# Identification of dominant hydrogeochemical processes for groundwaters in the Algerian Sahara supported by inverse modeling of chemical and isotopic data

Rabia Slimani, Abdelhamid Guendouz, Fabienne Trolard, Adnane Souffi

Moulla, Belhadj Hamdi-Aissa, Guilhem Bourrie

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## To cite this version:

Rabia Slimani, Abdelhamid Guendouz, Fabienne Trolard, Adnane Souffi Moulla, Belhadj Hamdi-Aissa, et al.. Identification of dominant hydrogeochemical processes for groundwaters in the Algerian Sahara supported by inverse modeling of chemical and isotopic data. Hydrology and Earth System Sciences, 2017, 21 (3), pp.1669-1691. 10.5194/hess-21-1669-2017. hal-01602771

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# Identification of dominant hydrogeochemical processes for groundwaters in the Algerian Sahara supported by inverse modeling of chemical and isotopic data

Rabia Slimani<fnref n="1" />, Abdelhamid Guendouz<fnref n="2" />, Fabienne Trolard<fnref n="3" />, Adnane Souffi Moulla<fnref n="4" />, Belhadj Hamdi-Aïssa<fnref n="1" />, and Guilhem Bourrié<fnref n="3" />

*Correspondence to:* Rabia Slimani (slm_rabia@yahoo.fr)

Received: 8 September 2015 – Discussion started: 1 February 2016

Revised: 31 October 2016 – Accepted: 20 November 2016 – Published: 21 March 2017

**Abstract.** Unpublished chemical and isotopic data taken in November 1992 from the three major Saharan aquifers, namely the Continental Intercalaire (CI), the Complexe Terminal (CT) and the phreatic aquifer (Phr), were integrated with original samples in order to chemically and isotopically characterize the largest Saharan aquifer system and investigate the processes through which groundwaters acquire their mineralization. Instead of classical Debye–Hückel extended law, a specific interaction theory (SIT) model, recently incorporated in PHREEQC 3.0, was used. Inverse modeling of hydrochemical data constrained by isotopic data was used here to quantitatively assess the influence of geochemical processes: at depth, the dissolution of salts from the geological formations during upward leakage without evaporation explains the transitions from CI to CT and to a first end member, a cluster of Phr (cluster I); near the surface, the dissolution of salts from sabkhas by rainwater explains another cluster of Phr (cluster II). In every case, secondary precipitation of calcite occurs during dissolution. All Phr waters result from the mixing of these two clusters together with calcite precipitation and ion exchange processes. These processes are quantitatively assessed by the PHREEQC model. Globally, gypsum dissolution and calcite precipitation were found to act as a carbon sink.

## 1 Introduction

A scientific study published in 2008 (OECD, 2008) showed that 85% of the world population lives in the driest half of the earth. More than 1 billion people residing in arid and semi-arid areas of the world have access to only few or nonrenewable water resources. The North-Western Sahara Aquifer System (NWSAS) is one of the largest confined reservoirs in the world and its huge water reserves are essentially composed of an old component. It is represented by two main deep aquifers, the Continental Intercalaire and the Complexe Terminal. This system covers a surface of more than 1 million km<fnref n="2" /> (700 000 km<fnref n="2" /> in Algeria, 80 000 km<fnref n="2" /> in Tunisia and 250 000 km<fnref n="2" /> in Libya). Due to the climatic conditions of Sahara, these formations are poorly renewed: about 1 billion m<fnref n="3" /> yr⁻<fnref n="1" /> essentially infiltrated in the piedmont of the Saharan Atlas in Algeria, as well as in the Jebel Dahar and Jebel Nafusa in Tunisia and Libya, respectively. However, the very large extension of the system as well as the great thickness of the aquifer layers has favored the accumulation of huge water reserves. Ouargla basin is located in the middle of the NWSAS and thus benefits from groundwater resources (Fig. 1) which are contained in the following three main reservoirs (UNESCO, 1972; Eckstein and Eckstein, 2003; OSS, 2003, 2008):

<footnote n="1">¹Ouargla University, Fac. des Sciences de la Nature et de la Vie, Lab. Biochimie des Milieux Désertiques, 30000 Ouargla, Algeria</footnote>

<footnote n="2">²Blida University, Science and Engineering Faculty, P.O. Box 270, Soumaâ, Blida, Algeria</footnote>

<footnote n="3">³INRA – UMR1114 EMMAH, Avignon, France</footnote>

<footnote n="4">⁴Algiers Nuclear Research Centre, P.O. Box 399, Alger-RP, 16000 Algiers, Algeria</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Location and schematic relations of aquifers in Ouargla. Blue lines represent limits between aquifers, and the names of aquifers are given in bold letters; as the limit between Senonian and Mio-Pliocene aquifers is not well defined, a dashed blue line is used. Names of villages and cities are given in roman (Bamendil, Ouargla, Sidi Khouiled), while geological/geomorphological features are in italic (glacis, sabkha, chott, dunes). Depths are relative to the ground surface. Letters “a” and “b” refer to the cross section (Fig. 2) and to the localization map (Fig. 3).</figcaption></figure>

- At the top, the phreatic aquifer (Phr), in the Quaternary sandy gypsum permeable formations of Quaternary, is almost unexploited, due to its extreme salinity (50 g L⁻¹).

- In the middle, the Complexe Terminal (CT) (Cornet and Gouscov, 1952; UNESCO, 1972) is the most exploited and includes several aquifers in different geological formations. Groundwater circulates in one or the two lithostratigraphic formations of the Eocene and Senonian carbonates or in the Mio-Pliocene sands.

- At the bottom, the Continental Intercalaire (CI), hosted in the lower Cretaceous continental formations (Barremian and Albian), mainly composed of sandstones, sands and clays. It is only partially exploited because of its significant depth.

The integrated management of these groundwaters is presently a serious issue for local water resources managers due to the large extension of the aquifers and the complexity of the relations between them. Several studies (Guendouz, 1985; Fontes et al., 1986; Guendouz and Moulla, 1996; Edmunds et al., 2003; Guendouz et al., 2003; Hamdi-Aïssa et al., 2004; Foster et al., 2006; OSS, 2008) started from chemical and isotopic information (²H, ¹⁸O, ²³⁴U, ²³⁸U, ³⁶Cl) to characterize the relationships between

aquifers. In particular, such studies focused on the recharge of the deep CI aquifer system. These investigations especially dealt with water chemical facies, mapped iso-contents of various parameters and reported typical geochemical ratios ([SO₄²⁻]/[Cl⁻], [Mg²⁺]/[Ca²⁺]) as well as other correlations. Minerals-solutions equilibria were checked by computing saturation indices with respect to calcite, gypsum, anhydrite and halite, but processes were only qualitatively assessed. The present study aims at applying for the first time ever in Algeria, inverse modeling to an extreme environment featuring a lack of data on a scarce natural resource (groundwater). New data were hence collected in order to characterize the hydrochemical and the isotopic composition of the major aquifers in the Saharan region of Ouargla. New possibilities offered by progress in geochemical modeling were used. The objective was also to identify the origin of the mineralization and the water-rock interactions that occur along the flow path. More specifically, inverse modeling of chemical reactions allows one to select the best conceptual model for the interpretation of the geochemical evolution of Ouargla aquifer system. The stepwise inversion strategy involves designing a list of scenarios (hypotheses) that take into consideration the most plausible combinations of geochemical processes that may occur within the studied medium. After resolving the scenarios in a stepwise manner, the one that provides the best conceptual geochemical model is then se-
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Geologic cross section in the region of Ouargla. The blue pattern used for chott and sabkha correspond to the limit of the saturated zone.</figcaption></figure>

lected, which allowed Dai et al. (2006) to optimize simul-
taneously transmissivities and geochemical transformations
in a confined aquifer. Inverse modeling with PHREEQC 3.0
was used here in a different way (only on geochemical data
but for several aquifers) to account for the modifications of
the composition of water along the flow path. At least two
chemical analyses of groundwater at different points of the
flow path, and a set of phases (minerals and/or gases) which
potentially react while water circulates, are needed to operate
the program (Charlton et al., 1996).

A number of assumptions are inherent to the application of
inverse geochemical modeling: (i) the two groundwater anal-
yses from the initial and the final boreholes should represent
groundwater that flows along the same flow path; (ii) dis-
persion and diffusion do not significantly affect groundwater
chemistry; (iii) a chemical steady state prevails in the ground-
water system during the time considered; and (iv) the mineral
phases used in the inverse calculation are or were present
in the aquifer (Zhu and Anderson, 2002). The soundness or
the validity of the results depends on a valid conceptualiza-
tion of the groundwater system, on the validity of the ba-
sic hydrogeochemical concepts and principles, on the accu-
racy of model input data and on the level of understanding of
the geochemical processes occurring in the area (Güler and
Thyne, 2004; Sharif et al., 2008). These requirements are ful-
filled in the region of Ouargla, which can be considered as
a “window” to the largest Saharan aquifer, and thus one of
the largest aquifers in the world in a semi-arid to hyper-arid
region subject to both global changes: urban sprawl and cli-
mate change. The methodology developed here and the data
collected can easily be integrated in the PRECOS framework
proposed for the management of environmental resources
(Trolard et al., 2016).

## 2 Methodology

2.1 Presentation of the study area

The study area is located in the northeastern desert of Algeria
(lower Sahara) (Le Houérou, 2009) near the city of Ouargla
(Fig. 1), 31°54′ to 32°1′ N and 5°15′ to 5°27′ E, with a mean
elevation of 134 (m a.s.l.). It is located in the quaternary val-
ley of Oued Mya basin. The present climate belongs to the
arid Mediterranean type (Dubief, 1963; Le Houérou, 2009;
ONM, 1975/2013), as it is characterized by a mean annual
temperature of 22.5 °C, a yearly rainfall of 43.6 mm yr⁻¹ and
a very high evaporation rate of 2138 mm yr⁻¹.

Ouargla's region and the entire lower Sahara has expe-
rienced during its long geological history alternating ma-
rine and continental sedimentation phases. During Secondary
era, vertical movements affected the Precambrian basement,
causing, in particular, collapse of its central part, along an
axis passing approximately through the Oued Righ Valley
and the upper portion of the Oued Mya Valley. According
to Furon (1960), a epicontinental sea spread to the Lower
Eocene of northern Sahara. After the Oligocene, the sea grad-
ually withdrew. It is estimated at present that this sea did
not reach Ouargla and transgression stopped at the edge of
the bowl (Furon, 1960; Lelièvre, 1969). The basin is carved
into Mio-Pliocene (MP) deposits, which alternate with red
sands, clays and sometimes marls; gypsum is not abundant
and dated from the Pontian era (during the MP) (Cornet and
Gouscov, 1952; Dubief, 1953; Ould Baba Sy and Besbes,
2006). The continental Pliocene consists of a local limestone
crust with puddingstone or lacustrine limestone (Fig. 2),
shaped by eolian erosion into flat areas (regs). The Quater-
nary formations are lithologically composed of alternating
layers of permeable sand and relatively impermeable marl
(Aumassip et al., 1972; Chellat et al., 2014).

The exploitation of Mio-Pliocene aquifer is ancient and at
the origin of the creation of the oasis (Lelièvre, 1969; Mou-
lias, 1927). The piezometric level was higher (145 m a.s.l.)
but overexploitation at the end of the 19th century led to a
catastrophic decrease of the resource, with presently more
than 900 boreholes (ANRH, 2011).

The exploitation of the Senonian aquifer dates back
to 1953 at a depth between 140 and 200 m, with a small ini-
tial rate of approximately 9 L s⁻¹; two boreholes have been
exploited since 1965 and 1969, with a total flow rate of ap-
proximately 42 L s⁻¹, for drinking water and irrigation.

The exploitation of the Albian aquifer dates back to 1956;
presently, two boreholes are exploited:

- El Hedeb I, 1335 m deep, with a flow rate of 141 L s⁻¹;

- El Hedeb II, 1400 m deep, with a flow rate of 68 L s⁻¹.

## 2.2 Sampling and analytical methods

The sampling programme consisted of collecting samples
along transects corresponding to directions of flow for both
the Phr and CT aquifers, while it was possible to collect only
eight samples from the CI. A total of 107 samples were col-
lected during a field campaign in 2013 along the main flow
path of Oued Mya. Of these, 67 were from piezometers tap-
ping the phreatic aquifer, 32 from CT wells and the last 8
from boreholes tapping the CI aquifer (Fig. 3). Analyses of
Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻ and HCO₃⁻ were per-
formed by ion chromatography at Algiers Nuclear Research
Center (CRNA). Previous and yet unpublished data (Guen-
douz and Moulla, 1996) sampled in 1992 are used here too:
59 samples for the Phr aquifer, 15 samples for the CT aquifer
and 3 samples for the CI aquifer for chemical analyses and
data of ¹⁸O and ³H (Guendouz and Moulla, 1996).

## 2.3 Geochemical method

PHREEQC was used to check minerals-solutions equilibria
using the specific interaction theory (SIT), i.e., the extension
of Debye-Hückel law by Scatchard and Guggenheim incor-
porated recently in PHREEQC 3.0 (Parkhurst and Appelo,
2013). Inverse modeling was used to calculate the number of
minerals and gases' moles that must, respectively, dissolve
or precipitate/degas to account for the difference in compo-
sition between initial and final water end members (Plum-
mer and Back, 1980; Kenoyer and Bowser, 1992; Deutsch,
1997; Plummer and Sprinckle, 2001; Güler and Thyne, 2004;
Parkhurst and Appelo, 2013). This mass balance technique
has been used to quantify reactions controlling water chem-
istry along flow paths (Thomas et al., 1989). It is also used to
quantify the mixing proportions of end-member components
in a flow system (Kuells et al., 2000; Belkhiri et al., 2010,
2012).

Inverse modeling involves designing a list of scenarios
(modeling setups) that take into account the most plausible
combinations of geochemical processes that are likely to oc-
cur in our system. For example, the way to identify whether
calcite dissolution/precipitation is relevant or not consists of

solving the inverse problem under two alternate scenarios:
(1) considering a geochemical system in which calcite is
present and (2) considering a geochemical system without
calcite. After simulating the two scenarios, it is usually possi-
ble to select the setup that gives the best results as the solution
to the inverse modeling according to the fit between the mod-
eled and observed values. Then one can conclude whether
calcite dissolution/precipitation is relevant or not. This step-
wise strategy allows us to identify the relevance of a given
chemical process by inversely solving the problem through
alternate scenarios in which the process is either participat-
ing or not (Dai et al., 2006).

In the geochemical modeling inverse, soundness of results
is dependent upon valid conceptualization of the system, va-
lidity of basic concepts and principles, accuracy of input data
and level of understanding of the geochemical processes. We
use the information from the lithology, general hydrochemi-
cal evolution patterns, saturation indices and mineral stability
diagrams to constrain the inverse models.

## 3 Results and discussion

Tables 1 to 4 illustrate the results of the chemical and the iso-
topic analyses. Samples are ordered according to an increas-
ing electric conductivity (EC), and this is assumed to provide
an order for increasing salt content. In both the phreatic and
CT aquifers, temperature is close to 25 °C, while for the CI
aquifer, temperature is close to 50 °C. The values presented
in Tables 1 to 5 are raw analytical data that were corrected for
defects of charge balance before computing activities with
PHREEQC. As analytical errors could not be ascribed to
a specific analyte, the correction was made proportionally.
The corrections do not affect the anion-to-anion mole ratios,
such as for [HCO₃⁻]/([Cl⁻] + 2[SO₄²⁻]) or [SO₄²⁻]/[Cl⁻],
whereas they affect the cation-to-anion ratio, such as for
[Na⁺]/[Cl⁻].

