Abstract

Water resources are economically and environmentally the most valuable for countries affected by aridity. This study is to identify the factors influencing the quality of the waters of the aquifer system of the Great Sebkha of Oran, one area that is already in a stress situation. The determination of the origin of the salinity of the waters was approached from an analysis of the chemical type. Water mineralization is mainly governed by the phenomena of dissolution and precipitation of minerals (calcite, dolomite, anhydrite, gypsum and halite).

INTRODUCTION

Scarcity of water, pollution load, political issues and rising population have drawn great attention to the proper management of water resources such as groundwater in the 21st century (Benouara et al. 2016).

The basin of the Great Sebkha of Oran is characterized by endorheic drainage. In this agricultural space, the most widely used irrigation system is drip. Water supply is one of the determining factors in agricultural production, both in crop intensification and extension of irrigation. Water deficit linked to climate semi-aridity has forced farmers to resort to the use of underground water of poor quality which is not without consequences for land degradation.

Environmental data are strongly characterized by inherent variability, and only limited understanding of the environmental distribution of contaminants can be gained from chemical analysis (Machiwal & Jha 2015). The complex and highly variable geological and lithological scenarios combine the content of the major chemical components and dissolved CO2, and will determine their healthy properties and uses. These factors create spatially different water types (i.e. hydrochemical facies) (Ciotoli & Guerra 2016).

Our study is based on the collection of samples of surface water and groundwater collected in pretty well-distributed points throughout the basin (Table 1 and Figure 1).

