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Volume 31 Issue 5
Oct.  2020
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Madene Elaid, Meddi Hind, Boufekane Abdelmadjid, Meddi Mohamed. Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers. Journal of Earth Science, 2020, 31(5): 993-1006. doi: 10.1007/s12583-020-1293-y
Citation: Madene Elaid, Meddi Hind, Boufekane Abdelmadjid, Meddi Mohamed. Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers. Journal of Earth Science, 2020, 31(5): 993-1006. doi: 10.1007/s12583-020-1293-y

Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers

doi: 10.1007/s12583-020-1293-y
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  • In the alluvial aquifers of Upper and Middle Cheliff (North-West Algeria), the groundwater quality is deteriorating. The objective of this study was to characterize the physical and chemical properties of these aquifers; and to evaluate the groundwater quality and its appropriateness for drinking and agricultural use. An investigation was carried out by estimating of the physiochemical parameters (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3-, Br- and TDS) to identify the chemical characteristics of groundwater. Morever, the isotopic composition was examined to identify the sources of recharge of these aquifers. The groundwater geochemistry for the high water level (May, 2012 and June, 2017) and low water level (November, 2012 and October, 2017) was studied. Accordingly, water samples from 39 water sampling points were collected (October, 2017 and June, 2018), for the purpose of analyzing stable isotopes (18O, 2H). The results show that the groundwater is mainly characterized by Ca-Cl and Na-Cl type. The chemical quality of the water is from fair to poor with the presence of nitrates used in agricultural and urban discharge. Also, the Br/Cl ratio gives indications on the origin of the salinity. This salinity is due to the leaching of chlorinated fertilizers, the dissolution of evaporite deposits and the rise of deep salty water by the fault of Chellif. While, the diagram of δ2H=f18O) indicates that the origin of the recharge of these aquifers is the Atlantic and Mediterranean oceanic meteoric rainwater.
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Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers

doi: 10.1007/s12583-020-1293-y
    Corresponding author: Meddi Hind, ORCID:0000-0001-6592-304X, h.meddi@ensh.dz

Abstract: In the alluvial aquifers of Upper and Middle Cheliff (North-West Algeria), the groundwater quality is deteriorating. The objective of this study was to characterize the physical and chemical properties of these aquifers; and to evaluate the groundwater quality and its appropriateness for drinking and agricultural use. An investigation was carried out by estimating of the physiochemical parameters (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3-, Br- and TDS) to identify the chemical characteristics of groundwater. Morever, the isotopic composition was examined to identify the sources of recharge of these aquifers. The groundwater geochemistry for the high water level (May, 2012 and June, 2017) and low water level (November, 2012 and October, 2017) was studied. Accordingly, water samples from 39 water sampling points were collected (October, 2017 and June, 2018), for the purpose of analyzing stable isotopes (18O, 2H). The results show that the groundwater is mainly characterized by Ca-Cl and Na-Cl type. The chemical quality of the water is from fair to poor with the presence of nitrates used in agricultural and urban discharge. Also, the Br/Cl ratio gives indications on the origin of the salinity. This salinity is due to the leaching of chlorinated fertilizers, the dissolution of evaporite deposits and the rise of deep salty water by the fault of Chellif. While, the diagram of δ2H=f18O) indicates that the origin of the recharge of these aquifers is the Atlantic and Mediterranean oceanic meteoric rainwater.

Madene Elaid, Meddi Hind, Boufekane Abdelmadjid, Meddi Mohamed. Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers. Journal of Earth Science, 2020, 31(5): 993-1006. doi: 10.1007/s12583-020-1293-y
Citation: Madene Elaid, Meddi Hind, Boufekane Abdelmadjid, Meddi Mohamed. Contribution of Hydrogeochemical and Isotopic Tools to the Management of Upper and Middle Cheliff Aquifers. Journal of Earth Science, 2020, 31(5): 993-1006. doi: 10.1007/s12583-020-1293-y
  • Groundwater is the second largest reservoir of freshwater, after glaciers, lakes and rivers, with volumes of the order 8 to 10 ×106 km3 potentially exploitable (UNESCO, 1978), making up 30% of the resources of drinking water (Gleick, 1996). However, this resource, which used to be in good quality, is now threatened by various points and diffuse sources of contamination. The distribution of water between the layers of the earth varies in space and time. Knowledge of the hydrological functioning of aquifers and the geochemistry of groundwater is crucial for assessing the quality and natural tracing of water using the isotopic composition. The natural chemical composition of groundwater responds to determinism partly due to the lithological nature of aquifers and superficial terrains traversed by water (Blum et al., 2001).

    Historically, the groundwater chemistry of Upper and Middle Cheliff has attracted the attention of many researchers. Most of these studies have focused on the interaction between groundwater and bedrock with a fundamental control on the chemical characteristic of groundwater (Bemiloud, 2017; Touhari, 2015; Bouzelboudjen, 1987), which led to the idea used in this study: the origin of geochemical groundwater can be traced and distinguish the origin of groundwater salinity using a stable isotopic composition. This approach has been used by many researchers around the world (Beal et al., 2019; Ogrinc et al., 2019). There was also discussion of studying water quality in our study area based on the most characteristic elements to determine the potability of this groundwater. This approach has been widely used in various basins around the world (Wali et al., 2019; Wright et al., 2019; Zheng et al., 2019; Cheng et al., 2017).

    The distinction between the different mechanisms of salinization makes it possible to reconstruct the origin of ground waters as well as their pathways, and to imagine their future evolutions. Among the geochemical criteria which can help identify the intrusion of surface or deep water as opposed to other sources of salinity in the aquifer, such as chemical elements (cations, anions and dry residue) and ionic ratios, such as Na+/Cl-, Br-/Cl-, Ca2+/Mg2+ and also the stable isotopes 18O and 2H ((Chenaker et al., 2018; Belkoum and Houha, 2017; Trabelsi et al., 2007)). This approach was used by Mehr et al. (2017), Zhao et al. (2016) and Ma et al. (2014).

    The use of bromide anion contents is an essential complement to chloride measurements in order to explain the salinity anomalies in groundwaters. Br=f(Cl) and to the molar ratio Br/Cl, it allows marine influence areas(the ocean, precipitation) to be distinguished from evaporitic or anthropogenic influence areas (Farid et al., 2015).

