Delineation of groundwater recharge zones in the Mitidja plain, north Algeria, using multi-criteria analysis

Groundwater is a vital natural resource; it is considered as a major factor in the development of urban, industrial and rural regions with more and more increasing demand. Groundwater presents about 34% of the whole yearly fresh water source worldwide (Boufekane et al. 2019). In order to identify possible new aquifer areas, the combination of multi-criteria decision analysis (MCDA) and geographical information system (GIS) techniques were used (Famiglietti 2014). The groundwater resources have the highest priority of preservation. The lithological properties of the aquifer formation and its porosity have great importance in determining the presence and estimating the amount of groundwater.

In recent years, water demand has intensively increased, mainly in the arid and semi-arid regions of the world, especially, in the Middle East and North Africa, causing the deficit of these important natural resources. The decreasing water level (overexploitation) due to human activity (industrial and agricultural activities, urbanization) and climate change increase the demand on groundwater resources (Tiwari et al. 2017). This critical situation is observed in the plain of Mitidja (north Algeria), which has experienced a reduction in natural recharge that has led to a significant drop in the piezometric levels reaching 40 m in some areas (Demmak 2008). This reduction is mainly due to the reduction of rainfall by about 20% since 1975 (Meddi & Hubert 2003).

Thus, in order to remedy this overexploitation and to control the groundwater pollution problem, it is necessary to pump groundwater in specific areas. Therefore, determining potential groundwater zones has become very significant in several regions of the world.

Recently, several models have been suggested to situate potential groundwater recharge zones, such as the artificial neural network (ANN) (Sokeng et al. 2016), frequency ratio (FR) (Jothibasu & Anbazhagan 2017), logistic regression (LR) (Ozdemir 2011), random forest (RF) (Zabihi et al. 2016), weights of evidence (WoE) (Tahmassebipoor et al. 2016), boosted regression tree (BRT) (Mousavi et al. 2017), support vector machine (SVM) (Lee et al. 2018), evidential belief function (EBF) (Pourghasemi & Beheshtirad 2015) and groundwater recharge mapping (GRM), which we develop and apply in this study.

Potential groundwater recharge mapping is useful to assist water resource managers to better plan water exploitation and management (Saidi et al. 2017). This time gaining method allows establishment of thematic maps such as geology, hydrology, geomorphology, lineaments and slope (Shaban et al. 2006). Nowadays, computer technology (programming or software) is used in hydrogeology studies. For this study, we used a combination between MCDA approach and GIS technique to delineate groundwater potential zones. During the last years, several studies have been carried out to assess groundwater potential mapping in different regions of the world with satisfactory results as shown by field surveys (Magesh et al. 2012); for example, Saidi et al. (2017) in Tunisia, Shaban et al. (2006) in Lebanon, Yeh et al. (2016) in Taiwan, Fashae et al. (2014) in Nigeria, Abdalla (2012) in Egypt, Mohebzadeh et al. (2012) in Iran, Şener et al. (2018) in Turkey, Mahmoud et al. (2014) in Saudi Arabia, Mandal et al. (2016) in India and Ait El Mekki & Laftouhi (2016) in Morocco.

The main purpose of this research is to provide a simple methodology to delineate the groundwater potential recharge zones in the Mitidja plain using a multi-criteria analysis that included eight hydrogeological parameters: lithology, drainage, rainfall, soil, slope, land use, permeability and water depth combined using remote sensing and GIS systems. Remote sensing is used to examine drainage, slope and land while the GIS is used to manage data in different thematic layers (lithology, rainfall, soil, permeability and water depth) which are taken into consideration for assessing groundwater potential zones in the Mitidja plain.

The results of this research may serve water management studies by helping water managers to choose the best potential zones.

The Mitidja plain covers an area of 1,450 km², is located in the northern part of Algeria and in the northeast is open to the Mediterranean Sea. It is situated between latitude 36° 22′ 30″ and 36° 48′ 58″ north and longitude 2° 32′ 07″ and 3° 28′ 14″ east (Figure 1). It has an extent of 100 km from east to west and between 8 and 18 km from north to south. This plain is divided into two parts: eastern Mitidja and western Mitidja. The relief of the study area varies between 0 m and 600 m above mean sea level. The medium rise of the region is 50 m which increases in a regular way to the piedmont of the Blidean Atlas to the south, and the ripples of the Sahel to the north, allowing the plain to have a basin form. Administratively, it spans four towns: Algiers, Blida, Boumerdes and Tipaza.

