Introduction

In arid and semi-arid regions, groundwater is the main source for use in irrigation and drinking water supply. The management of groundwater resources is a major challenge facing the populations of such regions, especially when the extracted resources exceed their capacity for renewal (natural recharge). Successive years of drought together with the pressure of population growth and harmful agricultural practices increase the groundwater supply and lead systematically to the decrease of this resource, putting these populations at a high risk to its availability.

In the Gafsa region, west-central Tunisia, groundwaters are exploited for agricultural and domestic purposes. Agricultural activities are particularly developed in the northeastern part of this region (acknowledged as Gafsa North), where groundwaters are mostly of good quality (salinity exceeds rarely 3.5 g/l) (CRDA- Gafsa 2016) and the number of industrial wells is still rising during the last decades, exceeding the planned abstraction rate. In fact, in Gafsa North, the exploitable groundwater resources are estimated to be about 33.1 millions of m3/year (CRDA- Gafsa 2016); however, the abstraction exceeds remarkably the renewing rates; by way of example and evidence, the abstracted groundwaters are evaluated to be about 38 millions of m3 in 2012 (CRDA- Gafsa 2016). In the few following years, the excessive abstraction for irrigation will result in an overexploitation of groundwater resources in this region if no emergency management and policies are planned.

In this study, we use geological and hydrogeological data together with the Schlumberger depth sounding method to elucidate the hydrogeology of the Garaat Sened aquifers (Fig. 1), in the southeastern part of Gafsa North, with emphasis on its hydrodynamics and their main controlling factors.

Fig. 1
figure 1

Simplified geologic map of the study area with locations of hydraulic wells, vertical electrical sounding, and geoelectrical profiles PI and PII of Fig. 7. The inset at the top-left shows the location of the study area in the western Mediterranean area

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Study area

Climatic and hydrological conditions

The region of Gafsa North is located in west-central Tunisia (Fig. 1). It is characterized by semi-arid climate with an irregular rainfall ranging between 124 and 222 mm/y. The study area is located in the southeastern part of the Gafsa North region and extends approximately between the longitudes of 9°04′E and 9°21′E and the latitudes of 34°28′N and 34°37′N. In this area, the aquifer system is encased in the plain located northward Gafsa city, between the highlands of Jebel Orbata to the southeast and Jebel El Goussa to the north (Fig. 1) and extending from Zannouch plain to the west to Kroumet El Gara to the east made of Cenomanian dolomites. The plains of Zannouch and Sened are separated by Ragoubet Meyal hill culminating to ~ 533 m and considered as a topographical water divide. The Sened plain matches with the restricted watershed of Garaat Sened enclosing a surface drainage system made of some intermittent and ephemeral streamflows. The main streams are Wadi Drag and Wadi Rwibta and their tributaries that drain this watershed from south to north following the general topographic slope. To the northeast, the Wadi Nadour represents the outlet of this watershed. It drains the plain between Kroumet El Gara and Jebel Majoura and extends eastward to join the Wadi Leben.