    The objective of this article is to understand the mineralization process and the hydrodynamics of the groundwater of the Upper and Middle Cheliff alluvial aquifers using chemical tools (major elements) and isotopic (18O and 2H). Then, a simulation tool based on the combination of the two geochemical and isotopic approaches is used to highlight the main plausible reaction mechanisms responsible for the mineralization, and the apparent age of the water. The variation of the isotopic composition is a very useful indicator of the origin and nature of the resource. Finally, the origin of water masses that recharge the aquifer and specify the mechanisms of this recharge is defined. These evaluations are crucial for sustainable exploitation in a semi-arid region with very limited groundwater resources.

  • The Upper and Middle Cheliff Basin, which is located in north western Algeria and occupies the north-east of the Cheliff Basin, has undergone significant agricultural development in recent years (Fig. 1).

    Figure 1.  Geographical location, geology of the study area and geological sections across the Upper and Middle Cheliff Basin.

    The groundwater resources of the Upper and Middle Cheliff plains have been assessed within the framework of the National Water Plan (1971). This evaluation is based on the hydrogeological studies conducted by ANRH and the rain/infiltration method. These plains are composed of coarse alluvium and occupy an area of 1 070 km2 (Upper Cheliff: 370 km2, Middle Eastern Cheliff: 360 km2, Middle Western Cheliff: 340 km2), whose potential in groundwater is estimated at 43 Hm3/year (DHW, 1971) (Upper Cheliff: 16 Hm3/year, Middle Eastern Cheliff: 16 Hm3/year, Middle Western Cheliff: 11 Hm3/year). This water potential is the first source for water supply and irrigation. The latter, combined with drought, caused drying up of the springs and a significant reduction of the alluvial aquifers. Water resources are not only subject to overexploitation, but also to a degradation of their quality due to wastewater discharges and salinization resulting from the leaching of evaporite formations occurring after the earthquake of October 10, 1980 by an important surface faulting. Indeed, this tectonic accident released deep water, and allowed their rise towards the alluvial water table in the region of El Attaf (Middle Chellif water table), which belongs to the sub-basin of the Tikazale wadi (IFES, 2002). These elements (interaction between deep salt water and alluvial groundwater, uncontrolled urban discharges, the return of irrigation water and over-exploitation of groundwater for irrigation purposes) modify the chemistry of the water and make it unsuitable for the desired uses.

    The study area is composed, according to the orohydrographic delimitations, of 11 sub-basins. They are drained by the Cheliff wadi which crosses them for a length of about 349 km. It covers 10 916.60 km2 (ANRH Blida, 2017).

    The plains of Upper and Middle Cheliff are characterized by a semi-arid climate with Saharan influences in summer and Mediterranean influences in winter. The rainfall distribution is very spatially marked. North of the study area, interannual precipitation is very important on the southern slopes of the Dahra and Zaccar mountains, with an interannual average of more than 600 mm. Precipitation decreases in the plains of Upper and Middle Cheliff where it varies between 300 and 400 mm. The total annual potential of evapotranspiration ranges from 1200 mm to 1 600 mm, more than double of the total rainfall. The Upper and Middle Chéliff Basin is characterized by an average interannual temperature oscillating from 13 to 20 ℃.

  • The upper and middle Cheliff watershed is located in the Tell Atlas of Algera and corresponds to a subsident intra-mountainous furrow. It located between the Boumaâd Massif, Beni Naceur Massif and Dahra in the north, and the Ouarsenis strongholds south according to Perrodon (1957) and Mattaeur (1958) (Fig. 1).

    From lithostratigraphic information, the Upper and Middle Cheliff depression is constituted as a whole by the Mio-Plio-Quaternary formations age (Achour and Bouzelboudjen, 1998).

    They are crossed from east to west by the Cheliff wadi which enters the plain of the Upper Cheliff by the threshold of Djendel and one leaves it by the threshold of the Doui (Djada, 1987) and himself who enters the plain of the Middle Eastern Cheliff by the threshold of the Doui and leaves by the threshold of Oum Drou (Pontéba) (Bouzelboudjen, 1987) and himself who enters the plain of the Middle Western Cheliff by the threshold of Oum Drou (Pontéba) and leaves by the threshold of Boukadir (Charon).

    These hydraulic thresholds correspond to upwelling of the clay-marl substratum, which is impermeable in the crossing, from which any underground flow is practically excluded.

    The intra-mountainous furrow that corresponds to the plains of Upper and Middle Cheliff were filled by Neogene deposits where Quaternary, Pliocene and Miocene sediments accumulated (Mattauer, 1958; Perrodon, 1957; Glangeaud, 1955). The Neogene formations of marine origin, of thickness that can reach 3 000 m (Meghraoui, 1982; Perrodon, 1957) begin Quaternary deposits. They are predominant in the plains where they are consisted of coarse alluvium (ancient Quaternary) and silt (Late Quaternary) placed on the Upper Pliocene, which is formed by sandstone and limestone elements as well as sand, and the Lower Pliocene (Marine Pliocene) beginning with a transgression on the gypsum series of the Late Miocene, to end with the Astian regression (Fig. 1).

    Upper Miocene (Vindobonian) and Lower Miocene (Burdigalian) are the last important phase of tangential tectonics forming a marly series. The neogenic deposits constitute the filling of the basin which includes three large plains; they can be distinguished from east to west as follows:

    The plain of El Khemis or the plain of Upper Cheliff; the plain of El Abadia-Amra or the plain of Middle Eastern Cheliff; the plain of Chlef or the plain of Middle Western Cheliff.

    The Cheliff Basin is a Neogene post-nappe basin. The Tellienne chain constitutes the substratum of this basin (Lepvrier, 1978, 1971), we distinguished an Early Neogene Tellian substratum and a Neogene post-nappe basin.

    The Tellian chain consists of a succession of reliefs parallelto the Mediterranean coast, formed mainly of Jurassic and Cretaceous lands (Obert and Lepvrier, 1976).

    The structures being elongated East-West, the tectonic is complex. The main elements of this chain are: (1) autocthon nucleis. The Doui, Rouina and Temoulga, which are epimetamorphic massives with Schistosity (Kirèche, 1993, 1977); (2) allochthonous sets (thrust sheets). Covering a large area, they form the major part of the formations that took place during the different alpine tectonic phases (Middle Eocene and Lower Miocene).