Table 1

Network measurement and sampling

NatureNameUTM
XYZ
Drilling Oued Tlélat OT2 Bis 720.654 3,933.795 107 
Drilling Oued Tlélat OT5 722.975 3,938.85 100 
Drilling Oued Tlélat OT7 719.975 3,939.495 90 
Wells TOUATI 696.875 3,938.55 92 
Wells Orangerie 696.7 3,939.95 112 
Wells BELHADRI Abdelkader 700.725 3,942.35 143 
Source Ain Beida 683.525 3,922.55 94 
Drilling Moulin Mokhtar el Oumda 695.975 3,920.87 113 
Wells CHEHEIDA Belgacem 697.25 3,923.05 109 
10 Drilling MOULAY Said 697.325 3,925.42 101 
11 Wells Kratsa 700.85 3,928.85 98 
12 Drilling Kratsa 701.47 3,928.457 99 
13 Wells DELLA-KRACHAI Boumediène 694.975 3,939 100 
14 Drilling BOUDINAR Ahmed 693.625 3,944.05 480 
15 Drilling Ain el Arba {Frères Aziz F1) 691.575 3,919.75 105 
16 Oued Oued el Ared 720.875 3,938.575 89 
17 Wells Bou Tlélis (Pépinière) 693.1 3,938.35 91 
18 Wells ABDELLI Said N°2 706.875 3,943.56 117 
19 Wells   707.34 3,943.275 110 
20 Wells HAMMADI Mohammed 707.475 3,943.175 104 
21 Wells Ex-Ferme Saint Pierre 707.8 3,943.175 108 
22 Wells CHETTIR Noureddin N°1 1 706.57 3,942 91 
23 Wells SENOUCI Ali N°1 708.775 3,942.75 88 
24 Wells MAHMOUDI Mohamed 705.87 3,942.425 98 
25 Wells CHIKHI 688.05 3,936.2 85 
26 Wells KARAMA Houari (Domaine Mimoun) 685.45 3,935.75 86 
27 Drilling DHEIMI Moulay 695,150 3,938.92 92 
28 Wells ABBOU Kouider 692.675 3,938.05 86 
29 Wells MOSTAGHALM (Rios) 693.2 3,938 89 
30 Drilling M'lèta N°2 718.895 3,934.508 96 
31 Wells   720.675 3,928.1 156 
32 Drilling Misserghin F9 701.047 3,941.592 122 
33 Drilling S sec 2 695.368 3,939.124 105 
34 Drilling S sec 3 698.794 3,940.176 99.8 
35 Drilling S, Sidi Salem 704.64 3,943.724 128 
36 Drilling S, de Misserghin syndic, 704.807 3,943.433 118 
37 Drilling S, Ban Lartigue 2 723.729 3,933.812 106 
38 Drilling SE7A 718.032 3,928.71 108.8 
39 Drilling S4A 717.921 3,928.723 107.3 
40 Drilling S, Douar Saida 702.647 3,919.456 134 
41 Drilling S, Hammam bou hadjar 687.452 3,918.822 140 
42 Drilling Ain Arbaa 691.526 3,919.41 109 
43 Wells Flaga Abdelkader 721.873 3,929.194 130 
44 Drilling Tafaraoui N°2 723.591 3,934.105 120 
45 Drilling S, Misserghin 703.52 3,943.157 115 
46 Wells Berial F4 697.836 3,940.1 104 
47 Wells Station de service Tamazourah 712.95 3,920.75 181.5 
48 Wells   717.685 3,929.55 98 
49 Wells   704.875 3,944.5 140 
50 Wells Station Tamazourah 2 712.962 3,920.775 181.5 
51 Oued Oued Rassoul 706.457 3,918.5 163 
52 Oued Oued Haimeur 701.613 3,916.599 165 
53 Oued Oued Tafaraoui 723.551 3,929.71 156 
54 Oued Oued Tamazourah 713.105 3,920.378 188 
55 Oued Oued Besbas (Oued Tleta) 697.614 3,917.024 153 
NatureNameUTM
XYZ
Drilling Oued Tlélat OT2 Bis 720.654 3,933.795 107 
Drilling Oued Tlélat OT5 722.975 3,938.85 100 
Drilling Oued Tlélat OT7 719.975 3,939.495 90 
Wells TOUATI 696.875 3,938.55 92 
Wells Orangerie 696.7 3,939.95 112 
Wells BELHADRI Abdelkader 700.725 3,942.35 143 
Source Ain Beida 683.525 3,922.55 94 
Drilling Moulin Mokhtar el Oumda 695.975 3,920.87 113 
Wells CHEHEIDA Belgacem 697.25 3,923.05 109 
10 Drilling MOULAY Said 697.325 3,925.42 101 
11 Wells Kratsa 700.85 3,928.85 98 
12 Drilling Kratsa 701.47 3,928.457 99 
13 Wells DELLA-KRACHAI Boumediène 694.975 3,939 100 
14 Drilling BOUDINAR Ahmed 693.625 3,944.05 480 
15 Drilling Ain el Arba {Frères Aziz F1) 691.575 3,919.75 105 
16 Oued Oued el Ared 720.875 3,938.575 89 
17 Wells Bou Tlélis (Pépinière) 693.1 3,938.35 91 
18 Wells ABDELLI Said N°2 706.875 3,943.56 117 
19 Wells   707.34 3,943.275 110 
20 Wells HAMMADI Mohammed 707.475 3,943.175 104 
21 Wells Ex-Ferme Saint Pierre 707.8 3,943.175 108 
22 Wells CHETTIR Noureddin N°1 1 706.57 3,942 91 
23 Wells SENOUCI Ali N°1 708.775 3,942.75 88 
24 Wells MAHMOUDI Mohamed 705.87 3,942.425 98 
25 Wells CHIKHI 688.05 3,936.2 85 
26 Wells KARAMA Houari (Domaine Mimoun) 685.45 3,935.75 86 
27 Drilling DHEIMI Moulay 695,150 3,938.92 92 
28 Wells ABBOU Kouider 692.675 3,938.05 86 
29 Wells MOSTAGHALM (Rios) 693.2 3,938 89 
30 Drilling M'lèta N°2 718.895 3,934.508 96 
31 Wells   720.675 3,928.1 156 
32 Drilling Misserghin F9 701.047 3,941.592 122 
33 Drilling S sec 2 695.368 3,939.124 105 
34 Drilling S sec 3 698.794 3,940.176 99.8 
35 Drilling S, Sidi Salem 704.64 3,943.724 128 
36 Drilling S, de Misserghin syndic, 704.807 3,943.433 118 
37 Drilling S, Ban Lartigue 2 723.729 3,933.812 106 
38 Drilling SE7A 718.032 3,928.71 108.8 
39 Drilling S4A 717.921 3,928.723 107.3 
40 Drilling S, Douar Saida 702.647 3,919.456 134 
41 Drilling S, Hammam bou hadjar 687.452 3,918.822 140 
42 Drilling Ain Arbaa 691.526 3,919.41 109 
43 Wells Flaga Abdelkader 721.873 3,929.194 130 
44 Drilling Tafaraoui N°2 723.591 3,934.105 120 
45 Drilling S, Misserghin 703.52 3,943.157 115 
46 Wells Berial F4 697.836 3,940.1 104 
47 Wells Station de service Tamazourah 712.95 3,920.75 181.5 
48 Wells   717.685 3,929.55 98 
49 Wells   704.875 3,944.5 140 
50 Wells Station Tamazourah 2 712.962 3,920.775 181.5 
51 Oued Oued Rassoul 706.457 3,918.5 163 
52 Oued Oued Haimeur 701.613 3,916.599 165 
53 Oued Oued Tafaraoui 723.551 3,929.71 156 
54 Oued Oued Tamazourah 713.105 3,920.378 188 
55 Oued Oued Besbas (Oued Tleta) 697.614 3,917.024 153 
Figure 1

Geological map (Benziane 2013) and location of sampling points.