Geological setting

The lithostratigraphic units outcropping within the study area are Lower Cretaceous (Hauterivian) to Quaternary in age. Oldest series outcrops along the folded structures of Orbata to the southeast; however, recent deposits occupy the lowlands and alluvial plains (Fig. 1). A synthetic lithostratigraphic column (Fig. 2) has been drawn on the basis of outcropping series and data from hydraulic wells. The Mio-Plio-Quaternary (MPQ) series acknowledged as Segui Formation are made of a heterogenic sequence of clays, silts, and conglomerates with gypsum. This Formation rests on a sandy sequence Miocene in age (Beglia Formation), which overlays unconformably the Maastrichtian limestones of Berda Formation (also called Abiod Formation) (Burollet 1956; Khessibi 1985). This latter is Campanian-Maastrichtian in age and it is made of two limestones members separated by a marly sequence. The Coniacian-Santonian series (Aleg Formation) comprise an alternation of light-green marls and clays with limestones and gypsum levels. Below, the Cenomanian-Turonian series (Gattar member) encompass a sequence of dolomitic limestones topped by a hardground surface and form a hard carbonates bar culminating along the Gafsa chain. This bar is also considered as the upper member of the Zebbag Formation (Burollet et al. 1952; Burollet 1956; Razgallah et al. 1994). The latter is Upper Albian-Cenomanian in age and it is complete in Jebel Orbata. It includes three distinctive members: a lower dolomitic member (Lower Zebbag) Vraconian in age; a middle member (Middle Zebbag) Cenomanian in age, which is made of anhydrites, gypsum, and dolomites; and the upper member described above as Gattar member. The Late Hauterivian to Late Aptian series form the Gafsa Group including three transgressive formations: the Orbata Formation Aptian in age and dominantly carbonated with intercalations of clays and gypsum, the sandy Sidi Aïch Formation (Late Barremian), and the Bou Hedma Formation (Late Hauterivian-Early Barremian) made of an heterogonous sequence of limestones, dolomites, clays, evaporitic rocks, and sandstones (Burollet 1956; M’Rabet 1987; M’rabet et al. 1995). The Valanginian-Early Hauterivian series form the Sened Group including two deltaic regressive formations (Meloussi and Boudinar) made of clays, sands, and carbonates (Fig.2).

Fig. 2
figure 2

Synthetic lithostratigraphic column of the study area

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The study area makes part of the central Atlasic domain of Tunisia and encloses some faults and folded structures (Fig.1). These folded structures are related to the Cenozoic compression of the Atlasic domain (Zargouni 1985; Boukadi et al. 1998; Ben Ayed 1986; Bédir 1995; Hlaiem 1998; Dhahri and Boukadi 2010; Amri Tanfous et al. 2017). However, faults occurred mostly during the Mesozoic as extensional structures, and then, they have been inverted during the Cenozoic as thrusts and strike-slip faults (Amri Tanfous et al. 2017; Masrouhi et al. 2019). To the south, three main south-verging anticlinal structures cored with Lower Cretaceous-Jurassic rocks make a long chain of ~ 80 km, extending from the Gafsa fault to the west and to the N-S axis to the east. These structures are respectively from west to east; the E-W oriented structure of Orbata, the NE-SW oriented structures of Tibouna-Darine, and the E-W structure of Bouhedma (Fig. 1). To the north of Garaat Sened watershed, Jebel Goussa constitutes the periclinal closure of the anticlinal structure of Majoura-Meloussi, which is a main Atlasic NE-SW oriented structure. The main faults affecting the study area are the E-W to NE-SW fault affecting the southern flanks of Orbata-Tibouna-Darine structures, the NW-SE faults of Gafsa and Mech and the N-S faults of Sidi Ali Ben Aoun to the northwest of the study area (Bédir 1995). The Mech fault passes between Bouhedma and Darine structures and affects also Jebel Goussa. It seems extending northwestward near Jebel Souinia. It is well underlined along carbonated and hard outcrops near anticlinal structures; however, it is not expressed alongside the land filled with MPQ soft sediments as much as in the Garaat Sened. This accident acted mainly as a dextral strike slip fault as much as the NW-SE fault of Gafsa and it exhibits also a reverse offset responsible for the vertical slip of the Abiod layers near the well 19333 (Fig. 1).

During our fieldwork, we highlighted another fault that is bounding the western side of the dolomitic outcrops of Kroumet El Gara and the hill eastern Wadi Rwibta. This fault is underlined by a subvertical fault plane NNW-SSE oriented, which exhibits vertical striation indicative of normal slip responsible for the subsidence of the western block (Fig. 3 a,b).