    This basin is characterized by intense neotectonics (Meghraoui et al., 1986) materialized on the surface by the seismic rift of Oued Fodda (earthquake of October 10th, 1980) causing notable changes on superficial and underground flows with a rise of nearly 2 m of groundwater level.

  • Examination of the piezometric maps has revealed no change in the morphology or the shape of the piezometric curves during the high and low water level periods in 2012 and 2017, which reflects the same flow regime. However, there is a decrease in the piezometric ratings of wells and piezometries during periods of low water level compared to the high water level periods of 0.6 m in 2012 to 4.3 m in 2017, due to the low recharge of the water table following the low annual rainfall and the intensive over-exploitation of the aquifer which ensures the irrigation of crops.

    The observation of the morphology of the piezometric map of the low-water period 2017 (Fig. 2) indicates that the piezometric levels are decreasing from east to west. It shows that the underground flow follows, generally, an east-west direction which coincides appreciably with the course of the Cheliff wadi. The isopipe curves reveal that the tablecloth is convergent and feeds the wadi in the center.

    Figure 2.  Piezometric map of alluvial layers of the Upper and Middle Chellif (year, 2017).

    The values of hydraulic gradient is low; vary from 1×10-3 to 3×10-3 between El Amra and El Abadia (in the Northern), between Ain Soltane and Arib (in the North-Eastern) and Chattia et Boukadir (in the South-Western). These low values show that the formations are more and more coarse and regular. They are related to the formation of gravels and pebbles dominant in this region. Their values change somewhat from one sector to another in this region and indicate a better permeability of the aquifer.

    In the upstream part between Ain Lechiakh and Barbouche and in the centre between Rouina and Ain Defla on the foothills of the Doui Massif and on the foothills of the Temoulga Massif the isopiezo curves are tightened indicating a strong hydraulic gradient of the order of 2.14×10-2 to 2.18×10-2. This variation of the hydraulic gradient is due, essentially, to the heterogeneity of the lithology through the rise of the Miocene substratum formed by the Helvetian age clays and pudding stones. This, as a matter of fact, is due to the decrease in the permeability of the grounds at this area.

    The piezometric surface is not deep enough which increases the effect of evaporation in the area, where the hydraulic gradient decreases, thus giving sufficient time to the interaction between groundwater and lithology.

    Wadi-water table exchanges can take place in both directions; thus the wadi has two roles: draining and feeding. The variations of the piezometric level in the aquifer are related to the rainfall and the level of the wadi of Cheliff, because the Cheliff wadi has dug its bed in the aquifer. This type of direct link exists in a certain way because wells are implanted in the minor bed of Cheliff wadi.

  • Samples were taken on a seasonal basis. Water samples for laboratory analyzes were taken from wells and piezometers along the months of May, June, October and November during the high and low water level periods of 2012 and 2017.

    Two sampling campaigns in 2012 involving 63 water samples (43 boreholes and 20 piezometers) during high water level in June and 71 water samples (47 boreholes and 24 piezometers) during low water level in November. In another series of two (2) sampling campaigns organized respectively in May and October of 2017, 40 water samples (26 boreholes and 14 piezometers) were collected during high water level periods and 48 water samples (33 boreholes and 15 piezometers). All water samples were collected after well pumping for a minimum of 10 to 15 min. These samples were preserved in 250 mL polyethylene bottles for physicochemical and isotopic analyzes, and stored in a cooler and analyzed just after the sampling campaign in the ANRH (National Water Resources Agency) and CRNA (Nuclear Research Center of Alger) laboratory.

    The physical parameters (temperature, pH and electrical conductivity) were measured in situ using a pH meter and a field conductivity meter. The hydrochemical analyzes (cation and anions) were carried out using ion chromatography (Dionex 120), a spectrometer, a turbidimeter and a colorimeter.

    The filtered and acidified samples (1% vol./vol.) HNO3 were analyzed for the cations (Ca2+, Mg2+, Na+, K+) by atomic absorption spectrophotometry and a Dionex 120 ion chromatograph. The methods used for the anion analyzes were the following: the mercury thiocyanate method for chlorides (Cl-), the turbidimetric method for sulphates (SO42-), the colorimetric method (colorimetric assays) to analyze nitrates (NO3-) in the ANRH laboratory in 2012 and using an ion chromatograph Dionex 120 in laboratory CRNA in the 2017 period.

    Bromine analyzes were carried out on 25 groundwater samples taken during the month of October 2017 from wells and piezometers of different salinity and distributed throughout the Upper and Middle Eastern Cheliff region. The bromine analyzes were carried out using ion chromatography (Dionex 120) in laboratory CRNA.

    Isotopic measurements (18O and 2H) were realized at the Laboratory of Nuclear Research Center of Algiers (CRNA) using a Laser Picarro-L2110i spectrometer. Analyzes were processed under LIMS (Laboratory Information Management System) from 39 groundwater samples respectively organized in October 2017 and June 2018. The analytical uncertainties on the measurements were 0.36‰ for deuterium (2H) and 0.05‰ for Oxygen-18 (18O).

  • According to Table S1, the temperatures of the samples collected show fairly homogeneous values, between 18 and 20 ℃ with an average of about 19 ℃.

    The pH values are between 5.1 (piezometric Pz9) and 9.4 (piezometric Pz15). The majority of the samples have a pH varying between 7 and 8.2; with an average value of 7.6.

    The electrical conductivities have a wide range of variations from 770 μS/cm (W105-91) to 17 150 μS/cm (piezometric Pz 15A). The lowest values are measured between El Amra and El Abadia, where they do not exceed 1 000 μS/cm. However, they have high values in the region of El Attaf (Jebel Témoulga), where they vary between 12 000 and 17 150 μS/cm. The evolution of these concentrations can be linked to several factors:

    (1) An intercommunication between Jurassic limestone waters (Triassic gypseous ground of Jebel Témoulga) and those of alluvium, through a tectonic accident affecting this region, reoccurring during the earthquake of October 10, 1980. This accident released deep salty water and allowed its rise to the alluvial aquifer (IFES, 2002).