Figure 1

Geological map (Benziane 2013) and location of sampling points.

The results show that pumping-induced hydraulic gradient changes and artificial connection of aquifers by well screens can mix chemically distinct groundwater (Ayotte et al. 2011). Correlations between major elements highlight the main mechanisms involved in the evolution of the salinity of the water in the different horizons of the aquifer system and map the vulnerability of soils.

GEOLOGICAL AND HYDROGEOLOGICAL CONTEXT

The basin of the Great Sebkha of Oran is located in northwestern Algeria (Figure 2). It extends over an area of 1,890 km2 of which 298 km2 are occupied by the Sebkha. This salt lake is over 40 km long and 6 to 13 km wide.

Figure 2

Location of the study area (Benziane 2013).

Figure 2

Location of the study area (Benziane 2013).

Geological studies have highlighted a structure consisting of three estates: a native bedrock ante nappe Mesozoic forming the backbone of the massif to the schistosity of Murdjadjo, to the north; a complex of allochthonous units put in place in the Miocene forming the Tessala mountains to the south; and sedimentation post nappe Neogene and Quaternary, subsiding in the central area. The formations thus defined generally have lateral variations in thickness and facies (Figure 1).

The aquifer system is made by stacking, depending on the location, two to three layers of aquifers: the Miocene limestone, sandstone and sandy Pliocene and Quaternary alluvium.

MATERIALS AND METHODS

For the purposes of the study of the geochemical characterization of the aquifer system, we have selected 55 water points pretty well distributed all around the lake of the Great Sebkha of Oran. This network is restricted compared with that constituted originally (93 points) for various reasons, namely the drying up of the oued and some sources, and the poor state or the abandonment of some wells. More than 95% of the network permanently retained and listed consists of wells for irrigation of crops and also for domestic use (Table 1 and Figure 1).

The collection of water samples was held during low flow, 4–20 July 2011. These samples were analyzed in the laboratories of applied geology at the University of Sciences and Technology of Oran and Centre University of Tlemcen according to methods described in Table 2.

Table 2

Analytical methods and apparatus used

ParameterMethod and apparatus
pH Analyzer multi-parameter portable Mark Hanna instrument model HI 9811 
Electrical conductivity 
 Volumetric 
Mg2+ 
Cl 
Ca2+ 
 Spectrometry, spectrometer Optizen 2120 UV 
Na+ Ionogram Easy lyte Na/K/Cl, 800 ml, MEDICA 001384-001 R2 Analyzer, 12160/12014-05 
K+ 
ParameterMethod and apparatus
pH Analyzer multi-parameter portable Mark Hanna instrument model HI 9811 
Electrical conductivity 
 Volumetric 
Mg2+ 
Cl 
Ca2+ 
 Spectrometry, spectrometer Optizen 2120 UV 
Na+ Ionogram Easy lyte Na/K/Cl, 800 ml, MEDICA 001384-001 R2 Analyzer, 12160/12014-05 
K+ 

RESULTS AND DISCUSSION

Critical analysis of the results

The results were verified by calculation of the ion balance (Table 3). In theory, a chemical analysis is considered as reliable only if the balance is less than or equal to 5%. The reliability of the data of the analysis has also been verified by simple linear regression between the sum of cations and anions, on the one hand, and between the electrical conductivity and the amount of ions, on the other hand. The balance calculated for all of the samples is surplus to 87% in anions. This surplus could, in the case of waters relatively loaded with dissolved salts, be explained by the existence other, unscanned cations, or erroneous results.