### 3.1 Characterization of chemical facies of the groundwater

Piper diagrams drawn for the studied groundwaters (Fig. 4)
broadly show a scatter plot dominated by a sodium chlo-
ride facies. However, when going into small details, the
widespread chemical facies of the Phr aquifer are closer to
the NaCl cluster than those of the CI and CT aquifers. Re-
spectively, CaSO₄, Na₂SO₄, MgSO₄ and NaCl are the most
dominant chemical species (minerals) that are present in the
phreatic waters. This sequential order of solutes is compa-
rable to that of other groundwater occurring in north Africa
and especially in the neighboring area of the chotts (depres-
sions where salts concentrate by evaporation) Merouane and
Melrhir (Vallès et al., 1997; Hamdi-Aïssa et al., 2004).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Location map of sampling points.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Piper diagram for the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers.</figcaption></figure>

## 3.2 Spatial distribution of the mineralization

The salinity of the phreatic aquifer varies considerably depending on the location (namely, the distance from wells or drains) and time (due to the influence of irrigation) (Fig. 5a).

Its salinity is low around irrigated and fairly well-drained areas, such as the palm groves of Hassi Miloud, just north of Ouargla (Fig. 3), that benefit from freshwater and are drained to the sabkha Oum El Raneb. However, the three lowest salinity values are observed in the wells of the Ouargla palm grove itself, where the Phr aquifer water table is deeper than 2 m.

Conversely, the highest salinity waters are found in wells drilled in the chotts and sabkhas (a sabkha is the central part of a chott where salinity is the largest) (Safioune and Oum er Raneb) where the aquifer is often shallower than 50 cm.

The salinity of the CT (Mio-Pliocene) aquifer (Fig. 5b) is much lower than that of the Phr aquifer and ranges from 1 to 2 g L⁻¹; however, its hardness is larger and it contains more sulfate, chloride and sodium than the waters of the Senonian formations and those of the CI aquifer. The salinity of the Senonian aquifer ranges from 1.1 to 1.7 g L⁻¹, while the average salinity of the CI aquifer is 0.7 g L⁻¹ (Fig. 5c).

A likely contamination of the Mio-Pliocene aquifer by phreatic groundwaters through casing leakage in an area where water is heavily loaded with salt, and therefore particularly aggressive, cannot be excluded.

## 3.3 Saturation indices

The calculated saturation indices (SIs) reveal that waters from CI at 50 °C are close to equilibrium with respect to calcite, except for three samples that are slightly oversaturated.

They are, however, all undersaturated with respect to gypsum (Fig. 6).

Moreover, they are oversaturated with respect to dolomite and undersaturated with respect to anhydrite and halite (Fig. 7).

Waters from the CT and phreatic aquifers show the same pattern, but some of them are more largely oversaturated with respect to calcite, at 25 °C.

However, several phreatic waters (P031, P566, PLX4, PL18, P002, P023, P116, P066, P162 and P036) that are located in the sabkhas of Safioune, Oum er Raneb, Bamendil and Aïn el Beïda's chott are saturated with gypsum and anhydrite. This is in accordance with highly evaporative environments found elsewhere (UNESCO, 1972; Hamdi-Aïssa et al., 2004; Slimani, 2006).

No significant trend of SI from south to north upstream and downstream of Oued Mya (Fig. 7) is observed. This suggests that the acquisition of mineralization is due to geochemical processes that have already reached equilibrium or steady state in the upstream areas of Ouargla.

## 3.4 Change of facies from the carbonated cluster to the evaporites' cluster

The facies shifts progressively from the carbonated cluster (CI and CT aquifers) to the evaporites' cluster (Phr aquifer) with an increase in sulfates and chlorides at the expense of carbonates (SI of gypsum, anhydrite and halite). This is illustrated by a decrease of the $[HCO_3^-]/([Cl^-] + 2[SO_4^{2-}])$ ratio (Fig. 8) from 0.2 to 0 and of the $[SO_4^{2-}]/[Cl^-]$ ratio from 0.8 to values smaller than 0.3 (Fig. 9) while salinity increases. Carbonate concentrations tend towards very small values, while it is not the case for sulfates. This is due to both gypsum dissolution and calcite precipitation. Chlorides in groundwater may come from three different sources: (i) ancient sea water entrapped in sediments, (ii) dissolution of halite and related minerals that are present in evaporite deposits and (iii) dissolution of dry fallout from the atmosphere, particularly in these arid regions (Matiatos et al., 2014; Hadj-Ammar et al., 2014).

The $[Na^+]/[Cl^-]$ ratio ranges from 0.85 to 1.26 for the CI aquifer, from 0.40 to 1.02 for the CT aquifer and from 0.13 to 2.15 for the Phr aquifer. The measured points from the three considered aquifers are linearly scattered with good approximation around the unity slope straight line that stands for halite dissolution (Fig. 10). The latter appears as the most dominant reaction occurring in the medium. However, at very high salinity, $Na^+$ seems to swerve from the straight line towards smaller values.

A further scrutiny of Fig. 10 shows that CI waters are very close to the 1:1 line. CT waters are enriched in both $Na^+$ and $Cl^-$ but slightly lower than the 1:1 line while phreatic waters are largely enriched and much more scattered. CT waters are closer to the seawater mole ratio (0.858), but some lower values imply a contribution from a source of
<table><caption>**Table 1.** Field and analytical data for the Continental Intercalaire aquifer.</caption><thead><tr><th rowspan="2">Locality</th><th rowspan="2">Lat.</th><th rowspan="2">Long.</th><th rowspan="2">Elev.</th><th rowspan="2">Date</th><th rowspan="2">EC<br>(mS cm<sup>-1</sup>)</th><th rowspan="2">T<br>(°C)</th><th rowspan="2">pH</th><th rowspan="2">Alk.</th><th rowspan="2">Cl<sup>-</sup></th><th rowspan="2">SO<sub>4</sub><sup>2-</sup></th><th rowspan="2">Na<sup>+</sup></th><th rowspan="2">K<sup>+</sup></th><th rowspan="2">Mg<sup>2+</sup></th><th rowspan="2">Ca<sup>2+</sup></th><th rowspan="2">Br<sup>-</sup></th></tr><tr><th>(mmol L<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Hedeb I</td><td>3 534 750</td><td>723 986</td><td>134</td><td>9 Nov 2012</td><td>2.0</td><td>46.5</td><td>7.6</td><td>3.5</td><td>5.8</td><td>6.8</td><td>10.7</td><td>0.6</td><td>2.5</td><td>3.3</td><td>0.034</td></tr><tr><td>Hedeb I</td><td>3 534 750</td><td>723 986</td><td>134</td><td>1992</td><td>1.9</td><td>49.3</td><td>7.3</td><td>0.4</td><td>5.8</td><td>1.1</td><td>5.7</td><td>0.2</td><td>0.8</td><td>0.5</td><td></td></tr><tr><td>Hedeb II</td><td>3 534 310</td><td>724 290</td><td>146</td><td>1992</td><td>2.0</td><td>47.4</td><td>7.6</td><td>0.6</td><td>6.2</td><td>1.2</td><td>5.1</td><td>0.2</td><td>1.3</td><td>0.8</td><td></td></tr><tr><td>Aouinet Moussa</td><td>3 548 896</td><td>721 076</td><td>132</td><td>1992</td><td>2.2</td><td>48.9</td><td>7.5</td><td>1.3</td><td>6.5</td><td>1.9</td><td>5.6</td><td>0.2</td><td>1.1</td><td>1.2</td><td></td></tr><tr><td>Aouinet Moussa</td><td>3 548 896</td><td>721 076</td><td>132</td><td>22 Feb 2013</td><td>2.2</td><td>48.9</td><td>7.5</td><td>3.2</td><td>9.8</td><td>3.9</td><td>6.3</td><td>0.7</td><td>5.7</td><td>1.3</td><td></td></tr><tr><td>Hedeb I</td><td>3 534 750</td><td>723 986</td><td>134</td><td>11 Dec 2010</td><td>2.2</td><td>49.3</td><td>7.3</td><td>1.9</td><td>12.4</td><td>4.6</td><td>10.7</td><td>0.7</td><td>3.8</td><td>2.3</td><td></td></tr><tr><td>Hedeb II</td><td>3 534 310</td><td>724 290</td><td>146</td><td>11 Dec 2010</td><td>2.2</td><td>47.4</td><td>7.6</td><td>2.1</td><td>13.1</td><td>5.5</td><td>13.9</td><td>0.5</td><td>4.5</td><td>1.4</td><td></td></tr><tr><td>Hassi Khfif</td><td>3 591 659</td><td>721 636</td><td>110</td><td>24 Feb 2013</td><td>2.4</td><td>50.5</td><td>6.8</td><td>2.9</td><td>14.3</td><td>5.2</td><td>10.8</td><td>0.8</td><td>3.4</td><td>4.6</td><td>0.033</td></tr><tr><td>Hedeb I</td><td>3 534 750</td><td>723 986</td><td>134</td><td>27 Feb 2013</td><td>2.0</td><td>46.5</td><td>7.6</td><td>3.5</td><td>15.1</td><td>7.7</td><td>11.8</td><td>0.5</td><td>5.6</td><td>5.2</td><td></td></tr><tr><td>Hassi Khfif</td><td>3 591 659</td><td>721 636</td><td>110</td><td>9 Nov 2012</td><td>2.2</td><td>50.1</td><td>7.6</td><td>3.3</td><td>15.3</td><td>7.8</td><td>12.2</td><td>0.6</td><td>5.8</td><td>4.9</td><td></td></tr><tr><td>El Bour</td><td>3 560 264</td><td>720 366</td><td>160</td><td>22 Feb 2013</td><td>2.9</td><td>54.5</td><td>7.3</td><td>2.6</td><td>18.6</td><td>6.2</td><td>20.6</td><td>0.7</td><td>4.8</td><td>1.4</td><td></td></tr></tbody></table>

chloride other than halite or from entrapped seawater. Conversely, a $[Na^+]/[Cl^-]$ ratio larger than 1 is observed for phreatic waters, which implies the contribution of another source of sodium, most likely sodium sulfate, that is present as mirabilite or thenardite in the chotts and the sabkha areas.

The $[Br^-]/[Cl^-]$ ratio ranges from $2 \times 10^{-3}$ to $3 \times 10^{-3}$. The value of this molar ratio for halite is around $2.5 \times 10^{-3}$, which matches the aforementioned range and confirms that halite dissolution is the most dominant reaction taking place in the studied medium.

In the CI, CT and Phr aquifers, calcium originates both from carbonate and sulfate (Figs. 11 and 12). Three samples from the CI aquifer are close to the $[Ca^{2+}]/[HCO_3^-]$ 1:2 line, while calcium sulfate dissolution explains the excess of calcium. However, nine samples from the Phr aquifer are depleted in calcium, and plotted under the $[Ca^{2+}]/[HCO_3^-]$ 1:2 line. This cannot be explained by precipitation of calcite, as some are undersaturated with respect to that mineral, while others are oversaturated.

In this case, a cation exchange process seems to occur and lead to a preferential adsorption of divalent cations, with a release of $Na^+$. This is confirmed by the inverse modeling that is developed below and which implies $Mg^{2+}$ fixation and $Na^+$ and $K^+$ releases.

Larger sulfate values observed in the phreatic aquifer (Fig. 12) with $[Ca^{2+}]/[SO_4^{2-}] < 1$ can be attributed to a $Na^+$-Mg sulfate dissolution from a mineral bearing such elements. This is, for instance, the case of bloedite.

### 3.5 Isotope geochemistry

The CT and CI aquifers exhibit depleted and homogeneous $^{18}$O contents, ranging from -8.32 to -7.85‰. This was already previously reported by many authors (Edmunds et al., 2003; Guendouz et al., 2003; Moulla et al., 2012). On the other hand, $^{18}$O values for the phreatic aquifer are widely dispersed and vary between -8.84 and 3.42‰ (Table 4). Waters located north of the virtual line connecting, approximately, Hassi Miloud to Sebkhet Safioune, are found more enriched in heavy isotopes and are thus more evaporated. In that area, the water table is close to the surface and mixing of both CI and CT groundwaters with phreatic ones through irrigation is nonexistent. Conversely, waters located south of Hassi Miloud up to Ouargla city show depleted values. This is the clear fingerprint of a contribution to the Phr waters from the underlying CI and CT aquifers (Gonfiantini et al., 1975; Guendouz, 1985; Fontes et al., 1986; Guendouz and Moulla, 1996).