    (2) The proximity of Miocene marl grounds between Rouina and Ain Defla and in the region of Ouled Fares which explains the increase of electrical conductivities.

    (3) The highest concentrations of electrical conductivities are observed also in the south of the plain of High Cheliff, on the left bank of Cheliff wadi near Massine wadi which can be explained by the outburst of trias in the form of white gypsum efflorescence rich in NaCl (Touhari, 2015).

  • In order to show the spatial distribution of the chemical elements, hydrochemical mapping was carried out. The distribution of the concentrations depends on several factors such as lithology, the hydrodynamic state of the aquifer and climatic conditions (Ghebouli and Bencheikh Elhocine, 2008).

    In this study, we will map the dominant chemical elements (Ca2+, Mg2+, K+, Na+, Cl-, SO42-, HCO3-) characterizing the chemical facies as well as the nitrates which have an influence on the water quality and which show a significant evolution during the two periods (high and low water levels) in 2012 and 2017.

    Figures 3a, 3b show that the spatial distribution of the total dissolved solid (TDS) during the two years of study is relatively low at the upstream compared to the downstream of the aquifer, between El Amra and El Abadia (Middle Cheliff groundwater) and between Ain Defla and Arib (Upper Chéliff groundwater). The values do not exceed 1 000 mg/L. On the contrary, it has strong values in the region of El Attaf (Jebel Témoulga) located west of the water table of Middle Eastern Cheliff and next to the city of Djelida at downstream of the plain of Upper Cheliff.

    Figure 3.  (a) Map of the total dissolved solid (TDS), low water periods 2012 and (b) map of the total dissolved solid (TDS), (c) chloride, (d) sulphate, (e) sbicarbonates, (f) nitrates, low water periods 2017.

    The elements Cl-, Ca2+, Mg2+, K+ and Na+ have relatively the same spatial distribution. The highest concentrations of these elements are observed in the region of El Attaf.

    Chloride levels range from 4 980 to 7 660 mg/L in 2012 and 570 to 4 720 mg/L in 2017. These are high in the groundwater of the alluvial aquifer in the region of Attaf (near Jebel Témoulga) (Fig. 3c). The high chloride content in the alluvial aquifer could be explained by the upwelling of deep saline waters, as a result of the active fault NW-SE truncating the eastern end of Jebel Témoulga (Triassic or Permo-triassics formations rooted on the massif) (IFES, 2002), which was provoked by the earthquake of October 10; 1980 that led to the opening just at the piedmont of Jebel Témoulga and which caused the groundwater to overflow from limestone by the Astian. At the upstream of the Middle Cheliff aquifers and in the Upper Chéliff aquifer, there have been significant concentrations of chlorides that exceed the Algerian standards set for (250 mg/L), the chlorides are more abundant in the groundwater of the plains; more than 90% of water points have a chlorinated chemical tendency. The abundance of chlorides results from evaporitic formations (the dissolution of the halite), and from the permanent contact of the groundwater with the marly substratum; the proximity of the marly Miocene soils and the effect of superficial leaching in times of heavy rains and evaporation due to the shallowness of the piezometric surface (where it is observed that the depths do not exceed 20 m in the plains and the semi-arid climate) or it is explained by the existence of another origin of this ion than the dissolution of the halite. The gypsum rich in NaCl deposits white efflorescence that has been observed upstream at the bottom of Massine wadi. The water points on the right (piezometric Pz 13A and piezometric Pz 15A), are characterized by high concentrations of chlorides, coming from the great depths, this water characteristic is to be linked with the presence of the triassic salt formations under the Jurassic limestones of Jebel Témoulga and their flow through the limestone formations of the Jurassic and the deep gypso-salt layers.

    The maximum values of sulphates are encountered at the level of oued Fodda and Chlef in the South-West and in the Djelida zone up to Djendal in the North-East (with some ranging greater than 550 mg/L) (Fig. 3d). They are due to the leaching of evaporite deposits (the solubility of gypseous formations) and/or the effect of anthropogenic inputs (leaching of sulphated chemical fertilizers). The maximum values of bicarbonates are found in the Middle Cheliff aquifers at the level of Ain Defla in the South-East and Fodda wadi in the South-West and in the Rouina zone in the South. This elevation is explained by the dissolution of carbonate minerals (Fig. 3e). The evolution of nitrate concentrations shows that only 40% of the points have values higher than the Algerian standard set by (50 mg/L), (Fig. 3f). The highest concentrations of NO-3 are found in the east near Djendel, in the south near Bir Ouled Khelifa and Rouina, in the north near Mekhatria and in the west in the Chlef region. They can be used as indicators of chemical pollution of water. As reported by various authors (Stadler et al., 2008; Girard and Hillaire-Marcel, 1997; Njitchoua et al., 1997), the presence of nitrates in high concentration, in aquifers in arid climates, is due to anthropogenic pollution. Since the study area is mainly agricultural. Low concentrations of these elements are observed north of the plain, between El Amra and El Abadia, on the right bank of Cheliff wadi.

    Figure 4 depicts the spatial variations in the physico-chemical parameters of groundwater in alluvial aquifers of the Upper and Middle Cheliff for the years 2012 and 2017, which shows the spatial dispersion of chemical elements contents and gives an idea of the unique or multiple origins of waters. Chlorides are highly variable, followed by sodium, calcium, magnesium and sulphate. This classification can be explained by various origins of water chemistry (dissolution of aquifer rock: gypsum, halite and dolomites) or anthropogenic origin (wastewater and fertilizers) or base exchange; i.e., these ions are acquired during the passage of water from several formations, namely halite, gypsum and marl (clays) or the infiltration of untreated wastewater or irrigation water loaded with salts and fertilizers. On the other hand, we notice that the box of bicarbonates is narrower, indicating a unique origin which is that of carbonate formation.

    Figure 4.  Box whisker plot for the groundwater quality. Q3. The 3rd quartile is the data in the series that separates the bottom 75% of the data; Max. the maximum concentration value of the chemical element in a series of samples; Min. the minimum concentration value of the chemical element in a series of samples; Q1. the first quartile is the data in the series that separates the bottom 25% of the data; Median. the median of a set of values is a value x which makes it possible to cut the set of values into two equal parts, i.e., represents 50% of the data.