Table 3

Measurements and chemical analysis results

T (°C)pHσCa+2Mg+2Na+K+Cl
(μS/cm)(mg/l)
34 6.7 3,450 380 764 3,630 310.5.07 2,157.7 307.7 
22 6.8 4,060 200 644 1,605 356.5.85 1,727.6 311.6 
33.5 6.3 6,880 200 836 1,901 715.7.02 2,485 310 
26 6.7 7,250 220 3,265 11,359 623.6.63 7,976 299.4 
27 5,170 240 536 1,417 602.5.85 2,094.5 300.6 
24 7.2 3,150 248 2,037 7,776 333.2.34 4,775 294.2 
24.5 7.4 6,080 180 692 12,084 4,687.31.59 5,994.5 297.2 
30 6.8 7,650 260 956 3,509 894.3.12 2,995 320.2 
22 6.6 15,820 288 2,396 1,356 818.36.27 7,810 368.7 
10 24 6.9 10,300 192 1,580 8,162 2,267.21.84 4,960 345.4 
11 22.5 6.7 11,370 176 2,036 9,528 2,037.6.24 5,551 400.4 
12 26 6.9 4,920 208 548 2,595 1,106.4.29 2,123.5 322.1 
13 25.5 6.9 7,810 340 1,040 4,242 871.8.19 2,378.5 302.1 
14 27 6.9 1,450 156 368 816 184 3.51 1,136 299.3 
15 6.9 3,500 280 896 3,191 623.13.65 3,094.5 295.7 
16 27 7.2 6,770 180 1,472 5,732 756.7.02 4,307.5 319.1 
17 26 6.3 12,850 460 1,388 3,086 1,642.12.48 5,970 315.2 
18 24.5 6.4 6,760 680 1,844 8,414 1,067.13.26 5,669 312.2 
19 24 6.5 6,920 380 2,444 11,000 1,117.13.26 6,684 320.7 
20 23 6.6 6,260 440 2,516 11,194 1,016.14.82 6,417.5 310.2 
21 25 6.4 6,240 520 2,516 10,904 1,025.15.99 6,607.5 308.8 
22 21.5 6.6 7,550 600 2,504 12,060 1,179.3.12 6,617.5 315.4 
23 22 6.9 2,300 320 344 421 384.0.858 1,775 297.2 
24 23.5 6.6 6,360 600 800 3,053 1,018.10.53 3,288.5 309 
25 32 6.8 6,790 300 1,040 3,090 535.8.58 2,685 315.5 
26 21 6.9 13,360 304 1,892 4,356 1,752.24.18 5,970 307.1 
27 27.5 6.8 8,140 236 1,000 2,036 848.8.19 3,372.5 313.7 
28 26 6.4 11,330 460 1,532 6,193 1,212.8.97 5,408 304.4 
29 26 6.5 8,760 348 1,244 4,481 800.8.19 3,662.5 328.7 
30 27 7.5 8,130 192 1,652 7,789 1,166.3.51 5,272 321.9 
31 31 6.8 4,460 184 1,532 6,521 713 1.95 4,952.5 307.8 
32 27 7.3 1,240 160 1,196 5,000 167.3.51 2,994 294.1 
33 6.9 5,310 316 1,364 3,462 181.2.73 2,965.5 301.5 
34 7.5 950 420 812 3,520 177.2.73 2,207 295.7 
35 7.5 730 200 596 1,634 158.1.95 1,100.5 302.9 
36 21 7.1 3,300 200 668 2,226 370.9.75 1,584.5 300.8 
37 40 6.9 4,350 340 1,124 5,595 372.6.24 3,065 318.8 
38 7.2 3,020 400 500 2,095 618.3.12 1,775 299.4 
39 29.5 6.9 3,610 460 4,125 14,330 342.3.51 9,100 308.1 
40 27 7.6 1,060 120 465 450 230 2.34 1,597.5 296.5 
41 24.5 5.1 9,350 420 644 2,541 1,345.42.51 3,324 299.4 
42 28.5 6.8 5,020 360 725 1,534 483 8.58 2,307.5 from 297.5 
43 25 6.9 6,040 380 656 2,246 979.3.9 2,591.5 311.1 
44 36 6.5 4,070 460 500 2,062 158.3.12 1,065 312.7 
45 7.4 720 184 212 1,616 1,000.11.7 1,455.5 296.1 
46 26 7.1 8,240 236 740 3,288 1,451.2.34 3,769 313.1 
47 25 6.8 4,680 180 740 470 692.6.63 3,017.5 323 
48 24.5 6.9 7,850 220 855 1,566 733.5.85 2,769 305.7 
49 21 7.4 1,030 216 1,415 941 124.1.95 3,550 297.6 
50 27.5 6.9 4,830 460 4,525 19,962 708.7.8 8,907.5 313.6 
51 6.5 5,000 360 300 2,038 805 5.46 1,520 343.6 
52 6.7 8,800 400 432 3,848 1,734.8.97 3,775 367.7 
53 7.1 4,630 160 408 1,495 630.3.12 1,420 342.3 
54 7.3 3,410 80 1,420 4,433 437 4.29 2,775 329.2 
