Contribution of isotopic techniques to the knowledge of the aquiferous system of the great Sebkha of Oran

In the Great Sebkha Basin of Oran, water flows, both surface and subterranean, originate on the slopes and converge toward the discharging area of the salt Lake Great Sebkha of Oran (≈ 80 m above sea level). Isotopic water tracing of 2H, 18O, 14C, and 13C was carried out on the main aquifer formations to clarify the phenomena of evaporation, dissolution, mixing of different water bodies, and their residence time in the aquifer system. The data concern: the salt Lake of the Great Sebkha of Oran, the Quaternary alluvial water table of the periphery, the limestone Miocene, the Brédéah pumping station (whose waters are polluted by brackish water intrusions), deepwater tabler table of the Mio-Pliocene complex of the Plain of Maflak, the Pliocene of the Plain of Tameira, and the Thermal Complex of Hammam Bou Hadjar. The d18O levels range from −9.61 to + 4.8 ‰ V-SMOW at the salt Lake of Great Sebkha of Oran. The 2H contents range from −70 to 20. Minimum values characterize run off from the northern slopes of the Tessala. The 13C contents of dissolved total inorganic carbon (TDI) measured ranged from −10.92 to −3.9 ‰ vs PDB. The activities of the 14C are divided between 2 and 51 PMC. The highest activity in 14C (≈50 PMC) is measured on samples taken from the Miocene limestones of Djebel Murdjadjo.


Introduction
The Great Sebkha of Oran, because of its proximity to the second metropolis of Algeria, is an important concern of the public authorities since the second half of the nineteenth century. Many scientific studies of the environment for its development have thus been carried out. Nevertheless, none of the proposed technical schemes has emerged. This endorheic space has become, over time, an experimental ground of a predilection for researchers working within the framework of the protection of the environment and the valorization of the site.
Geochemical and isotopic tracers have been effectively applied in groundwater examination. Since, systematic monitoring of water quality is essential to better preserve this environment, especially geological and hydrogeological (Hachemi et al. 2020).
Geochemical and isotopic studies were done to characterize the aquifer and to distinguish the factors that control the mineralization (Sacchi et al. 2021;Freidman and O'Neil 1977). They are punctual and relate mainly to the water table. The origin of the waters and the relationships between the different aquifer levels of the system have not been addressed until then.
This study aims to be a contribution, using an overall sampling of surface and underground water Pliocene outcrops forms taken from the main aquifer levels of the drainage Basin, of a certain veil on certain ideas relating to the salinization of the environment carried by the population and providing some clarification of the salinity of the area. In particular, it exposes and discusses the first results of isotopic techniques using natural tracing and groundwater dating.

The aquifer system
The Neogene Basin of the Great Sebkha of Oran is characterized by the stacking of two to three aquifer layers in its central zone. This hydrogeological structure forms an aquifer system in which the slopes of the reliefs of the perimeter constitute the recharge zone and the Lake of the Sebkha, the discharge zone, at least for surface water and underground groundwater (Benziane 2013).
The formations of the substratum are characterized by the circulation of water of lesser importance in the few permeable levels or networks of fractures. The Neogene (Miocene and Pliocene) and Quaternary formations are the most important groundwater reservoirs in the Basin. The miocene The Messinian is composed of seaweed limestone flushing on the southern slopes of Djebel Murdjadjo and the northern Tessala Mountains. These reef limestones develop on both sides of the Sebkha-M'léta zone and constitute the summit of Djebel Tessalah. Their extension under the Lake remains hypothetical (Fig. 1).

The pliocene
The Pliocene is in the area of El Kerma and south of Ain el Arba. The outcrops forms and slopes are characterized by good infiltration capacity, easily absorbing precipitation. The infiltrated water is not drained by any source. It must flow northeast, under the Plain of the Figuier, and to the south, under the Plain of the M'lèta. The Pliocene sandstones contain, at depth, an important captive aquifer whose quality varies according to the geographical sectors. To the south of the Plateau de la M'léta, the Pliocene outcrops form a more or less narrow strip along the slope of the Tessala Mountains. They occur under carbonate facies. Under the plains of M'léta and Maflak, the Pliocene is rather sandy and associated with the Miocene with which it forms an aquifer complex recognized and captured by deep drilling (300-500 m). This relatively powerful assembly (150 m), rests locally on the impermeable allochthonous.