    It should also be noted that chlorides with the extent of their boxes reveal a high dispersion than other minerals. These statistics represented by the box plots clearly and easily confirm our results.

  • The different water samples have been classified according to their chemical composition using the Piper diagram (Djabri, 1996).

    The different water samples have been classified according to their chemical composition using the Piper diagram (Piper, 1944). This diagram (Fig. 5) shows that the overall chemical character falls within the following two water types: (1) Chloride-calcium facies is prominent and the most dominant eastward, westward of each aquifer with a presentation number of 20 and 19 samples during low water level periods of 2012 and 2017, respectively. The chlored-calcium facies is important and the most dominant and spreads eastward, west of the tablecloth. It can be explained by the dissolution of the Mio-Plio-Quaternary alluvial formations and gypsiferous marls combined with the phenomenon of Base Exchange where Ca2+ ions are released by the clay minerals of the alluvial aquifers against adsorbed Na ions.

    Figure 5.  Presentation of Upper and Middle Cheliff waters on the Piper diagrams (red points: high water period 2012 and 2017; green points: low water period 2012 and 2017).

    (2) The chlored-sodium facies develops to the west of the Middle Cheliff East nappe and to the south of the High Cheliff nappe, on the left bank of the Cheliff wadi. This confirms that our waters are influenced by the process of dissolution of saliferous formations (halite [NaCl]) diffused in fine-textured sediments (clays) or found in Triassic formations that increase the concentration of Na+ and Cl- ions after the infiltration of water from heavy rains (Massin wadi) or upwelling of deep water (near Gebel Témoulga).

    The study of the correlations established by the binary graphs (the logarithmic diagrams) between the concentrations of the principal major elements (SO42-, Mg2+, Cl-, Ca2+, Na+, HCO3-) made it possible to identify the various mechanisms and processes that contribute to the mineralization of the sampled waters. The Ca2+, Mg2+ and HCO3- ions are formed in water during processes like dissolution, precipitation, dolomitization of carbonate rocks (Bathurst, 1971).

    There is a correlation between sulphate-calcium which confirms the evaporitic origin of sulphates by dissolution of anhydrite and gypsum. The projection of Ca2+ as a function of SO42-shows that the majority of the water points existing above the line (Fig. 6a) show an excess of Ca2+ compared to SO42-.This excess of Ca2+ is due to the phenomenon of base exchange, as the clays in the substratum can release Ca2+ ions after having fixed the Na+.

    Figure 6.  Relationships between the major elements, period 2017 (red points: high water period 2017; green points: low water period 2017).

    The contents of Ca2+ and Mg2+ (Fig. 6c) have very variable concentrations, since these ions are involved in various processes of dissolution/precipitation of gypsum, calcite and dolomite and again the phenomenon of base-exchange. The increase in Ca2+ levels that accompanied the low Mg2+ levels during the 2017 low water level period is due to the base exchange phenomenon, because the substratum clays can release Ca2+ ions after having fixed the Na+. This indicates that the origin of Ca2+ is not only the dissolution of calcite and gypsum, and thus confirms the hypothesis of a contribution of Ca2+ by ion exchange. In 2012, the water points show a stoichiometric distribution of Ca2+ ions with Mg2+ ions indicating the dominance of the dolomitization phenomenon where the Ca2+ ions are fixed and the Mg2+ ions are released. The dolomitization exchange reaction has been reported as the main cause of decreasing the Mg/Ca ratio in the waters of carbonate basins. This decrease is progressive according to the increasing age of aquifers, and it is controlled by the balance between calcite and dolomite and strongly dependent on temperature (Fidelibus and Tulipano, 1986). Base water exchange is called hard water, the groundwater in which the alkaline earths (Ca2+, Mg2+) have been exchanged by Na+ ions (Gupta et al., 2008).

    The HCO3- ion is weakly correlated with Ca2+ and Mg2+, meaning that calcium and magnesium evolve independently of bicarbonates, indicating that the dissolution of carbonate rocks (calcite, dolomite) is not the only source for these elements, because they also come from the dissolution of evaporites (Figs. 6b, 6f).The correlation between Na+ and Cl- (Fig. 6d) shows a notable characteristic of the alluvial layers of Upper and Middle Cheliff; it is its richness in Cl- compared to Na+. This is due to the phenomenon of base exchange, as the clays in the substrate can release Ca2+ ions after fixing Na+. However, we can group our waters into two groups; in the first group, the water points have a stoichiometric distribution of Na+ ions with Cl- ions. This confirms that our waters are influenced by the dissolution process of saline formations (halite [NaCl]) diffused in fine-textured sediments (clays) or found in Triassic formations that increase the concentration of Na+ and Cl- ions after infiltration of rain-laden water or upwelling of deep waters. The second group contains water points located above the line of slope equal to 1 indicating an excess of Cl- over Na+. This excess could be explained by the existence of another origin for Cl- ions (other than halites). This is related to another origin for this ion other than the dissolution of the halite. It may also have an anthropogenic origin (waste water).

    Mineral saturation indices, which are very useful for assessing the extent to which water chemistry is controlled by solid phase equilibrium (Appelo and Postma, 1993), have been used to calculate the indices.

    The use of the geochemical program of PHREEQ, integrated in the diagram program of the hydrochemical calculations (Simler, 2005), has allowed us to calculate the saturation indice of the calcite, the dolomite, the aragonite, the gypsum, the anhydrite and the halite.

    The saturation index (SI) is calculated by comparing the chemical activities of the dissolved ions of the mineral (ion activity product, IAP) with their solubility product (KT). In equation form

    The under-saturation of a mineral can contribute to its dissolution in water leading to an increase in the saline load in its presence. The state of equilibrium or super-saturation causes, on the other hand, a possible precipitation of the mineral.

    The calculations of these indices show that the carbonate minerals (calcite, aragonite and dolomite) have different degrees of saturation.

