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ISSN: 2157-7587
Hydrology: Current Research
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Determination the Origin of Mineralization in the Coastal Aquifer Northeast Tunisia by Isotopic Method

Mzoughi Aroua1*, Ben Hamouda Mohamed Fethi2, and Bouhlel Salah1

1Laboratory of Mineral Resources and Environment, Faculty of Sciences of Tunis, University of Tunis El Manar, Tunis, Tunisia

2Hydrology and Isotopic Geochemistry Unit, CNSTN Sidi-Thabet Technology Center, Tunisia

*Corresponding Author:
Mzoughi Aroua
Unit of Geology and Applied Geochemistry (UR11ES16)
Department of Geology, Faculty of Sciences of Tunis
University of Tunis El Manar, 2092 Tunis
Tunisia
Tel: +21621482388
E-mail: [email protected]

Received date: April 05, 2017; Accepted date: April 27, 2017; Published date: May 04, 2017

Citation: Aroua M, Fethi BHM, Salah B (2017) Determination the Origin of Mineralization in the Coastal Aquifer Northeast Tunisia by Isotopic Method. Hydrol Current Res 8: 273. doi: 10.4172/2157-7587.1000273

Copyright: © 2017 Aroua M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

A geochemical and isotopic techniques were undertaken to characterize groundwater in Northeast Tunisia. Hydrogeochemical investigations demonstrated that groundwater can be classified into different water facies. The Ras Djebel-RafRaf aquifer showed a (Ca-Cl-SO4) and (Na-Cl-NO3) water type. Data inferred from 18O and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower δ18O and δ2H values is interpreted as recharged by non-evaporated rainfall originating from Mediterranean air masses from Mediterranean air masses at higher altitude. However, water with relatively enriched δ18O and δ2H contents is thought to reflect the occurrence of an evaporation process related to the long-term practice of flood irrigation.

Keywords

Hydrogeology; Geochemistry; Coastal aquifer; Tunisia

Introduction

The region of Bizerte (Figure 1) is situated in boundary Northen of Tunisia. It forms with the Cap Blanc the most advanced point of Africa. It is exposed to humid winds from the northwest and West, and receives an average rainfall of 570 mm/year [1], This coastal occupies an area of 50 Km2. It is limited by Djebels bouchoucha, Touchela and Bab Banzart in NW, Djebels Ennadour and Demna in South, the Mediterranean Sea in the North and NE.

hydrology-current-research-RafRaf-Ras

Figure 1: Geological map of RafRaf-Ras Djebel plain.

This zone is influenced by a sub-humid Mediterranean climate. The annual temperature is around 18° C. The ETP, about 1197 mm/year. This basin is drained by many wadis. The use of water is essential since the Ras Djebel is an agricultural region by excellence, in Fact the major part of population practise agricultural activities.

In result, this aquifer undergoes increased anthropic pressure through overexploitation and construction of two dams at Wadi Beni Ata and Wadi Shaab Eddoud.

Geology Setting

From a geologic standpoint, the main geologic features of this study area are [2,3].

(a) Upper Cretaceous, represented by the lower senonian and the lower campanian which is divided into two distinct entities: the marls senonian and the calcareous marl campanian.

(b) Lower Paleocene, represented by grey marls.

(c) Eocene, formed by limestone.

(d) Mio-Pliocene, formed by continental materials: Marls, clays, conglomerates, gypsum and sands.

(e) Quaternary, represented by the villafranchian, formed essentially by limestone.

Materials and Methods

For the evaluation of groundwater quality, 50 water samples were collected from the shallow aquifer of Ras Djebel plain during October (Figure 1), the groundwater samples were analysed for chemical and isotopic composition.

Measurement of temperature, electrical conductivity and pH were measured in the field. Major elements (SO42-, Cl-, NO3-, Ca2+, Mg2+, Na+ and K+) were analysed at the laboratory of National Center for Nuclear Sciences and Technologies. These elements are expressed in mg/L-1. Salinity was analyzed by evaporating a specified volume of water sample in a graduated capsule for 24 h in an incubator at 105°C.

The dry residue concentration was determined by subtracting the final mass from the initial mass of the sample that was placed in the incubator. Ca2+ and Mg2+ were measured by Liquid Ion Chromatography while K+ and Na+ were measured by flame photometer.

The results of chemical analyses of the investigated groundwater samples are presented in Table 1.