The quaternary
The Quaternary, made up of alluvial deposits deposited on the plains around the Great Sebkha of Oran, contains a water table fed by its impluvium and by infiltrations of runoff from the reliefs. The water in this aquifer is generally highly charged and its mineralization increases as one approach the salt Lake. This water table, whose flows generally converge toward the Sebkha, is captured by farmer wells for irrigation purposes (Benziane 2013).

Sampling network
An inventory survey carried out mainly along a direction transverse to the Basin axis, from the reliefs to the Sebkha (landfill site), made it possible to select water points to constitute the sampling network. These water points, numbering 50 (Table 1), are distributed in localized sources on the flanks of mountains, wells, boreholes, and Lakes on the plains. The samples for chemical and isotopic analyzes were carried out from February 2002 to May 2003.

Analytical techniques
Physicochemical measurements were taken out in situ. The analyzes carried out in the laboratory concern the chemistry of the major elements as well as the isotopic contents of the molecule of water ( 2 H, 18 O) and dissolved inorganic carbon ( 14 C, 13 C) ( Table 1, Fig. 1).
The chemical analyzes were carried out by volumetric methods and atomic absorption in the Laboratories of Applied Geology of the University of Sciences and Technology of Oran (USTO) and Chemistry of the National Agency of the Hydraulic Resources. This was achieved using standard methods as suggested by Rodier (1996) and Degremont (2005). The isotopic analyzes were carried out, within the framework of the CMEP interuniversity cooperation project, at the Laboratory of Hydrology and isotopic geochemistry at the University of Paris-South.
Samples for the analysis of the stable isotopes of the water molecule ( 2 H and 18 O) have undergone gas extraction on preassembled lines before being measured by the VG isotech mass spectrometer.
Six water points collecting different geological formations in the Basin space were sampled for 13 C and 14 C analysis. This analysis was carried out on bicarbonates precipitated in the form of BaCO 3 using BaCl 2 in a medium rendered basic (pH > 8) by the addition of sodium hydroxide (NaOH). The d 13 C contents were measured on the CO 2 obtained by an acid attack (H 3 PO 4 ) of the dissolved total inorganic carbon CITD obtained from the precipitate of BaCO 3 . The 14 C activities were measured by β-count using the Quantulus liquid scintillation spectrometer on benzene previously synthesized from CITD.

Physicochemical data
-The temperature varies according to the period and depth of water. A wide range, therefore, characterizes the waters of the Basin. For surface water, the temperature is direct, related to that of the air. It varies from 9 to 15 °C in the wet season (November to March). In May, it is 28.5 °C at Lake Grande Sebkha of Oran. It is generally found for groundwater between 18 and 25 °C. Higher temperatures, linked to the geothermal gradient, concern water whose depth of the deposit is more than 300 m (33.7 °C at the Oued Tlélat OT5 drilling). The maximum temperature (65.2 °C) is encountered at the South-West

Water balance with the matrix
A program for simulating chemical distributions and water-mineral equilibria, developed under the name WATEQ (Plummer et al. 1984;Parkhurst et al. 1980;Ball and Nordstrom, 1991) was applied to the waters of the Great Sebkha aquifer system in Oran. The data provided to this program are the physicochemical parameters (temperature, conductivity, and pH) and the ionic concentrations of the major elements (Table 1).

Mechanisms of water salinization
The waters derive their composition from the rains and the reactions with the minerals of the soil and the rocks more or less resistant to the alteration. The evolution of the chemical composition of these waters is controlled essentially by three phenomena: dissolution of minerals, precipitation of other minerals, and concentration by evaporation. Irreversible processes, such as dissolution and evaporation, should be distinguished from reversible processes, such as balances between aqueous species and precipitation of minerals (Dever 1985). The salinity of the waters is thus governed by their initial composition, the processes that intervene at the entrance of the hydrological system, and the solution of the salts in contact with the aquifer rocks. The fundamental constituent of rainwater is carbonic acid (H 2 CO 3 ). This is due to the solution of carbon dioxide (CO 2 ) in the atmosphere. Sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ) can also be dissolved in rainwater, especially in industrial areas. These acids are the main agents of the alteration of carbonates and silicates (Salomon and Mook 1986). The acidity of the water can be increased in the soil by dissolving CO 2 , resulting from the degradation of the organic matter, and by a supply of sulfuric acid, resulting from the oxidation of sulfides to sulfates.
Rainwater can also dissolve aerosols such as sodium chloride before entering the soil, as observed in Ben Ziane's Sebkha study (Benziane 1983).
Finally, if the aquifer contains evaporate rocks (gypsum, halite, etc.), these, due to their high solubility, can yield appreciable quantities of dissolved salts.
The origin of the salinity can be discussed according to the geochemical equilibrium and the variation of the concentrations concerning a conserved element, little affected by precipitation of minerals, or biological and chemical processes (Fritz and Fontes 1980).