    If we assume that the equilibrium state is in the range of -0.2 to +0.2; we can say that the three minerals raised are in a state of oversaturation for the majority of the water points, except for water points (Pz5, Pz11, Pz13 and Pz15) which show a state of under saturation (Fig. 7). The concentrations of the three minerals have the same evolution, which confirms the carbonated origin. Evaporitic minerals (gypsum, anhydrite and halite) show lower degrees of saturation than carbonate minerals. This indicates that the groundwater is very under saturated with respect to this mineral. The calculation of the saturation index of the different minerals in the water indicates that only the carbonate minerals tend to reach a state of oversaturation. On the other hand, the evaporitic minerals are always in the state of under saturation in spite of the high concentrations which they acquire. The solution is saturated or supersaturated with respect to the mineral, which tends to precipitate. On the other hand, the solution is undersaturated with respect to the mineral that tends to dissolve.

    Figure 7.  Variation of Minerals Saturation Index in the low water level periods in 2012 and 2017.

    In summary, the thermodynamic interpretation has shown the influence of evaporite minerals on the chemistry of water. Under saturation in gypsum, anhydrite and halite causes continuity in the dissolution and enrichment of water in these elements. In addition, carbonate minerals are sometimes close to equilibrium, often in oversaturation, and tend to precipitate in the form of calcite and dolomite.

  • The Br-/Cl- ratio provides information on the origin of salinity (Takrouni, 2003; Fedrigoni et al., 2001; Edmunds, 1996) and to characterize the origin of chlorine, and hence the origin of water and its mineralization (Hsissou et al., 1999; Andreasen and Fleck, 1997; Marjoua, 1995). The only sources of chlorides and bromides in groundwater are seawater (which has a weight ratio of 3.47×10-3) and the dissolution of halite (which has a weight ratio of 0.183×10-3) (Marjoua, 1995; Meybeck, 1984) and anthropogenic (wastewater) (Han et al., 2014; Stoecker et al., 2013). Normed with Chloride, the ratio Br-/Cl-, compared to that of the current seawater, can be used. From the results of our waters (Fig. 8a), the relationship between bromide and chloride ions shows a linear correlation (r=0.61). The Br-/Cl- ratio is often the most relevant for clarifying the origin of chlorides in groundwater (Rittenhouse, 1967); the Br-/Cl- weight ratios vary between 1.43‰ and 8.55‰. They can be grouped into two types:

    Figure 8.  (a) Relationship between Br and Cl and (b) evolution of the Br/Cl ratio according to the EC during the low water level period 2017.

    The first type: presented by points of waters where the ratio Br-/Cl- is lower than that of the sea; therefore the waters are influenced by the dissolution of the halite and the leaching of chlorinated fertilizers or the infiltration of untreated sewage.

    The second type: presented by the water points where the ratio of Br-/Cl- is near or exceeds that of the sea, these points are (w85-38, w83-34, Pz5A, Pz6A, Pz9A, Pz10A and Pz11A). Additionally, and according to Fisher's and Mullican's research in 1997, the origin of this salinity is explained, as that in the alluvial aquifers probably, as being derived from an old precipitation of the inland seawater that the region has known.

    The interpretation of the graph of the evolution of the Br-/Cl- ratio according to the electrical conductivity of the groundwater (Fig. 8b) has shown that most of the points are characterized by a Br-/Cl- ratio lower than 3.47‰ with a high concentration of EC.

  • In the Upper Cheliff water table some water points also indicate a very high concentration of nitrates, which is not associated with a large increase in chloride. These points indicate local pollution, probably of agricultural origin (chemical fertilizers). Chemical fertilizers are generally characterized by high levels of nitrogen and low chlorides. Domestic and animal effluents, on the other hand, have high Cl- concentrations and low NO3-/Cl- ratios (Li et al., 2013; Liu et al., 2006). Finally, the ratio NO3-/Cl- decreases in case of denitrification or consumption of nutrients by the plants without this modifying the concentrations of Cl-.

    Some points have a very high concentration of sulphates but contain only a few nitrates. They are due to the leaching of evaporite deposits (gypsum).

  • Deuterium (2H) and oxygen 18 (18O) are two stable isotopes of hydrogen and oxygen, and it is an integral part of the water molecule, hence the interest of their use as natural tracers of groundwater in the region. Stable isotopes are a powerful tool for determining the origin and history of waters, recharge areas and the relationships between aquifers. The results of isotopic analyses of oxygen 18 and deuterium are summarized in Table S1.

  • In order to better understand the hydrodynamics of the groundwater in the study area, thirty-nine (39) groundwater samples were collected in two periods (Table S1).

    Low water level period: the month of October 2017 and High water level period: the month of June 2018.

    To interpret the isotopic data of the study area, the stable isotope contents (oxygen 18 and deuterium) of the water molecule are shown in a δ18O/δ2H diagram, with reference to the input function (rainwater) which is represented by the Global Meteoric Water Line (GMWL). According to the results of analysis:

    For the 2017 low water level period, the groundwater of the Upper and Middle Cheliff alluvial aquifer has levels of Oxygen-18 ranging from -6.78‰ to -4.51‰ vs. Standard Mean Ocean Water (SMOW) with an average of -5.91‰ vs. SMOW. The deuterium content varies between -42.50‰ and -31.20‰ vs. SMOW with an average of -37.76‰ vs. SMOW.

    For the 2018 high water level period, oxygen 18 levels ranged from -6.44‰ to -4.23‰ vs. SMOW with an average of -5.60‰ vs. SMOW, while deuterium levels ranged from -42.40‰ and -29.70‰ vs. SMOW with an average of -37.22‰ vs. SMOW.

    The first reading of the analyzed samples shows that the most enriched values are recorded at the wells located along the Cheliff Valley near the piedmonts of the Doui Massif, Ouarsenis and Zaccar, which confirms the presence of a recent major local recharge realized from the Upper Middle Cheliff Basin relief.

    The results obtained (Table S1) were plotted on the δ2H=f18O) diagram (Figs. 9a, 9b).

    Figure 9.  Oxygen 18-deuterium relationship in the groundwater of the Upper and Middle Cheliff; ((a) Low water level period 2017; (b) high water level period 2018) and (c) relationship between chlorine levels and groundwater 18O levels in Upper and Middle Eastern Cheliff aquifers during the 2017 low water level period.