Results and Discussion

Hydrochemistry

Physical-chemical parameters: The overall chemical characteristics of groundwater are presented in Table 1. The pH values range from 6, 88 to 8, 3 with an average value of 7, 3. The temperature values are between 20, 2°C and 25°C. The values measured on the surface water are influenced by the surface air temperatures.

The total dissolved solids and the Electrical Conductivity (EC) values range from 1, 03 g/L to 12, 96 and 0, 2 to 0, 7 μS/cm. The spatial distribution maps (Figure 2) shows that salinity increase in the direction of groundwater flow. The Electrical Conductivity (EC) is well correlated with the total dissolved solids (TDS).

hydrology-current-research-Map-salinity

Figure 2: Map of salinity.

The factor of correlation is in the order of 1.

Major ion chemistry:

N°èch pH Cond sec Ca Mg Na K SO4 Cl NO3 HCO3
1 7.2 2.41 1567 220 40.32 328.9 8.19 324.52 479.25 23.3 351.54
2 6.9 5.37 3491 368 86.64 749.8 9.75 845.24 955.7 70.59 423.46
3 7.1 2.4 1560 180 32.88 280.6 28.47 333.26 323.1 97.4 572.26
4 7.2 2.15 1398 200 32.4 230 8.59 349.32 323.15 152.1 353.4
5 7.3 2.46 1599 142 62.16 331.2 8.97 467.31 370.23 86 523.9
6 7.7 2.4 1560 106 65.52 354.2 15.6 402.26 388.57 144.4 416.64
7 7.1 2.49 1619 156 48.48 299 30.81 354.42 455.79 90.87 407.34
8 8 2.92 1898 201 66.48 386.4 9.75 573.34 556.17 19.11 391.84
9 7.1 3.82 2483 137 104.64 616.4 7.02 492.12 753 40.17 701.84
10 7.5 4.04 2626 196 47.76 464.6 31.98 534.34 657.9 29.15 502.2
11 7.2 1.66 1079 89 28.32 149.5 72.15 202.16 238.48 49.92 221.34
12 7.2 3.56 2314 114 59.76 595.7 24.57 464.89 648.78 21.35 513.98
13 7 6.7 4355 319 67.44 1299.5 85.8 986.48 1567.7 41.24 544.36
14 7 5.37 3491 401 104.64 832.6 93.6 854.89 1327.5 12.68 564.82
15 7.2 2.93 1905 222 26.64 503.7 3.51 452.14 670.24 45.24 550.56
16 7.2 2.81 1827 260 30.96 411.7 2.34 335.22 660.9 47.58 412.3
17 7 3.36 2184 252 30.96 496.8 6.24 521.76 608.74 15.11 626.2
18 7.1 3.22 2093 178 66.72 397.9 31.98 254.72 632.97 48.26 574.12
19 7 3.25 2113 270 138.72 328.9 38.22 443.93 733.48 57.82 515.84
20 7.1 3.3 2145 250 104.88 368 23.79 459.82 806.07 54.21 493.52
21 6.9 2.97 1931 205 86.88 393.3 17.16 304.88 744.33 34.42 512.74
22 7 3.31 2152 263 119.04 342.7 7.41 350.4 843.33 53.24 550.56
23 7.3 1.44 936 66 39.36 138 18.33 160.01 147.61 73.22 288.3
24 7.2 3.4 2210 240 47.52 446.2 13.26 526.93 560.96 60.16 600.78
25 6.9 7.09 4609 542 207.84 984.4 8.19 987.69 1742.3 99.94 485.46
26 7 4.03 2620 277 124.08 517.5 3.12 499.09 971.78 99.84 512.12
27 7.1 3.66 2379 315 0.48 471.5 3.9 466.06 561.69 82.49 396.18
28 7.1 3.6 2340 322 14.16 427.8 15.99 380.74 691.25 22.72 458.18
29 7.1 5 3250 357 58.32 611.8 27.3 550.1 1046 77.03 522.66
30 7.1 3.55 2308 338 24 577.3 19.11 282.92 1019.3 83.07 556.14
31 7.1 4.17 2711 318 62.64 738.3 16.38 599.52 1118.6 47.97 515.22
32 6.9 3.94 2561 386 14.64 489.9 8.19 629.57 821.75 46.12 434.62
33 7.2 2.94 1911 200 15.6 411.7 30.42 386.99 550.79 17.36 408.58
34 7 3.75 2438 480 10.8 397.9 6.24 275.25 1145.7 45.14 521.42
35 7.2 3.56 2314 354 17.28 473.8 7.8 393.06 724.18 43.58 535.06
36 7.6 3.22 2098 282 19.92 473.8 5.46 408.41 803.41 16.58 480.5
37 7.1 2.78 1807 251 73.44 202.4 9.75 297.5 398.4 59.48 442.68
38 7.1 3.6 2340 205 81.6 395.6 13.65 339.69 579.64 61.23 555.52
39 7.3 2.31 1502 136 45.6 273.7 13.65 169.04 368.54 17.55 391.84
40 7.8 0.27 174 50 7.68 6.9 8.19 53.45 42.5 11.41 65.1
41 7.4 2.98 1937 244 98.64 404.8 7.02 429.47 671.63 7.51 559.24
42 8.3 0.99 607 83 20.4 92 24.96 130.58 147.25 46.51 132.06
43 7.3 2.28 1482 113 15.36 292.1 9.36 235.8 252.96 45.05 499.72
44 7.1 3.46 2249 282 119.04 342.7 16.38 623.95 540.14 11.31 532.58
45 7.4 4.43 2880 208 139.44 719.9 17.16 744.19 1025.3 16.58 516.46
46 7.5 3.42 2223 222 13.44 427.8 24.18 514.97 668.82 32.08 301.32
47 7.3 4.2 2730 300 42.72 625.6 31.59 570.49 933.48 92.63 420.36
48 7.1 6.3 4095 253 74.88 1016.6 56.94 328.8 1674.2 40.07 603.26
49 7.2 4.79 3114 228 28.08 901.6 64.35 274.27 1041.3 58.79 539.4
50 7.4 1.56 1014 111 15.12 142.6 107.64 152.75 163.99 126.95 272.18