Spatial distribution of global mineralization
Due to their nature (Lake, springs, wells, and boreholes), the analyzed waters have high dispersed contents of major components. The dry residue values range from 500 mg/l at the Belhadri Abdelkader well, located on the southern slope of Djebel Murdjadjo, and collecting the Messinian limestone formation at 67 880 mg/l at the Sebkha Lake. Flows in the subsoil, as shown by piezometric maps, tend to be directed toward the saline Lake discharge area, which is the lowest area of the Basin (approximately 80 m altitude) (Benziane 2013). The saline charge of the waters increases in the same direction with, however, two main areas. The first corresponds to the Sebkha-Mleta axial zone (sens strict Lake and the Quaternary alluvial layer of its perimeter) where the salinity is very high. The second includes the waters of the northern and southern margins, taken from the Miocene and Pliocene formations with low and fairly constant mineralization.

Saturation indices
The saturation index (or state) of an aqueous solution for a mineral is the ratio: Log Q/K where Q is the product of ion activity and K, balanced (Table 2). If we consider that the waters are saturated from the index IS = ± 1, we can see that certain waters of the Basin are in equilibrium with the main minerals (calcite, gypsum, and halite). Others such as the Sebkha Lake, the source of Aïn Témimounet (emerging in the Tessala), the Cheheida Belgacem well, the Kratsa drilling, and the Oued Kef el Ogab and Oued Besbes seem to be undersaturated of carbonate minerals. The water points catching the Miocene and Pliocene formations are undersaturated with gypsum and anhydrite. Nevertheless, all the waters, both surface and subterranean, are largely undersaturated to the halite (Table 2).

Geochemical classification of waters
The representation of chemical compositions in the Piper diagram confirms that the surface and subterranean waters are taken from the different aquifer levels of the Basin are generally characterized by facies dominated by chloride, sulfate, Sodium, and calcium. These waters are divided between three hydrochemical poles (Fig. 2):-a chloride-sodium pole representing the majority of the waters of the Basin, particularly the waters of the Lake of the Sebkha and those of the groundwater;-a chloride and sulfate pole calcium and magnesium characteristic mainly of waters of Oueds and springs coming from the slope of the Tessala Mountains;-a calcium bicarbonate pole characteristic of the waters originating from the Miocene limestone formations of Djebel Murdjadjo. The surface waters of the main Oueds flowing during rainfall events on the saline and gypsiferous slopes of the Tessala Mountains are characterized by sulfatesodium facies. The waters of the Oued Kef el Ogab and Oued El Ared whose permanent flows seem to drain aquifer levels and those of the salt Lake of the Great Sebkha of Oran with high values at the approach of the dry season (167.4 g/l of a dry residue on a sample taken on 11/05/03) are chloride-sodic.

