Introduction
A scientific study published in 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 km2
(700 000 km2 in Algeria, 80 000 km2 in Tunisia and
250 000 km2 in Libya). Due to the climatic conditions of Sahara, these
formations are poorly renewed: about 1 billion m3 yr-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. ) which are
contained in the following three main reservoirs :
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)
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.
Geologic cross section in the region of Ouargla. The blue pattern
used for chott and sabkha correspond to the limit of the saturated
zone.
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
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 selected, which allowed
to optimize simultaneously 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 .
A number of assumptions are inherent to the application of inverse
geochemical modeling: (i) the two groundwater analyses from the initial and
the final boreholes should represent groundwater that flows along the same
flow path; (ii) dispersion and diffusion do not significantly affect
groundwater chemistry; (iii) a chemical steady state prevails in the
groundwater system during the time considered; and (iv) the mineral phases
used in the inverse calculation are or were present in the aquifer
. The soundness or the validity of the results depends on a
valid conceptualization of the groundwater system, on the validity of the
basic hydrogeochemical concepts and principles, on the accuracy of model
input data and on the level of understanding of the geochemical processes
occurring in the area . These requirements are
fulfilled 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 climate change. The methodology developed
here and the data collected can easily be integrated in the PRECOS framework
proposed for the management of environmental resources .
Methodology
Presentation of the study area
The study area is located in the northeastern desert of Algeria
(lower Sahara) near the city of Ouargla
(Fig. ), 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 valley of Oued Mya basin. The present climate
belongs to the arid Mediterranean type , 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 experienced during its
long geological history alternating marine 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
gradually withdrew. It is estimated at present that this sea did not reach
Ouargla and transgression stopped at the edge of the bowl .
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) .
The continental Pliocene consists of a local limestone crust with
puddingstone or lacustrine limestone (Fig. ), shaped by
eolian erosion into flat areas (regs). The Quaternary formations are
lithologically composed of alternating layers of permeable sand and
relatively impermeable marl .
The exploitation of Mio-Pliocene aquifer is ancient and at the origin of the
creation of the oasis . 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 .
The exploitation of the Senonian aquifer dates back to 1953 at a depth between
140 and 200 m, with a small initial rate of approximately 9 L s-1; two
boreholes have been exploited since 1965 and 1969, with a total flow rate
of approximately 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.
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 collected during a field campaign in 2013 along the main flow path of
Oued Mya. Of these, 67 were from piezometers tapping the phreatic aquifer,
32 from CT wells and the last 8 from boreholes tapping the CI aquifer
(Fig. ). Analyses of Na+, K+, Ca2+,
Mg2+, Cl-, SO42- and HCO3- were performed by ion
chromatography at Algiers Nuclear Research Center (CRNA). Previous and yet
unpublished data 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 .
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 incorporated recently in PHREEQC 3.0
. 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 composition between
initial and final water end members .
This mass balance technique has been used to
quantify reactions controlling water chemistry along flow paths
. It is also used to quantify the mixing proportions
of end-member components in a flow system .
Location map of sampling points.
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 occur 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 possible to select the setup that gives the best
results as the solution to the inverse modeling according to the fit between
the modeled and observed values. Then one can conclude whether calcite
dissolution/precipitation is relevant or not. This stepwise 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
participating or not .
Piper diagram for the Continental Intercalaire (filled squares),
Complexe Terminal (open circles) and phreatic (open
triangles) aquifers.
In the geochemical modeling inverse, soundness of results is dependent upon
valid conceptualization of the system, validity 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
hydrochemical evolution patterns, saturation indices and mineral stability
diagrams to constrain the inverse models.
Results and discussion
Tables to illustrate the results of the
chemical and the isotopic analyses. Samples are ordered according to an
increasing 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-].
Characterization of chemical facies of the groundwater
Piper diagrams drawn for the studied groundwaters (Fig. )
broadly show a scatter plot dominated by a sodium chloride 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.
Respectively, 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 comparable to that of other groundwater
occurring in north Africa and especially in the neighboring area of the
chotts (depressions where salts concentrate by evaporation) Merouane and
Melrhir .
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. a).
Its salinity is low around irrigated and fairly well-drained areas, such as
the palm groves of Hassi Miloud, just north of Ouargla
(Fig. ), 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. b)
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. c).
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.
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. ).
Moreover, they are oversaturated with respect to dolomite and undersaturated
with respect to anhydrite and halite (Fig. ).
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.
Field and analytical data for the Continental Intercalaire aquifer.
Locality
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Hedeb I
3 534 750
723 986
134
9 Nov 2012
2.0
46.5
7.6
3.5
5.8
6.8
10.7
0.6
2.5
3.3
0.034
Hedeb I
3 534 750
723 986
134
1992
1.9
49.3
7.3
0.4
5.8
1.1
5.7
0.2
0.8
0.5
Hedeb II
3 534 310
724 290
146
1992
2.0
47.4
7.6
0.6
6.2
1.2
5.1
0.2
1.3
0.8
Aouinet Moussa
3 548 896
721 076
132
1992
2.2
48.9
7.5
1.3
6.5
1.9
5.6
0.2
1.1
1.2
Aouinet Moussa
3 548 896
721 076
132
22 Feb 2013
2.2
48.9
7.5
3.2
9.8
3.9
6.3
0.7
5.7
1.3
Hedeb I
3 534 750
723 986
134
11 Dec 2010
2.2
49.3
7.3
1.9
12.4
4.6
10.7
0.7
3.8
2.3
Hedeb II
3 534 310
724 290
146
11 Dec 2010
2.2
47.4
7.6
2.1
13.1
5.5
13.9
0.5
4.5
1.4
Hassi Khfif
3 591 659
721 636
110
24 Feb 2013
2.4
50.5
6.8
2.9
14.3
5.2
10.8
0.8
3.4
4.6
0.033
Hedeb I
3 534 750
723 986
134
27 Feb 2013
2.0
46.5
7.6
3.5
15.1
7.7
11.8
0.5
5.6
5.2
Hassi Khfif
3 591 659
721 636
110
9 Nov 2012
2.2
50.1
7.6
3.3
15.3
7.8
12.2
0.6
5.8
4.9
El Bour
3 560 264
720 366
160
22 Feb 2013
2.9
54.5
7.3
2.6
18.6
6.2
20.6
0.7
4.8
1.4
Field and analytical data for the Complexe Terminal aquifer.