Table 1: Geochemical data for the studied aquifer.

*Chloride and sodium

Concentrations of sodium and chlorides vary respectively between 6, 9 to 1299, 5 mg/L and 42, 5 to 1742 mg/L. For both Na+ and Cl-, the highest concentration levels characterize the wells situated on the border of the sea (P13, P25 and P48). The spatial distribution of chloride and sodium concentrations illustrate a similar evolution to that salinity: they both increase in the direction of groundwater flow.

*Calcium

Ca2+ concentrations ranged between 50 and 542 mg/L. The most elevated value was measured in well P25, while lowest level was recorded in well P40. The water of this coastal aquifer is moderately rich in calcium. The mostly analyzed waters are pure, which is in conformity with pH values of these waters. Calcium and pH have an influence on water aggressiveness [4].

Ech RDJ1 RDJ2 RDJ3 RDJ5 RDJ6 RDJ8 RDJ9 RDJ10 RDJ11 RDJ12 RDJ13 RDJ14 RDJ15 RDJ16 RDJ17 RDJ18 RDJ21 RDJ23 RDJ24 RDJ26
δ2H -27,7 -26,7 -27,6 -30,8 -30,1 -32,8 -31,1 -25,1 -31,1 -31,6 -26,7 -24,3 -27,2 -27,0 -25,7 -28,2 -23,4 -31,8 -28,5 -26,6
δ18O -4,4 -3,6 -4,5 -5,0 -4,6 -5,9 -5,0 -4,0 -4,9 -5,0 -4,4 -4,0 -4,5 -4,1 -4,1 -4,2 -4,2 -5,3 -4,8 -4,1

Table 2: Isotopic data for the studied aquifer.

*Magnesium

Mg2+ concentrations in Ras Djebel-RafRaf aquifer fluctuated between 0, 48 and 207 mg/L. The highest values were recorded for well P25, whereas the lower one was recorded for well P40.

*Potassium

K+ concentrations are relatively low compared to the concentrations of other cations which values vary between 2, 34 and 107, 64.

*Sulfates

The sulphates concentrations of the water in the Ras Djebel- RafRaf aquifer varied between 53, 45 and 987, 69 mg/L. The highest concentrations were measured for well P25 and P13 while the lowest value is characterized by well P40.

*Nitrates

NO3- concentrations varied between 7, 51 and 144, 4 mg/L. The highest concentration was measured for well P6 while the lowest value was characterized by well P41.

Forty two percent of groundwater samples taken during this study, show nitrate concentrations exceeding the maximum European admissible nitrate concentration limit in drinking water (50 mg/L).

The examination of nitrate distribution map (Figure 3) reveals that high nitrate concentrations appear to be related to the fertilizer application and long term flood irrigation practises, highlighting the significant contribution of the return flow of irrigation waters to the degradation of groundwater [5-8].

hydrology-current-research-salinity

Figure 3: Map of salinity.