Isotopes of the water molecule ( 2 H and 18 O)
The d 18 O levels range from −9.61 to + 4.8 ‰ V-SMOW at the salt Lake of Great Sebkha of Oran; this enrichment in the heavy isotope is certainly linked to an intense evaporation phenomenon which marks, as early as May, the beginning of the drying up of the Lake (Craig 1957(Craig , 1961bFontes 1985Fontes , 1976Majoube 1971)). The 2 H contents range from −70 to 20. Minimum values characterize run off from the northern slopes of the Tessala. The 2 H enrichment concerns the waters loaded with dissolved salts, especially that of the salt Lake with a maximum value of 20 (Table 3).
In detail, the figure shows that this tendency results from the juxtaposition of a set of points grouped around the global line of the meteoric waters of an evaporation line. Nevertheless, we see a dispersion of the points representing the surface waters, with deviations Δδ 2 H ≈ 90 ‰ and Δδ 18 O ≈14 ‰. These waters have, however, a notable enrichment to the Salt Lakes of the Grande Sebkha of Oran and the Daïet el Bagrat. As for the Oueds, which are fed by runoff during rainy periods, the stable isotope contents of the water molecule are low. Only the Oued Kef el Ogab, draining an aquifer level whose saline load is estimated to be more than 55 g/l with a perennial flow, presents relatively enriched water. The majority of the collected groundwater samples are grouped and located below the local meteoric lines of the Algiers δ 2 H = 7.15 δ 18 O + 7.92) and global, thus representing evaporated water (Craig 1961a). Some waters are registered on the World Meteorological Right δ 2 H = 8 δ 18 O + 10). Average values are around −6 ‰ for d 18 O and −42 ‰ for 2 H. However, some waters differ from these average values. These are the thermal waters of Hammam Bou Hadjar, sources n° 22 and 23 draining the tellian allochthonous formations of the Tessala Mountains. The source Aïn el Bagra feeding the thermal complex seems to be of deep origin. It is a resurgence that goes back to the surface through artesianism, through fractures and/or ascending diffusion. The well of Sidi Ali Bou Tlèlis, capturing the Quaternary aquifer, constitutes an isolated case of certainly the fact that its exploitation is abandoned. Although there is not much difference between the two lines (global and local Algiers), and given the depletion of heavy isotopes of the Oueds, one can conclude that the rainfall of Oranian is aligned with the world's meteoric right. The confirmation of this conclusion should be made later by a sampling of precipitated rainwater on the Oran Tell.

Relation δ 18 O-Global mineralization
Flows in the subsoil, as shown by the piezometric maps, tend to move toward the saline Lake discharge area, which is the lowest area of the Basin. In addition, the concentrations of the water increase in the same direction, as we approach it. In addition to evaporative phenomena on open water bodies, the hypothesis of a dissolution of the salts cannot be excluded. The distribution of the points on a correlation diagram between the global mineralization and the 18 O content shows that the values relative to the surface waters are fairly dispersed with an 18 O enrichment linked to relatively high values of the global mineralization (Fig. 4). This applies in particular to the waters of Lake Grande Sebkha of Oran and Oued Kef el Ogab; which may suggest that the mechanism responsible for salt concentration is evaporation. On the other hand, the 18 O contents of runoff from the Oued Besbes, El Rassoul, and Tamzourah, from the Tessala Mountains, during a limited rainfall episode, remain quite low. For groundwater, the points are more or less grouped on the diagram. It would seem that the acquisition of their mineralization is made, at least for the deep horizons, by dissolution. Evaporation, therefore, does not play the main role in the increase in mineralization that is due, in large part, to the solution of gypsum, anhydrite, or halite. The phenomena of dissolution and evaporation are linked in superficial horizons.  A source of 13 C -enriched carbon appears to be contributing to the CITD, particularly in the deep waters of the Basin. The carbonate nature of the outcrops that constitute the recharge zone, at least for the northern part of the Basin, represents a potential source of "dead" carbon. We, therefore, consider that the carbon in solution in the waters of the aquifer system of the Great Sebkha of Oran is the product of an isotopic mixture between a gaseous phase and a mineral phase according to the following equation: δ 13 C CITD = δ 13 C biogenic + δ 13 C mineral.

Fig. 2 Piper diagram
Carbon -14 14 C activity is defined as a percent of modern carbon (PMC = percent modern carbon). The accepted value of the period is T = 5730 years. This value corresponds to the calculation of the residence time, expressed in years B.P (before present). By convention also, the present corresponds to the year 1950.
The activities of the 14 C are divided between 2 and 51 PMC (Table 4, Fig. 1). The highest activity in 14 C (≈50 PMC) is measured on samples taken from the Miocene limestones of Djebel Murdjadjo.
Relation A 14 C/δ d 13 C A good correlation of the points emerges as showing well an enrichment in d 13 C on the edges of the Basin with however different activities according to the geological nature of the formations. The A 14 C values are higher on the slope of Djebel Murdjadjo, which argues in favor of a rapid recharge in the Miocene cracked karst limestones. Karst aquifers exhibit a high degree of anisotropy and heterogeneity relative to the complexity of hydraulic aquifer proprieties (Ayadi et al. 2018;Dorr and Munnich 1979) Point 45 corresponds to the waters of the drilling of Oued Sebbah OS1 capturing, on the opposite slope, sandstones, and sands of the marine Pliocene. In both cases, carbon mineralization may be due to the role of carbon from biogenic origin resulting from transpiration and degradation of plants and oxidation of organic matter (Table 4).
Estimated initial activity (A 0 of 14 C) The activity of the carbon dissolved in the form of bicarbonates should therefore be considered as the result of an equiatomic mixture of the two carbon species: CO 2 carries 14 C and solid carbonate generally devoid of radiocarbon. With 100% of modern carbon in CO 2 of biological origin and 0% in mineral carbon, a theoretical initial activity of 50% is readily obtained for dissolved bicarbonate (Fontes 1976;Daoud et al. 1995).

