Introduction

Water resources are under serious pressure worldwide, and their efficient management and protection constitute a key issue for the future (Wildemeersch et al. 2010). This plays a vital role in the socioeconomic development in arid and semiarid regions (Tlili-Zrelli et al. 2013). Understanding the local hydrogeological system helps manage and protect groundwater resources (Wildemeersch et al. 2010). The chemical composition of groundwaters depends on the available precipitation, bedrock geology, and various geochemical processes, and their concentrations vary with residence time. Numerous studies were carried out to evaluate the impact of hydrogeochemical processes on water resources as this is a frequent objective of hydrogeological research (Abderamane et al. 2013; Kumar et al. 2013; Thivya et al. 2013; Tlili-Zrelli et al. 2013; Wang et al. 2013; Wildemeersch et al. 2010; Wu et al. 2013; Zhai et al. 2013).

A very suitable natural laboratory to study these processes is the Figuig Basin in Eastern Morocco, where the deterioration of groundwater quantity and quality is significant. This deterioration is evidenced by the decrease in the water output of springs, the increase of water salinity, and the contamination by wastewaters (Jilali 2014a, b; Jilali et al. 2015a). For this reason, geological and hydrogeological research in the Figuig region has been the focus of the studies of Jilali et al. (2015b) and Jilali and Zarhloule (2015). The main objective of the present work is to improve the understanding of hydrogeochemical processes in this region. To achieve this objective, we used stable isotopes (D and 18O), radioactive isotopes (3H and 14C), and major ion chemistry (K+, Na+, Ca2+, Mg2+, HCO3 −, Cl−, SO4 2−, NO3 −). We have applied various graphical representations and maps to classify and interpret the geochemical data we have obtained.

Study area

The Figuig Region study area is located near the Morocco-Algeria border, in the Eastern High Atlas of Morocco, and is bound by several mountains (Jbels Grouz, el Haïmeur, Mélias, Zenaga Tarhla, and Sidi Youssef). The most important wadi (river) is Zouzfana located to the east of the Figuig Oasis. It lies between latitude 32° 4′ N and 32° 9′ N and longitude 1° 20′ and 1° 10′ W, and covers an area of about 60 km2 (Fig. 1). The climate of the region is arid, with average rainfall of about 120 mm/year (Jilali 2014a, b; Jilali and Zarhloule 2015; Jilali et al. 2015c).

Fig. 1
figure 1

Location of the study area

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Geological and hydrogeological background

The geology of the region is formed by Mesozoic and Cenozoic formations. The principal Mesozoic sedimentation occurred in the Triassic and Jurassic (Lias and Dogger). The Cretaceous (InfraCenomanian) is only present on the Algerian side of the Morocco-Algeria border, and the region is covered by Quaternary formations. The stratigraphic sequence consists of (1) red-green clays and basaltic tuffs of Triassic age, (2) Hettengian-Pliensbachian formations of limestones and dolostones, (3) a Toarcian formation composed of alternating limestones and marls, (4) Aalenian limestone formations, (5) a Bajocian formation composed of alternating limestones and marls, followed by a sequence of limestones, and (6) a Quaternary formation composed of alluviums, sand, and travertine (Jilali 2014a; Jilali et al. 2015b; Jilali and Zarhloule 2015). This area is very much fractured and presents two principal fault systems oriented 70° N and 140° N (Fig. 2). A more detailed geological description can be found in Jilali (2014a). Jilali et al. (2015b). and Jilali and Zarhloule (2015).

Fig 2
figure 2

Geological and hydrogeological map of the study area and groundwater samples. J.G Jbel Grouz, J.H Jbel el Haïmeur, Z.S.A Zrigat Sidi Abdelkader, T.B Tiniet el Bida, T Tachroumt, W.B Wadi Bouchalikane, W.L Wadi Lakbir, W.T Wadi Tazoukart, J.M Jbel Mélias, J.Z Jbel Zenaga, J.T Jbel Tarhla, J.S.Y Jbel Sidi Youssef (Jilali 2014; Jilali et al. 2015; Jilali and Zarhloule 2015). modified

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The Figuig Region is a good example of an unconfined multilayer aquifer (Jilali 2014a; Jilali and Zarhloule 2015). It is exploited by wells, boreholes, and springs, essentially in the Quaternary and Dogger layers. The Lias aquifer is exploited on the mountain of el Haïmeur by two boreholes providing table water supply and by another borehole in Tiniet el Bida, where the water is used for irrigation. The water flow has two main directions: from north to south and from north-west to south-east (Fig. 2). The groundwater level is at a depth of between 10 and 20 m in the south and 40–60 m in the west and north (Jilali 2014a, b; Jilali et al. 2015b; Jilali and Zarhloule 2015).