Locality
Site
Aquifer
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Bamendil
D7F4
M
3 560 759
720 586
296
20 Jan 2013
2.0
20.1
7.9
1.6
10.1
5.8
9.9
0.7
3.9
2.5
Bamendil
D7F4
M
3 560 759
720 586
296
1992
2.0
21.1
8.2
0.9
10.6
3.5
10.6
0.1
2.3
1.8
Ifri
D1F151
S
3 538 891
721 060
204
1992
2.7
23.5
7.0
1.3
10.7
2.7
8.0
0.7
2.3
2.1
Said Otba
D2F66
S
3 540 257
720 085
216
1992
2.3
24.0
8.0
1.4
11.0
4.7
11.5
0.2
2.1
3.3
Oglat Larbaâ
D6F64
M
3 566 501
729 369
177
1992
2.3
18.0
7.9
1.4
11.4
6.8
11.6
2.3
2.0
4.6
El Bour
D4F94
M
3 536 245
722 641
100
27 Jan 2013
3.1
26.2
7.4
1.6
12.8
6.8
5.2
1.9
1.6
9.1
Said Otba I
D2F71
S
3 557 412
718 272
211
1992
2.3
24.2
8.2
1.5
13.5
5.7
15.0
0.3
3.3
2.6
Debiche
D6F61
M
3 547 557
717 067
173
26 Jan 2013
2.2
23.9
7.7
1.8
14.2
8.4
12.6
0.7
5.4
4.4
Rouissat III
D3F10
S
3 535 068
722 352
248
1992
3.1
26.1
7.3
2.4
14.3
6.9
13.1
0.4
3.4
5.4
Said Otba I
D2F71
S
3 557 412
718 272
212
26 Jan 2013
5.6
25.1
7.3
2.4
14.3
6.9
13.1
0.4
3.4
5.4
0.034
Rouissat III
D3F10
S
3 535 068
722 352
248
20 Jan 2013
2.3
18.9
8.0
1.6
15.2
8.6
12.6
1.6
5.8
4.3
Ifri
D1F151
S
3 538 891
72 1060
204
27 Jan 2013
2.4
22.9
7.8
1.7
15.4
8.3
13.7
0.2
5.2
4.8
Said Otba
D2F66
S
3 540 257
720 085
216
31 Jan 2013
2.4
24.9
7.9
2.2
16.1
8.6
16.5
0.7
4.9
4.3
Oglat Larbaâ
D6F64
M
3 566 501
729 369
177
31 Jan 2013
2.4
23.7
7.6
2.3
16.3
8.6
13.6
0.7
5.9
5.0
SAR Mekhadma
D1F91
S
3 536 757
717 822
221
3 Feb 2013
2.5
25.8
7.7
3.4
16.5
8.5
16.1
0.7
5.3
4.9
Sidi Kouiled
D9F12
S
3 540 855
729 055
329
24 Jan 2013
2.6
21.3
8.1
4.6
16.8
8.8
16.1
0.8
6.2
5.0
Aïn N'Sara
D6F50
S
3 559 323
716 868
255
25 Jan 2013
3.4
25.7
7.4
2.0
16.9
9.7
15.9
0.3
3.4
7.9
0.033
A. Louise
D4F73
S
3 537 523
721 904
310
26 Jan 2013
2.6
24.0
7.5
2.0
17.4
9.1
13.9
2.0
5.8
5.1
Ghâzalet A. H
D6F79
M
3 598 750
720 356
119
2 Feb 2013
2.8
22.5
7.5
3.5
17.4
9.3
16.6
0.6
6.2
4.9
Aïn Moussa II
D9F30
S
3 537 814
719 665
220
2 Feb 2013
7.5
23.9
7.5
2.4
17.5
8.2
17.3
0.4
3.1
6.5
0.033
Aïn N'Sara
D6F50
S
3 559 323
716 868
255
2 Feb 2013
2.6
23.8
7.6
2.1
17.7
9.2
15.5
1.1
6.1
4.7
H. Miloud
D1F135
M
3 547 557
717 067
173
3 Feb 2013
2.8
21.6
7.5
3.3
17.9
9.2
16.5
1.0
6.2
4.9
El Bour
D6F97
S
3 540 936
715 816
169
25 Jan 2013
2.6
19.9
8.0
2.1
17.9
9.3
15.8
1.6
5.8
4.7
H. Miloud
D1F135
M
3 547 557
717 067
173
1992
2.1
22.7
8.1
2.8
18.1
5.7
16.6
0.5
3.6
4.3
N'Goussa El Hou
D6F51
S
3 556 256
718 979
198
31 Jan 2013
2.9
22.9
7.5
2.0
18.4
9.6
17.1
0.5
6.2
5.0
El Koum
D6F67
S
3 573 694
721 639
143
21 Jan 2013
3.1
22.9
8.1
3.5
18.4
9.7
17.9
0.3
6.5
5.1
El Koum
D6F67
S
3 573 694
721 639
143
1992
2.5
25.0
7.6
1.5
18.8
7.2
10.2
3.4
5.0
5.8
Itas
D1F150
M
3 536 186
717 046
93
21 Jan 2013
3.7
23.9
7.5
1.5
18.8
7.1
10.1
3.4
5.0
5.8
Aïn Moussa V
D9F13
M
3 538 409
718 680
210
8 Feb 2013
2.4
25.3
7.2
2.3
19.4
9.4
18.8
0.4
3.3
7.6
0.034
El Bour
D4F94
M
3 536 245
722 641
100
1992
2.3
21.2
7.9
1.6
20.1
7.2
12.1
2.6
5.8
5.2
Rouissat I
D3F18
M
3 535 564
722 498
80
26 Jan 2013
3.1
23.0
8.1
3.2
21.2
11.1
19.6
0.9
7.1
6.0
Rouissat I
D3F18
M
3 535 564
722 498
80
1992
2.0
20.0
7.8
1.7
21.7
8.5
17.7
1.2
5.1
6.0
Station de Pompage chott
D5F80
S
3 541 656
723 521
224
4 Feb 2013
3.3
24.5
8.2
3.9
22.1
11.9
19.9
2.1
7.6
6.3
Chott Palmeraie
D5F77
S
3 538 219
725 541
243
5 Feb 2013
3.4
24.6
7.5
3.3
22.3
12.1
20.9
1.2
8.3
5.8
Bour el Aïcha
D1F134
M
3 545 533
720 391
86
5 Feb 2013
3.4
22.2
7.3
4.1
23.2
12.2
21.2
1.5
8.6
6.0
Abazat
D2F69
M
3 552 504
712 786
137
3 Feb 2013
3.5
24.6
7.6
2.2
24.7
12.7
21.1
1.7
8.5
6.5
Garet Chemia
D1F113
S
3 536 174
716 808
213
28 Jan 2013
4.1
28.0
7.3
2.2
25.9
9.5
25.4
0.6
3.6
7.2
0.037
Frane
D6F62
M
3 570 175
717 133
167
27 Jan 2013
3.8
24.2
7.9
2.3
25.9
13.5
22.6
0.6
8.9
7.2
Continued.