Estimated residence time
The calculated age represents the average time to renew the reserve. It is, in fact, the final state of a mass of water, probably derived from a more complex form of flow between the two extremes.
The study of the d 13 C DIC contents allows us to admit the existence of a dilution of the dissolved total inorganic carbon of biogenic origin by a "dead" carbon resulting from the dissolution of the carbonate matrix. The result of this dilution appears in the form of apparent aging of the waters. The estimation of the "real" residence times of water in the aquifer then consists in calculating a value of the initial activity A0 corrected for the phenomena of mixing and chemical and isotopic exchanges between the water and the matrix. The correction models applied in this study are those of Pearson, Fontes & Garnier, AIEA, Evans, Eichinger. The 14C activity of soil gas is estimated to be 100% assuming that the fraction before 1950 is dominant. The values of the 14C activity of the samples and the calculated "ages" are shown in Table 5.
The results of calculations obtained on the initial activities and the corresponding residence times show that the models which are based on an isotopic mixture between the biogenic CO 2 and the carbonate matrix seem to be the most suitable for this study. As for the Tamers model, it adapts less well than the Pearson model; its application gives very little initial activity around 50%. Carbon dilution is practiced equally, between an abiogenic pole and a mineral pole, which is the principle of the model. On the whole, models based on isotopic mixing, except for Mook's, which offers negative initial activities, give fairly consistent values. The values corresponding to the samples taken from the limestone formations of the northern edge indicate that more recent waters (< 500 years) are preferred. On the other hand, on the southern edge of the Basin, the waters are relatively older due to slower circulation in a porous medium represented by sandstones and sands. The case of the well of the Brédeah pumping station, although located on the boundary of the calcareous outcrops, gives a relatively older age in comparison with the structures receiving the same formation. Its low activity may well confirm the mixing of waters (Miocene limestone-Quaternary alluvium and even deepwater) caused by intensive pumping. The waters taken from the drilling of Oued Tlélat OT5, located in the axis of the Basin and capturing the formations of the Mio-Pliocene complex at a depth of more than 300 m, are the oldest after those of the thermal spring of Hammam Bou Hadjar. The waters of the latter, the oldest in the Basin, appear to emerge with vertical drainage after a long journey in the deep subsoil (Table 5).

Relationship ages -δ d 18 O
We note a palaeoclimatic effect due to the aging of the waters of the thermal complex of Hammam Bou Hadjar (n° 47) is a diagram showing the mean ages, calculated according to the correction models described above, about the oxygen-18 contents, with depletion in d 18 O (Fig. 1). The diagram allows us to distinguish, in addition, four deposits of water ( Fig. 6): • a deposit of present waters (n° 16 and 38) located on the southern slope of Jebel Murdjadjo, protected from any sensitive contamination; • a subsurface water deposit at the Brédeah pumping station (n° 2) ( Fig. 1), supported by current refills, with an average age of a few thousand years, aging is certainly due to a mixture of masses of water coming from three distinct aquifer formations under the effect of intensive pumping; • a relatively old deepwater deposit in the Miocene and Pliocene formations; • a deposit, represented by the thermal waters of Hammam Bou Hadjar, certainly the oldest (≅ 18 000 years BP) of the Basin (n ° 47) (Fig. 1). Figure 7 shows a linear relationship showing growth of 14 C activities with NO 3 − ions, indicative of organic contamination by recent waters, particularly at points on the northern edge of the Basin. This pollution is much more important at point 2, which stands out from the alignment. It is the Brédeah pumping station, located downstream of a precarious habitat located on vulnerable limestone soil without sanitation networks (Fig. 7).