Phreatic waters result from a mixing of two end members. Evidence for this is given by considering the $[Cl^-]$ and $^{18}$O relationship (Fig. 13). The two clusters are (i) a first cluster of $^{18}$O-depleted groundwater (Fig. 14), and (ii) another cluster of $^{18}$O-enriched groundwater with positive values and a high salinity. The latter is composed of phreatic waters occurring in the northern part of the study region.
<table><caption>**Table 2.** Field and analytical data for the Complexe Terminal aquifer.</caption><thead><tr><th>Locality</th><th>Site</th><th>Aquifer</th><th>Lat.</th><th>Long.</th><th>Elev.</th><th>Date</th><th>EC<br>(mS cm<sup>-1</sup>)</th><th>T<br>(°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr></thead><tbody><tr><td>Bamendil</td><td>D7F4</td><td>M</td><td>3 560 759</td><td>720 586</td><td>296</td><td>20 Jan 2013</td><td>2.0</td><td>20.1</td><td>7.9</td><td>1.6</td><td>10.1</td><td>5.8</td><td>9.9</td><td>0.7</td><td>3.9</td><td>2.5</td></tr><tr><td>Bamendil</td><td>D7F4</td><td>M</td><td>3 560 759</td><td>720 586</td><td>296</td><td>1992</td><td>2.0</td><td>21.1</td><td>8.2</td><td>0.9</td><td>10.6</td><td>3.5</td><td>10.6</td><td>0.1</td><td>2.3</td><td>1.8</td></tr><tr><td>Ifri</td><td>D1F151</td><td>S</td><td>3 538 891</td><td>721 060</td><td>204</td><td>1992</td><td>2.7</td><td>23.5</td><td>7.0</td><td>1.3</td><td>10.7</td><td>2.7</td><td>8.0</td><td>0.7</td><td>2.3</td><td>2.1</td></tr><tr><td>Said Otba</td><td>D2F66</td><td>S</td><td>3 540 257</td><td>720 085</td><td>216</td><td>1992</td><td>2.3</td><td>24.0</td><td>8.0</td><td>1.4</td><td>11.0</td><td>4.7</td><td>11.5</td><td>0.2</td><td>2.1</td><td>3.3</td></tr><tr><td>Oglat Larbaâ</td><td>D6F64</td><td>M</td><td>3 566 501</td><td>729 369</td><td>177</td><td>1992</td><td>2.3</td><td>18.0</td><td>7.9</td><td>1.4</td><td>11.4</td><td>6.8</td><td>11.6</td><td>2.3</td><td>2.0</td><td>4.6</td></tr><tr><td>El Bour</td><td>D4F94</td><td>M</td><td>3 536 245</td><td>722 641</td><td>100</td><td>27 Jan 2013</td><td>3.1</td><td>26.2</td><td>7.4</td><td>1.6</td><td>12.8</td><td>6.8</td><td>5.2</td><td>1.9</td><td>1.6</td><td>9.1</td></tr><tr><td>Said Otba I</td><td>D2F71</td><td>S</td><td>3 557 412</td><td>718 272</td><td>211</td><td>1992</td><td>2.3</td><td>24.2</td><td>8.2</td><td>1.5</td><td>13.5</td><td>5.7</td><td>15.0</td><td>0.3</td><td>3.3</td><td>2.6</td></tr><tr><td>Debiche</td><td>D6F61</td><td>M</td><td>3 547 557</td><td>717 067</td><td>173</td><td>26 Jan 2013</td><td>2.2</td><td>23.9</td><td>7.7</td><td>1.8</td><td>14.2</td><td>8.4</td><td>12.6</td><td>0.7</td><td>5.4</td><td>4.4</td></tr><tr><td>Rouissat III</td><td>D3F10</td><td>S</td><td>3 535 068</td><td>718 272</td><td>212</td><td>26 Jan 2013</td><td>5.6</td><td>25.1</td><td>7.3</td><td>2.4</td><td>14.3</td><td>6.9</td><td>13.1</td><td>0.4</td><td>3.4</td><td>5.4</td></tr><tr><td>Said Otba I</td><td>D2F71</td><td>S</td><td>3 557 412</td><td>718 272</td><td>212</td><td>26 Jan 2013</td><td>5.6</td><td>25.1</td><td>7.3</td><td>2.4</td><td>14.3</td><td>6.9</td><td>13.1</td><td>0.4</td><td>3.4</td><td>5.4</td></tr><tr><td>Rouissat III</td><td>D3F10</td><td>S</td><td>3 535 068</td><td>722 352</td><td>248</td><td>1992</td><td>3.1</td><td>26.1</td><td>7.3</td><td>2.4</td><td>14.3</td><td>6.9</td><td>13.1</td><td>0.4</td><td>3.4</td><td>5.4</td></tr><tr><td>Ain N'Sara</td><td>D6F50</td><td>S</td><td>3 539 323</td><td>716 868</td><td>255</td><td>25 Jan 2013</td><td>3.4</td><td>25.7</td><td>7.4</td><td>2.0</td><td>16.9</td><td>9.7</td><td>15.9</td><td>0.3</td><td>3.4</td><td>7.9</td></tr><tr><td>Ain N'Sara</td><td>D6F50</td><td>S</td><td>3 539 323</td><td>716 868</td><td>255</td><td>25 Jan 2013</td><td>3.4</td><td>25.7</td><td>7.4</td><td>2.0</td><td>16.9</td><td>9.7</td><td>15.9</td><td>0.3</td><td>3.4</td><td>7.9</td></tr><tr><td>A. Louise</td><td>D4F73</td><td>S</td><td>3 537 523</td><td>721 904</td><td>310</td><td>26 Jan 2013</td><td>2.6</td><td>24.0</td><td>7.5</td><td>2.0</td><td>17.4</td><td>9.1</td><td>13.9</td><td>2.0</td><td>5.8</td><td>5.1</td></tr><tr><td>Glâralet A. H</td><td>D6F79</td><td>M</td><td>3 598 750</td><td>720 356</td><td>119</td><td>2 Feb 2013</td><td>2.8</td><td>22.5</td><td>7.5</td><td>3.5</td><td>17.4</td><td>9.3</td><td>16.6</td><td>0.6</td><td>6.2</td><td>4.9</td></tr><tr><td>Ain Moussa II</td><td>D9F30</td><td>S</td><td>3 537 814</td><td>719 665</td><td>220</td><td>2 Feb 2013</td><td>7.5</td><td>23.9</td><td>7.5</td><td>2.4</td><td>17.5</td><td>8.2</td><td>17.3</td><td>0.4</td><td>3.1</td><td>6.5</td></tr><tr><td>Ain N'Sara</td><td>D6F50</td><td>S</td><td>3 559 323</td><td>716 868</td><td>255</td><td>2 Feb 2013</td><td>2.6</td><td>23.8</td><td>7.6</td><td>2.1</td><td>17.7</td><td>9.2</td><td>15.5</td><td>1.1</td><td>6.1</td><td>4.7</td></tr><tr><td>H. Miloud</td><td>D1F135</td><td>M</td><td>3 547 557</td><td>717 067</td><td>173</td><td>3 Feb 2013</td><td>2.8</td><td>21.6</td><td>7.5</td><td>3.3</td><td>17.9</td><td>9.2</td><td>16.5</td><td>1.0</td><td>6.2</td><td>4.9</td></tr><tr><td>El Bour</td><td>D6F97</td><td>S</td><td>3 540 936</td><td>715 816</td><td>169</td><td>25 Jan 2013</td><td>2.6</td><td>19.9</td><td>8.0</td><td>2.1</td><td>17.9</td><td>9.3</td><td>15.8</td><td>1.6</td><td>5.8</td><td>4.7</td></tr><tr><td>H. Miloud</td><td>D1F135</td><td>M</td><td>3 547 557</td><td>717 067</td><td>173</td><td>1992</td><td>2.1</td><td>22.7</td><td>8.1</td><td>2.8</td><td>18.1</td><td>5.7</td><td>16.6</td><td>0.5</td><td>3.6</td><td>4.3</td></tr><tr><td>N'Goussa El Hou</td><td>D6F51</td><td>S</td><td>3 556 256</td><td>718 979</td><td>198</td><td>31 Jan 2013</td><td>2.9</td><td>22.9</td><td>7.5</td><td>2.0</td><td>18.4</td><td>9.6</td><td>17.1</td><td>0.5</td><td>6.2</td><td>5.0</td></tr><tr><td>El Koum</td><td>D6F67</td><td>S</td><td>3 573 694</td><td>721 639</td><td>143</td><td>21 Jan 2013</td><td>3.1</td><td>22.9</td><td>8.1</td><td>3.5</td><td>18.4</td><td>9.7</td><td>17.9</td><td>0.3</td><td>6.5</td><td>5.1</td></tr><tr><td>El Koum</td><td>D6F67</td><td>S</td><td>3 573 694</td><td>721 639</td><td>143</td><td>1992</td><td>2.5</td><td>25.0</td><td>7.6</td><td>1.5</td><td>18.8</td><td>7.2</td><td>10.2</td><td>3.4</td><td>5.0</td><td>5.8</td></tr><tr><td>Itas</td><td>D1F150</td><td>M</td><td>3 536 186</td><td>717 046</td><td>93</td><td>21 Jan 2013</td><td>3.7</td><td>23.9</td><td>7.5</td><td>1.5</td><td>18.8</td><td>7.1</td><td>10.1</td><td>3.4</td><td>5.0</td><td>5.8</td></tr><tr><td>Ain Moussa V</td><td>D9F13</td><td>M</td><td>3 538 409</td><td>718 680</td><td>210</td><td>8 Feb 2013</td><td>2.4</td><td>25.3</td><td>7.2</td><td>2.3</td><td>19.4</td><td>9.4</td><td>18.8</td><td>0.4</td><td>3.3</td><td>7.6</td></tr><tr><td>El Bour</td><td>D4F94</td><td>M</td><td>3 536 245</td><td>722 641</td><td>100</td><td>1992</td><td>2.3</td><td>21.2</td><td>7.9</td><td>1.6</td><td>20.1</td><td>7.2</td><td>12.1</td><td>2.6</td><td>5.8</td><td>5.2</td></tr><tr><td>Rouissat I</td><td>D3F18</td><td>M</td><td>3 555 564</td><td>722 498</td><td>80</td><td>26 Jan 2013</td><td>3.1</td><td>23.0</td><td>8.1</td><td>3.2</td><td>21.2</td><td>11.1</td><td>19.6</td><td>0.9</td><td>7.1</td><td>6.0</td></tr><tr><td>Rouissat I</td><td>D3F18</td><td>M</td><td>3 555 564</td><td>722 498</td><td>80</td><td>1992</td><td>2.0</td><td>20.0</td><td>7.8</td><td>1.7</td><td>21.7</td><td>8.5</td><td>17.7</td><td>1.2</td><td>5.1</td><td>6.0</td></tr><tr><td>Station de Pompage chott</td><td>D5F80</td><td>S</td><td>3 541 656</td><td>723 521</td><td>224</td><td>4 Feb 2013</td><td>3.3</td><td>24.5</td><td>8.2</td><td>3.9</td><td>22.1</td><td>11.9</td><td>19.9</td><td>2.1</td><td>7.6</td><td>6.3</td></tr><tr><td>Chott Palmeraie</td><td>D5F77</td><td>S</td><td>3 538 219</td><td>725 541</td><td>243</td><td>5 Feb 2013</td><td>3.4</td><td>24.6</td><td>7.5</td><td>3.3</td><td>22.3</td><td>12.1</td><td>20.9</td><td>1.2</td><td>8.3</td><td>5.8</td></tr><tr><td>Bour el Aïcha</td><td>D1F134</td><td>M</td><td>3 545 533</td><td>720 391</td><td>86</td><td>5 Feb 2013</td><td>3.4</td><td>22.2</td><td>7.3</td><td>4.1</td><td>23.2</td><td>12.2</td><td>21.2</td><td>1.5</td><td>8.6</td><td>6.0</td></tr><tr><td>Abazat</td><td>D2F69</td><td>M</td><td>3 552 504</td><td>712 786</td><td>137</td><td>3 Feb 2013</td><td>3.5</td><td>24.6</td><td>7.6</td><td>2.2</td><td>24.7</td><td>12.7</td><td>2.1</td><td>1.7</td><td>8.5</td><td>6.5</td></tr><tr><td>Garet Chemia</td><td>D1F113</td><td>S</td><td>3 536 174</td><td>716 808</td><td>213</td><td>28 Jan 2013</td><td>4.1</td><td>28.0</td><td>7.3</td><td>2.2</td><td>25.9</td><td>9.5</td><td>25.4</td><td>0.6</td><td>3.6</td><td>7.2</td></tr><tr><td>Frane</td><td>D6F62</td><td>M</td><td>3 570 175</td><td>717 133</td><td>167</td><td>27 Jan 2013</td><td>3.8</td><td>24.2</td><td>7.9</td><td>2.3</td><td>25.9</td><td>13.5</td><td>22.6</td><td>0.6</td><td>8.9</td><td>7.2</td></tr></tbody></table>
<table><caption>Table 2. Continued.</caption><thead><tr><th>Locality</th><th>Site</th><th>Aquifer</th><th>Lat.</th><th>Long.<br>(m)</th><th>Elev.</th><th>Date</th><th>EC<br>(mS cm<sup>-1</sup>)</th><th>T<br>(°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr></thead><tbody><tr><td>Oum er Raneb</td><td>D6 F69</td><td>M</td><td>3 540 451</td><td>721 919</td><td>216</td><td>25 Jan 2013</td><td>4.2</td><td>24.1</td><td>7.0</td><td>2.6</td><td>27.9</td><td>8.7</td><td>22.9</td><td>0.6</td><td>4.4</td><td>8.0</td><td>0.035</td></tr><tr><td>N'Goussa El Hou</td><td>D6F51</td><td>S</td><td>3 556 256</td><td>718 979</td><td>198</td><td>1992</td><td>3.1</td><td>23.2</td><td>8.0</td><td>2.6</td><td>28.4</td><td>8.6</td><td>23.1</td><td>0.6</td><td>4.5</td><td>8.0</td><td></td></tr><tr><td>H. Miloud Benyaza</td><td>D1F138</td><td>M</td><td>3 551 192</td><td>717 042</td><td>89</td><td>28 Jan 2013</td><td>3.8</td><td>25.2</td><td>7.6</td><td>2.4</td><td>28.4</td><td>14.2</td><td>23.9</td><td>1.7</td><td>10.0</td><td>7.1</td><td></td></tr><tr><td>Ain Larbaâ</td><td>D6F49</td><td>M</td><td>3 558 822</td><td>716 799</td><td>156</td><td>28 Jan 2013</td><td>3.9</td><td>23.7</td><td>7.3</td><td>2.2</td><td>28.9</td><td>9.0</td><td>23.9</td><td>0.5</td><td>5.0</td><td>7.7</td><td>0.037</td></tr><tr><td>H. Miloud Benyaza</td><td>D1F138</td><td>M</td><td>3 551 192</td><td>717 042</td><td>89</td><td>1992</td><td>2.9</td><td>22.8</td><td>7.5</td><td>2.2</td><td>28.9</td><td>9.1</td><td>23.9</td><td>0.5</td><td>5.0</td><td>7.7</td><td></td></tr><tr><td>Rouissat</td><td>D3F8</td><td>M</td><td>3 545 470</td><td>732 837</td><td>332</td><td>3 Feb 2013</td><td>4.4</td><td>25.4</td><td>7.5</td><td>1.7</td><td>29.8</td><td>8.3</td><td>22.8</td><td>1.2</td><td>6.2</td><td>6.1</td><td></td></tr><tr><td>Rouissat</td><td>D3F8</td><td>M</td><td>3 545 470</td><td>732 837</td><td>332</td><td>1992</td><td>6.2</td><td>25.3</td><td>7.2</td><td>1.7</td><td>29.8</td><td>8.3</td><td>22.9</td><td>1.2</td><td>6.2</td><td>6.1</td><td></td></tr><tr><td>Ain El Arch</td><td>D3F26</td><td>M</td><td>3 534 843</td><td>723 381</td><td>93</td><td>1992</td><td>5.1</td><td>25.1</td><td>7.4</td><td>1.6</td><td>34.7</td><td>8.9</td><td>24.0</td><td>0.9</td><td>8.4</td><td>6.5</td><td></td></tr><tr><td>Station de Pompage chott</td><td>D5F80</td><td>S</td><td>3 541 656</td><td>723 521</td><td>224</td><td>1992</td><td>3.7</td><td>25.4</td><td>7.7</td><td>2.3</td><td>42.2</td><td>13.5</td><td>36.8</td><td>1.1</td><td>7.4</td><td>9.7</td><td></td></tr></tbody></table>

M = Mio-Pliocene aquifer; S = Senonian aquifer.

Cluster I represents the waters from CI and CT whose isotopic composition is depleted in $^{18}$O (average value around -8.2‰) (Fig. 13). They correspond to an old water recharge (paleo-recharge), whose age, estimated by means of $^{14}$C, exceeds 15 000 yr BP (Guendouz, 1985; Guendouz and Michelot, 2006). Thus, it is not a water body that is recharged by recent precipitation. It consists of CI and CT groundwaters and partly of phreatic waters and can be ascribed to an upward leakage favored by the extension of faults near Amguid El Biod dorsal.

Cluster II, observed in Sebkhet Safioune, can be ascribed to the direct dissolution of surficial evaporitic deposits conveyed by evaporated rainwater.

Evaporation alone cannot explain the distribution of data that is observed (Fig. 13). Evidence for this is given in a semi-logarithmic plot (Fig. 14), as classically obtained according to the simple approximation of the Rayleigh equation (cf. Appendix):

$$ \begin{aligned} \delta^{18}\text{O} & \approx 1000 \times (1 - \alpha) \log[\text{Cl}^{-}] + k, \\ & \approx -\epsilon \log[\text{Cl}^{-}] + k, \end{aligned} \quad (1) $$

where $\alpha$ is the fractionation factor during evaporation, $\epsilon \equiv -1000 \times (1 - \alpha)$ is the enrichment factor and $k$ is a constant (Ma et al., 2010; Chkir et al., 2009). CI and CT waters are better separated in the semi-logarithmic plot because they are differentiated by their chloride content. According to Eq. (1), simple evaporation gives a straight line (solid line in Fig. 14). The value of $\epsilon$ used is the value at 25 °C, which is equal to -73.5.