Locality
Site
Aquifer
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Oum er Raneb
D6 F69
M
3 540 451
721 919
216
25 Jan 2013
4.2
24.1
7.0
2.6
27.9
8.7
22.9
0.6
4.4
8.0
0.035
N'Goussa El Hou
D6F51
S
3 556 256
718 979
198
1992
3.1
23.2
8.0
2.6
28.4
8.6
23.1
0.6
4.5
8.0
H. Miloud Benyaza
D1F138
M
3 551 192
717 042
89
28 Jan 2013
3.8
25.2
7.6
2.4
28.4
14.2
23.9
1.7
10.0
7.1
Aïn Larbaâ
D6F49
M
3 558 822
716 799
156
28 Jan 2013
3.9
23.7
7.3
2.2
28.9
9.0
23.9
0.5
5.0
7.7
0.037
H. Miloud Benyaza
D1F138
M
3 551 192
717 042
89
1992
2.9
22.8
7.5
2.2
28.9
9.1
23.9
0.5
5.0
7.7
Rouissat
D3F8
M
3 545 470
732 837
332
3 Feb 2013
4.4
25.4
7.5
1.7
29.8
8.3
22.8
1.2
6.2
6.1
Rouissat
D3F8
M
3 545 470
732 837
332
1992
6.2
25.3
7.2
1.7
29.8
8.3
22.9
1.2
6.2
6.1
Aïn El Arch
D3F26
M
3 534 843
723 381
93
1992
5.1
25.1
7.4
1.6
34.7
8.9
24.0
0.9
8.4
6.5
Station de Pompage chott
D5F80
S
3 541 656
723 521
224
1992
3.7
25.4
7.7
2.3
42.2
13.5
36.8
1.1
7.4
9.7
M = Mio-Pliocene aquifer; S = Senonian aquifer.
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 .
No significant trend of SI from south to north upstream and downstream of
Oued Mya (Fig. ) 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.
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 [HCO3-] / ([Cl-] + 2[SO42-])
ratio (Fig. ) from 0.2 to 0 and of the
[SO42-] / [Cl-] ratio from 0.8 to values smaller than 0.3
(Fig. ) 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 .
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. ). 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. 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 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.
Field and analytical data for the phreatic aquifer.
Locality
Site
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Bou Khezana
P433
3 597 046
719 626
118
20 Jan 2013
2.1
22.7
9.2
1.6
12.0
7.3
13.0
1.0
4.3
2.8
Bou Khezana
P433
3 597 046
719 626
118
1992
2.0
22.1
8.9
1.5
12.0
6.9
11.6
0.9
4.4
2.9
Hassi Miloud
P059
3 547 216
718 358
124
27/01/2013
2.1
23.9
8.2
1.9
13.0
7.3
12.6
1.3
4.4
3.4
0.024
Aïn Kheir
PL06
1992
4.0
23.8
7.5
1.9
14.2
17.9
15.9
0.6
10.6
7.5
Hassi Naga
PLX3
3 584 761
717 604
125
20 Jan 2013
2.9
23.0
8.1
2.0
17.7
9.4
16.6
0.9
5.8
5.0
0.031
LTP 30
1992
4.1
23.7
7.1
5.3
18.2
10.0
24.3
0.4
1.4
8.1
Maison de culture
PL31
3 537 988
720 114
124
1992
2.5
23.8
8.1
1.5
18.9
7.8
26.1
0.6
2.1
3.0
El Bour
P006
3 564 272
719 421
161
1992
3.0
23.4
7.9
1.3
19.0
7.7
12.4
2.7
5.3
5.3
Hassi Miloud
P059
3 547 216
718 358
124
1992
2.8
23.5
7.8
2.3
20.8
9.4
34.2
4.3
1.4
0.9
Oglat Larbaâ
P430
3 567 287
730 058
139
24 Jan 2013
4.5
27.5
8.3
3.3
22.1
12.4
21.8
2.6
8.6
5.5
Maison de culture
PL31
3 537 988
720 114
124
28 Jan 2013
3.7
22.2
8.2
4.2
22.6
8.6
28.4
2.2
4.0
3.2
Frane El Koum
P401
3 572 820
719 721
112
20 Jan 2013
3.4
27.5
7.5
2.2
23.3
13.4
21.8
1.9
8.3
6.3
0.032
Gherbouz
PL15
3 537 962
718 744
134
1992
2.5
23.5
7.7
3.0
23.5
14.0
50.6
2.8
1.0
0.3
Bour El Haïcha
P408
3 544 999
719 930
110
1992
2.4
23.5
7.8
2.4
24.2
13.2
41.9
6.1
2.3
0.8
Station d'épuration
PL30
3 538 398
721 404
130
1992
5.5
23.8
7.4
3.0
24.3
21.2
24.3
0.9
20.2
2.2
Frane Ank Djemel
P422
3 575 339
718 875
109
20 Jan 2013
4.1
24.2
8.4
4.4
25.3
9.5
23.7
1.8
4.2
7.9
0.025
Route Aïn Beïda
PLX2
3 537 323
724 063
127
1992
4.7
23.6
7.2
2.0
25.7
10.4
14.8
0.2
9.3
7.4
H. Chegga
PLX4
3 577 944
714 428
111
20 Jan 2013
4.1
25.2
7.6
3.0
26.2
9.8
24.0
2.3
5.0
7.5
0.033
Hassi Miloud
P058
3 547 329
716 520
129
27 Jan 2013
3.7
24.6
8.1
3.0
27.7
10.6
19.0
2.3
9.1
6.6
0.033
Route Aïn Moussa
P057
3 548 943
717 353
133
1992
5.3
23.4
7.7
1.3
28.2
11.5
17.6
2.0
11.5
5.8
Route El Goléa