55 7.3 5,710 240 480 2,214 929.5.85 2,100 325 
T (°C)pHσCa+2Mg+2Na+K+Cl
(μS/cm)(mg/l)
34 6.7 3,450 380 764 3,630 310.5.07 2,157.7 307.7 
22 6.8 4,060 200 644 1,605 356.5.85 1,727.6 311.6 
33.5 6.3 6,880 200 836 1,901 715.7.02 2,485 310 
26 6.7 7,250 220 3,265 11,359 623.6.63 7,976 299.4 
27 5,170 240 536 1,417 602.5.85 2,094.5 300.6 
24 7.2 3,150 248 2,037 7,776 333.2.34 4,775 294.2 
24.5 7.4 6,080 180 692 12,084 4,687.31.59 5,994.5 297.2 
30 6.8 7,650 260 956 3,509 894.3.12 2,995 320.2 
22 6.6 15,820 288 2,396 1,356 818.36.27 7,810 368.7 
10 24 6.9 10,300 192 1,580 8,162 2,267.21.84 4,960 345.4 
11 22.5 6.7 11,370 176 2,036 9,528 2,037.6.24 5,551 400.4 
12 26 6.9 4,920 208 548 2,595 1,106.4.29 2,123.5 322.1 
13 25.5 6.9 7,810 340 1,040 4,242 871.8.19 2,378.5 302.1 
14 27 6.9 1,450 156 368 816 184 3.51 1,136 299.3 
15 6.9 3,500 280 896 3,191 623.13.65 3,094.5 295.7 
16 27 7.2 6,770 180 1,472 5,732 756.7.02 4,307.5 319.1 
17 26 6.3 12,850 460 1,388 3,086 1,642.12.48 5,970 315.2 
18 24.5 6.4 6,760 680 1,844 8,414 1,067.13.26 5,669 312.2 
19 24 6.5 6,920 380 2,444 11,000 1,117.13.26 6,684 320.7 
20 23 6.6 6,260 440 2,516 11,194 1,016.14.82 6,417.5 310.2 
21 25 6.4 6,240 520 2,516 10,904 1,025.15.99 6,607.5 308.8 
22 21.5 6.6 7,550 600 2,504 12,060 1,179.3.12 6,617.5 315.4 
23 22 6.9 2,300 320 344 421 384.0.858 1,775 297.2 
24 23.5 6.6 6,360 600 800 3,053 1,018.10.53 3,288.5 309 
25 32 6.8 6,790 300 1,040 3,090 535.8.58 2,685 315.5 
26 21 6.9 13,360 304 1,892 4,356 1,752.24.18 5,970 307.1 
27 27.5 6.8 8,140 236 1,000 2,036 848.8.19 3,372.5 313.7 
28 26 6.4 11,330 460 1,532 6,193 1,212.8.97 5,408 304.4 
29 26 6.5 8,760 348 1,244 4,481 800.8.19 3,662.5 328.7 
30 27 7.5 8,130 192 1,652 7,789 1,166.3.51 5,272 321.9 
31 31 6.8 4,460 184 1,532 6,521 713 1.95 4,952.5 307.8 
32 27 7.3 1,240 160 1,196 5,000 167.3.51 2,994 294.1 
33 6.9 5,310 316 1,364 3,462 181.2.73 2,965.5 301.5 
34 7.5 950 420 812 3,520 177.2.73 2,207 295.7 
35 7.5 730 200 596 1,634 158.1.95 1,100.5 302.9 
36 21 7.1 3,300 200 668 2,226 370.9.75 1,584.5 300.8 
37 40 6.9 4,350 340 1,124 5,595 372.6.24 3,065 318.8 
38 7.2 3,020 400 500 2,095 618.3.12 1,775 299.4 
39 29.5 6.9 3,610 460 4,125 14,330 342.3.51 9,100 308.1 
40 27 7.6 1,060 120 465 450 230 2.34 1,597.5 296.5 
41 24.5 5.1 9,350 420 644 2,541 1,345.42.51 3,324 299.4 
42 28.5 6.8 5,020 360 725 1,534 483 8.58 2,307.5 from 297.5 
43 25 6.9 6,040 380 656 2,246 979.3.9 2,591.5 311.1 
44 36 6.5 4,070 460 500 2,062 158.3.12 1,065 312.7 
45 7.4 720 184 212 1,616 1,000.11.7 1,455.5 296.1 
46 26 7.1 8,240 236 740 3,288 1,451.2.34 3,769 313.1 
47 25 6.8 4,680 180 740 470 692.6.63 3,017.5 323 
48 24.5 6.9 7,850 220 855 1,566 733.5.85 2,769 305.7 
49 21 7.4 1,030 216 1,415 941 124.1.95 3,550 297.6 
50 27.5 6.9 4,830 460 4,525 19,962 708.7.8 8,907.5 313.6 
51 6.5 5,000 360 300 2,038 805 5.46 1,520 343.6 
52 6.7 8,800 400 432 3,848 1,734.8.97 3,775 367.7 
53 7.1 4,630 160 408 1,495 630.3.12 1,420 342.3 
54 7.3 3,410 80 1,420 4,433 437 4.29 2,775 329.2 
55 7.3 5,710 240 480 2,214 929.5.85 2,100 325 