    The representative points of the different samples align well with a representation on a line of equation δ2H=4.73×(δ18O)-9.81 for the low water level period of 2017 and an equation line δ2H=4.38×(δ18O)-12.64 for the high water level period of 2018. These two lines are different from that of the global meteoric waters defined by Craig (1961). The oxygen 18-deuterium relationship over all sampled waters shows a very good correlation for the two periods (R2=0.86 for the low-water period 2017 and R2=0.77 for the high-water period 2018).

    On the other hand, in this diagram (18O/2H), the waters of the Upper and Middle Cheliff alluvial aquifer, are divided into two groups of water in both periods. The first group represents non-evaporated waters, located near the Global Meteoric Water Line (GMWL): δ2H=8δ18O+10‰ (SMOW, Craig, 1961). These waters come from a rapid infiltration of meteoric waters without any modification of their isotopic content and without having undergone evaporation phenomena in a semi-arid climate. However, the second group represents slightly evaporated waters, located below the global meteoric right (water points are: w84-8, w84-14, w84-39, w84-63, w84-155, w83-41, w83-48, w83-52, w83-56) for both periods. They are located in the southern part of the Cheliff Valley on the edges of the Jurassic-Primary massif of Ouarsenis (piedmont of the Amrouna Massif) and Doui with some points located at the northern part of the Cheliff Valley on the borders of Jurassico-Cretaceous massif of Zaccar and Djebel Gontas. These water points, deviating from the DMM, witness an isotopic fractionation due to evaporation, thus explaining the relative enrichment in 18O and 2H. These sampled waters are distributed around straight lines. This isotopic enrichment is related to the evaporation of rainwater before their infiltration.

    The intersection of the GMWL with these evaporation lines makes it possible to define the main origin of the underground waters of Upper and Middle Cheliff. It displays a value that ranges from -6.20‰ to -6.10‰ vs. SMOW for oxygen 18 and -40 to -38‰ vs. SMOW for deuterium. These values could represent the initial isotopic composition of the precipitation waters which contributed to the recharge of the aquifer before their infiltration and before any evaporation, indicating a recharge from relatively high reliefs (heights) such as the massifs of Ouarsenis (Djebel Amrouna) and Zaccar indicating the existence of an old groundwater reserve.

    It is also observed that the water points (Pz6, Pz7 and w84-185) are close to the Western Mediterranean Meteoric Water Line (WMMWL) δ2H=8δ18O+13.7 (Celle, 2000), and this demonstrates that the recharge of the water table in this part was made from rainwater of Mediterranean origin. The first group of water points is influenced by the coexistence of rains of Atlantic and Mediterranean origin.

    The temporal variations of the isotopic contents of the groundwater are extremely low for all the monitored waters, with some exceptions such as the piezometry (Pz 6A) where the excess in deuterium exceeds 13. The excess in deuterium "d" is between 3.04 and 14.62 in low water level in 2017 and between 2.44 and 13 in high water level in 2018. This decrease of the isotopic levels in oxygen 18 is accompanied by a significant decrease in mineralization of 833 mg/L. No seasonal variation is observed, not even of response to particular climatic events (dry year 2017 and wet year 2018).

    This temporal homogeneity of the isotopic signature of groundwater may be indicative of a relatively high residence time of water within the aquifer.

    On the other hand, it has been shown that the excess of deuterium in groundwater is more important in the period of low water level in 2017 than in high water level in 2018. The period of low water level in 2017 which makes summer dry and hot (this induces a strong evapotranspiration) and the period of high water level in 2018 which makes winter a rainy period (favorable to the infiltration of waters). Excess in deuterium may vary from one point to another and depends on local climatic and topographic contexts (Gonfiantini, 1996).

  • Chloride levels and δ18O values were correlated to verify whether the enrichment of the isotopic contents of the samples is related to an evaporation process or not (Bouchaou et al., 2008).

    The results of the representation of the 18O content according to the chloride concentrations (Fig. 9c) show that the waters of the Upper and Middle Cheliff aquifers can be subdivided into three distinct groups of water corresponding to a mixture of three types of water.

    The waters of the first group: have well homogenized waters with a content that varies between 1 and 30 mg/L of chlorides and δ18O values between -7‰ and -5.5‰.

    The second group presents the evaporated waters which are characterized by an enrichment in oxygen 18 with a content which varies between 1 and 30 mg/L of chlorides and δ18O values between -5.2‰ and -4.5‰. These are slightly evaporated waters corresponding to the water mass of the alluvial aquifers sampled in the points located in the southern part of the Cheliff Valley on the edge of the Jurassico-Primary massif of Ouarsenis, Doui and the border of Jurassico-Cretaceous massive of Zaccar, which proves that even if the evaporation exists its effect on the salinization of the waters is very limited.

    The third group presents water that is highly charged with chloride contents between 80 and 160 mg/L but isotopically non-evaporated with δ18O values included between -6.5‰ and -6‰. This category of water corresponds to the mass of alluvial groundwater in the Attaf region near Jebel Témoulga. The lack of correlation between chlorine contents and 18O values shows that the concentration of solutions by evaporation is not the cause of the increase in salinity of the water. This character of the waters is to be connected with the presence of the triassic salt formations under Jurassic limestones of Jebel Témoulga and their flow through Jurassic limestone formations and deep gypso-salt layers can explain the strong mineralization of the waters, which then contaminates the proximal alluvial aquifer.

  • The spatial distribution of these δ18O levels suggests a zonation of the water bodies in the aquifer system. In fact, we notice that the water enriched in stable isotopes is located in the parts where the depth of the static level of the aquifer is about 10 to 15 m maximum (Table S1). When the static level approaches the ground, this shows the impact of the evaporation process in the Upper Cheliff and Middle Eastern Cheliff plain, where the static levels are less profound. It is observed that the relationship between oxygen 18 and the depth of the water table is important until 10 to 12 m deep. The comparison of the piezometry between the period of low water 2017 and the period of the high water 2018 in relation with the oxygen 18 allowed to observe that the depth of the static level evolves according to several parameters such as: the precipitation, the effect of evaporation (high temperature), the effect of pumping, the nature of the roof of the aquifer (presence of clay) and the reduced permeability of formations that delay the infiltration of water to the depth.

    In general, deuterium depletion varies between -1‰ and -4‰ δ2H per 100 m and oxygen 18 depletion ranges from -0.15‰ to -0.5‰ δ18O/100 m (Clark and Fritz, 1997). The determination of average water infiltration altitudes is based on the use of the stable oxygen isotope of the water molecule (Blavoux and Létolle, 1995).