P115
3 533 586
714 060
141
1992
2.6
23.7
7.6
2.8
28.8
14.5
58.7
0.1
0.8
0.7
Mekhadma
PL05
3 537 109
718 419
137
1992
23.9
7.8
1.7
30.9
16.7
24.9
1.0
15.7
4.5
Polyclinique Bel Abbès
PL18
3 537 270
721 119
119
31/01/2013
4.7
22.2
7.9
1.8
31.2
15.4
21.3
3.9
11.2
8.4
H. Chegga
PLX4
3 577 944
714 428
111
1992
4.5
23.7
7.6
1.5
31.5
10.1
20.1
5.9
7.5
6.5
Route El Goléa
P116
3 532 463
713 715
117
1992
5.6
23.7
7.6
1.4
31.9
12.8
22.2
0.8
10.6
8.0
Gherbouz
PL15
3 537 962
718 744
134
21 Jan 2013
4.7
23.3
8.2
1.8
32.4
14.6
27.8
0.8
6.8
10.8
Route El Goléa
P117
3 531 435
713 298
111
1992
4.7
23.7
7.7
1.5
32.8
12.8
30.2
1.0
9.2
5.7
Route Aïn Moussa
P057
3 548 943
717 353
133
26 Jan 2013
5.7
26.2
7.6
2.5
33.5
11.9
27.7
5.9
6.0
7.6
Ecole Paramédicale
PL32
3 538 478
720 170
131
21 Jan 2013
5.7
22.9
8.2
2.0
33.6
12.1
29.2
3.3
6.4
8.2
Direction des Services Agricoles
PL10
3 537 055
719 746
114
1992
6.1
23.7
7.7
1.3
35.0
13.5
8.6
1.9
19.4
7.2
Route El Goléa
P117
3 531 435
713 298
111
3 Feb 2013
5.5
25.0
7.7
3.3
35.4
13.8
37.1
3.0
8.4
5.7
Route El Goléa
P116
3 532 463
713 715
117
3 Feb 2013
5.8
22.5
8.1
1.7
36.3
11.6
28.5
3.2
6.8
8.4
Station d'épuration
PL30
3 538 398
721 404
130
31 Jan 2013
5.3
25.1
7.8
4.1
38.4
14.6
28.5
4.5
11.6
8.1
Hassi Debiche
P416
3 581 097
730 922
106
24 Jan 2013
5.5
23.7
8.8
0.3
38.6
18.0
22.3
0.9
4.8
21.3
Direction des Services Agricoles
PL10
3 537 055
719 746
114
28 Jan 2013
5.5
24.6
8.4
2.4
38.8
16.9
36.9
1.9
9.1
9.2
Hôpital
LTPSN2
3 538 292
720 442
132
27 Jan 2013
6.1
25.4
7.8
1.6
39.7
11.7
36.0
8.4
5.1
6.0
Parc SONACOM
PL28
3 536 077
719 558
134
21 Jan 2013
6.1
24.5
8.1
1.8
39.8
11.8
30.6
5.2
7.1
8.5
Bour El Haïcha
P408
3 544 999
719 930
110
27 Jan 2013
6.2
23.1
8.1
1.8
42.0
19.1
27.5
13.2
13.4
8.1
Route Aïn Moussa
P056
3 549 933
717 022
128
1992
7.6
23.6
7.9
0.6
42.1
10.7
18.9
1.9
12.6
9.3
Route Aïn Moussa
P056
3 549 933
717 022
128
26 Jan 2013
6.0
24.6
7.6
2.2
42.5
17.9
32.1
8.0
12.5
8.1
Continued.
Locality
Site
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Ecole Okba B. Nafaa
PL41
3 538 660
719 831
127
31 Jan 2013
6.3
24.1
7.7
2.1
44.9
13.2
36.2
11.8
6.3
6.7
Parc Hydraulique
P419
3 539 494
725 605
132
31 Jan 2013
7.0
26.4
7.8
2.1
45.1
14.4
41.4
10.8
6.0
6.9
Parc Hydraulique
PL13
3 536 550
720 200
123
21 Jan 2013
7.2
24.5
7.5
3.2
47.8
14.5
44.4
10.6
6.4
6.6
Mekhadma
PL25
3 536 230
718 708
129
21 Jan 2013
7.6
27.1
7.9
1.8
48.0
14.5
42.9
6.6
7.4
7.6
Said Otba
P506
3 535 528
725 075
126
4 Feb 2013
8.3
24.3
8.1
1.7
52.6
14.6
42.8
11.0
7.5
7.8
Said Otba
P506
3 535 528
725 075
126
1992
6.7
23.3
7.5
1.8
54.4
17.6
33.3
4.1
22.2
5.2
Mekhadma
P566
3 540 433
719 661
115
27 Jan 2013
9.0
24.6
7.6
1.7
62.5
15.2
71.6
3.0
4.6
6.1
Mekhadma
PL17
3 536 908
718 511
130
21 Jan 2013
9.4
24.5
8.1
3.4
63.2
15.6
77.2
2.5
4.1
5.1
Palm. Gara Krima
P413
3 530 116
722 775
130
4 Feb 2013
10.1
30.2
7.9
1.6
63.6
21.5
88.3
4.1
4.2
4.7
Mekhadma
PL25
3 536 230
718 708
129
1992
9.5
23.7
8.0
0.6
75.6
10.6
10.2
2.6
32.9
9.5
Said Otba (Bab Sbaa)
P066
3 542 636
718 957
126
1992
7.8
23.5
7.6
1.5
80.2
12.5
45.9
2.5
23.6
5.9
CEM Malek Bennabi
PL03
3 540 010
725 738
130
1992
7.3
23.9
7.6
3.1
84.1
30.6
108.6
2.2
10.2
9.0
Entreprise nationale de télévision
PL21
3 536 074
721 268
128
1992
9.7
23.8
7.3
4.5
84.3
23.7
61.6
3.8
33.5
1.9
Hôtel Transat
PL23
3 538 419
720 950
126
28 Jan 2013
15.0
24.2
8.2
4.5
86.6
16.7
79.9
3.2
14.5
6.9
Entreprise nationale de télévision
PL21
3 536 074
721 268
128
28 Jan 2013
16.4
25.7
7.5
2.0
99.9
17.4
85.5
5.7
15.7
7.6
Mekhadma
PL05
3 537 109
718 419
137
21 Jan 2013
16.8
24.8
7.6
2.0
101.3
17.7
85.9
5.9
16.7
7.6
Beni Thour
PL44
3 536 039
721 673
