Hydrologic and geologic factors controlling groundwater geochemistry in the Turonian aquifer (southern Tunisia) Kamel Abid & Kamel Zouari & Marek Dulinski & Najiba Chkir & Brahim Abidi Abstract Water in the fissured limestone and dolomite of the Turonian aquifer of Tunisia has been investigated using geochemical (major ions) and isotopic (δ18O, δ2H, 14C) data. To carry out a characterization of aquifer behaviour, 48 representative samples were collected at the end of the humid season. The evolution of chemical composition of groundwater from recharge areas to discharge areas is characterized by increasing sodium, chloride and sulphate contents as a result of leaching of evaporite rock. In the study, three distinct chemical trends in groundwater were identified. The major reactions responsible for the chemical evolution of groundwater in the investigated area fall into three categories: (1) calcite precipitation, (2) gypsum and halite dissolution, and (3) ion exchange. The stable isotope composition of water samples indicates largescale interaction between the Continental Intercalaire and the Turonian aquifer and the presence of a young local component which probably enters the system via faults and/or fractures. Keywords Tunisia . Precipitation . Dissolution . Ion exchange . Mixing process

Received: 3 September 2009 / Accepted: 13 October 2010 Published online: 11 November 2010 * Springer-Verlag 2010 K. Abid ()) : K. Zouari : N. Chkir École Nationale d’Ingénieurs de Sfax, BP W 3038, Sfax, Tunisia e-mail: [email protected] Tel.: +216-74275595 Fax: +216-74275595 M. Dulinski Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al. Mickiewicza 30, 30-059, Krakow, Poland B. Abidi Arrondissement des Ressources en Eau, Commissariat Régionale au Développement Agricole de Gabès Route de Médenine, 6019, Gabés, Tunisia Hydrogeology Journal (2011) 19: 415–427

Introduction In arid zones, high salinity typifies most groundwater. A perennial problem for agriculture in these areas is the scarcity of water due to low and irregular rainfall. In the last century, expansion of the global population has increased the pressure on water resources. In southern Tunisia, most groundwater is taken from deep aquifers via more than 800 shallow and deep wells (OSS 2003). Most aquifers show signs of advanced overexploitation, revealed by springs drying, decreasing piezometric surfaces and degradation in water quality (ERESS 1972; Mamou 1990; OSS 2003). The regional groundwater system in southern Tunisia is characterized by two principal multi-layer aquifer sequences: the Continental Intercalaire (CI) overlain by the Complexe Terminal (CT). Several studies (ERESS 1972; OSS 2003) have been carried out on these aquifers, which were focused primarily on predicting their long-term behaviour through numerical modelling of groundwater flow. Reconnaissance studies of chemical and isotopic composition of groundwater confirmed the existence of paleowaters in the two sequences (Guendouz et al. 2002; Edmunds et al. 1997, 2003). Indeed, the depleted heavy isotope content and negligible radiocarbon concentration measured in waters of these aquifers (Gonfiantini et al. 1974; Fontes et al. 1983; Edmunds et al. 2003; Zouari et al. 2003) confirmed the presence of old waters recharged during the late Pleistocene period. Unconfined or semi-confined CT discharges in the Chotts regions of Tunisia. Several aquifers are composed of lagoonal and marine units of the CT sequence, and the Turonian aquifer, being the object of this study, is one of them, though it has not been well defined until now. This aquifer extends over the major part of the sedimentary basin in southern Tunisia. In the southern part and Djerid regions, the water resources of the Turonian aquifer are very limited and therefore groundwater is not exploited in this area. In the presented study, hydrogeological, hydrochemical and isotopic information from the groundwater system is integrated and used to determine the main factors and mechanisms controlling the chemistry of Turonian groundwater in the investigated area. Geochemical data are particularly useful for evaluating groundwater evoluDOI 10.1007/s10040-010-0668-z

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tion and recharge in the arid regions of North Africa where groundwater resources are generally non-renewable. Despite the water development schemes, caused by urban and agricultural expansions, an acute environmental problem of groundwater salinization appeared in these area. The main issues that will be addressed in this report include: (1) processes affecting salinization of groundwater; (2) relative importance of hydrological and geological factors controlling water chemistry; (3) location of recharge zones, and (4) identification of hydraulic interconnection between the aquifer systems.

Study area The study area (7°30′−10°30′E, 30°10′–34°10′N) is situated in northern Africa, between the Algerian border in the west, the Mediterranean Sea in the east, the Atlas Saharan Mountains in the north, and Libyan-Algerian border in the south (Fig. 1). Southern Tunisia is characterized by an arid to semiarid climate. The amount of precipitation varies considerably from year to year. The maximum precipitation (250 mm/year) is recorded along the Golf of Gabes and the high zones of Dahar Mountains (INM 1998). The amount of precipitation in the southern part of the basin is

less than 100 mm/year (INM 1998). Most of the rainwater is lost by evaporation (Trabelsi et al. 2008).