P115 is the only sample that appears on the straight evaporation line (Fig. 14). It should be considered as an outlier since the rest of the samples are all well aligned on the logarithmic fit derived from the mixing line of Fig. 13.

The phreatic waters that are close to cluster I (Fig. 13) correspond to groundwaters occurring in the edges of the basin (Hassi Miloud, piezometer P433) (Fig. 14). They are low mineralized and acquire their salinity via two processes, namely dissolution of evaporites along their underground transit up to Sebkhet Safioune and dilution through upward leakage by the less-mineralized waters of the CI and CT aquifers (for example, Hedeb I for CI and D7F4 for CT) (Fig. 14) (Guendouz, 1985; Guendouz and Moulla, 1996).

The rates of the mixing that are due to upward leakage from CI to CT towards the phreatic aquifer can be calculated by means of a mass balance equation. It only requires knowing the $\delta$ values of each fraction that is involved in the mixing process.

The $\delta$ value of the mixture is given by

$$ \delta_{\text{mix}} = f \times \delta_1 + (1 - f) \times \delta_2, \quad (2) $$

where $f$ is the fraction of the CI aquifer, $1 - f$ the fraction of the CT and $\delta_1, \delta_2$ are the respective isotope contents.
<table><caption>**Table 3.** Field and analytical data for the phreatic aquifer.</caption><thead><tr><th>Locality</th><th>Site</th><th>Lat.</th><th>Long.</th><th>Elev.</th><th>Date</th><th>EC (mS cm<sup>-1</sup>)</th><th>T (°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr><tr><td></td><td></td><td colspan="5" style="text-align:center;">(m)</td><td colspan="5" style="text-align:center;">(mS cm<sup>-1</sup>)</td><td colspan="4" style="text-align:center;">(mmol L<sup>-1</sup>)</td></tr></thead><tbody><tr><td>Bou Khezana</td><td>P433</td><td>3 597 046</td><td>719 626</td><td>118</td><td>20 Jan 2013</td><td>2.1</td><td>22.7</td><td>9.2</td><td>1.6</td><td>12.0</td><td>7.3</td><td>13.0</td><td>1.0</td><td>4.3</td><td>2.8</td></tr><tr><td>Bou Khezana</td><td>P433</td><td>3 597 046</td><td>719 626</td><td>118</td><td>1992</td><td>2.0</td><td>22.1</td><td>8.9</td><td>1.5</td><td>12.0</td><td>6.9</td><td>11.6</td><td>0.9</td><td>4.4</td><td>2.9</td></tr><tr><td>Hassi Miloud</td><td>P059</td><td>3 547 216</td><td>718 358</td><td>124</td><td>27/01/2013</td><td>2.1</td><td>23.9</td><td>8.2</td><td>1.9</td><td>13.0</td><td>7.3</td><td>12.6</td><td>1.3</td><td>4.4</td><td>3.4</td><td>0.024</td></tr><tr><td>Aïn Kheir</td><td>PL06</td><td></td><td></td><td></td><td>1992</td><td>4.0</td><td>23.8</td><td>7.5</td><td>1.9</td><td>14.2</td><td>17.9</td><td>15.9</td><td>0.6</td><td>10.6</td><td>7.5</td><td></td></tr><tr><td>Hassi Naga</td><td>PLX3</td><td>3 584 761</td><td>717 604</td><td>125</td><td>20 Jan 2013</td><td>2.9</td><td>23.0</td><td>8.1</td><td>2.0</td><td>17.7</td><td>9.4</td><td>16.6</td><td>0.9</td><td>5.8</td><td>5.0</td><td>0.031</td></tr><tr><td></td><td>LTP 30</td><td></td><td></td><td></td><td>1992</td><td>4.1</td><td>23.7</td><td>7.1</td><td>5.3</td><td>18.2</td><td>10.0</td><td>24.3</td><td>0.4</td><td>1.4</td><td>8.1</td><td></td></tr><tr><td>Maison de culture</td><td>PL31</td><td>3 537 988</td><td>720 114</td><td>124</td><td>1992</td><td>2.5</td><td>23.8</td><td>8.1</td><td>1.5</td><td>18.9</td><td>7.8</td><td>26.1</td><td>0.6</td><td>2.1</td><td>3.0</td><td></td></tr><tr><td>El Bour</td><td>P006</td><td>3 564 272</td><td>719 421</td><td>161</td><td>1992</td><td>3.0</td><td>23.4</td><td>7.9</td><td>1.3</td><td>19.0</td><td>7.7</td><td>12.4</td><td>2.7</td><td>5.3</td><td>5.3</td><td></td></tr><tr><td>Hassi Miloud</td><td>P059</td><td>3 547 216</td><td>718 358</td><td>124</td><td>1992</td><td>2.8</td><td>23.5</td><td>7.8</td><td>2.3</td><td>20.8</td><td>9.4</td><td>34.2</td><td>4.3</td><td>1.4</td><td>0.9</td><td></td></tr><tr><td>Oglat Larbaâ</td><td>P430</td><td>3 567 287</td><td>730 058</td><td>139</td><td>24 Jan 2013</td><td>4.5</td><td>27.5</td><td>8.3</td><td>3.3</td><td>22.1</td><td>12.4</td><td>21.8</td><td>2.6</td><td>8.6</td><td>5.5</td><td></td></tr><tr><td>Maison de culture</td><td>PL31</td><td>3 537 988</td><td>720 114</td><td>124</td><td>28 Jan 2013</td><td>3.7</td><td>22.2</td><td>8.2</td><td>4.2</td><td>22.6</td><td>8.6</td><td>28.4</td><td>2.2</td><td>4.0</td><td>3.2</td><td></td></tr><tr><td>Frane El Koum</td><td>P401</td><td>3 572 820</td><td>719 721</td><td>112</td><td>20 Jan 2013</td><td>3.4</td><td>27.5</td><td>7.5</td><td>2.2</td><td>23.3</td><td>13.4</td><td>21.8</td><td>1.9</td><td>8.3</td><td>6.3</td><td>0.032</td></tr><tr><td>Gherbouz</td><td>PL15</td><td>3 537 962</td><td>718 744</td><td>134</td><td>1992</td><td>2.5</td><td>23.5</td><td>7.7</td><td>3.0</td><td>23.5</td><td>14.0</td><td>50.6</td><td>2.8</td><td>1.0</td><td>0.3</td><td></td></tr><tr><td>Bour El Haïcha</td><td>P408</td><td>3 544 999</td><td>719 930</td><td>110</td><td>1992</td><td>2.4</td><td>23.5</td><td>7.8</td><td>2.4</td><td>24.2</td><td>13.2</td><td>41.9</td><td>6.1</td><td>2.3</td><td>0.8</td><td></td></tr><tr><td>Station d'épuration</td><td>PL30</td><td>3 538 398</td><td>721 404</td><td>130</td><td>1992</td><td>5.5</td><td>23.8</td><td>7.4</td><td>3.0</td><td>24.3</td><td>21.2</td><td>24.3</td><td>0.9</td><td>20.2</td><td>2.2</td><td></td></tr><tr><td>Frane Ank Djemel</td><td>P422</td><td>3 575 339</td><td>718 875</td><td>109</td><td>20 Jan 2013</td><td>4.1</td><td>24.2</td><td>8.4</td><td>4.4</td><td>25.3</td><td>9.5</td><td>23.7</td><td>1.8</td><td>4.2</td><td>7.9</td><td>0.025</td></tr><tr><td>Route Aïn Beïda</td><td>PLX2</td><td>3 537 323</td><td>724 063</td><td>127</td><td>1992</td><td>4.7</td><td>23.6</td><td>7.2</td><td>2.0</td><td>25.7</td><td>10.4</td><td>14.8</td><td>0.2</td><td>9.3</td><td>7.4</td><td></td></tr><tr><td>H. Chegga</td><td>PLX4</td><td>3 577 944</td><td>714 428</td><td>111</td><td>20 Jan 2013</td><td>4.1</td><td>25.2</td><td>7.6</td><td>3.0</td><td>26.2</td><td>9.8</td><td>24.0</td><td>2.3</td><td>5.0</td><td>7.5</td><td>0.033</td></tr><tr><td>Hassi Miloud</td><td>P058</td><td>3 547 329</td><td>716 520</td><td>129</td><td>27 Jan 2013</td><td>3.7</td><td>24.6</td><td>8.1</td><td>3.0</td><td>27.7</td><td>10.6</td><td>19.0</td><td>2.3</td><td>9.1</td><td>6.6</td><td>0.033</td></tr><tr><td>Route Aïn Moussa</td><td>P057</td><td>3 548 943</td><td>717 353</td><td>133</td><td>1992</td><td>5.3</td><td>23.4</td><td>7.7</td><td>1.3</td><td>28.2</td><td>11.5</td><td>17.6</td><td>2.0</td><td>11.5</td><td>5.8</td><td></td></tr><tr><td>Route El Goléa</td><td>PL15</td><td>3 533 586</td><td>714 060</td><td>141</td><td>1992</td><td>2.6</td><td>23.7</td><td>7.6</td><td>2.8</td><td>28.8</td><td>14.5</td><td>58.7</td><td>0.1</td><td>0.8</td><td>0.7</td><td></td></tr><tr><td>Mekhadma</td><td>PL05</td><td>3 537 109</td><td>718 419</td><td>137</td><td>1992</td><td>23.9</td><td>7.8</td><td>1.7</td><td>30.9</td><td>16.7</td><td>24.9</td><td>1.0</td><td>15.7</td><td>4.5</td><td></td><td></td></tr><tr><td>Polyclinique Bel Abbès</td><td>PL18</td><td>3 537 270</td><td>721 119</td><td>119</td><td>31/01/2013</td><td>4.7</td><td>22.2</td><td>7.9</td><td>1.8</td><td>31.2</td><td>15.4</td><td>21.3</td><td>3.9</td><td>11.2</td><td>8.4</td><td></td></tr><tr><td>H. Chegga</td><td>PLX4</td><td>3 577 944</td><td>714 428</td><td>111</td><td>1992</td><td>4.5</td><td>23.7</td><td>7.6</td><td>1.5</td><td>31.5</td><td>10.1</td><td>20.1</td><td>5.9</td><td>7.5</td><td>6.5</td><td></td></tr><tr><td>Route El Goléa</td><td>PL16</td><td>3 532 463</td><td>713 715</td><td>117</td><td>1992</td><td>5.6</td><td>23.7</td><td>7.6</td><td>1.4</td><td>31.9</td><td>12.8</td><td>22.2</td><td>0.8</td><td>10.6</td><td>8.0</td><td></td></tr><tr><td>Gherbouz</td><td>PL15</td><td>3 537 962</td><td>718 744</td><td>134</td><td>21 Jan 2013</td><td>4.7</td><td>23.3</td><td>8.2</td><td>1.8</td><td>32.4</td><td>14.6</td><td>27.8</td><td>0.8</td><td>6.8</td><td>10.8</td><td></td></tr><tr><td>Route El Goléa</td><td>PL17</td><td>3 531 435</td><td>713 298</td><td>111</td><td>1992</td><td>4.7</td><td>23.7</td><td>7.7</td><td>1.5</td><td>32.8</td><td>12.8</td><td>30.2</td><td>1.0</td><td>9.2</td><td>5.7</td><td></td></tr><tr><td>Route Aïn Moussa</td><td>P057</td><td>3 548 943</td><td>717 353</td><td>133</td><td>26 Jan 2013</td><td>5.7</td><td>26.2</td><td>7.6</td><td>2.5</td><td>33.5</td><td>11.9</td><td>27.7</td><td>5.9</td><td>6.0</td><td>7.6</td><td></td></tr><tr><td>Ecole Paramédicale</td><td>PL32</td><td>3 538 478</td><td>720 170</td><td>131</td><td>21 Jan 2013</td><td>5.7</td><td>22.9</td><td>8.2</td><td>2.0</td><td>33.6</td><td>12.1</td><td>29.2</td><td>3.3</td><td>6.4</td><td>8.2</td><td></td></tr><tr><td>Direction des Services Agricoles</td><td>PL10</td><td>3 537 055</td><td>719 746</td><td>114</td><td>1992</td><td>6.1</td><td>23.7</td><td>7.7</td><td>1.3</td><td>35.0</td><td>13.5</td><td>8.6</td><td>1.9</td><td>19.4</td><td>7.2</td><td></td></tr><tr><td>Route El Goléa</td><td>PL17</td><td>3 551 435</td><td>713 298</td><td>111</td><td>3 Feb 2013</td><td>5.5</td><td>25.0</td><td>7.7</td><td>3.3</td><td>35.4</td><td>13.8</td><td>37.1</td><td>3.0</td><td>8.4</td><td>5.7</td><td></td></tr><tr><td>Route El Goléa</td><td>PL16</td><td>3 532 463</td><td>713 715</td><td>117</td><td>3 Feb 2013</td><td>5.8</td><td>22.5</td><td>8.1</td><td>1.7</td><td>36.3</td><td>11.6</td><td>28.5</td><td>3.2</td><td>6.8</td><td>8.4</td><td></td></tr><tr><td>Station d'épuration</td><td>PL30</td><td>3 538 398</td><td>721 404</td><td>130</td><td>31 Jan 2013</td><td>5.3</td><td>25.1</td><td>7.8</td><td>4.1</td><td>38.4</td><td>14.6</td><td>28.5</td><td>4.5</td><td>11.6</td><td>8.1</td><td></td></tr><tr><td>Hassi Debiche</td><td>P416</td><td>3 581 097</td><td>730 922</td><td>106</td><td>24 Jan 2013</td><td>5.5</td><td>23.7</td><td>8.8</td><td>0.3</td><td>38.6</td><td>18.0</td><td>22.3</td><td>0.9</td><td>4.8</td><td>21.3</td><td></td></tr><tr><td>Direction des Services Agricoles</td><td>PL10</td><td>3 537 055</td><td>719 746</td><td>114</td><td>28 Jan 2013</td><td>5.5</td><td>24.6</td><td>8.4</td><td>2.4</td><td>38.8</td><td>16.9</td><td>36.9</td><td>1.9</td><td>9.1</td><td>9.2</td><td></td></tr><tr><td>Hôpital</td><td>LTPSN2</td><td>3 538 292</td><td>720 442</td><td>132</td><td>27 Jan 2013</td><td>6.1</td><td>25.4</td><td>7.8</td><td>1.6</td><td>39.7</td><td>11.7</td><td>36.0</td><td>8.4</td><td>5.1</td><td>6.0</td><td></td></tr><tr><td>Parc SONACOM</td><td>PL28</td><td>3 536 077</td><td>719 558</td><td>134</td><td>21 Jan 2013</td><td>6.1</td><td>24.5</td><td>8.1</td><td>1.8</td><td>39.8</td><td>11.8</td><td>30.6</td><td>5.2</td><td>7.1</td><td>8.5</td><td></td></tr><tr><td>Bour El Haïcha</td><td>P408</td><td>3 544 999</td><td>719 930</td><td>110</td><td>27 Jan 2013</td><td>6.2</td><td>23.1</td><td>8.1</td><td>1.8</td><td>42.0</td><td>19.1</td><td>27.5</td><td>13.2</td><td>13.4</td><td>8.1</td><td></td></tr><tr><td>Route Aïn Moussa</td><td>P056</td><td>3 549 933</td><td>717 022</td><td>128</td><td>1992</td><td>7.6</td><td>23.6</td><td>7.9</td><td>0.6</td><td>42.1</td><td>10.7</td><td>18.9</td><td>1.9</td><td>12.6</td><td>9.3</td><td></td></tr><tr><td>Route Aïn Moussa</td><td>P056</td><td>3 549 933</td><td>717 022</td><td>128</td><td>26 Jan 2013</td><td>6.0</td><td>24.6</td><td>7.6</td><td>2.2</td><td>42.5</td><td>17.9</td><td>32.1</td><td>8.0</td><td>12.5</td><td>8.1</td><td></td></tr></tbody></table>
<table><caption>Table 3. Continued.</caption><thead><tr><th>Locality</th><th>Site</th><th>Lat.</th><th>Long.</th><th>Elev.</th><th>Date</th><th>EC<br>(mS cm<sup>-1</sup>)</th><th>T<br>(°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup><br>(mmol L<sup>-1</sup>)</th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr></thead><tbody><tr><td>Ecole Okba B. Nafaa</td><td>PL41</td><td>3 538 660</td><td>719 831</td><td>127</td><td>31 Jan 2013</td><td>6.3</td><td>24.1</td><td>7.7</td><td>2.1</td><td>44.9</td><td>13.2</td><td>36.2</td><td>11.8</td><td>6.3</td><td>6.7</td></tr><tr><td>Parc Hydraulique</td><td>P419</td><td>3 539 494</td><td>725 605</td><td>132</td><td>31 Jan 2013</td><td>7.0</td><td>26.4</td><td>7.8</td><td>2.1</td><td>45.1</td><td>14.4</td><td>41.4</td><td>10.8</td><td>6.0</td><td>6.9</td></tr><tr><td>Parc Hydraulique</td><td>PL13</td><td>3 536 550</td><td>720 200</td><td>123</td><td>21 Jan 2013</td><td>7.2</td><td>24.5</td><td>7.5</td><td>3.2</td><td>47.8</td><td>14.5</td><td>44.4</td><td>10.6</td><td>6.4</td><td>6.6</td></tr><tr><td>Mekhadma</td><td>PL25</td><td>3 536 230</td><td>718 708</td><td>129</td><td>21 Jan 2013</td><td>7.6</td><td>27.1</td><td>7.9</td><td>1.8</td><td>48.0</td><td>14.5</td><td>42.9</td><td>6.6</td><td>7.4</td><td>7.6</td></tr><tr><td>Said Otba</td><td>P506</td><td>3 535 528</td><td>725 075</td><td>126</td><td>4 Feb 2013</td><td>8.3</td><td>24.3</td><td>8.1</td><td>1.7</td><td>52.6</td><td>14.6</td><td>42.8</td><td>11.0</td><td>7.5</td><td>7.8</td></tr><tr><td>Said Otba</td><td>P506</td><td>3 535 528</td><td>725 075</td><td>126</td><td>1992</td><td>6.7</td><td>23.3</td><td>7.5</td><td>1.8</td><td>54.4</td><td>17.6</td><td>33.3</td><td>4.1</td><td>22.2</td><td>5.2</td></tr><tr><td>Mekhadma</td><td>P566</td><td>3 540 433</td><td>719 661</td><td>115</td><td>27 Jan 2013</td><td>9.0</td><td>24.6</td><td>7.6</td><td>1.7</td><td>62.5</td><td>15.2</td><td>71.6</td><td>3.0</td><td>4.6</td><td>6.1</td></tr><tr><td>Mekhadma</td><td>PL17</td><td>3 536 908</td><td>718 511</td><td>130</td><td>21 Jan 2013</td><td>9.4</td><td>24.5</td><td>8.1</td><td>3.4</td><td>63.2</td><td>15.6</td><td>77.2</td><td>2.5</td><td>4.1</td><td>5.1</td></tr><tr><td>Palm. Gara Krima</td><td>P413</td><td>3 530 116</td><td>722 775</td><td>130</td><td>4 Feb 2013</td><td>10.1</td><td>30.2</td><td>7.9</td><td>1.6</td><td>63.6</td><td>21.5</td><td>88.3</td><td>4.1</td><td>4.2</td><td>4.7</td></tr><tr><td>Mekhadma</td><td>PL25</td><td>3 536 230</td><td>718 708</td><td>129</td><td>1992</td><td>9.5</td><td>23.7</td><td>8.0</td><td>0.6</td><td>75.6</td><td>10.6</td><td>10.2</td><td>2.6</td><td>32.9</td><td>9.5</td></tr><tr><td>Said Otba (Bab Sbaa)</td><td>P066</td><td>3 542 636</td><td>718 957</td><td>126</td><td>1992</td><td>7.8</td><td>23.5</td><td>7.6</td><td>1.5</td><td>80.2</td><td>12.5</td><td>45.9</td><td>2.5</td><td>23.6</td><td>5.9</td></tr><tr><td>CEM Malek Bennabi</td><td>PL03</td><td>3 540 010</td><td>725 738</td><td>130</td><td>1992</td><td>7.3</td><td>23.9</td><td>7.6</td><td>3.1</td><td>84.1</td><td>30.6</td><td>108.6</td><td>2.2</td><td>10.2</td><td>9.0</td></tr><tr><td>Entreprise nationale de télévision</td><td>PL21</td><td>3 536 074</td><td>721 268</td><td>128</td><td>1992</td><td>9.7</td><td>23.8</td><td>7.3</td><td>4.5</td><td>84.3</td><td>23.7</td><td>61.6</td><td>3.8</td><td>33.5</td><td>1.9</td></tr><tr><td>Hôtel Transat</td><td>PL23</td><td>3 538 419</td><td>720 950</td><td>126</td><td>28 Jan 2013</td><td>15.0</td><td>24.2</td><td>8.2</td><td>4.5</td><td>86.6</td><td>16.7</td><td>79.9</td><td>3.2</td><td>14.5</td><td>6.9</td></tr><tr><td>Entreprise nationale de télévision</td><td>PL21</td><td>3 536 074</td><td>721 268</td><td>128</td><td>28 Jan 2013</td><td>16.4</td><td>25.7</td><td>7.5</td><td>2.0</td><td>99.9</td><td>17.4</td><td>85.5</td><td>5.7</td><td>15.7</td><td>7.6</td></tr><tr><td>Mekhadma</td><td>PL05</td><td>3 537 109</td><td>718 