Saturation Index

Water saturation states for minerals as anhydrite (SI anh), gypsum (SI gyp), aragonite (arg) and calcite (cal) were computed. If the SI is < 0, dissolution is considered as the dominant process for the related mineral and when is >0, precipitation of mineral is likely to be occurring in the system [9].

The majority of samples are supersaturated towards aragonite from which the carbonates are precipitated. This supersaturation of water towards these carbonates minerals such as aragonite and calcite shows that the possibility of dissolution of these minerals by the waters is not possible and that cannot contribute to the acquisition of mineralization [10]. In addition, the majority of samples of this groundwater are under saturated towards gypsum.

Piper diagram

The piper diagram [3] (Figure 4) was determined to precisely recognize the different groundwater facies. Nitrate concentration was taken into account when plotting this diagram because of its relative abundance in the groundwater.

hydrology-current-research-Piper-diagram

Figure 4: Piper diagram.

The data represented in this groundwater samples are classified into two major groups: (Ca-Cl-SO4) and (Na-Cl-NO3) water type.

Relationships between TDS and major elements

Plots of various major elements as a function of TDS value were drawn to determine the contributing elements to groundwater mineralization and the major hydrogeochemical process. Shows that Cl-, SO42- and Na+ are well correlated with TDS presenting very close values of 1 (respectively: 0, 88; 0, 67 and 0,87). In result chlorides, sulphates and sodium are the main contributors to groundwater mineralization.

However, the contents of calcium and bicarbonate participate significantly is not determining role in the acquisition of mineralization. In addition to TDS, the correlation matrix established between sulphates and Cl- ion (Figure 5A) shows that the majority of points are situated above the line (1:1). The increase of Cl- in comparison to sulphates proves that the Cl- has probably two origins: It is linked to a marine intrusion or dissolution of the halite. This hypothesis is confirmed by the relation between the Cl- and Na+ ions (Figure 5B).

hydrology-current-research-sulphates

Figure 5: Relationship between sulphates, Sodium, and chlorides.

The relationship between Ca2+ and Cl- ions (Figure 5C) shows excess of Cl- against Ca2+, which demonstrates a process of precipitation of gypsum (CaSO4, 2H2O) or anhydrite (CaSO4). Besides, the effect of precipitation is felt in the relationship between the SO42- and Ca2+ ions (Figure 5D) where the sulphate deficiency is related to the ion exchange.

Isotopic composition of groundwater

Isotope geochemistry techniques are valuable tools in investigating many problems in hydrology and evaluating hydrogeological and hydro chemical controlling mechanisms in any groundwater system [11,12].

They provide significant insights into water origin, recharge circumstances (time and location), water flow directions and residence times [11].

*Stable isotope of water (δ18O, δ2H):

Stable isotopes (δ18O, δ2H) have conservative proprieties and provide information on the groundwater recharge process (Table 2) [13]. They can offer an evaluation of physical process that affects water masses, such as evaporation and mixing [14].

The isotopic compositions of the groundwater samples from the Grombalia shallow aquifer range from -5, 3‰ to -3, 6‰ for δ18O and from -31‰ to -24, 3‰ for δ2H.

The correlation diagram of δ18O/δ2H (Figure 6) shows the position of all samples relative to the Global Meteoric Water Line (GMWL: δ2H=8δ18O+10) [15] and the Local Meteoric Water Line of the Tunis- Carthage (LMWL: δ2H= 8δ18O+12, 4) [16,17].

hydrology-current-research-Isotope-composition

Figure 6: Isotope composition of groundwater.

Nevertheless, these groundwater samples (Figure 6A) can be further divided into two groups:

• The first group, which is placed below the GMWL is probably the consequence of the infiltration of an evaporated component likely deriving from the return flow of irrigation water.

• The second group, placed between the GMWL and the LMWL, shows that the precipitation ensuring the recharge of aquifer from a mixture of oceanic and Mediterranean vapour masses.

The plot of chloride versus δ18O (Figure 6B) indicates the different processes responsible for the variation of groundwater salinity.

Some water samples show a correlation between these two elements. It is probable that the dissolution mechanism takes precedence over evaporation in the acquisition of the mineralization.

Conclusion

Based on hydrogeological characteristics the aquifer of Ras Djebel Northest Tunisia can be classified into two groups: Ca-Cl-SO4 and Na- Cl-NO3 water type. The acquisition of mineralization mainly through natural mineralization mechanisms such as the ion exchange and the geology of groundwater. Nevertheless, anthropogenic process related to agricultural practices also plays an important role in the salinization of groundwater.

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