Methodology

Sample collection and analysis

A total of 72 groundwater samples (F: borehole; P: well and S: spring) were collected from the study area (Table 1). The electric conductivity (EC), major ion chemistry (K+, Na+, Ca2+, Mg2+, HCO3 −, Cl−, SO4 2−, NO3 −), 2H, 18O, 3H, 14C, and 13C analyses (vintage 1983 and 2004) were done by the Hydraulic Basin Agency of Moulouya (HBAM) except for the 1995 vintage data, which were done by Assou (1996). The methods of analysis used for the major ions were SKALAR and Rodier (1984). whereas the isotope analysis was performed in the CNESTEN (National Center of Energy Sciences and Nuclear Techniques of Morocco). The precision of the chemical analyses was defined by checking ionic charge balance. The acceptable error on the ion balance is taken at a maximum of 10 %.

Table 1 Electrical conductivity (EC in μS/Cm), concentrations of the main chemical species (in mg/l) and isotopic geochemistry of groundwater samples collected
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Statistical analysis & geochemical modeling

The Principal Component Analysis (PCA) multivariate statistical technique was used in our research. This method is a quantitative and independent approach for the classification of groundwater samples according to their geochemical characteristics and may simplify and organize large data sets in order to make useful groupings of similar samples (Kumar et al. 2013; Tlili-Zrelli et al. 2013; Wu et al. 2013). The STATISTICA software was used to process the 1995 vintage data analyses. For this, the analyses of EC, K+, Na+, Ca2+, Mg2+, HCO3 −, Cl−, SO4 2−, and NO3 − were used as variables. To investigate thermodynamic controls of mineral-water interactions, the geochemical modeling software PHREEQC (Parkhurst and Appelo 1999) was used to calculate the mineral saturation index (SI).

Results and discussion

Groundwater chemistry

According to Table 1, the EC of groundwater ranges from 1530 to 13,100 μS/Cm. It is low in the western and northern part of the study area and high in the southern. Figure 3 and Table 2 show major cations and anions for underlining possible correlations between host rock lithology and the observed hydrogeochemical facies. The majority of groundwater samples as identified using a Piper diagram and the AquaChem software are Na-Ca-Cl-HCO3 (Jurassic carbonates formations) and Na-K-Cl (Quaternary formations) type waters (Table 2). The latter water type is probably related to anthropogenic activities (irrigation and wastewater) taking place in the southern area.

Fig. 3
figure 3

Piper diagram for the major ions for different years

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Table 2 Water type
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Saturation index

The Saturation Indices (SI) of the minerals calcite, dolomite, gypsum, aragonite, and anhydrite were calculated using the PHREEQC code. The results of the modeling are shown in Table 3. Saturation indices less than zero indicate that the groundwater is undersaturated with respect to that particular mineral, those equal to zero that the groundwater is saturated, whereas a Saturation Index superior to zero indicates a supersaturation of the groundwater with respect to that particular mineral phase, and that the water is incapable of dissolving more of the mineral (Kortatsi 2006). The calculated SI values for gypsum, anhydrite, and for most of the dolomite were less than zero (Table 3), indicating the undersaturation of groundwater in these minerals, which is in good agreement with the inferred dissolution of gypsum (Zhai et al. 2013). In contrast, the SI of calcite and aragonite (CaCO3) indicated that the groundwater was supersaturated with these carbonate minerals, a fact corroborated by the presence of calcareous rocks.

Table 3 Saturation indices of Figuig groundwater
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The plots of Cl− and Na+ ions with EC show that the chloride and sodium were strongly correlated with EC with R 2 = 0.973 and 0.868, respectively (Fig. 4). These indicate that groundwater salinities were mainly controlled by these ions. Thus, a plot of chloride versus sodium shows a high correlation coefficient of 0.837 (Fig. 4) indicating that these ions have the same origin, which can be (1) the dissolution of halite present in the sedimentary rocks and (2) the derivation from saline surface deposits.