    The estimation of the recharge levels of the Upper and Middle Cheliff aquifers are based on the calculation of the altitudinal gradient (δ18O/Alt); yet, there is a lack of data in δ18O and δ2H on rainfall for the study area.

    Stable isotope levels at the national and international levels of the entire Mediterranean Basin (Algeria, Tunisia, Morocco and southern Europe) are mainly negative. The numerous studies carried out in the region include (Chkir and Zouari, 2008 (Morocco and Tunisia); Mohamed, 2012 (Mauritania); Kamel et al., 2006 (Tunisia); Mustapha et al., 2012 (Morocco); Aouidane, 2017 (Algeria); Belkoum and Houha, 2017 (Algeria)). The waters of the aquifer system of the entire Mediterranean Basin have very variable isotopic contents in space with locations where the water is enriched in stable isotopes (18O and 2H). They are centered on the following values: δ18O varies between -9‰ to -1‰ and δ2H varies between -45‰ to -29‰. The first indicator is the same climate (semi-arid), the content of different stable isotopes, the isotopic levels of groundwater in the same country vary in a range: from -6.78‰ to -4.23‰ for 18O in the Upper and Middle Chéliff Basin and from -8.83‰ to -1.62‰ δ18O in the Ain Oussara Basin (Mebrouk et al., 2003) and from -11.51‰ to -10.03‰ δ18O in the Remila Basin (W. Khenchela) (Aouidane, 2017) and in different countries for the Djérid Basin (Tunisia) of -4.90‰ to -3.19‰ δ18O (Kamel et al., 2006) and from -4.90‰ to -3.19‰ δ18O in the plain of Bahira (Morocco) (Mustapha et al., 2012). The second indicator is the same geological formation (Moi-Plio-Quaternary) in the basins of Upper and Middle Chéliff (Algeria), Essaouira (Morocco) and Jeffara (Tunisia), the content of different stable isotopes, In the Essaouira basin, the isotopic contents of plio-quaternary waters are between -3.7‰ and -4.7‰ δ18O and the Jeffara Basin, show oxygen-18 levels that vary between -7.34‰ and -4.08‰. This spatial heterogeneity in stable isotope levels makes even the structural elements that control flows and climate, but recharge areas are deferential. Isotope analyzes were carried out at a regional level to see the renewal of the aquifers between the southern part of the Zaccar, Boumaad and Dahra piedmont and the northern part of the Ouarsenis chain with the Upper and Middle Chéliff alluvial aquifers.

  • The application of geochemistry and isotopic techniques in north western Algeria is an important tool to understand the chemical composition, the origin and nature of waters, the understanding of the mechanisms governing chemistry water from the alluvial layer of Upper and Middle Cheliff and specify the mechanisms of this recharge.

    The groundwater of the Upper and Middle Cheliff alluvial aquifers exhibit significant variations in mineralization. The use of the chemical approach namely the major elements (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3- and NO3-) besides some trace elements (Br-) and stable isotope (18O, 2H), helped to understand the process of mineralization of water. The main origins of this mineralization are: (1) dissolution of evaporitic (halite) aquifer rock in the plains or evaporation of water before and during infiltration; (2) the possible existence of deep salty water upward in fault and flexure zones as a result of the rejection of the NW-SE fault truncating the eastern end of the Jebel Témoulga in Middle Eastern Cheliff alluvial beds; and (3) the infiltration of charged irrigation water in salts and fertilizers in the irrigated perimeters of the plain of Upper and Middle Cheliff and the phenomenon of base exchange.

    The waters are characterized by high salinity and the presence of two facies according to the Piper method, which are the calcium chloride facies, and the sodium chloride. This characteristic of the waters is to be connected with the presence of the triassic salt formations under Jurassic limestones of Jebel Témoulga and their flow through the Jurassic limestone formations and the deep gypso-salt layers (The fault throw in 1980 overflowed these waters) can explain the strong mineralization of the waters, which then contaminates the proximal alluvial aquifer.

    The use of the chemical approach (Br/Cl) allowed us to exclude the hypothesis of an increase in groundwater salinity of Upper and Middle Eastern Chéliff aquifers by dissolution of halite, or by evaporation of water before and during infiltration. This increase in water salinity does not appear to come from marine contamination. The high salinity of the waters, mainly due to chlorides, is due to the leaching of chlorinated fertilizers, salts not used by plants (especially chlorides) and the dissolution of evaporite deposits.

    The waters of the region of Upper and Middle Western Chéliff, shows a pollution by nitrates, where contents exceeding 50 mg/L are recorded. These high levels of nitrates can be explained by the presence of various sources of pollution mainly related to agriculture, livestock and urban practices (domestic and industrial discharges without treatment).

    The results of the isotopic analysis of the oxygen 18/deuterium relations which are at our disposal, showed generally that the existence of two types of fresh water: water that is the result of a current recharge coming from a rapid infiltration meteoric waters; and old waters, which could be in relation with exchanges with the aquifers near the massif of Ouarsenis.

    The 18O versus Cl- contents of these groundwaters would thus reflect a mixture of the recharge waters by the current rainfall on the basin and the saline waters coming up from the deep aquifer, of deep origin, as a result of the active fault NW-SE which truncates the eastern end of Jebel Témoulga.

    The isotopic contents seem to vary according to the depth of the static level of the water table, and would be more "positive" at the perimeter.

    This hydrochemical and isotopic study makes it possible to declare that the processes of mineralization and the origin of the salinity of groundwater are linked on the one hand to the lithological nature of the traversed geological formations, and, on the other hand, to the climatic phenomena causing evaporation of water from the water table.

  • The authors warmly thank the National Agency for Water Resources (ANRH), Nuclear Research Center of Algeria (CRNA) for the multiform support provided for the realization of this study. Our also thank Dr. Meddi Mohamed (Director of DAPGRS-ENSH) for Logistics and Dr. Cherchali Mohamed El-Hocine (Laboratory Manager CRNA) for performing isotope analyzes as well as to anonymous evaluators who have greatly helped to improve the quality of this manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1293-y.

    Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at http://doi.org/10.1007/s12583-020-1293-y.

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