134
1992
4.7
23.9
7.2
2.7
109.8
67.2
134.7
5.7
42.0
8.8
Tazegrart
PLSN1
3 537 675
71 9416
125
22 Jan 2013
17.1
24.9
8.0
3.4
114.2
18.1
92.9
12.8
16.9
7.8
CEM Malek Bennabi
PL03
3 540 010
725 738
130
27 Jan 2013
10.8
23.1
7.5
3.3
117.3
14.7
116.4
2.1
9.0
7.2
El Bour
P006
3 564 272
719 421
161
3 Feb 2013
18.3
23.6
7.8
6.3
131.9
18.1
96.3
8.6
27.1
8.0
Aïn Moussa
P015
3 551 711
720 591
103
1992
12.4
23.6
7.7
2.4
134.7
28.2
73.0
3.1
52.4
6.3
Station de Pompage
PL04
3 541 410
723 501
138
27 Jan 2013
19.0
26.4
7.9
4.0
138.0
16.7
108.8
13.1
19.5
8.7
Drain Chott Ouargla
D. Ch
1992
23.9
7.7
2.7
142.2
24.5
96.31
3.2
44.2
3.0
Beni Thour
PL44
3 536 039
721 673
134
28 Jan 2013
20.2
25.8
7.8
5.0
153.0
17.7
125.9
6.3
22.8
8.1
CNMC (Société nationale des
PL27
3 535 474
718 407
126
21 Jan 2013
21.2
24.8
8.1
1.7
169.4
18.4
130.3
4.9
27.8
8.6
matériaux de construction)
Bamendil
P076
3 540 137
716 721
118
26 Jan 2013
22.3
27.2
7.6
4.3
171.5
17.1
130.8
6.3
28.0
8.8
N'Goussa
P041
3 559 563
716 543
135
26 Jan 2013
25.9
24.5
8.2
8.0
208.6
13.4
198.9
3.6
11.8
8.8
N'Goussa
P009
3 559 388
717 707
123
26 Jan 2013
27.5
28.4
8.4
11.5
208.8
15.8
195.1
2.7
18.7
9.0
LTP16
1992
11.5
23.8
7.5
3.8
213.4
48.6
147.9
7.5
75.3
4.3
P100
1992
17.2
23.6
7.6
3.4
235.0
46.4
264.8
4.7
25.6
5.6
Chott Adjadja Aven
PLX1
3 540 758
726 115
132
28 Jan 2013
32.9
23.4
8.0
4.4
245.6
20.9
141.4
26.9
44.6
17.7
Route Frane
P003
3 569 043
721 496
134
2 Feb 2013
31.0
23.5
8.0
6.9
252.7
17.9
208.2
9.4
30.0
10.0
El Bour–N'Goussa
P007
3 562 236
718 651
129
26 Jan 2013
30.1
28.4
7.8
5.4
254.7
15.5
209.2
10.4
28.8
7.5
Route Aïn Beïda
PLX2
3 537 323
724 063
127
21 Jan 2013
43.3
25.7
8.1
5.2
262.2
93.0
270.4
15.5
62.8
21.7
Aïn Moussa
P015
3 551 711
720 591
103
25 Jan 2013
32.0
22.7
8.0
2.9
263.0
15.4
206.9
6.6
32.1
9.9
Aïn Moussa
P402
3 549 503
721 514
138
25 Jan 2013
60.0
28.7
8.6
7.7
313.2
93.9
442.8
23.3
12.6
10.2
Route Frane
P001
3 572 148
722 366
127
1992
23.6
8.4
4.0
323.6
58.1
331.4
5.0
49.8
4.0
Aïn Moussa
P014
3 551 466
719 339
131
1992
23.4
7.3
4.0
337.0
64.3
328.7
5.5
62.4
5.5
Continued.
Locality
Site
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
N'Goussa
P019
3 562 960
717 719
113
2 Feb 2013
60.6
27.8
7.7
6.0
356.2
96.0
432.5
29.8
21.0
26.2
N'Goussa
P018
3 562 122
716 590
110
26 Jan 2013
61.1
26.2
8.4
6.5
372.4
82.3
347.1
22.6
60.7
26.6
Aïn Moussa
P014
3 551 466
719 339
131
25 Jan 2013
49.0
25.2
7.9
1.8
399.7
21.1
389.3
2.4
19.0
7.4
Route Sedrata
P113
3 535 586
714 576
105
3 Feb 2013
62.2
24.8
8.2
6.0
414.8
83.8
362.7
33.3
70.2
26.5
N'Goussa
P009
3 559 388
717 707
123
1992
23.3
7.8
2.4
426.9
57.8
393.8
9.1
59.1
12.0
Route Frane
P001
3 572 148
722 366
127
2 Feb 2013
66.2
28.3
7.2
6.5
468.7
101.5
350.3
26.0
116.2
35.3
Sebkhet Safioune
P031
3 577 804
720 172
120
1992
23.8
7.3
6.3
481.8
43.4
326.8
12.6
94.2
23.6
Sebkhet Safioune
P031
3 577 804
72 0172
120
2 Feb 2013
76.0
27.9
8.1
5.9
500.3
110.3
470.5
28.7
79.1
35.5
Route Frane
P002
3 570 523
722 028
108
1992
23.8
7.8
6.3
522.4
183.0
653.8
10.0
104.7
11.0
Sebkhet Safioune
P030
3 577 253
721 936
130
1992
23.5
7.7
4.4
527.7
123.5
533.8
11.6
106.2
10.7
Oum Raneb
P012
3 554 089
718 612
114
25 Jan 2013
64.1
30.3
7.8
7.8
534.3
20.9
529.6
6.4
19.7
4.7
Oum Raneb
P012
3 554 089
718 612
114
1992
23.4
7.5
2.7
539.4
60.6
413.6
5.6
112.8
9.4
Ank Djemel
P423
3 540 881
723 178
102
31 Jan 2013
90.8
23.5
7.5
6.2
636.5
101.3
495.5
38.3
125.8
30.3
Said Otba Chott
P096
3 540 265
724 729
111
1992
23.6
7.7
3.7
645.1
78.5
357.3
6.0
208.4
12.9
Sebkhet Safioune
P030
3 577 253
721 936
130
03/02/2013
64.7
23.1
7.8
3.7
671.8
90.3
742.9
16.0
41.5
7.7
N'Goussa
P017
3 560 256
715 781