Changes in factors such as groundwater pH, ion exchange, and ion complexation, may be initiated or enhanced by agricultural inputs and irrigation, which also can substantially change the major-ion composition of the groundwater (Dubrovsky et al. 1993; Fujii & Swain 1995; Szabo et al. 1997; Seiler et al. 2003; Böhlke et al. 2007).

The determination of the monovalent cations Na+ and K+ by the method of electrolytes is likely doubtful. Indeed, these results compared with those of previous analyses by the technique of atomic absorption on the same points of water are completely different, calling into question the method practiced in the laboratory of the CHU of Tlemcen.

Dissolution and electrical conductivity of materials

The electrical conductivity of a complex saline solution is the sum of the conductivities attributed to each of the ions it contains (Schoeller 1962). Measurements of electrical conductivity, operated in situ, reflect a concentration of salts dissolved in the different horizons of the aquifer system in the study area. The recorded values show sizeable variations, ranging from 720 to 15,820 μS/cm at points 45 and 9 (Table 3). These values, denoting the upstream to downstream growth, increase in measure as we approach the Sebkha Lake. This variation is certainly linked to the lithological nature of the aquifer and the depth of the deposit of water. The waters of the limestone formations, circulating in karst aquifer networks, have low values. High values are recorded at water points serving alluvial levels, characterized by many lateral passages and vertical facies (Figure 3). This is, indeed, a continental sedimentation detrital, heterogeneous from a size point-of-view. In general, the permeability in these environments is directly related to the grain size of the sediments. The more the granulometry is thin, the more the flows are slow and the mineral matrix–water contact time is long, which translates into a high concentration of mineralization.

Figure 3

Map showing curves of iso-values of electrical conductivity (μS/cm).

Figure 3

Map showing curves of iso-values of electrical conductivity (μS/cm).

Overall, these conductivity values are, in the majority, superior to the limits of the fixed potability standards in 2011 (180–1,000 μs/cm for WHO; ≤2,800 μS/cm for Algeria).

The dominant chemical facies are chloride–sodium, and chlorinated and sulfated forms of calcium and magnesium (Figure 4).

Figure 4

Piper diagram (Benziane 2013).

Figure 5 establishes the relationship between electrical conductivity and the anions. Overall, sulfate concentration increases, from the outset, and then stabilizes, which is accounted for by the blocking of these ions by organic reduction. As chlorides, they grow with the increase of conductivity, which translates into the relative enrichment of the waters in chlorides and the precipitation of carbonate minerals.

Figure 5

Relationship of versus conductivity.

Figure 5

Relationship of versus conductivity.

Relationship of Na+ vs Cl

Several types of reactions between groundwater and rocks can be identified using traces of correlation with chlorine (Nordstrom et al. 1989). The latter is an item stored, not participating in water–rock interactions; He characterised the origin of the salinity of the water and is a mixture tracer (Fidelibus & Tulipano 1996). The origin of this element is linked primarily to the dissolution of salt formations and evaporation in closed environments (endorheic systems). In natural waters, the presence of both Na and Cl is attributed to the dissolution of halite that is found in the Triassic formations. Levels of Na+ and Cl should be balanced. However the determination of the cation Na+ obtained by the method of electrolytes gives a value deficit compared with the anion Cl. The ion Na+ measured differently by the method of atomic absorption that seems to give more reliable values is taken into account in the delimitation of the right of the correlation (Benziane 2013). The relationship between these two ions is characterized by a strong correlation coefficient (Figure 6). This relationship shows that all the points are more or less aligned on the right equal to 1 indicating that these two elements have, in the majority of cases, the same origin. Overall, the reported Na+/Cl is less than 1, indicating that the process of dissolution of alluvium salts (especially halite) is responsible for sodium content. However, the chlorides such as sodium may, exceptionally, have other origins (natural or anthropogenic). The deficit in Na which is from the phenomenon of inverse ion exchange between the water and the aquifer translates to an adsorption of Na+ and a release of Ca++. This can be explained by the deposition of salts under semi-arid to arid climatic conditions (low rainfall and high temperatures) or by the leaching of the marl land by rainwater. Understanding water storage changes in the basin and the related impacts of climate variability is an essential step in managing its water resources (Awange et al. 2014).