419</td><td>137</td><td>21 Jan 2013</td><td>16.8</td><td>24.8</td><td>7.6</td><td>2.0</td><td>101.3</td><td>17.7</td><td>85.9</td><td>5.9</td><td>16.7</td><td>7.6</td></tr><tr><td>Beni Thour</td><td>PL44</td><td>3 536 039</td><td>721 673</td><td>134</td><td>1992</td><td>4.7</td><td>23.9</td><td>7.2</td><td>2.7</td><td>109.8</td><td>67.2</td><td>134.7</td><td>5.7</td><td>42.0</td><td>8.8</td></tr><tr><td>Tazegrart</td><td>PL2N1</td><td>3 537 675</td><td>71 9416</td><td>125</td><td>22 Jan 2013</td><td>17.1</td><td>24.9</td><td>8.0</td><td>3.4</td><td>114.2</td><td>18.1</td><td>92.9</td><td>12.8</td><td>16.9</td><td>7.8</td></tr><tr><td>CEM Malek Bennabi</td><td>PL03</td><td>3 540 010</td><td>725 738</td><td>130</td><td>27 Jan 2013</td><td>10.8</td><td>23.1</td><td>7.5</td><td>3.3</td><td>117.3</td><td>14.7</td><td>116.4</td><td>2.1</td><td>9.0</td><td>7.2</td></tr><tr><td>El Bour</td><td>P006</td><td>3 564 272</td><td>719 421</td><td>161</td><td>3 Feb 2013</td><td>18.3</td><td>23.6</td><td>7.8</td><td>6.3</td><td>131.9</td><td>18.1</td><td>96.3</td><td>8.6</td><td>27.1</td><td>8.0</td></tr><tr><td>Ain Moussa</td><td>P015</td><td>3 551 711</td><td>720 591</td><td>103</td><td>1992</td><td>12.4</td><td>23.6</td><td>7.7</td><td>2.4</td><td>134.7</td><td>28.2</td><td>73.0</td><td>3.1</td><td>52.4</td><td>6.3</td></tr><tr><td>Station de Pompage</td><td>PL04</td><td>3 541 410</td><td>723 501</td><td>138</td><td>27 Jan 2013</td><td>19.0</td><td>26.4</td><td>7.9</td><td>4.0</td><td>138.0</td><td>16.7</td><td>108.8</td><td>13.1</td><td>19.5</td><td>8.7</td></tr><tr><td>Drain Chott Ouargla</td><td>D. Ch</td><td></td><td></td><td></td><td>1992</td><td>23.9</td><td>7.7</td><td>2.7</td><td>142.2</td><td>24.5</td><td>96.31</td><td>3.2</td><td>44.2</td><td>3.0</td><td></td></tr><tr><td>Beni Thour</td><td>PL44</td><td>3 536 039</td><td>721 673</td><td>134</td><td>28 Jan 2013</td><td>20.2</td><td>25.8</td><td>7.8</td><td>5.0</td><td>153.0</td><td>17.7</td><td>125.9</td><td>6.3</td><td>22.8</td><td>8.1</td></tr><tr><td>CNMC (Société nationale des matériaux de construction)</td><td>PL27</td><td>3 535 474</td><td>718 407</td><td>126</td><td>21 Jan 2013</td><td>21.2</td><td>24.8</td><td>8.1</td><td>1.7</td><td>169.4</td><td>18.4</td><td>130.3</td><td>4.9</td><td>27.8</td><td>8.6</td></tr><tr><td>Barnendil</td><td>P076</td><td>3 540 137</td><td>716 721</td><td>118</td><td>26 Jan 2013</td><td>22.3</td><td>27.2</td><td>7.6</td><td>4.3</td><td>171.5</td><td>17.1</td><td>130.8</td><td>6.3</td><td>28.0</td><td>8.8</td></tr><tr><td>N'Goussa</td><td>P041</td><td>3 559 563</td><td>716 543</td><td>135</td><td>26 Jan 2013</td><td>25.9</td><td>24.5</td><td>8.2</td><td>8.0</td><td>208.6</td><td>13.4</td><td>198.9</td><td>3.6</td><td>11.8</td><td>8.8</td></tr><tr><td>N'Goussa</td><td>P009</td><td>3 559 388</td><td>717 707</td><td>123</td><td>26 Jan 2013</td><td>27.5</td><td>28.4</td><td>8.4</td><td>11.5</td><td>208.8</td><td>15.8</td><td>195.1</td><td>2.7</td><td>18.7</td><td>9.0</td></tr><tr><td>LTP16</td><td></td><td></td><td></td><td></td><td>1992</td><td>11.5</td><td>23.8</td><td>7.5</td><td>3.8</td><td>213.4</td><td>48.6</td><td>147.9</td><td>7.5</td><td>75.3</td><td>4.3</td></tr><tr><td>P100</td><td></td><td></td><td></td><td></td><td>1992</td><td>17.2</td><td>23.6</td><td>7.6</td><td>3.4</td><td>235.0</td><td>46.4</td><td>264.8</td><td>4.7</td><td>25.6</td><td>5.6</td></tr><tr><td>Chott Adjadja Aven</td><td>PLX1</td><td>3 540 758</td><td>726 115</td><td>132</td><td>28 Jan 2013</td><td>32.9</td><td>23.4</td><td>8.0</td><td>4.4</td><td>245.6</td><td>20.9</td><td>141.4</td><td>26.9</td><td>44.6</td><td>17.7</td></tr><tr><td>Route Frane</td><td>P003</td><td>3 569 043</td><td>721 496</td><td>134</td><td>2 Feb 2013</td><td>31.0</td><td>23.5</td><td>8.0</td><td>6.9</td><td>252.7</td><td>17.9</td><td>208.2</td><td>9.4</td><td>30.0</td><td>10.0</td></tr><tr><td>El Bour-N'Goussa</td><td>P007</td><td>3 562 236</td><td>718 651</td><td>129</td><td>26 Jan 2013</td><td>30.1</td><td>28.4</td><td>7.8</td><td>5.4</td><td>254.7</td><td>15.5</td><td>209.2</td><td>10.4</td><td>28.8</td><td>7.5</td></tr><tr><td>Route Ain Beida</td><td>PLX2</td><td>3 537 323</td><td>724 063</td><td>127</td><td>21 Jan 2013</td><td>43.3</td><td>25.7</td><td>8.1</td><td>5.2</td><td>262.2</td><td>93.0</td><td>270.4</td><td>15.5</td><td>62.8</td><td>21.7</td></tr><tr><td>Ain Moussa</td><td>P015</td><td>3 551 711</td><td>720 591</td><td>103</td><td>25 Jan 2013</td><td>32.0</td><td>22.7</td><td>8.0</td><td>2.9</td><td>263.0</td><td>15.4</td><td>206.9</td><td>6.6</td><td>32.1</td><td>9.9</td></tr><tr><td>Ain Moussa</td><td>P042</td><td>3 549 503</td><td>721 514</td><td>138</td><td>25 Jan 2013</td><td>60.0</td><td>28.7</td><td>8.6</td><td>7.7</td><td>313.2</td><td>93.9</td><td>442.8</td><td>23.3</td><td>12.6</td><td>10.2</td></tr><tr><td>Route Frane</td><td>P001</td><td>3 572 148</td><td>722 366</td><td>127</td><td>1992</td><td></td><td>23.6</td><td>8.4</td><td>4.0</td><td>323.6</td><td>58.1</td><td>331.4</td><td>5.0</td><td>49.8</td><td>4.0</td></tr><tr><td>Ain Moussa</td><td>P014</td><td>3 551 466</td><td>719 339</td><td>131</td><td>1992</td><td></td><td>23.4</td><td>7.3</td><td>4.0</td><td>337.0</td><td>64.3</td><td>328.7</td><td>5.5</td><td>62.4</td><td>5.5</td></tr></tbody></table>
<table><caption>Table 3. Continued.</caption><thead><tr><th>Locality</th><th>Site</th><th>Lat.</th><th>Long.</th><th>Elev.</th><th>Date</th><th>EC</th><th>T</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr><tr><td></td><td></td><td colspan="3">(m)</td><td></td><td colspan="3">(mS cm<sup>-1</sup>)</td><td colspan="3">(°C)</td><td colspan="4">(mmol L<sup>-1</sup>)</td></tr></thead><tbody><tr><td>N'Goussa</td><td>P019</td><td>3562 960</td><td>717 719</td><td>113</td><td>2 Feb 2013</td><td>60.6</td><td>27.8</td><td>7.7</td><td>6.0</td><td>356.2</td><td>96.0</td><td>432.5</td><td>29.8</td><td>21.0</td><td>26.2</td></tr><tr><td>N'Goussa</td><td>P018</td><td>3562 122</td><td>716 590</td><td>110</td><td>26 Jan 2013</td><td>61.1</td><td>26.2</td><td>8.4</td><td>6.5</td><td>372.4</td><td>82.3</td><td>347.1</td><td>22.6</td><td>60.7</td><td>26.6</td></tr><tr><td>Ain Moussa</td><td>P014</td><td>3551 466</td><td>719 339</td><td>131</td><td>25 Jan 2013</td><td>49.0</td><td>25.2</td><td>7.9</td><td>1.8</td><td>399.7</td><td>21.1</td><td>389.3</td><td>2.4</td><td>19.0</td><td>7.4</td></tr><tr><td>Route Sedrata</td><td>P113</td><td>3535 586</td><td>714 576</td><td>105</td><td>3 Feb 2013</td><td>62.2</td><td>24.8</td><td>8.2</td><td>6.0</td><td>414.8</td><td>83.8</td><td>362.7</td><td>33.3</td><td>70.2</td><td>26.5</td></tr><tr><td>N'Goussa</td><td>P009</td><td>3559 388</td><td>717 707</td><td>123</td><td>2 Feb 2013</td><td>66.2</td><td>23.3</td><td>7.8</td><td>2.4</td><td>426.9</td><td>57.8</td><td>393.8</td><td>9.1</td><td>59.1</td><td>12.0</td></tr><tr><td>Route Frane</td><td>P001</td><td>3572 148</td><td>722 366</td><td>127</td><td>2 Feb 2013</td><td>66.2</td><td>23.3</td><td>7.2</td><td>6.5</td><td>468.7</td><td>101.5</td><td>350.3</td><td>26.0</td><td>116.2</td><td>35.3</td></tr><tr><td>Sebkhet Safioune</td><td>P031</td><td>3577 804</td><td>720 172</td><td>120</td><td>1992</td><td>76.0</td><td>27.9</td><td>8.1</td><td>5.9</td><td>500.3</td><td>110.3</td><td>470.5</td><td>28.7</td><td>79.1</td><td>35.5</td></tr><tr><td>Sebkhet Safioune</td><td>P031</td><td>3577 804</td><td>720 172</td><td>120</td><td>2 Feb 2013</td><td>76.0</td><td>27.9</td><td>8.1</td><td>5.9</td><td>500.3</td><td>110.3</td><td>470.5</td><td>28.7</td><td>79.1</td><td>35.5</td></tr><tr><td>Route Frane</td><td>P002</td><td>3570 523</td><td>722 028</td><td>108</td><td>1992</td><td>76.0</td><td>27.9</td><td>8.1</td><td>5.9</td><td>500.3</td><td>110.3</td><td>470.5</td><td>28.7</td><td>79.1</td><td>35.5</td></tr><tr><td>Sebkhet Safioune</td><td>P030</td><td>3577 253</td><td>721 936</td><td>130</td><td>1992</td><td>76.0</td><td>27.9</td><td>8.1</td><td>5.9</td><td>500.3</td><td>110.3</td><td>470.5</td><td>28.7</td><td>79.1</td><td>35.5</td></tr><tr><td>Oum Raneb</td><td>P012</td><td>3554 089</td><td>718 612</td><td>114</td><td>25 Jan 2013</td><td>64.1</td><td>30.3</td><td>7.8</td><td>7.8</td><td>534.3</td><td>20.9</td><td>529.6</td><td>6.4</td><td>19.7</td><td>4.7</td></tr><tr><td>Oum Raneb</td><td>P012</td><td>3554 089</td><td>718 612</td><td>114</td><td>1992</td><td>64.1</td><td>30.3</td><td>7.8</td><td>7.8</td><td>534.3</td><td>20.9</td><td>529.6</td><td>6.4</td><td>19.7</td><td>4.7</td></tr><tr><td>Ank Djemel</td><td>P423</td><td>3540 881</td><td>723 178</td><td>102</td><td>31 Jan 2013</td><td>90.8</td><td>23.5</td><td>7.5</td><td>6.2</td><td>636.5</td><td>101.3</td><td>495.5</td><td>38.3</td><td>125.8</td><td>30.3</td></tr><tr><td>Said Otba Chott</td><td>P096</td><td>3540 265</td><td>724 729</td><td>111</td><td>1992</td><td>64.7</td><td>23.1</td><td>7.8</td><td>3.7</td><td>671.8</td><td>90.3</td><td>742.9</td><td>16.0</td><td>41.5</td><td>7.7</td></tr><tr><td>Sebkhet Safioune</td><td>P030</td><td>3577 253</td><td>721 936</td><td>130</td><td>03/02/2013</td><td>64.7</td><td>23.1</td><td>7.8</td><td>3.7</td><td>671.8</td><td>90.3</td><td>742.9</td><td>16.0</td><td>41.5</td><td>7.7</td></tr><tr><td>N'Goussa</td><td>P017</td><td>3560 256</td><td>715 781</td><td>130</td><td>26 Jan 2013</td><td>100.1</td><td>31.0</td><td>7.1</td><td>3.8</td><td>679.3</td><td>114.1</td><td>597.8</td><td>10.7</td><td>125.9</td><td>26.3</td></tr><tr><td>Ank Djemel</td><td>P021</td><td>3573 943</td><td>723 161</td><td>105</td><td>1992</td><td>23.6</td><td>7.4</td><td>7.5</td><td>4.2</td><td>700.8</td><td>154.5</td><td>605.7</td><td>53.6</td><td>163.1</td><td>14.2</td></tr><tr><td>Station de Pompage</td><td>PL04</td><td>3541 410</td><td>723 501</td><td>138</td><td>1992</td><td>23.6</td><td>7.4</td><td>7.5</td><td>6.2</td><td>636.5</td><td>101.3</td><td>495.5</td><td>38.3</td><td>125.8</td><td>30.3</td></tr><tr><td>Route Frane</td><td>P002</td><td>3570 523</td><td>722 028</td><td>108</td><td>2 Feb 2013</td><td>62.8</td><td>26.9</td><td>7.6</td><td>1.7</td><td>748.5</td><td>62.6</td><td>651.5</td><td>14.7</td><td>77.7</td><td>27.3</td></tr><tr><td>Said Otba chott</td><td>P096</td><td>3540 265</td><td>724 729</td><td>111</td><td>3 Feb 2013</td><td>68.3</td><td>25.9</td><td>8.7</td><td>1.2</td><td>771.0</td><td>53.1</td><td>615.9</td><td>23.5</td><td>69.6</td><td>50.4</td></tr><tr><td>N'Goussa</td><td>P019</td><td>3562 960</td><td>717 719</td><td>113</td><td>1992</td><td>23.3</td><td>7.7</td><td>7.4</td><td>2.4</td><td>779.1</td><td>77.1</td><td>711.5</td><td>9.2</td><td>95.6</td><td>12.1</td></tr><tr><td>Said Otba (Bab Sbaa)</td><td>P066</td><td>3542 636</td><td>718 957</td><td>126</td><td>3 Feb 2013</td><td>150.6</td><td>26.2</td><td>7.2</td><td>12.3</td><td>799.1</td><td>283.0</td><td>1249.7</td><td>19.0</td><td>37.6</td><td>18.1</td></tr><tr><td>Ank Djemel</td><td>P021</td><td>3573 943</td><td>723 161</td><td>105</td><td>24 Jan 2013</td><td>82.3</td><td>29.6</td><td>7.6</td><td>2.4</td><td>800.4</td><td>94.4</td><td>824.0</td><td>11.0</td><td>53.4</td><td>25.4</td></tr><tr><td>N'Goussa</td><td>P018</td><td>3562 122</td><td>716 590</td><td>110</td><td>1992</td><td>23.3</td><td>7.5</td><td>1.2</td><td>818.7</td><td>81.0</td><td>244.2</td><td>49.5</td><td>319.4</td><td>24.8</td><td>24.8</td></tr><tr><td>Oum Raneb</td><td>P162</td><td>3546 133</td><td>725 129</td><td>98</td><td>25 Jan 2013</td><td>160.0</td><td>30.7</td><td>7.2</td><td>2.4</td><td>842.8</td><td>289.9</td><td>1309.9</td><td>13.3</td><td>33.5</td><td>17.7</td></tr><tr><td>Route Sedrata</td><td>P113</td><td>3535 586</td><td>714 576</td><td>105</td><td>1992</td><td>23.7</td><td>7.7</td><td>7.2</td><td>2.4</td><td>842.8</td><td>289.9</td><td>1309.9</td><td>13.3</td><td>33.5</td><td>17.7</td></tr><tr><td>Oum Raneb</td><td>PZ12</td><td>3547 234</td><td>722 931</td><td>110</td><td>5 Feb 2013</td><td>114.9</td><td>27.4</td><td>7.4</td><td>2.9</td><td>980.1</td><td>15.5</td><td>930.8</td><td>7.5</td><td>23.9</td><td>14.2</td></tr><tr><td>Hôtel Transat</td><td>P23</td><td>3538 419</td><td>720 950</td><td>126</td><td>1992</td><td>23.5</td><td>7.4</td><td>3.0</td><td>1103.3</td><td>94.5</td><td>707.8</td><td>19.1</td><td>270.9</td><td>13.3</td><td>13.3</td></tr><tr><td>Sebkhet Safioune</td><td>P023</td><td>3577 198</td><td>725 726</td><td>99</td><td>1992</td><td>23.3</td><td>7.4</td><td>2.3</td><td>1177.0</td><td>91.1</td><td>1058.2</td><td>11.7</td><td>133.5</td><td>12.4</td><td>12.4</td></tr><tr><td>Sebkhet Safioune</td><td>P034</td><td>3579 698</td><td>725 633</td><td>97</td><td>5 Feb 2013</td><td>130.0</td><td>34.9</td><td>8.1</td><td>1.8</td><td>1189.1</td><td>14.7</td><td>1055.1</td><td>18.3</td><td>56.4</td><td>17.4</td></tr><tr><td>Sebkhet Safioune</td><td>P023</td><td>3577 198</td><td>725 726</td><td>99</td><td>5 Feb 2013</td><td>117.9</td><td>29.4</td><td>8.2</td><td>1.9</td><td>1209.3</td><td>15.6</td><td>1129.4</td><td>8.4</td><td>42.9</td><td>10.2</td></tr><tr><td>Chott Adjadia</td><td>PLX1</td><td>3540 758</td><td>726 115</td><td>132</td><td>1992</td><td>23.6</td><td>8.0</td><td>3.8</td><td>1296.7</td><td>134.0</td><td>1458.7</td><td>5.2</td><td>48.0</td><td>4.3</td><td>4.3</td></tr><tr><td>Sebkhet Safioune</td><td>P063</td><td>3545 586</td><td>725 667</td><td>99</td><td>1992</td><td>23.5</td><td>7.5</td><td>1.9</td><td>1379.4</td><td>139.6</td><td>1257.4</td><td>18.6</td><td>182.3</td><td>10.0</td><td>10.0</td></tr><tr><td>LTP06</td><td>P076</td><td>3540 137</td><td>716 721</td><td>118</td><td>1992</td><td>1992</td><td>23.8</td><td>7.6</td><td>7.8</td><td>1638.7</td><td>712.1</td><td>2621.6</td><td>41.6</td><td>190.5</td><td>13.3</td></tr><tr><td>Bamendil</td><td>P007</td><td>3545 236</td><td>718 651</td><td>129</td><td>5 Feb 2013</td><td>23.5</td><td>7.7</td><td>5.7</td><td>1743.6</td><td>143.4</td><td>1321.9</td><td>26.9</td><td>331.4</td><td>12.3</td><td>12.3</td></tr><tr><td>El Bour-N'Goussa</td><td>P063</td><td>3545 586</td><td>725 667</td><td>99</td><td>5 Feb 2013</td><td>178.9</td><td>26.7</td><td>7.7</td><td>1.4</td><td>1887.9</td><td>92.9</td><td>1455.8</td><td>26.7</td><td>282.9</td><td>13.4</td></tr><tr><td>Sebkhet Safioune</td><td>P044</td><td></td><td></td><td></td><td>1992</td><td>23.4</td><td>7.8</td><td>4.5</td><td>2106.1</td><td>18.3</td><td>1765.5</td><td>27.3</td><td>171.2</td><td>6.5</td><td>6.5</td></tr><tr><td></td><td>P093</td><td></td><td></td><td></td><td>1992</td><td>23.6</td><td>7.5</td><td>1.5</td><td>2198.6</td><td>182.1</td><td>1957.5</td><td>29.5</td><td>278.2</td><td>10.4</td><td>10.4</td></tr><tr><td></td><td>P042</td><td></td><td></td><td></td><td>1992</td><td>23.4</td><td>7.6</td><td>1.1</td><td>2330.9</td><td>101.2</td><td>1963.7</td><td>52.2</td><td>248.1</td><td>11.2</td><td>11.2</td></tr><tr><td></td><td>P068</td><td></td><td></td><td></td><td>1992</td><td>23.5</td><td>7.5</td><td>3.4</td><td>2335.7</td><td>222.1</td><td>2302.3</td><td>26.8</td><td>219.9</td><td>7.2</td><td>7.2</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Contour maps of the salinity (expressed as global mineralization) in the aquifer system of the (a) phreatic aquifer and (b, c) Com-plexe Terminal (with b indicating Mio-Pliocene and c indicating Senonian); figures are isovalues of global mineralization (values in g L⁻¹).</figcaption></figure>