130
26 Jan 2013
100.1
31.0
7.1
3.8
679.3
114.1
597.8
10.7
125.9
26.3
Ank Djemel
P021
3 573 943
723 161
105
1992
23.6
7.4
4.2
700.8
154.5
605.7
53.6
163.1
14.2
Station de Pompage
PL04
3 541 410
723 501
138
1992
23.6
7.4
2.4
716.3
34.8
560.1
7.0
99.6
11.0
Route Frane
P002
3 570 523
722 028
108
2 Feb 2013
62.8
26.9
7.6
1.7
748.5
62.6
651.5
14.7
77.7
27.3
Said Otba chott
P096
3 540 265
724 729
111
3 Feb 2013
68.3
25.9
8.7
1.2
771.0
53.1
615.9
23.5
69.6
50.4
N'Goussa
P019
3 562 960
717 719
113
1992
23.3
7.7
2.4
779.1
77.1
711.5
9.2
95.6
12.1
Said Otba (Bab Sbaa)
P066
3 542 636
718 957
126
3 Feb 2013
150.6
26.2
7.2
12.3
799.1
283.0
1249.7
19.0
37.6
18.1
Ank Djemel
P021
3 573 943
723 161
105
24 Jan 2013
82.3
29.6
7.6
2.4
800.4
94.4
824.0
11.0
53.4
25.4
N'Goussa
P018
3 562 122
716 590
110
1992
23.3
7.5
1.2
818.7
81.0
244.2
49.5
319.4
24.8
Oum Raneb
P162
3 546 133
725 129
98
25 Jan 2013
160.0
30.7
7.2
2.4
842.8
289.9
1309.9
13.3
33.5
17.7
Route Sedrata
P113
3 535 586
714 576
105
1992
23.7
7.7
2.8
954.9
124.9
997.5
13.3
86.7
11.7
Oum Raneb
PZ12
3 547 234
722 931
110
5 Feb 2013
114.9
27.4
7.4
2.9
980.1
15.5
930.8
7.5
23.9
14.2
Hôtel Transat
PL23
3 538 419
720 950
126
1992
23.5
7.4
3.0
1103.3
94.5
707.8
19.1
270.9
13.3
Sebkhet Safioune
P023
3 577 198
725 726
99
1992
23.3
7.4
2.3
1177.0
91.1
1058.2
11.7
133.5
12.4
Sebkhet Safioune
P034
3 579 698
725 633
97
5 Feb 2013
130.0
34.9
8.1
1.8
1189.1
14.7
1055.1
18.3
56.4
17.4
Sebkhet Safioune
P023
3 577 198
725 726
99
5 Feb 2013
117.9
29.4
8.2
1.9
1209.3
15.6
1129.4
8.4
42.9
10.2
Chott Adjadja
PLX1
3 540 758
726 115
132
1992
23.6
8.0
3.8
1296.7
134.0
1458.7
5.2
48.0
4.3
Sebkhet Safioune
P063
3 545 586
725 667
99
1992
23.5
7.5
1.9
1379.4
139.6
1257.4
18.6
182.3
10.0
LTP06
1992
23.8
7.6
7.8
1638.7
712.1
2621.6
41.6
190.5
13.3
Bamendil
P076
3 540 137
716 721
118
1992
23.5
7.7
5.7
1743.6
143.4
1321.9
26.9
331.4
12.3
El Bour–N'Goussa
P007
3 562 236
718 651
129
1992
23.3
7.7
1.4
1860.5
91.6
1434.7
26.2
278.8
13.3
Sebkhet Safioune
P063
3 545 586
725667
99
5 Feb 2013
178.9
26.7
7.7
1.4
1887.9
92.9
1455.8
26.7
282.9
13.4
P044
1992
23.4
7.8
4.5
2106.1
18.3
1765.5
27.3
171.2
6.5
P093
1992
23.6
7.5
1.5
2198.6
182.1
1957.5
29.5
278.2
10.4
P042
1992
23.4
7.6
1.1
2330.9
101.2
1963.7
52.2
248.1
11.2
P068
1992
23.5
7.5
3.4
2335.7
222.1
2302.3
26.8
219.9
7.2
Continued.
Locality
Site
Lat.
Long.
Elev.
Date
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
Br-
(m)
(mS cm-1)
(∘C)
(mmol L-1)
Oum Raneb
PZ12
3 547 234
722 931
110
1992
23.3
7.6
2.2
2405.6
109.9
2178.6
25.2
199.4
12.7
Hassi Debiche
P416
3 581 097
730 922
106
1992
23.3
7.8
4.3
2433.7
178.9
2361.1
24.3
196.1
9.2
N'Goussa
P041
3 559 563
716 543
135
1992
23.4
7.9
2.1
2599.7
324.6
2879.0
44.6
152.8
11.0
Sebkhet Safioune
P034
3 579 698
725 633
97
1992
23.3
7.8
1.9
2752.0
134.1
2616.8
24.4
180.1
10.5
P039
1992
23.4
6.9
1.9
4189.5
201.4
4042.6
17.9
257.8
9.2
Sebkhet Safioune
P074
1992
23.5
6.5
4.2
4356.5
180.9
2759.9
57.4
930.1
22.6
Sebkhet Safioune
P037
1992
23.4
6.9
1.5
4953.8
184.5
4611.1
2.9
347.6
7.9
Sebkhet Safioune
P036
1992
23.4
7.5
1.4
4972.8
108.1
4692.2
36.8
221.1
9.6
For longitude and latitude, the reference is UTM 31 projection for
north Sahara 1959 (CLARKE 1880 ellipsoid).
The [Br-] / [Cl-] ratio ranges from 2 × 10-3 to 3 × 10-3. The value
of this molar ratio for halite is around 2.5 × 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. and ). Three samples from the CI
aquifer are close to the [Ca2+] / [HCO3-] 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
[Ca2+] / [HCO3-] 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 Mg2+ fixation and Na+ and K+ releases.