Figure 6

Relationship of Na+ versus Cl.

Figure 6

Relationship of Na+ versus Cl.

Relationship of SO4 vs Cl

The geochemical control of sulfate is the most complex to interpret. The delimitation of the right of the theoretical evolution of solutions is problematic since the Cl and are not simultaneously at their maximum dilution rate. One can think in this case of a sulfate reduction to the level of sediment, confirmed in the area by evaporitic formations. This phenomenon would explain the huge annual consumption of the element sulfate accompanied by production of bicarbonate according to the general equation:  
formula
(1)
where Fe2O3 is a red clay, residue of the dissolution of limestone, with depletion of SiO2 and in Fe2O3 enrichment.

The relationship between chloride and sulfate shows a dispersion of points indicating a change of the two elements having a salt origin, common gypsiferous (Figure 7). The predominance of sulfates over chlorides or vice versa depends essentially on the state of the minerals that generate these ions in water (equilibrium, oversaturated, undersaturated). The importance of the excess of one or the other element determines the dominance of the facies of that element over the other.

Figure 7

Relationship of , Mg+2, and Ca+2 versus Cl.

Figure 7

Relationship of , Mg+2, and Ca+2 versus Cl.

The levels of chlorides present values more important than sulfates, due to the special characteristics of that element. The chloride facies does not fit into the phenomena of chemical precipitation, was not adsorbed by the geological formations and is very mobile. The atmospheric origin assumes that sulfates and chlorides evolve together and in a similar way. An increase in chloride should be accompanied by that of sulfates. However, in the majority of our samples, this evolution is not highlighted. This concentration difference between chloride and sulfate does not seem to depend essentially on the nature of the land. Sulfate would mainly come from the dissolution of gypsum, widely represented in the basin, and secondary phenomena of oxidation–reduction of mineral and organic matter.

The graphical representation shows that the points have a reported () greater than 1, indicating a dominance of the chloride ions over sulfate. This difference can be explained by the solubility of material washed away during the rainy episodes; halite is more soluble than gypsum.

In the plains from the edge of the lake, the origin of sulfate can come from the land application of fertilizers in intensive agriculture.

Origin of calcium

The calcium originates from carbonate minerals and gypsum. The determination of the origin of each Ca++ concentration is needed to understand the mechanisms of the affinity of the groundwater. Calcium release by the dissolution of the gypseous rocks as well as following the attack of rock carbonate by carbonic acid from the water–carbon dioxide reaction. Figure 8 represents the calcium bicarbonate versus sulfate relationship. The graph shows that the origin of the calcium is rather carbonate than evaporitic; the reported is always less than 1.

Figure 8

Determination of the origin of calcium.

Figure 8

Determination of the origin of calcium.

The relationship between the ions Ca++ and is characteristic of the course of the groundwater. The low value of the ratio (between 0.05 and 10) translated to the depletion of water in Ca++ causing an increase in the content of . High concentration of HCO3 can also be attributed to the CO2 present in the soil. This CO2 can also result from the water–rock interaction as is confirmed by the reported /Σ anions (0.01–0.1) (Kortatsi et al. 2008).

Oxidation of organic matter by microbes generates CO2, which then combines with water to form carbonic acid and dissociates to H and HCO3 ions (Satyanarayanan et al. 2016).  
formula
(2)
Low concentrations of Ca++ are due to the precipitation of carbonate of calcium in the summer. Taking into account the correlation between calcium and magnesium, an exchange of base is likely between these two elements (Brinis et al. 2004). The waters are more magnesian than calcium. The study of the reported Ca++/Mg++ waters of this region advocated the dissolution of calcite and dolomite present in the Miocene aquifer. If the reported Ca++/Mg++ ≤ 1, the dissolution of dolomite should occur; a higher ratio (>1) is indicative of a contribution of more calcite (Mayo & Loucks 1995).