A Mediterranean-type climate dominates in the Mitidja plain. It is characterized by hot and dry summers and rainy winters. The average temperature varies between 6 °C in winter and 33 °C in summer, while the mean annual rainfall ranges from 500 to 700 mm.

Its topography and the favourable climate conditions make it an excellent agricultural area (Imache et al. 2007). The industry is developed and based in the industrial areas located in the central and eastern regions (Blida, Arabaa, Boufarik, El Harrach, Rouiba-Reghaia).

The hydrology of the study area is characterized by an intense hydrographic density with four important rivers: wadi Reghaia, wadi Hamiz, wadi El Harrach in the eastern Mitidja and wadi Mazafran in the western Mitidja. The tributaries of these rivers are derived from the Blidean Atlas with a significant flow in a south–north direction.

This coastal aquifer is formed by subsidence and sedimentation. In 1969, geophysical studies showed the presence of two superimposed aquifers in the plain. In order to understand the lithology variation a hydrogeological cross section was established. The AA’ section with a NNW–SSE direction correlates six drilling wells (Figure 2). This correlation shows the presence of: (i) the Pliocene (Astian) aquifer is confined, formed by sandstone and sandy limestones (Sbargoud et al. 2017). Its substratum is trained by the blue marls of the Plaisancien and its surface is developed by a semi-permeable yellow marl layer named marls of El Harrach. It is located between a depth of 250–300 m, from the ground surface; (ii) the quaternary alluvial aquifer is composed of sand, gravel and rollers alternating with silts and clays. In the western part (apart from the Mazafran zone), this aquifer is unconfined and founded on the yellow marls of El Harrach. Its thickness oscillates between 100 and 200 m. The piezometric level is encountered between 5 and 35 m.

Based on bibliographic data, piezometric measurements and pumping tests, the direction of groundwater flow is from south to north (Freeze & Cherry 1979; Toubal 1989; Ait-Ouali 2007; Zamiche et al. 2018). In general, the groundwater depth ranges from 5 to more than 130 m and the transmissivity varies from 1.5 × 10⁻² m²/s to 2.1 × 10⁻² m²/s. Rainfall is an important source of the aquifer recharge and this is in addition to underground supply that is sourced from the Blidean Atlas. In fact, the aquifer is exploited by more than 250 boreholes and wells.

In the present study, the groundwater prospective zone mapping and the thematic maps are prepared from several sources including (i) satellite images, two scenes of Landsat 8, Operational Land Imager (OLI/TIRS C1 level-1) were acquired from Landsat's official website (hearthexplorer.usgs.gov), the first scene covering the cities of Algiers, eastern part of Tipaza and western part of Boumerdes, dated 7 February 2017 and the second scene covering the city of Blida, eastern part of Ain Defla and northern part of Medea, dated 7 February 2017; (ii) the SRTM image DEM at 90 m spatial resolution of the study area which was downloaded from CGIAR-CSI GeoPortal (srtm.csi.cgiar.org); (iii) the field data and secondary data were collected from different government agencies: the National Agency of Hydrous Resources (ANRH) and the National Institute of Land, Irrigation and Drainage (INSID).

The contribution of these sources for each parameter is presented in the section ‘Thematic maps’ generation’. Furthermore, in order to make the various study zones' thematic maps and the final map for the estimation of the probable aquifer recharge zones, ArcGIS software (Version 10.5) was utilized.

Eight hydrogeological parameters influencing the aquifers' recharge have been considered: lithology (L), drainage (D), rainfall (R), soil (S), slope (Sp), land use (Lu), permeability (P) and water depth (W).

This approach is based on a multi-criteria decision-making technique that was originally developed by Saaty (1987).

To delineate the groundwater potential zones' map of the study area, the eight thematic layers' maps were reclassified and overlaid with weighted overlay in ArcGIS 10.5 software. The weight of each parameter was calculated on the basis of normalized weight using pair-wise correlation matrix according to Igwe et al. (2020). The PRindex weights are assigned from 1 to 5 according to the importance of the thematic layers. The normalized and assigned weights are presented in Table 1.