Larger sulfate values observed in the phreatic aquifer (Fig. )
with [Ca2+] / [SO42-] < 1 can be attributed to a Na-Mg sulfate
dissolution from a mineral bearing such elements. This is, for instance, the case of bloedite.
Isotope geochemistry
The CT and CI aquifers exhibit depleted and homogeneous 18O contents,
ranging from -8.32 to -7.85 ‰. This was already previously
reported by many authors . On
the other hand, 18O values for the phreatic aquifer are widely
dispersed and vary between -8.84 and 3.42 ‰ (Table ).
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 .
Phreatic waters result from a mixing of two end members. Evidence for this
is given by considering the [Cl-] and 18O relationship
(Fig. ). The two clusters are (i) a first cluster of 18O-depleted
groundwater (Fig. ), and (ii) another cluster of
18O-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.
Cluster I represents the waters from CI and CT whose isotopic composition is
depleted in 18O (average value around -8.2 ‰)
(Fig. ). They correspond to an old water recharge
(paleo-recharge), whose age, estimated by means of 14C, exceeds
15 000 yr BP . 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.
Contour maps of the salinity (expressed as global mineralization) in
the aquifer system of the (a) phreatic aquifer and (b, c) Complexe
Terminal (with b indicating Mio-Pliocene and c indicating Senonian); figures are
isovalues of global mineralization (values in
g L-1).
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{Ca2+} + log{CO32-} = logKsp
for calcite and
log{Ca2+} + 2log{H2O} + log{SO42-} = logKsp
for gypsum.
Variation of saturation indices with distance from south to north in
the region of Ouargla.
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. ). Evidence for this is given in a semi-logarithmic
plot (Fig. ), as classically obtained according to the
simple approximation of the Rayleigh equation (cf. Appendix):
δ18O≈1000×(1-α)logCl-+k,≈-ϵlogCl-+k,
where α is the fractionation factor during evaporation,
ϵ ≡ -1000 × (1 - α) is the enrichment factor and k is a
constant . CI and CT waters are better separated in
the semi-logarithmic plot because they are differentiated by their chloride
content. According to Eq. (), simple evaporation
gives a straight line (solid line in Fig. ). The value of ϵ
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. ). 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. .
The phreatic waters that are close to cluster I (Fig. )
correspond to groundwaters occurring in the edges of the basin (Hassi Miloud,
piezometer P433) (Fig. ). 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. ) .
Isotopic data 18O and 3H and chloride concentration in the Continental
Intercalaire, Complexe Terminal and phreatic aquifers (sampling campaign in 1992).
Piezometer
Cl-
δ18O
3H
Piezometer
Cl-
δ18O
3H
Piezometer
Cl-
δ18O
3H
(mmol L-1)
(‰)
(UT)
(mmol L-1)
(‰)
(UT)
(mmol L-1)
(‰)
(UT)
Phreatic aquifer
P007
1860.5
-2.5
0
PL15
23.5
-7.85
0.6(1)
P074
4356.4
3.4
6.8(8)
P009
426.9
-6.6
1.2(3)
P066
80.2
-8.1
0.8(1)
PL06
14.2
-8.1
1.0(2)
P506
54.4
-6.8
1.6(3)
PL23
1103.3
-6.1
0
PL30
24.3
-7.48
2.4(4)
P018
818.7
-2.9
6.2(11)
P063
1379.3
-3.4
8.7(15)
P002
522.4
-5.7
0.6(1)
P019
779.1
-4.7
5.6(9)
P068
2335.6
-3.1
8.8(14)
PL21
84.3
-7.7
1.2(2)
PZ12
2405.5
-2.3
8.1(13)
P030
527.7
-6.6
2.4(4)
PL31
18.9
-7.4
1.6(3)
P023
1176.9
-2.6
0.2(1)
P076
1743.5
-5.6
2.8(5)
P433
12.0
-8.8
0
P416
2433.7
-7.9
5.9(9)
P021
700.7
-5.2
2.6(4)
PL03
84.1
-7.4
1.7(3)
P034
2752.0
-1.8
5.7(9)
PL04
716.3
-2.9
PL44
109.8
-8.8
1.0(2)
P036
4972.7
3.3
2.1(4)
P093
2198.5
-2.6
5.1(8)
PL05
30.9
-7.4
1.9(3)
P037
4953.8
3.1
1.8(3)
P096
645.1
-6.1
4.8(8)
P408
24.2
-7.9
0
P039
4189.5
1.0
2.2(4)
PLX1
1296.6
-5.6
1.1(2)
P116
31.9
-7.2
1.1(2)
P041
2599.7
-0.6
7.3(13)
PLX2
25.7
-7.6
1.3(2)
LTP 16
213.4
-7.5
1.6(3)
P044
2106.1
-4.5
2.7(5)
P015
134.7
-6.8
3.0(5)
P117
32.8
-6.9
0.1
P014
336.9
-6.9
2.8(5)
P001
323.6
-4.7
2.5(4)
PL10
35.0
-7.3
0.2(1)
P012
539.3
-6.4
2.2(4)
P100
235.0
-5.8
0
PL25
75.6
-7.4
0.9(2)
P042
2330.8
2.1
6.0(10)
P056
42.1
-7.0
2.9(5)
LTP30
18.2
-7.5
1.1(2)
P006
19.0
-6.6
0.5(1)
P113
954.9
-4.8
0.8(2)
LTP06
1638.6
-1.9
2.8(5)
P057
28.2
-7.3
1.1(2)
PLX4
31.5
-7.1
0.3(1)
P031
481.8
-6.1
3.0(5)
P059
20.8
-7.8
0
P115
28.8
-2.5
6.8(12)
Borehole
Cl-
δ18O
3H
Borehole
Cl-
δ18O
3H
Borehole
Cl-
δ18O
3H
(mmol L-1)
(‰)
(UT)
(mmol L-1)
(‰)
(UT)
(mmol L-1)
(‰)
(UT)
Complexe Terminal aquifer
D5F80
42.2
-7.9
D1F138
28.9
-8.1
0.7(1)
D2F71
13.5
-8.2
0.6(1)
D3F8
29.8
-8.1
1.4(2)
D3F18
21.7
-8.2
0.2(1)
D7F4
10.6
-8.3
0.1(1)
D3F26
34.7
-8.0
0.8(1)
D3F10
14.3
-7.9
1.5(2)
D2F66
11.0
-8.3
D4F94
20.1
-8.2
0.6(1)
D6F51
28.4
-7.9
0.7(1)
D1F151
10.8
-8.3
0.4(1)
D6F67
18.8
-8.2
3.7(6)
D1F135
18.1
-8.1
1.1(2)
D6F64
11.4
-8.3
4.3(7)
Continental Intercalaire aquifer
Hadeb I
5.8
-8.0
0
Hadeb II
6.2
-7.9
0.1(1)
Aouinet Moussa
6.5
-7.9
1.1(2)
Change from carbonate facies to evaporite from the Continental
Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles)
aquifers.