Average values of mixing fractions from each aquifer to the phreatic waters computed by means of Eq. (2) gave the rates of 65 % for the CI aquifer and 35 % for the CT aquifer.

A mixture of a phreatic water component that is close to cluster I (i.e., P433) with another component which is rather close to cluster II (i.e., P039) (Figs. 13 and 14), for an in-termediate water with a δ¹⁸O signature ranging from −5 to −2‰, gives mixture fraction values of 52 % for cluster I and 48 % for cluster II. Isotope results will be used to indepen-dently cross-check the validity of the mixing fractions de-rived from an inverse modeling involving chemical data (see Sect. 3.6).

Turonian evaporites are found to lie in between the CI deep aquifer and the Senonian and Miocene formations bearing the CT aquifer. CT waters can thus simply originate from ascending CI waters that dissolve Turonian evaporites, a pro-cess which does not involve any change in ¹⁸O content. Con-versely, phreatic waters result, to a minor degree, from evap-oration and mostly from dissolution of sabkha evaporites by ¹⁸O-enriched rainwater and mixing with CI/CT waters.

## Tritium content of water

Tritium contents of the Phr aquifer are relatively small (Ta-ble 4), they vary between 0 and 8 TU. Piezometers PZ12, P036 and P068 show values close to 8 TU; piezometers P018, P019, P416, P034, P042 and P093 exhibit values ranging be-tween 5 and 6 TU; and the rest of the samples' concentrations are lower than 2 TU.

These values are dated back to November 1992, so they are old values and they are considered high comparatively to what is expected to be found nowadays. In fact, at present times, tritium figures have fallen lower than 5 TU in precipi-tation measured in the northern part of the country.