A rating scale was determined based on the range of variation of the contribution factors to determine homogenous ratings for all the parameters (Ait El Mekki & Laftouhi 2016). The rating of each parameter is assigned on a scale from 1 to 10 (Table 2). Moreover, the factors were classified according to this scale to obtain independent ratings for each factor (Table 3).

Several processing parameters are considered in this work:

• The lithology (L): the lithology of geological formations is the important parameter for the delineation of potential groundwater recharge zones. However, the hydraulic conductivity of these geological formations directly influences the infiltration. In addition, the aquifer recharge was influenced by lithology and that is done by ruling the water flow purification (El-Baz & Himida 1995).

• The drainage (D): the drainage density is considered as one of the important parameters to the aquifer recharge evaluation as it is attached directly to the runoff and infiltration (Boughariou et al. 2014). It is the overall length of the whole rivers existing in a discharge basin split by the whole zone of the discharge basin. The thematic analysis of a drainage network aids value to the features of a groundwater recharge area.

• The rainfall (R): rainfall is the most important influencing parameter in the groundwater potentiality of any zone because the groundwater recharge increases with the water quantity of precipitation (Ibrahim-Bathis & Ahmed 2016). The zones with low precipitation were assigned a low rating value while the highest rating value was appointed to zones receiving the highest precipitation.

• The soil (S): the type of soil is an important factor for delineating the potential groundwater recharge zones via the study of pedological data (soil resource inventory, soil map, soil profiles). Fine-grained soils limit infiltration due to the low permeability, conversely, coarse-grained soil materials favour infiltration (Fashae et al. 2014).

• The slope (Sp): the slope is the parameter that affects runoff and infiltration. A steeper slope leads to less infiltration but increases the runoff and the rate of erosion (sediment transport). On the other side, in the low slope area, the surface runoff is low allowing more time for infiltration of rainwater.

• The land use (Lu): the land use of a region performs an important role in runoff, infiltration and recharge of aquifers. Nowadays, the high spatial resolution of satellite images indicates very small details in urban or farmland zones (Jin & Davis 2005).

• The permeability (P): the permeability presents the ability of the aquifer to transmit water, enabling the rate at which groundwater flow is controlled under a specified hydraulic gradient (Sinan & Razack 2009). In fact, a higher permeability favours increased groundwater yields by infiltration.

• The water depth (W): the depth to the groundwater table depends on geological and hydrological characteristics of the area. In general, shallow aquifers display a high groundwater potentiality. However, the deeper aquifers present less groundwater potential.

• • The lithology (L): the classification of this area was based on 180 drilling logs for each well, and on the Mitidja geological report and synthesis map provided by the National Agency of Hydrous Resources (ANRH). It was classified into five categories:

• • - Very permeable: gravel and sand

  • - Permeable: conglomerate, sand, gravel and silt

  • - Low permeable: sandstone and sandy limestone

  • - Semi-permeable: yellow marl, gravel, silt and clay

  • - Impermeable: blue marls.

Figure 3 shows the distribution of lithology in the study area.

• The drainage (D): the drainage of the study area was checked with satellite images and calculated after digitization of all drainage existing in the study area. The high drainage density area indicates low-infiltration rate whereas the low-density areas are favourable to high-infiltration rate. The values for the drainage density ranged from 0 to 2.2 km/km² and were grouped into five classes (Figure 4).

• The rainfall (R): observation data from ten regional rainfall stations (ANRH stations) over a period of 33 years (1971–2004) indicated an annual average rainfall of about 616 mm. The average for the station with the lowest rainfall (Hadjout) was 502 mm/year and that for the station with the highest rainfall (Attatba) was 812 mm/year. To prepare a rainfall map of the study area, inverse distance-weighted (IDW) interpolation method of ArcGIS Spatial Analyst was applied using the annual rainfall. The resulting map was divided into five rainfall classes (Figure 5).

• The soil (S): the soil parameter was obtained by digitizing the existing soil maps (1:50,000) of ANRH, which cover the entire region, established by Ecrement & Seghir (1971). The analysis highlighted five soil types (Figure 6): sandy gravelly, sandy, sandy loamy, loamy and clay loam.