Fig. 3
figure 3

Field photos showing some hydrogeological characteristics of the study area (explanation within photos and in the main text)

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Hydrogeological setting

From hydrogeological point of view, the study area encloses several hydrogeological domains; highlands (Orbata-Bouhedma chain, Majoura-Melloussi mountains, and some other moderate hills made of carbonated outcrops) are mostly hard rock domains and some alluvial valleys, and intermediate basins domains are developed inland of the core of large-scale anticlinal structures (such as Orbata-Bouhedma, Goussa, and Majoura); alluvial plain domains occupy some lowlands restricted between surrounding high ground (such Garaat Sened); in addition, some regional sedimentary basin domains occupy the large-scale synclinal depressions of Central Tunisia farther northward.

Garaat Sened plain encompasses two aquifer systems (Farhat and Moumni 1989): an upper shallow to deep aquifer system encased within several sandy levels of the MPQ deposits considered as a single aquifer since there is no intercalation of impermeable level within and occupying an area of ~ 125 km2 and a deep hard rock aquifer encased within two distinctive Upper Cretaceous carbonated formations: the Campanian-Maastrichtian white limestones of Abiod Formation separated from the MPQ aquifer by impermeable 35- to 40-m thick strata of red silty clays, which are reduced to about 10 to 15 m near the high grounds bordering Garaat Sened, and the Turonian-Coniacian dolomites of Gattar member. The MPQ constitutes an unconfined aquifer because its permeable detrital material extends to the land surface (Fig.3f). Its recharge is ensured naturally by precipitation and streams, which drain a large area of high permeable terrains and exhibit remarkably nearly flat surface (Figs. 3g,h and 4), which decreases the speed of drainage and promotes infiltration. Nevertheless, deep percolation of water from irrigated areas can also improve the recharge of the MPQ aquifer. In a hydrogeological system, recharge can be ensured by the sum of several processes depending on the climate, soil surface, and landscape conditions. In the studied region, the climatic conditions are random, the agricultural activities are anarchic, and irrigated areas vary randomly every year. In these conditions, the whole recharge mechanism is complex (Fig. 4) and the quantification of the amount of recharge is really a serious challenge that provides many years of observations. However, the discharge of this aquifer is done by means of several shallow wells used for irrigation and drinking water supply due to their good quality. Chemical analyses of water samples from these wells showed that the dry residue ranges from 2 (near the recharge zones) to 3.5 g/l (in the central part of Garaat Sened).

Fig. 4
figure 4

Two 3D blocks illustrating the whole recharge mechanism in the study area. The bottom inset shows the locations and the observed sides of the blocks

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The deep hard rock aquifers yield relatively small water supply; nevertheless, they are encased in intensively fractured carbonated formations. The exposed fissures and fractures of both Abiod and Gattar carbonates (Fig. 3e) are privileged area for recharge to groundwater; the fracture network ensures direct infiltration during rainfall over hard high ground; in addition, the passages that streams originating from high ground have cut through these fractured hard rocks participate also in the aquifer recharge since these rocks are at the surface or overlain by thin alluvial formations (Figs. 3c, 4). The lower carbonated member of Abiod Formation is made of polygenic conglomeratic limestone with good apparent matrix and fissure porosity (Fig.3e). However, the upper member is made of fractured bedded limestones. The Gattar member that outcrops to the east of Sened plain shows two distinctive units: a lower massive carbonate unit and an upper conglomeratic dolomitic unit with very high porosity due to fractures and karstification (Fig. 3d). This unit exhibits good reservoir qualities and is subsequently an interesting target for groundwater exploration in this region.

The topographical surface water divide observed to the west of Garaat Sened around Ragoubet Meyal hill is also confirmed in subsurface for the Campanian-Maastrichtian aquifer system by means of the piezometric map of Fig. 5. This map shows also that the direction of groundwater flow is from south to north as described by Farhat and Moumni (1989). The chemical analyses of water samples obtained from available wells drilled through this aquifer showed that pumped waters are of good quality with a salinity ranging from 0.4 (upstream the watershed) to 1.9 g/l (downstream) (Fig. 6).