Tritium content of precipitation was measured as 16 TU in 1992 on a single sample that was collected from the Na-tional Agency for Water Resources station in Ouargla. A ma-jor part of this rainfall evaporates back into the atmosphere that is unsaturated in moisture. Consequently, enrichment in tritium happens as water evaporates back. The lightest frac-tions (isotopes) are the ones that escape first, enriching the remaining fraction in tritium. The 16 TU value would thus
<table><caption>Table 3. Continued.</caption><thead><tr><th>Locality</th><th>Site</th><th>Lat.</th><th>Long.</th><th>Elev.</th><th>Date</th><th>EC<br>(mS cm<sup>-1</sup>)</th><th>T<br>(°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Br<sup>-</sup></th></tr></thead><tbody><tr><td>Oum Raneb</td><td>PZ12</td><td>3547234</td><td>722931</td><td>110</td><td>1992</td><td>23.3</td><td>7.6</td><td>2.2</td><td>2405.6</td><td>109.9</td><td>2178.6</td><td>25.2</td><td>199.4</td><td>12.7</td><td></td></tr><tr><td>Hassi Debiche</td><td>P416</td><td>3581097</td><td>730922</td><td>106</td><td>1992</td><td>23.3</td><td>7.8</td><td>4.3</td><td>2433.7</td><td>178.9</td><td>2361.1</td><td>24.3</td><td>196.1</td><td>9.2</td><td></td></tr><tr><td>N'Goussa</td><td>P041</td><td>3559563</td><td>716543</td><td>135</td><td>1992</td><td>23.4</td><td>7.9</td><td>2.1</td><td>2599.7</td><td>324.6</td><td>2879.0</td><td>44.6</td><td>152.8</td><td>11.0</td><td></td></tr><tr><td>Sebkhet Safioune</td><td>P034</td><td>3579698</td><td>725633</td><td>97</td><td>1992</td><td>23.3</td><td>7.8</td><td>1.9</td><td>2752.0</td><td>134.1</td><td>2616.8</td><td>24.4</td><td>180.1</td><td>10.5</td><td></td></tr><tr><td>Sebkhet Safioune</td><td>P039</td><td></td><td></td><td></td><td>1992</td><td>23.4</td><td>6.9</td><td>1.9</td><td>4189.5</td><td>201.4</td><td>4042.6</td><td>17.9</td><td>257.8</td><td>9.2</td><td></td></tr><tr><td>Sebkhet Safioune</td><td>P074</td><td></td><td></td><td></td><td>1992</td><td>23.5</td><td>6.5</td><td>4.2</td><td>4356.5</td><td>180.9</td><td>2759.9</td><td>57.4</td><td>930.1</td><td>22.6</td><td></td></tr><tr><td>Sebkhet Safioune</td><td>P037</td><td></td><td></td><td></td><td>1992</td><td>23.4</td><td>6.9</td><td>1.5</td><td>4953.8</td><td>184.5</td><td>4611.1</td><td>2.9</td><td>347.6</td><td>7.9</td><td></td></tr><tr><td>Sebkhet Safioune</td><td>P036</td><td></td><td></td><td></td><td>1992</td><td>23.4</td><td>7.5</td><td>1.4</td><td>4972.8</td><td>108.1</td><td>4692.2</td><td>36.8</td><td>221.1</td><td>9.6</td><td></td></tr></tbody></table>

For longitude and latitude, the reference is UTM 31 projection for north Sahara 1959 (CLARKE 1880 ellipsoid).

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Equilibrium diagrams of calcite (top panel) and gypsum (bottom panel) for the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers. Equilibrium lines are defined as $log(Ca^{2+}) + log(CO_3^{2-}) = log K_{sp}$ for calcite and $log(Ca^{2+}) + 2log(H_2O) + log(SO_4^{2-}) = log K_{sp}$ for gypsum.</figcaption></figure>

correspond to a rainy event that had happened during the field campaign (5 and 6 November 1992). It is the most representative value for that region and for that time. Unfortunately, all the other stations (Algiers, Ankara and Tenerife) (Martinelli et al., 2014) are subject to a completely different climatic regime and (besides the fact that they have more recent values) can absolutely not be used for our case. Therefore, all the assumptions based on recent tritium rain values do not apply to this study.

Depleted contents in $^{18}$O and low tritium concentrations for phreatic waters fit the mixing scheme well and confirm the contribution from the older and deeper CI/CT groundwaters. The affected areas were clearly identified in the field and correspond to locations that are subject to a recycling and a return of irrigation waters whose origin are CI/CT boreholes. Moreover, the mixing that is clearly brought to light by
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Variation of saturation indices with distance from south to north in the region of Ouargla.</figcaption></figure>

the $Cl^-$ versus $^{18}$O diagrams (Figs. 13 and 14) could partly
derive from an ascending drainage from the deep and con-
fined CI aquifer (exhibiting depleted homogenous $^{18}$O con-
tents and very low tritium), a vertical leakage that is favored
by the Amguid El Biod highly faulted area (Guendouz and
Moulla, 1996; Edmunds et al., 2003; Guendouz et al., 2003;
Moulla et al., 2012).

**3.6 Inverse modeling**

We assume that the relationship between $^{18}$O and $Cl^-$ data obtained in 1992 is stable with time, which is a logical assumption as times of transfer from CI to both CT and Phr are very long. Considering both $^{18}$O and $Cl^-$ data, CI, CT and Phr data populations can be categorized. The CI and CT do not show appreciable $^{18}$O variations and can be considered as a single population. The Phr samples consist, however, of different populations: cluster I, with $\delta^{18}$O values close to -8 and small $Cl^-$ concentrations, more specifically less
<table><caption>**Table 4.** Isotopic data <sup>18</sup>O and <sup>3</sup>H and chloride concentration in the Continental Intercalaire, Complexe Terminal and phreatic aquifers (sampling campaign in 1992).</caption><thead><tr><th>Piezometer</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th><th>Piezometer</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th><th>Piezometer</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th></tr></thead><tbody><tr><td colspan="11" style="text-align:center; font-weight:bold;">Phreatic aquifer</td></tr><tr><td>P007</td><td>1860.5</td><td>-2.5</td><td>0</td><td>PL15</td><td>23.5</td><td>-7.85</td><td>0.6(1)</td><td>P074</td><td>4356.4</td><td>3.4</td><td>6.8(8)</td></tr><tr><td>P009</td><td>426.9</td><td>-6.6</td><td>1.2(3)</td><td>P066</td><td>80.2</td><td>-8.1</td><td>0.8(1)</td><td>PL06</td><td>14.2</td><td>-8.1</td><td>1.0(2)</td></tr><tr><td>P506</td><td>54.4</td><td>-6.8</td><td>1.6(3)</td><td>PL23</td><td>1103.3</td><td>-6.1</td><td>0</td><td>PL30</td><td>24.3</td><td>-7.48</td><td>2.4(4)</td></tr><tr><td>P018</td><td>818.7</td><td>-2.9</td><td>6.2(11)</td><td>P063</td><td>1379.3</td><td>-3.4</td><td>8.7(15)</td><td>P002</td><td>522.4</td><td>-5.7</td><td>0.6(1)</td></tr><tr><td>P019</td><td>779.1</td><td>-4.7</td><td>5.6(9)</td><td>P068</td><td>2335.6</td><td>-3.1</td><td>8.8(14)</td><td>PL21</td><td>84.3</td><td>-7.7</td><td>1.2(2)</td></tr><tr><td>PZ12</td><td>2405.5</td><td>-2.3</td><td>8.1(13)</td><td>P030</td><td>527.7</td><td>-6.6</td><td>2.4(4)</td><td>PL31</td><td>18.9</td><td>-7.4</td><td>1.6(3)</td></tr><tr><td>P023</td><td>1176.9</td><td>-2.6</td><td>0.2(1)</td><td>P076</td><td>1743.5</td><td>-5.6</td><td>2.8(5)</td><td>P433</td><td>12.0</td><td>-8.8</td><td>0</td></tr><tr><td>P416</td><td>2433.7</td><td>-7.9</td><td>5.9(9)</td><td>P021</td><td>700.7</td><td>-5.2</td><td>2.6(4)</td><td>PL03</td><td>84.1</td><td>-7.4</td><td>1.7(3)</td></tr><tr><td>P034</td><td>2752.0</td><td>-1.8</td><td>5.7(9)</td><td>PL04</td><td>716.3</td><td>-2.9</td><td></td><td>PL44</td><td>109.8</td><td>-8.8</td><td>1.0(2)</td></tr><tr><td>P036</td><td>4972.7</td><td>3.3</td><td>2.1(4)</td><td>P093</td><td>2198.5</td><td>-2.6</td><td>5.1(8)</td><td>PL05</td><td>30.9</td><td>-7.4</td><td>1.9(3)</td></tr><tr><td>P037</td><td>4953.8</td><td>3.1</td><td>1.8(3)</td><td>P096</td><td>645.1</td><td>-6.1</td><td>4.8(8)</td><td>P408</td><td>24.2</td><td>-7.9</td><td>0</td></tr><tr><td>P039</td><td>4189.5</td><td>1.0</td><td>2.2(4)</td><td>PLX1</td><td>1296.6</td><td>-5.6</td><td>1.1(2)</td><td>P116</td><td>31.9</td><td>-7.2</td><td>1.1(2)</td></tr><tr><td>P041</td><td>2599.7</td><td>-0.6</td><td>7.3(13)</td><td>PLX2</td><td>25.7</td><td>-7.6</td><td>1.3(2)</td><td>LTP 16</td><td>213.4</td><td>-7.5</td><td>1.6(3)</td></tr><tr><td>P044</td><td>2106.1</td><td>-4.5</td><td>2.7(5)</td><td>P015</td><td>134.7</td><td>-6.8</td><td>3.0(5)</td><td>P117</td><td>32.8</td><td>-6.9</td><td>0.1</td></tr><tr><td>P014</td><td>336.9</td><td>-6.9</td><td>2.8(5)</td><td>P001</td><td>323.6</td><td>-4.7</td><td>2.5(4)</td><td>PL10</td><td>35.0</td><td>-7.3</td><td>0.2(1)</td></tr><tr><td>P012</td><td>539.3</td><td>-6.4</td><td>2.2(4)</td><td>P100</td><td>235.0</td><td>-5.8</td><td>0</td><td>PL25</td><td>75.6</td><td>-7.4</td><td>0.9(2)</td></tr><tr><td>P042</td><td>2330.8</td><td>2.1</td><td>6.0(10)</td><td>P056</td><td>42.1</td><td>-7.0</td><td>2.9(5)</td><td>LTP30</td><td>18.2</td><td>-7.5</td><td>1.1(2)</td></tr><tr><td>P006</td><td>19.0</td><td>-6.6</td><td>0.5(1)</td><td>P113</td><td>954.9</td><td>-4.8</td><td>0.8(2)</td><td>LTP06</td><td>1638.6</td><td>-1.9</td><td>2.8(5)</td></tr><tr><td>P057</td><td>28.2</td><td>-7.3</td><td>1.1(2)</td><td>PLX4</td><td>31.5</td><td>-7.1</td><td>0.3(1)</td><td>P031</td><td>481.8</td><td>-6.1</td><td>3.0(5)</td></tr><tr><td>P059</td><td>20.8</td><td>-7.8</td><td>0</td><td>P115</td><td>28.8</td><td>-2.5</td><td>6.8(12)</td><td></td><td></td><td></td><td></td></tr><tr><th>Borehole</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th><th>Borehole</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th><th>Borehole</th><th>Cl<sup>-</sup><br>(mmol L<sup>-1</sup>)</th><th>δ<sup>18</sup>O<br>(‰)</th><th><sup>3</sup>H<br>(UT)</th></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">Complexe Terminal aquifer</td></tr><tr><td>D5F80</td><td>42.2</td><td>-7.9</td><td></td><td>D1F138</td><td>28.9</td><td>-8.1</td><td>0.7(1)</td><td>D2F71</td><td>13.5</td><td>-8.2</td><td>0.6(1)</td></tr><tr><td>D3F8</td><td>29.8</td><td>-8.1</td><td>1.4(2)</td><td>D3F18</td><td>21.7</td><td>-8.2</td><td>0.2(1)</td><td>D7F4</td><td>10.6</td><td>-8.3</td><td>0.1(1)</td></tr><tr><td>D3F26</td><td>34.7</td><td>-8.0</td><td>0.8(1)</td><td>D3F10</td><td>14.3</td><td>-7.9</td><td>1.5(2)</td><td>D2F66</td><td>11.0</td><td>-8.3</td><td></td></tr><tr><td>D4F94</td><td>20.1</td><td>-8.2</td><td>0.6(1)</td><td>D6F51</td><td>28.4</td><td>-7.9</td><td>0.7(1)</td><td>D1F151</td><td>10.8</td><td>-8.3</td><td>0.4(1)</td></tr><tr><td>D6F67</td><td>18.8</td><td>-8.2</td><td>3.7(6)</td><td>D1F135</td><td>18.1</td><td>-8.1</td><td>1.1(2)</td><td>D6F64</td><td>11.4</td><td>-8.3</td><td>4.3(7)</td></tr><tr><td colspan="11" style="text-align:center; font-weight:bold;">Continental Intercalaire aquifer</td></tr><tr><td>Hadeb I</td><td>5.8</td><td>-8.0</td><td>0</td><td>Hadeb II</td><td>6.2</td><td>-7.9</td><td>0.1(1)</td><td>Aouinet Moussa</td><td>6.5</td><td>-7.9</td><td>1.1(2)</td></tr></tbody></table>

than 35 mmol L<sup>-1</sup>; cluster II, with δ<sup>18</sup>O values larger than 3 and very large Cl<sup>-</sup> concentrations, more specifically larger than 4000 mmol L<sup>-1</sup> (Table 5); intermediate Phr samples resulting from mixing between clusters I and II (mixing line in Fig. 13, mixing curve in Fig. 14) and from evaporation of cluster I (evaporation line in Fig. 14). The mass-balance modeling has shown that relatively few phases are required to derive observed changes in water chemistry and to account for the hydrochemical evolution in Ouargla's region. The mineral phases' selection is based upon geological descriptions and analysis of rocks and sediments from the area (OSS, 2003; Hamdi-Aïssa et al., 2004).

The inverse model was constrained so that mineral phases from evaporites including gypsum, halite, mirabilite, glauberite, sylvite and bloedite were set to dissolve until they reach saturation, and calcite and dolomite were set to precipitate once they reached saturation. Cation exchange reactions of Ca<sup>2+</sup>, Mg<sup>2+</sup>, K<sup>+</sup> and Na<sup>+</sup> on exchange sites were

included in the model to check which cations are adsorbed or desorbed during the process. Dissolution and desorption contribute as positive terms in the mass balance, as elements are released in solution. On the other hand, precipitation and adsorption contribute as negative terms, while elements are removed from the solution. CO<sub>2</sub>(g) dissolution is considered by PHREEQC as a dissolution of a mineral, whereas CO<sub>2</sub>(g) degassing is dealt with as if it were a mineral precipitation.