• The slope (Sp): the slope map of the study area was prepared based on the SRTM 90 m data following standard GIS routines using ArcGIS software. The Mitidja plain was divided into five slope classes. For this study, it has been proposed that the better class indicates the areas having 0–2% slope (nearly flat) which give a high infiltration rate. Figure 7 illustrates the slope map of the study area with classes ranging from 0 to 12%.

• The land use (Lu): the land use of the study area is assessed in accordance with the map established by the National Institute of Land, Irrigation and Drainage (INSID). This map (Figure 8) focuses on the definition of five areas containing: water bodies, irrigated areas, forest rocky outcrops and bare soil, and urban areas. Around 45% of the total zone is under cultivation and 40% is an urban area.

• The permeability (P): based on the literature (Castany 1982), the permeability classification is based on the hydraulic conductivity. In this study, the permeability values are obtained from pumping test results of 81 wells (Toubal 1989). These values were interpolated using ordinary kriging method via the Surfer software (Version 13.4) to plot the permeability map (Figure 9). It was classified into five classes with values ranging from 0.5 to 2.5 × 10⁻⁴ m/s.

• The water depth (W): the groundwater depth data were collected from 43 observation wells provided by ANRH in May 2017. These data were exploited to establish the groundwater level contour map. Generally, in this region, shallow groundwater levels vary between 10 m and 20 m, indicating medium groundwater potentiality (Figure 10).

New effective weighting factors have been obtained by the use of the potential groundwater recharge map which are used for sensitivity analysis. The calculated mean effective weights based on the previously explained formula are shown in Table 3. Obviously, we notice that there are some significant differences compared to the theoretical values proposed by Ait El Mekki & Laftouhi (2016) and Igwe et al. (2020) as the weighting value of parameters changed, because the calculated values using weighting are based on the eight hydrogeological parameters influencing aquifers' recharge. The results in Table 4 show that:

• The lithology is considered as the most important parameter in the potential groundwater recharge zone.

• The lithology, the slope, the land use, the permeability and the water depth parameters' effective contribution weights are respectively, 5.31, 2.30, 2.10, 2.25 and 1.34 instead of the standard weights of, respectively, 5, 2, 2 and 1.

• Unlikely, the drainage, the rainfall and the soil parameters are over-estimated. They show, respectively, the following effective weights of 3.56, 2.58 and 2.30 instead of standard weights of, respectively, 4, 3 and 3.

The potential recharge map (Figure 11) was prepared by compiling and integrating the eight thematic maps for the various individual factors using Equation (1). It is defined as a map estimating the probability that groundwater will occur in a study area (Kim et al. 2018).

According to Ibrahim-Bathis & Ahmed's (2016) work based on the relationship of area statistics, weightage values and their corresponding rankings for potential groundwater recharge zone, the groundwater potential zone of this study area can be divided into three classes: 16–38 (low), 38.1–60 (moderate) and 60.1–73 (high).

About 26% of the total area is included in the low class, 47% belongs to the moderate class and 27% of the study area is located in the high groundwater potential zone with 1,450 km² (Table 5). The low class occupies the areas corresponding to steep slopes, with low rainfall and low permeability soil. The classes of moderate to high capacity of groundwater correspond to relatively flat areas associated with important rainfall and infiltration potentialities. The best groundwater potential zone is located in the alluvium formations.

To valid the potential groundwater recharge map, we used the results of groundwater productivity data (transmissivity) obtained from the pumping test results carried out at 102 boreholes in the study area by Toubal (1989). The transmissivity values range from 1.40 × 10⁻³ to 63.60 × 10⁻³ m²/s. Furthermore, according to the classification proposed by various authors (Lasm 2000; Dibi et al. 2004; Yao et al. 2010): low (T < 10⁻⁵ m²/s), moderate (10⁻⁵ < T < 10⁻⁴ m²/s) and high (T > 10⁻⁴ m²/s), the transmissivity values are classified in the high class. The boreholes' locations are shown in Figure 11.

The comparison of transmissivity values (boreholes) with potential groundwater recharge map (Figure 11 and Table 6) shows that 55 values (boreholes) correspond to high recharge area, 38 values (boreholes) coincide with moderate recharge area and 9 values (boreholes) with low recharge zone.