The reported (between 0.007 and 1.72) indicates that these two ions are at the origin of the dissolution of the gypseous and dolomitic formations. The study of the reported Ca++/Mg++ of the waters of this region advocated the dissolution of this dolomite at the level of the Miocene aquifer.

Mineral saturation indices

Using saturation index and activity diagrams, it is possible to predict the mineralogical reaction from groundwater data without collecting samples of the solid phase and mineralogical analysis (Deutsch 1997). Figure 9 shows that from the beginning of the concentration, the solutions are saturated to supersaturated in calcite and aragonite, overall saturated to balanced compared with dolomite and undersaturated with respect to anhydrite, gypsum and halite, which means that evaporate mineral phases are undersaturated and therefore favorable to dissolution along the path of groundwater flow.

Figure 9

Variation of mineral saturation index.

Figure 9

Variation of mineral saturation index.

The calculation of the minerals in the water saturation index shows that only carbonate minerals tend to precipitate, mostly in the form of dolomite (Figure 10). Evaporates are always in the state of undersaturation, resulting in their rapid dissolution and allowing the ions Na+, Cl and to stand in water with high concentrations. In the basin of the Great Sebkha of Oran, the main minerals constituting evaporates are gypsum, anhydrite, and halite.

Figure 10

Evolution of indices of saturation and pH.

Figure 10

Evolution of indices of saturation and pH.

For the study of calco-carbonic equilibrium, the graph of Figure 11 shows that indices of saturation of carbonate minerals have mean values of the order of 0.80 for calcite, 0.65 for aragonite and 1.49 for dolomite. These indices are mostly positive for calcite and aragonite, which involves precipitation processes. Only point 41 is marked by the dissolution of calcite. This last is a fast reaction and water can reach saturation through the unsaturated zone (Appelo & Postma 2005). The dissolution of the dolomite is slower than that of calcite (on the order of a few months). Note that they tend to become positive in the vicinity of a slightly alkaline pH (Figure 10).

Figure 11

IS halite–Cl concentration relationship.

Figure 11

IS halite–Cl concentration relationship.

The average values calculated for the index of saturation of the anhydrite and gypsum are respectively of the order −1.92 and −1.71. All samples are undersaturated in evaporites. The dissolution of gypsum promotes an increase in the concentration of calcium and thereby the reported Ca++/Mg++. Beyond 0.5, it causes the phenomenon of de-dolomitization, which seems to mark the water chemistry at points 23, 51 and 52 (Ammary 2007).

Halite is distinguished by its strong dissolution compared to that of gypsum and anhydrite. Figure 11 shows that the original brine binds a solution probably supersaturated in halite and the saturation index increases with the concentration of chloride or halite-rich evaporites. Some processes are responsible for the losses of a quantity of sodium.

CONCLUSION

The geochemical study of the Water Company confirms mineralization is variable in relation to the lithological nature of the slopes and aquifers in the basin of the Great Sebkha of Oran. Generally, the salinity of the water increases upstream to downstream in the direction of the Sebkha. The alluvial aquifer waters distinguish themselves particularly compared with the other aquifers of the system by their relatively high concentration in dissolved salts (15,820 μS/cm at point 22). The exploitation of the results of analyses and their representation on appropriate diagrams highlight two dominant chemical facies: chloride-sodium, and calcium and magnesium chloride and sulfate. These waters, of poor quality, are unfit for domestic consumption. They are often used on sites for irrigation of parcels of land.

The origin of this natural salinity is attributed to the slopes of the basin, particularly leaching at Tessala where outcroppings of gypsiferous marls and salt occupy large spaces. The groundwater contained in the alluvial formations is, moreover, subject to exchanges with the atmosphere (evaporation and evapotranspiration) that tend in a closed space to concentrate more salts dissolved in the water. This degradation of the environment is often supported by anthropic action, also in crop intensification and extension of irrigation into the drinking water supply. Intensive and prolonged pumping is the source of intrusions, in the Miocene aquifer limestone of the Murdjadjo, of bodies of brackish water from the overlying alluvial medium. Geochemical evaluation has been instrumental in describing these problems (Edmunds 2009).

Finally the Sebkha, due to its altitude, its position and its role as an evaporating machine, would be at the origin of the salinity of the waters of the basin. This area requires special attention, as non-persistent pollutants may influence water quality (Vidal Montes et al. 2016). Often, when quality is not within the required standards and it is not used adequately, water can be a determining factor in baking for obtaining the desired dough and final product characteristics (Sinani et al. 2014).

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