Inverse modeling leads to a quantitative assessment of the different solutes' acquisition processes and a mass balance for the salts that are dissolved or precipitated from Cl, CT and Phr groundwaters (Fig. 14, Table 6), as follows:

- Transition from Cl to CT involves gypsum, halite and sylvite dissolution, and some ion exchange, namely calcium and potassium fixation on exchange sites against magnesium release, with a very small and quite negligible amount of CO<sub>2</sub>(g) degassing. The maximum elemental concentration fractional error equals 1%. The
<table><caption>**Table 5.** Statistical parameters for the Continental Intercalaire (CI), Complexe Terminal (CT) and phreatic (Phr) aquifer samples selected on the basis of δ¹⁸O and Cl⁻ data (see text).</caption><thead><tr><th>Aquifer</th><th>Size</th><th>Parameter</th><th>EC (mS cm<sup>-1</sup>)</th><th>T (°C)</th><th>pH</th><th>Alk.</th><th>Cl<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup> (mmol L<sup>-1</sup>)</th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th></tr></thead><tbody><tr><td>CI</td><td>11</td><td>Average</td><td>2.2</td><td>49.0</td><td>7.5</td><td>2.3</td><td>11.0</td><td>4.7</td><td>10.3</td><td>0.5</td><td>3.6</td><td>2.4</td></tr><tr><td>CI</td><td>11</td><td>SD</td><td>0.3</td><td>2.0</td><td>0.2</td><td>1.0</td><td>4.6</td><td>2.5</td><td>4.6</td><td>0.2</td><td>2.0</td><td>1.8</td></tr><tr><td>CT</td><td>50</td><td>Average</td><td>3.2</td><td>23.0</td><td>7.8</td><td>2.3</td><td>20.0</td><td>8.9</td><td>17.0</td><td>1.0</td><td>5.5</td><td>5.6</td></tr><tr><td>CT</td><td>50</td><td>SD</td><td>1.1</td><td>2.4</td><td>0.4</td><td>0.8</td><td>7.0</td><td>2.6</td><td>6.0</td><td>0.8</td><td>2.2</td><td>1.7</td></tr><tr><td>Phr cluster I</td><td>30</td><td>Average</td><td>3.9</td><td>24.0</td><td>7.9</td><td>2.3</td><td>24.7</td><td>11.8</td><td>24.2</td><td>2.1</td><td>7.2</td><td>5.3</td></tr><tr><td>Phr cluster I</td><td>30</td><td>SD</td><td>1.3</td><td>1.3</td><td>0.4</td><td>1.0</td><td>6.9</td><td>3.4</td><td>11.0</td><td>1.7</td><td>5.0</td><td>2.7</td></tr><tr><td>Phr cluster II</td><td>3</td><td>Average</td><td></td><td>23.4</td><td>7.0</td><td>2.4</td><td>4761.0</td><td>158.0</td><td>4021.0</td><td>32.4</td><td>500.0</td><td>13.0</td></tr><tr><td>Phr cluster II</td><td>3</td><td>SD</td><td></td><td>0.1</td><td>0.5</td><td>1.6</td><td>350.0</td><td>43.0</td><td>1093.0</td><td>28.0</td><td>378.0</td><td>8.0</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Change from carbonate facies to evaporite from the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Change from sulfate facies to chloride from the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers.</figcaption></figure>

model consists of a minimum number of phases (i.e., six solid phases and CO₂(g)); another model also implies dolomite precipitation with the same fractional error.

- Transition from CT to an average water component of cluster I involves dissolution of halite, sylvite and bloedite from Turonian evaporites, with a very tiny calcite precipitation. The maximum fractional error in elemental concentration is 4%. Another model implies CO₂(g) escape from the solution, with the same fractional error. Large amounts of Mg²⁺ and SO₄²⁻ are released within the solution (Sharif et al., 2008; Li et al., 2010; Carucci et al., 2012).

- The formation of Phr cluster II can be modeled as being a direct dissolution of salts from the sabkha by rainwater with positive δ¹⁸O; the most concentrated water (P036 from Sebkhet Safioune) is taken here for cluster II, and pure water is taken as rainwater. In a descending order of amount, halite, sylvite, gypsum and huntite are the minerals that are the most involved in the dissolution process. A small amount of calcite precipitates while some Mg²⁺ is released versus K⁺ fixation on exchange sites. The maximum elemental fractional error in the concentration is equal to 0.004%. Another model implies dolomite precipitation with some more huntite dissolving, instead of calcite precipitation, but salt dissolution and ion exchange are the same. Huntite, dolomite and calcite stoichiometries are linearly related, so both models can fit field data, but calcite precipitation is preferred compared to dolomite precipitation at low temperature.

- The origin of all phreatic waters can be explained by a mixing in variable proportions of cluster I and cluster II. For instance, waters from cluster I and cluster II can easily be separated by their δ¹⁸O, respectively, close to
<figure><img src="image_13.png" /></figure>

**Figure 10.** Correlation between Na⁺ and Cl⁻ concentrations in the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers. Seawater composition (star) is [Na⁺] = 459.3 mmol L⁻¹ and [Cl⁻] = 535.3 mmol L⁻¹ (Stumm and Morgan, 1999, p. 899).

<figure><img src="image_14.png" /></figure>

**Figure 11.** Calcium versus HCO₃⁻ diagram in the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers. Seawater composition (star) is [Ca²⁺] = 10.2 mmol L⁻¹ and [HCO₃⁻] = 2.38 mmol L⁻¹ (Stumm and Morgan, 1999, p. 899).

<figure><img src="image_15.png" /></figure>

**Figure 12.** Calcium versus SO₄²⁻ diagram in the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers. Seawater composition (star) is [Ca²⁺] = 10.2 mmol L⁻¹ and [SO₄²⁻] = 28.2 mmol L⁻¹ (Stumm and Morgan, 1999, p. 899).

<figure><img src="image_16.png" /></figure>

**Figure 13.** Chloride concentration versus δ¹⁸O in the Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers from Ouargla.

−8 and +3.5‰ (Figs. 13 and 14). Mixing the two clusters is of course not an inert reaction, but rather results in the dissolution and the precipitation of minerals. Inverse modeling is then used to compute both mixing rates and the extent of matter exchange between soil and solution. For example, a phreatic water (piezometer P068) with intermediate values (δ¹⁸O = −3 and [Cl⁻] ≈ 2 M) is explained by the mixing of 58 % water from cluster I and 42 % from cluster II. In addition, calcite precipitates, Mg²⁺ fixes on exchange sites against Na⁺ and K⁺, and gypsum dissolves, as does a minor amount of huntite (Table 6). The maximum elemental concentration fractional error is 2.5 % and the mixing fractions’ weighted δ¹⁸O is −3.17‰, which is very close to the measured value (−3.04‰). All the other models, making use of a minimum number of phases, and not taking into consid-
<figure><img src="image_17.png" /><figcaption>**Figure 14.** Log [Cl⁻] concentration versus δ¹⁸O in Continental Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles) aquifers from Ouargla.</figcaption></figure>

eration ion exchange reactions are not found compatible with isotope data. Mixing rates obtained with such models are, for example, 98 % of cluster I and 0.9 % of cluster II, which leads to a δ¹⁸O = (-7.80‰) which is quite far from the real measured value (-3.04‰).

The main types of groundwaters occurring in the Ouargla basin are thus explained and could quantitatively be reconstructed. An exception is, however, sample P115, which is located exactly on the evaporation line of Phr cluster I. Despite numerous attempts, it could not be quantitatively rebuilt. Its ³H value (6.8) indicates that it is derived from a more or less recent water component with very small salt content, most possibly affected by rainwater and some preferential flow within the piezometer. As this is the only sample on this evaporation line, there remains doubt regarding its significance.

Globally, the summary of mass transfer reactions occurring in the studied system (Table 6) shows that gypsum dissolution results in calcite precipitation and CO₂(g) dissolution, thus acting as an inorganic carbon sink.

## 4 Conclusions

Two of the aquifers studied in this work, Complexe Terminal and Continental Intercalaire, are the main aquifers of Sahara, by extent (thousands of kilometers from the recharge area to the Gulf of Gabès) and time of transfer (thousands of years). The last one, the phreatic aquifer, is a shallow aquifer.

The chemical facies of these aquifers have long been qualitatively described. Our results quantitatively explain, for the first time, the processes that occur during upward leakage through interaction between solution and the mineral constituents of the aquifers and ultimately by mixing with surface waters. The hydrochemical study of the aquifer system occurring in Ouargla's basin allowed us to identify the origin of its mineralization. Waters exhibit two different facies: sodium chloride and sodium sulfate for the phreatic aquifer (Phr), sodium sulfate for the Complexe Terminal (CT) aquifer and sodium chloride for the Continental Intercalaire (CI) aquifer. Calcium carbonate precipitation and evaporite dissolution explain the facies change from carbonate to sodium chloride or sodium sulfate that is recorded. However, reactions imply many minerals with common ions, deep reactions without evaporation, as well as shallow processes affected by both evaporation and mixing. Those processes are separated by considering both chemical and isotopic data, and quantitatively explained making use of an inverse geochemical modeling. The latter was applied, for the first time ever, in Algeria, to an extreme environment featuring a lack of data on a scarce natural resource such as Saharan groundwater. The populations of the region rely on this resource for their daily drinkable water as well as for agriculture, which mainly consists of date production and some vegetables that grow within the date-palm groves. Results obtained through inverse modeling could help water resources managers, both at the local and the regional scales, to gather the necessary information for an integrated management of that vital resource. Moreover, and regarding the large geographic scale of the aquifers, such a pilot study could be taken as a supporting work to further investigations elsewhere in similar regions. The present study leads to the main result that phreatic waters do not originate simply from infiltration of rainwater and dissolution of salts from the sabkhas. Conversely, Phr waters are largely influenced by the upwardly mobile deep CT and CI groundwaters, with fractions of the latter interacting with evaporites from the Turonian formations. Phreatic water occurrence is explained as a mixture of two end-member components: cluster I, which is very close to CI and CT, and cluster II, which is highly mineralized and results from the dissolution by rainwater of salts from the sabkhas. At depth, CI leaks upwardly and dissolves gypsum, halite and sylvite, with some ion exchange, to give waters of the CT aquifer composition. CT transformation into Phr cluster I waters involves the dissolution of Turonian evaporites (halite, sylvite and bloedite) with minor calcite precipitation. At the surface, direct dissolution by rainwater of salts from sabkhas (halite, sylvite, gypsum and some huntite) with precipitation of calcite and Mg²⁺/K⁺ ion exchange results in cluster II Phr composition. All phreatic groundwaters result from a mixing of cluster I and cluster II water that is accompanied by calcite precipitation, fixation of Mg²⁺ on ion exchange sites against the release of K⁺ and Na⁺. Moreover, some CO₂(g) escapes from the solution at depth, but dissolves much more at the surface.
<table><caption>**Table 6.** Summary of mass transfer for geochemical inverse modeling. Phases and thermodynamic database are from PHREEQC 3.0 (Parkhurst and Appelo, 2013).</caption><thead><tr><th>Phases</th><th>Stoichiometry</th><th>CI/CT</th><th>CT/Phr I</th><th>Rainwater/P036</th><th>PhrI/PhrII<br>60/40 %</th></tr></thead><tbody><tr><td>Calcite</td><td>CaCO<sub>3</sub></td><td>–</td><td>-6.62 × 10<sup>-6</sup></td><td>-1.88 × 10<sup>-1</sup></td><td>-2.26 × 10<sup>-1</sup></td></tr><tr><td>CO<sub>2</sub>(g)</td><td>CO<sub>2</sub></td><td>-6.88 × 10<sup>-5</sup></td><td>–</td><td>8.42 × 10<sup>-4</sup></td><td>5.77 × 10<sup>-4</sup></td></tr><tr><td>Gypsum</td><td>CaSO<sub>4</sub>·2H<sub>2</sub>O</td><td>4.33 × 10<sup>-3</sup></td><td>–</td><td>1.55 × 10<sup>-1</sup></td><td>1.67 × 10<sup>-1</sup></td></tr><tr><td>Halite</td><td>NaCl</td><td>7.05 × 10<sup>-3</sup></td><td>3.76 × 10<sup>-3</sup></td><td>6.72 × 10<sup>0</sup></td><td>1.28 × 10<sup>0</sup></td></tr><tr><td>Sylvite</td><td>KCl</td><td>2.18 × 10<sup>-3</sup></td><td>1.08 × 10<sup>-3</sup></td><td>4.02 × 10<sup>-1</sup></td><td>–</td></tr><tr><td>Bloedite</td><td>Na<sub>2</sub>Mg(SO<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O</td><td>–</td><td>1.44 × 10<sup>-3</sup></td><td>–</td><td>–</td></tr><tr><td>Huntite</td><td>CaMg<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub></td><td>–</td><td>–</td><td>4.74 × 10<sup>-2</sup></td><td>5.65 × 10<sup>-2</sup></td></tr><tr><td>Ca ion exchange</td><td>CaX<sub>2</sub></td><td>-1.11 × 10<sup>-3</sup></td><td>–</td><td>–</td><td>–</td></tr><tr><td>Mg ion exchange</td><td>MgX<sub>2</sub></td><td>1.96 × 10<sup>-3</sup></td><td>–</td><td>1.75 × 10<sup>-1</sup></td><td>-2.02 × 10<sup>-1</sup></td></tr><tr><td>Na ion exchange</td><td>NaX</td><td>–</td><td>–</td><td>–</td><td>3.92 × 10<sup>-1</sup></td></tr><tr><td>K ion exchange</td><td>KX</td><td>-1.69 × 10<sup>-3</sup></td><td>–</td><td>-3.49 × 10<sup>-1</sup></td><td>1.20 × 10<sup>-2</sup></td></tr></tbody></table>

Values are in mol kg⁻¹ H₂O. Positive-phase (mass entering solution) and negative-phase (mass leaving solution) mole transfers indicate dissolution and precipitation, respectively; this indicates no mass transfer.

The most complex phenomena occur during the dissolution of Turonian evaporites while CI leaks upwardly towards CT, and from Phr I to Phr II, while the transition from CT to Phr I implies a very limited number of phases. Globally, gypsum dissolution and calcite precipitation processes both act as an inorganic carbon sink.

## 5 Data availability

The Supplement related to this article is available online at doi:10.5281/zenodo.32287.
Appendix A

According to a simple Rayleigh equation, the evolution of the heavy isotope ratio in the remaining liquid R₁ is given by

$$R_1 \approx R_{1,0} \times f_1^{\alpha-1}, \quad (A1)$$

where f₁ is the fraction of remaining liquid and α the fractionation factor.

The fraction of remaining liquid is derived from chloride concentration, as chloride can be considered conservative during evaporation: all phreatic waters are undersaturated with respect to halite, which precipitates only in the last stage. Hence, the following equation holds:

$$f_1 \equiv \frac{n_{w,1}}{n_{w,0}} = \frac{[\mathrm{Cl}^-]_0}{[\mathrm{Cl}^-]_1}. \quad (A2)$$

By taking natural logarithms, one obtains

$$\ln R_1 \approx (1 - \alpha) \times \ln [\mathrm{Cl}^-] + \text{constant.} \quad (A3)$$

As, by definition,

$$R_1 \equiv R_{SD} \times \left( 1 + \frac{\delta^{18}\mathrm{O}}{1000} \right), \quad (A4)$$

one has

$$\ln R_1 \equiv \ln R_{SD} + \ln \left( 1 + \frac{\delta^{18}\mathrm{O}}{1000} \right), \\ \approx \ln R_{SD} + \frac{\delta^{18}\mathrm{O}}{1000}. \quad (A5)$$

Hence, with base 10 logarithms,

$$\delta^{18}\mathrm{O} \approx 1000(1 - \alpha) \log [\mathrm{Cl}^-] + \text{constant}, \\ \approx -\epsilon \log [\mathrm{Cl}^-] + k, \quad (A6)$$

where, as classically defined, $\epsilon = 1000(\alpha - 1)$ is the enrichment factor.
*Competing interests.* The authors declare that they have no conflict of interest.

*Acknowledgements.* The authors wish to thank the staff members of the National Agency for Water Resources in Ouargla (ANRH) and the Laboratory of Algerian Waters (ADE) for the support provided to the Technical Cooperation programme within which this work was carried out. Analyses of ¹⁸O were funded by the project CDTN/DDHI (Guendouz and Moulla, 1996). The support of University of Ouargla and of INRA for travel grants of R. Slimani and G. Bourrié are gratefully acknowledged too.

Edited by: M. Giudici

Reviewed by: G. Martinelli and one anonymous referee

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