These validation results demonstrate that around 91% of the boreholes accurately match with the zonation of moderate–high groundwater potential map. The high groundwater potential zones coincide with groundwater productivity areas (west, south, south-west and east central parts of the study area) due to favourable conditions such as alluvial plain, shallow groundwater depth, low slope, flat topography, optimum rainfall, favourable soil porosity condition and permanent wadis. Thus, considering the results of this study, it can be concluded that this model for groundwater potential zone mapping gives very good results in the present research.

The results obtained in this study show that the best groundwater recharge potential area is situated in the southern region (piedmont of the Blidean Atlas) due to the presence of the alluvium formations, the zone being nearly flat and the high rainfall with a high infiltration capacity. Additionally, in this zone, the important flow of wadis (presence of continuous springs) also helps the stream flow recharge towards the groundwater system.

Quantitatively (high potential groundwater recharge), the most favourable zones to recharge are those of the extreme south between Bouinane and Arabaa cities and the southwest one (Chiffa). These zones, with a high recharge capacity, are eligible for recovery practices such as artificial recharge from the hydrographic network, for example, using the water of wadi El Harach in the region of Bouinane, the water of wadi Djemaa (principal affluent of wadi El Harach) in the region of Arabaa and the water of wadi Mazafran in the region of Chiffa.

To control the quality and quantity of water used for artificial recharge in this region, a follow-up was carried out in the upstream part of wadi El Harrach, precisely at the Rocher Pigeons station, for the period 2009–2013.

Table 7 shows that the water quantity from wadi El Harrach, in its upstream part, is largely sufficient to feed the Mitidja aquifer artificially, especially during flood periods. Generally, groundwater is profoundly linked to the presence of nearby water sources where the properties of the wadi linked to the groundwater are the width and depth of the river, slope and the flow velocity (Taormina & Chau 2015).

However, since runoff values and some flood values are high, which could be affected by high sediment transport, we proposed the achievement of infiltration basins in the upstream part of wadi El Harrach.

The analysis results of the superficial water (Table 8) from wadi El Harrach, in its upstream part, show that this water has good quality that can be used as a source for the Mitidja groundwater recharge.

In this study, a mapping approach has been proposed to identify the potential areas of groundwater recharge using remote sensing and GIS techniques in the Mitidja plain, northern Algeria. This approach consists of the delimitation of groundwater potential recharge zones in order to protect the water resources from the point of view of quantity and quality in this region.

Eight hydrogeological parameters, including lithology, drainage, rainfall, soil, slope, land use, permeability and water depth were taken into consideration and evaluated to determine potential recharge zones.

These hydrogeological factors are created and superposed (thematic layers) to develop the best model in order to establish a thematic map of potential recharge. The obtained results show that the final groundwater potential zone map was divided into three classes: high, moderate and low. Also, the results indicate that 47% of the study area has moderate groundwater potential; about 27% of the total area was identified as high potential zone for groundwater recharge corresponding to the southern part of the study area, exactly in the piedmont of the Blidean Atlas, near to wadi El Harach in the region of Bouinane, wadi Djemaa in the region of Arabaa and wadi Mazafran in the region of Chiffa. These areas are recommended to create artificial groundwater recharge structures to store rainwater and surface runoff, especially during flood periods.

The low potential groundwater recharge zones which are considered unsuitable areas for groundwater recharge processes are located especially in the northern and eastern areas, corresponding to a low permeability soil.

This approach showed the best suitable artificial recharging sites in the Mitidja plain, in order to improve the groundwater level and to confront the seawater intrusion problem in the coastal zone situated in the northeastern part of the study area. The final map obtained by this approach can be used by water sector managers in selecting suitable sites for groundwater recharge and even for groundwater resources management, e.g., drilling new boreholes and wells.

The obtained results will especially serve water management works in Mitidja plain as this reservoir is the largest freshwater aquifer in Algeria and forms part of the socio-economic development of Algiers and surrounding cities (Blida, Boumerdes and Tipaza).

This work was carried out as part of the project ERANETMED_WATER-13-166, Groundwater Resilience to Climate Change and High Pressure within an IWRM Approach. We are grateful to Directorate General for Scientific Research and Technology Development staff (DGRSDT-MESRS) for funding, support and help with this project.

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