Fig. 5
figure 5

Piezometric map of Garaat Sened area established based on the available hydraulic wells

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Fig. 6
figure 6

Salinity map of Garaat Sened area (actual setting)

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Data and methodology

Multiple sets of data have been used in this survey; geological and hydrogeological data are obtained from field works, hydraulic wells, and previous works. Chemical data are obtained after analyzing 14 groundwater samples collected from available wells. Geoelectrical data were obtained by vertical electrical sounding (VES) using the Schlumberger depth sounding method (Keller and Frischknecht 1966; Van and Cook 1966; Astier 1971; Koefoed 1979; Parasnis 1997).

The VES method is characterized by simple instrumentation, easy field operations, and economical analysis of data, and therefore, it has been largely used for investigating groundwater (Sikandar et al. 2010; Andrieu and Ricolvi 1972; Borgi and Ben Baccar 1980; Sehli 1984; Ekine and Osobonye 1996; Ebraheem et al. 1997; Gasmi et al. 2001;Gasmi 2002; Mhamdi et al. 2004; Mhamdi et al. 2005a, b, c, d; Gasmi et al. 2005, Mhamdi et al. 2006; Gouasmia 2008; Mhamdi, 2010; Mhamdi et al. 2011; Mhamdi et al. 2015).

Using this method, an electrical current is artificially transmitted into the ground using two grounded electrodes (A and B) while the difference of potential produced by the circulation of this current into the geological layers is measured with two potential electrodes (M and N). The current penetration depth is proportional to the length of the AB current electrode spacing; for Schlumberger soundings, the greater the current, the greater the depth of investigation. The nature and geometry of the underground layers are subsequently deduced on the basis of the variations in the calculated resistivity (Loke and Barker 1995; Dahlin 2001). In fact, VES data allow to draw sounding curves in which apparent resistivity is expressed as a function of electrode spacing AB/2. When VES are plotted in a longitudinal profile, lateral variations in resistivity are best detected and then can image the ground.

In this survey, 14 VES have been carried out in the study area using a SYSCAL R2 resistivity meter from IRIS Instruments (Orleans, France). VES were done with a current electrode spacing (AB) of 800 to 2000 m in order to reach a depth of investigation of 200 to 500 m (Astier 1971; Koefoed 1979; Parasnis 1997). Lithological data from four hydraulic wells referred as 19333, 20473, 19721, and 20670 in Fig. 1 were used to calibrate measured values of true resistivity with corresponding lithological units and to establish a geoelectrical model for the studied aquifers.

Results and discussions

VES calibration and processing

The processing and the interpretation of the VES curves are usually used to determine the depth of investigation and the distribution of the resistivity in the subsurface. Since the resistivity interpretations are limited by the principles of “equivalence” and “suppression” (Kunetez 1966), rigorous geophysical and geological controls are required to evaluate the quality of such interpretations and to discriminate between valid alternative resistivity interpretations (Mhamdi et al. 2015).

In this step, measured values of true resistivity have been correlated with their corresponding lithological units after a series of calibration tests using standard VES data with theoretical curve corresponding to the studied lithological units (Fig. 7). The following geoelectrical model was established:

  • Plio-Quaternary deposits are characterized by variable values of true resistivity since they are composed of heterogeneous detrital sediments as much as sands, clays, silts, and conglomerates with occasional gypsum intercalations.

  • Mio-Pliocene deposits are mainly made of conglomerates, sands, and clays and are characterized by true resistivity values ranging from 20 to 30 Ohm m.

  • Campanian-Maastrichtian limestones of Berda Formation have resistivity values ranging from 70 to 100 Ohm m.

  • The Coniacian-Santonian alternation of marls and clays with limestones and gypsum levels (Aleg Formation) is considered a conductive level with true resistivity values ranging from 8 to 10 Ohm m.

  • The Cenomanian-Turonian dolomitic limestones exhibit generally true resistivity values ranging from 88 to 120 Ohm m.