Change from sulfate facies to chloride from the Continental Intercalaire
(filled squares), Complexe Terminal (open circles) and phreatic (open
triangles) aquifers.
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
p. 899.
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
p. 899.
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 δ values of each fraction that is
involved in the mixing process.
The δ value of the mixture is given by
δmix=f×δ1+(1-f)×δ2,
where f is the fraction of the CI aquifer, 1 - f the fraction of the CT and
δ1, δ2 are the respective isotope contents.
Average values of mixing fractions from each aquifer to the phreatic waters
computed by means of Eq. () 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. and ), for an
intermediate 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 independently
cross-check the validity of the mixing fractions derived 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
process which does not involve any change in 18O content. Conversely,
phreatic waters result, to a minor degree, from evaporation 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 (Table ),
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 between 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
precipitation 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 National Agency for Water Resources
station in Ouargla. A major 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 fractions (isotopes)
are the ones that escape first, enriching the remaining fraction in
tritium. The 16 TU value would thus 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)
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 18O 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 the Cl- versus 18O
diagrams (Figs. and ) could partly derive from
an ascending drainage from the deep and confined CI aquifer (exhibiting
depleted homogenous 18O contents and very low tritium), a vertical
leakage that is favored by the Amguid El Biod highly faulted area
.
Inverse modeling
We assume that the relationship between 18O 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
18O and Cl- data, CI, CT and Phr data populations can be
categorized. The CI and CT do not show appreciable 18O variations
and can be considered as a single population. The Phr samples consist,
however, of different populations: cluster I, with δ18O values close to -8
and small Cl- concentrations, more specifically less than
35 mmol L-1; cluster II, with δ18O values larger than 3
and very large Cl- concentrations, more specifically larger than
4000 mmol L-1 (Table ); intermediate Phr samples resulting
from mixing between clusters I and II (mixing line in Fig. ,
mixing curve in Fig. ) and from evaporation of cluster I
(evaporation line in Fig. ). 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 .
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
Ca2+, Mg2+, K+ and Na+ 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. CO2(g) dissolution is considered by
PHREEQC as a dissolution of a mineral, whereas CO2(g) degassing is
dealt with as if it were a mineral precipitation.
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).
Aquifer
Size
Parameter
EC
T
pH
Alk.
Cl-
SO42-
Na+
K+
Mg2+
Ca2+
(mS cm-1)
(∘C)
(mmol L-1)
CI
11
Average
2.2
49.0
7.5
2.3
11.0
4.7
10.3
0.5
3.6
2.4
CI
11
SD
0.3
2.0
0.2
1.0
4.6
2.5
4.6
0.2
2.0
1.8
CT
50
Average
3.2
23.0
7.8
2.3
20.0
8.9
17.0
1.0
5.5
5.6
CT
50
SD
1.1
2.4
0.4
0.8
7.0
2.6
6.0
0.8
2.2
1.7
Phr cluster I
30
Average
3.9
24.0
7.9
2.3
24.7
11.8
24.2
2.1
7.2
5.3
Phr cluster I
30
SD
1.3
1.3
0.4
1.0
6.9
3.4
11.0
1.7
5.0
2.7
Phr cluster II
3
Average
23.4
7.0
2.4
4761.0
158.0
4021.0
32.4
500.0
13.0
Phr cluster II
3
SD
0.1
0.5
1.6
350.0
43.0
1093.0
28.0
378.0
8.0
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
p. 899.
Chloride concentration versus δ18O in the Continental
Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles)
aquifers from Ouargla.
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 CI, CT and Phr groundwaters (Fig. ,
Table ), as follows:
Transition from CI 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
CO2(g) degassing. The maximum elemental concentration fractional error
equals 1 %. The 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 .
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 -8 and +3.5 ‰ (Figs. and ). 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 ). 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
consideration 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 ‰).
Summary of mass transfer for geochemical inverse modeling. Phases and
thermodynamic database are from PHREEQC 3.0 .
Phases
Stoichiometry
CI/CT
CT/Phr I
Rainwater/P036
PhrI/PhrII
60/40 %
Calcite
CaCO3
–
-6.62 × 10-6
-1.88 × 10-1
-2.26 × 10-1
CO2(g)
CO2
-6.88 × 10-5
–
8.42 × 10-4
5.77 × 10-4
Gypsum
CaSO4 ⋅ 2H2O
4.33 × 10-3
–
1.55 × 10-1
1.67 × 10-1
Halite
NaCl
7.05 × 10-3
3.76 × 10-3
6.72 × 100
1.28 × 100
Sylvite
KCl
2.18 × 10-3
1.08 × 10-3
4.02 × 10-1
–
Bloedite
Na2Mg(SO4)2 ⋅ 4H2O
–
1.44 × 10-3
–
–
Huntite
CaMg3(CO3)4
–
–
4.74 × 10-2
5.65 × 10-2
Ca ion exchange
CaX2
-1.11 × 10-3
–
–
–
Mg ion exchange
MgX2
1.96 × 10-3
–
1.75 × 10-1
-2.02 × 10-1
Na ion exchange
NaX
–
–
–
3.92 × 10-1
K ion exchange
KX
-1.69 × 10-3
–
-3.49 × 10-1
1.20 × 10-2
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.
Log [Cl-] concentration versus δ18O in Continental
Intercalaire (filled squares), Complexe Terminal (open circles) and phreatic (open triangles)
aquifers from Ouargla.
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 ) shows that gypsum dissolution results in
calcite precipitation and CO2(g) dissolution, thus acting as an
inorganic carbon sink.
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. 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.