Pollution of water by nitrates
In addition, the binding effects of demineralization of the waters of Brédeah station will be taken into account. Indeed, this station rejected its brines in the nearest outlet. These natural salts as sodium chloride would give their high solubility, spread on soils, and dissolve in plain water. This catastrophic situation, which has lasted for a few years, gave a serious blow to the environment, particularly to farms (Boualla et al. 2019).

Conclusions
The waters thus analyzed have, given their nature (Lake, springs, wells, and boreholes), contents of major components that are fairly dispersed. Dry residue values ranged from 500 mg/l at the Belhadri Abdelkader well located on the southern slope of Djebel Murdjadjo and captured Messinian limestones at 67 880 mg/l at the Sebkha Lake. The salinity of the waters is governed by their initial composition, the processes involved in the entry of the hydrological system, and the solution of the salts during the transit of the solution to the rocks in the aquifer (Oster 2001). The salinization of water and soil is intimately linked to their deposit and circulation conditions. Although close to the sea, the endorheic Basin of the Great Sebkha of Oran is composed of quite a varied lithological facies. In this environment, where a semiarid climate prevails, the evaporation phenomenon is relatively intense. The waters of the Sebkha, under the effect of temperature, evaporate, leaving, in places in summer, the place to crusts of salt.
Groundwater flows, as shown by piezometric maps, tend to be directed toward the saline Lake discharge area, which is the lowest area of the Basin (approximately 80 m altitude). The saline charge of the waters increases in the same direction, as we approach it. For this slick, concentration by evaporation, under the same climate, remains the main cause. This phenomenon is often supported by anthropogenic action. Indeed, the residents of Salt Lake continue to exploit this water table by intensive pumping for irrigation purposes (Rhoades 1992).
Along the Murdjadjo slope, the initial chemical composition of the calcium bicarbonate type, directly related to the calcareous aquifer, was enriched in chloride and sodium ions following a mixture due to prolonged intensive pumping. This is particularly true of the sources of Brédéah, which have been transformed into a drainage station since 1880. These waters, once famous for their good quality with less than one gram per liter of salts, have become brackish over time Of 7 g/l of dissolved salts.
The deep aquifer of the Mio-Pliocene complex of the M'léta and Maflak plains contains relatively charged waters in contact with evaporate levels. These rocks, due to their high solubility (264 g of halite/kg of a solution, 2 g of gypsum/kg of a solution, etc.), can provide appreciable quantities of dissolved salts. For the waters of these aquifers, we retain the dissolution of salts and minerals as a source of concentration and possibly mixtures of waters operated by the phenomena of drainage.
The isotopic aspect allows us to specify more precisely the conditions of the deposit and circulation of water. Tracing by the stable isotopes of the water molecule shows, in the majority of cases, evaporated waters aligned on the world meteoric right with however a notable enrichment on those of the Salt Lakes of the Grande Sebkha of Oran and the Daïet el Bagrat.
Carbon-14 (A 14 C,%) activities of groundwater vary between 2.2% and 50.7%, which represents a significant difference in residence time in reservoirs. The waters of Miocene limestones flush on the southern slope of Djebel Murdjadjo have the highest values of 14C activity. The recharging of these cracked and karstic formations is therefore faster.
This activity decreases by 30% at the Brédeah pumping station (A ≅ 20%) whose wells originally received the same formation. This decrease could be explained by the invasion of brackish waters resulting from the overlying alluvial formation and probably other older waters. The measured activity probably reflects the mixing of different water masses (freshwater from calcareous-brackish water from deep alluvial deposits), as a result of intensive pumping, the permeability in a porous medium being lower than that of the cracked calcareous medium.
The deepwater deposit in the Mio-Pliocene complex is marked by an average residence time of more than 10.000 years BP. On the other hand, there is a not inconsiderable proportion of recent waters on the northern edge of the Basin, where it is, in fact, a succession of refills spreading between the Current and the Holocene. In the deep horizons, we also observe the intervention of the carbonate matrix in the mineralization, by dissolution, of the groundwater.
Finally, the extreme case of the thermal spring of Ain el Bagra, feeding the complex of Hammam Bou Hadjar and having old waters of more than 18 000 years BP, needs to be further specified in a future study which Would take into account the geochemical (major elements and trace elements) and isotopic aspects. included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.