil aquifer treatment (SAT) effect while enhancing social acceptance.

In this study, environmental, socio-economic and hydrological criteria were utilized in the development of six thematic maps to generate a MAR suitability map as a tool for decision-making. The application of the analysis resulted in the development of MAR suitability areas in the northeast part that seemed to be optimum for reclaimed water infiltration using spreading method techniques. The obtained results proved to be a good match with the field results that were obtained from existing MAR sites.

The developed methodology reflected the importance of specific determining factors (i.e., slope and depth to the water table) that govern the successful implementation of infiltration basins and maximizes the benefits from SAT effects when taken in conjunction with other hydrogeological and socio-economic variables. It will therefore assist decision-makers in improving future crop mapping by connecting current accessible resources with potential new non-conventional resources while taking future population projections and food security demands into account. With the obtained results, more environmental and beneficial use of reclaimed water is proposed in the context of balancing water demand and supply in the West Delta Region.

Further development of the methodology presented in this paper would include adapting other MAR techniques and conducting a sensitivity analysis to better advise on potential trade-offs toward reducing over-exploitation and improving groundwater resource management in the region. Additionally, a simulation-optimization model on the local level should be built to complement the above findings and calculate the optimal recharging rates in the selected sites based on the treatment quality of the recharged effluent and according to national environmental limits.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.