Fig. 7
figure 7

Calibration of true resistivity values and lithology. Apparent resistivity data are marked by black point symbol. Solid black curve represents the apparent resistivity curve. Solid red block line represents the modeled resistivity (synthetic resistivity). The current electrode spacing (AB/2) in meters is represented in the horizontal axis, and the resistivity in ohm meter is in the vertical axis

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Interpretation of geoelectrical profiles

The quantitative interpretation of VES curves for a horizontally stratified structure ascertains the number of lithological layers and their relative resistivities. The obtained result is deduced intuitively from the shape of the VES curve based on the number of slopes and their tilt angle. However, considering the principles of “equivalence” and “suppression” (Kunetez 1966), this result is still in doubt.

In this study, we performed two geoelectrical profiles (Fig. 8) based on rigorous calibrations based on wells data, electrical logs, and lithostratigraphical data to avoid possible confusions that can emerge from inherent problems of equivalence and suppression.

Fig. 8
figure 8

Geoelectrical profiles (see Fig.1 for location) established based on hydraulic wells and Schlumberger depth sounding method

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The PI profile (Fig.8, Profile I) is N-S oriented. It shows that the limestone aquifer of the Abiod Formation is drilled in shallow depths in Henchir Nadhour to the Nord; however, it deepens toward the center of Garaat Sened since it is not reached at ~ 260 m in 20473 well. In this area, the MPQ aquifer is relatively thick and it is the main exploited target. In 19333 well, the top of Abiod Formation is reached at a depth of ~ 110 m. This is due to the fact that this well is located near the northwestern flank of the folded structure of Darine (Fig. 1). In this well, we have noticed that the Mio-Pliocene levels (gravels, sands, and clays) are encountered twice onto and below the Abiod limestone. This observation is confirmed by dating of some rock samples. In fact, the 19333 well is located near the Mech fault, which seems responsible for this situation; the composite offset of this reverse strike-slip fault lifted up a hanging wall made of the Mio-Pliocene, Abiod, and Turonian-Santonian series that slip over the Mio-Pliocene levels, which explain the situation.

The PII profile (Fig.8, Profile II) is E-W oriented. It shows that the MPQ aquifer is relatively thick in the eastern part of the Sened plain, which is collapsed by the normal fault that underlines the western side of Kroumet El Gara (Fig. 1). The thickness of this aquifer is reduced to the north of Ragoubet Meyal near the VES 9, which is emplaced near the axis of the anticlinal structure of Jebel Goussa. In this area, the Abiod strata exhibit slight divergent dip to make a subsurface water divide, which is also observed within the piezometric map (Fig. 3).

Conclusions

The hydrogeology of the Garaat Sened aquifers, in the southeastern part of Gafsa North, and its hydrodynamics have been studied using geological and hydrogeological data together with the Schlumberger depth sounding method. This area encompasses two types of aquifer systems: (i) an upper shallow to deep aquifer system made of very porous and permeable detrital material that occupies an area of ~ 125 km2 and (ii) two deep fractured Upper Cretaceous carbonated aquifers in age that both exhibit good reservoir qualities with significant apparent matrix and fissure porosity. The hydrogeological conditions of all of these aquifers seem to be favorable for rapid and considerable natural recharge, which is made exclusively by rainfall. However, Garaat Sened area is characterized by semi-arid climate with an irregular rainfall ranging between 124 and 222 mm/y. This limited amount of rainfall and the small number of streams draining the area impacted consequently the capacity for renewal of its groundwaters’ resources.

In fact, in semi-arid Mediterranean regions of North Africa, the absence of high rainfall increases considerably the development of groundwater resources. In addition, the development of harmful agricultural practices and the pressure of population growth together with successive years of drought engender a high risk to groundwater availability in these regions. In such condition, appropriate management and policies should be planned to maintain the balance between demand and renewable resources. This can be solved by adopting rigorous legislation and public education, by improving water conservation measures, and by monitoring hydrological data.