Fig. 4
figure 4

Plots of: Na vs EC, Cl vs Ec and Cl vs Na

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PCA

The variables for PCA were EC, K+, Na+, Ca2+, Mg2+, HCO3 −, Cl−, SO4 2−, and NO3 − (Table 4). Two independent factors were extracted, which explained 84.93 % of the total variation (TV). The first one (F1) was responsible for 73.91 % of TV and was represented by EC, K+, Na+, Ca2+, Mg2+, Cl−, and NO3 −. The second factor (F2) explained 11.02 % of the TV and was mainly represented by HCO3 − and SO4 2−. The spatial distribution of the variables and individuals in the systems F1–F2 (Figs. 5, 6, and 7) shows the presence of three groups of waters. Group 1 corresponds to the waters of springs originating in the fault hosted by Upper Bajocian limestone formations. Group 2 corresponds to the waters of wells and boreholes situated in the irrigation zone, where agricultural activities take place (palm trees), hosted by quaternary alluvial formations in the south. Finally, group 3, in the north, characterizes the waters of wells located in Lower Bajocian and Liassic beds, consisting of alternating limestones, marls, and other carbonates.

Table 4 Correlation coefficients of physicochemical parameters
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Fig. 5
figure 5

Spatial distribution of the variables in the axes system F1–F2

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Fig. 6
figure 6

Spatial distribution of the individuals in the axes system F1–F2

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Fig. 7
figure 7

Zoom of the spatial distribution of the individuals in the axes system F1–F2

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18O and 2H stable isotopes

The isotopes 2H and 18O are excellent tracers, which can be used to determine the origin of groundwater and are widely used to study the circulation of natural water and groundwater movement (Vasanthavigar et al. 2013). The isotope compositions of well and spring waters that we obtained are plotted in Fig. 8. The WMWL is δD = 8 δ18O + 10 ‰ (Craig 1961). The relationship between δD and δ18O in groundwater samples suggests that the chemical composition was significantly modified by evaporation.

Fig. 8
figure 8

δ18O-δD diagram of groundwater

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The δ18O and δD values are in the range of −9.12 to −8.3 and −62.12 to −54.88 ‰, respectively. The δ18O value decreases from north to south (high to low altitude). The linear correlation between δ18O and δD of 1988 (Assou 1996) is similar to that of 2004. The regional altitude isotopic gradient of the Figuig oasis is estimated to be −0.9 ‰ per 100 m from samples collected in springs and wells, at different altitudes ranging from 856 to 941 m (Fig. 9). The low value of 18O suggests that the paleo-recharge is of Holocene age.

Fig. 9
figure 9

Altitude isotopic gradient of the Figuig oasis

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13C, 14C, and 3H geochemistry

The 13C, 14C, and 3H values measured are in the ranges of −8.07 to −7.4 ‰, 22.31 to 33.76 pCm and 0.3 to 2.9 TU, respectively (Table 1). The different models used to estimate the age of groundwater (Eichinger 1983; Fontes and Garnier 1979; Ingerson and Pearson 1964) yield ages ranging between 2495 and 10,696 years (Holocene), see Table 5. These are in good agreement with the low values of 18O and 3H.

Table 5 Mean residence time of the water based on different models
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Conclusions

Water scarcity and degradation are significant problems in the Figuig oasis. Our research has identified two major types of groundwater chemistry: Na-Ca-Cl-HCO3 and Na-Cl dominant. The EC of groundwater is low in the western and northern part of the study area and high in the southern part. We interpret the origin of salinity to be partly linked to the lithology of the geological formations and, to a lesser extent, to the irrigation/evaporation processes in the palm groves. Multivariate statistical analysis carried out using PCA of major hydrochemical ions has identified two major geochemical processes with 84.93 % of TV. The relationship of 2H and 18O shows that the initial isotopic composition has been modified by the evaporation processes during local water infiltration. Finally, we have been able to constrain the age of the groundwater as ranging from 2495 to 10,696 years. Our findings are important, as they provide the basis for a much better understanding and, therefore, much better management of the vital water resources of the Figuig aquifer.