Geology and hydrogeology A representative West–East cross section of the study area is shown in Fig. 2. The regional geology has been discussed by several authors (e.g. Busson 1970; Bouaziz 1995). The age of geological formations cropping out in southern Tunisia extends from the upper Permian to the Quaternary, with an unconformity from Palaeocene to Eocene (Fig. 3). The catchment area is composed of limestone, dolomite, gypsum, halite, clayey sand and conglomerates (Figs. 2 and 3). The relation between stratigraphic and hydrogeologic units (Mamou 1990; Bouaziz 1995) is presented in Fig. 3. As shown in Figs. 2 and 3, the CI aquifer is situated within the continental formations of Lower Cretaceous (Neocomian, Barremian, Aptian and Albian). The aquifer is formed by a complex succession of detrital sediments separated by clay-rich strata, with the maximum aquifer thickness exceeding 1,500 m (Edmunds et al. 2003). The CT aquifer is located in the Upper Cretaceous and Tertiary formations. Lateral continuation of the CT in the coastal plain forms the Djeffara aquifer. Tectonic movements

Fig. 1 Network of wells in the study area Hydrogeology Journal (2011) 19: 415–427

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Fig. 2 Hydrogeological cross section in the study area. Location of individual cores as in Fig. 1

have significantly influenced hydrodynamic functioning of these aquifers. In the southeast part of the basin (Djeffara of Medenine and Tataouine), Cretaceous deposits are missing (Fig. 2). Stratigraphic gaps in these areas reflect several factors such as the irregularity of the paleotopography, eustatic movements and tectonic instabilities (Castany 1954; Ellouze 1984; Bouaziz 1995). The Turonian carbonates constitute the principal aquifer formation in Matmatas, north of Gabes and Nefzaoua regions. The aquifer consists of dolomites and fissured limestones, whose thickness rarely exceeds 100 m (Bouaziz 1995). Although its thickness is limited, the Turonian aquifer is characterized by high transmissivity, generally around 100×10−3 m2/s. In the study area, the Turonian aquifer is largely affected by several normal faults being the result of Hydrogeology Journal (2011) 19: 415–427

tectonic activity (Bouaziz 1995). This extensive tectonic phase (Upper Cretaceous), that was described also in central Tunisia (Rabhi 1987), caused lateral compartmentalization and the isolation of some parts of the Turonian reservoir from overlying aquifers (OSS 2003). The general groundwater flow directions (Fig. 4) are from the south to the north (from Libya to the Chotts region), from the south-west to the north-east (from Dahar upland to the Gulf of Gabes) and from the south-east to the north-west (from Dahar upland to Chotts region). Tectonic activity and changes in facies and in thickness of sedimentary deposits seem to play an important role in groundwater flow and hydraulic continuity. This is visible in the Nefzaoua region, where thickening of the units and lithological changes had an important influence on DOI 10.1007/s10040-010-0668-z

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Fig. 3

Relation between stratigraphic and hydrogeologic units in the extreme south and Djerid regions (after Mamou 1990; Bouaziz 1995)

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Fig. 4 Piezometric map of the Turonian aquifer

piezometry levels and chemical composition of water (Mamou 1990; Edmunds et al. 1997). The Turonian aquifer reaches its maximum thickness in the Chotts basin (110 m), where it is considered as the most important aquifer layer of the CT aquifer system, and diminishes towards the edges of the basin. In this area, the CI aquifer is principally located in the sandstones of the Kebar el Hadj formation (Mamou 1990; Abidi and Ben Baccar 2001; Edmunds et al. 2003). Thickness of the sandstone series frequently exceeds 1,500 m. Towards the Chott el Fedjej region, the CI formations are shallower and cover depths between 500 and 900 m (Mamou 1990; Edmunds et al. 2003; Abid et al. 2009). The variations in thickness of the CI formations produce an artesian pressure head within the range 180 to 225 m above ground surface level in the Nefzaoua region, and more than 50 m in the Chott el Fedjej region (OSS 2003; Edmunds et al. 2003). The temperature of groundwaters in these areas varies from 55 to 75°C (Abid et al. 2009). Hydraulic continuity between the CI and the Turonian aquifer has been demonstrated through the differences in hydraulic heads and a set of boreholes showing concordant hydrogeologic and hydrodynamic parameters. The hydraulic heads of the CI aquifer indicate W–E flow, in Hydrogeology Journal (2011) 19: 415–427

the direction of Gulf of Gabes, with an artesian pressure head of about 200 m above ground surface (OSS 2003). On the other hand, the Turonian aquifer shows a general SW–NE flow, from the Matmata relief and Saharan platform towards the Mediterranean Sea (Fig. 4). Near El Hamma faults, the artesian pressure head is in the order of 50 m, and is explained by the ascension of CI groundwater through El Hamma faults (OSS 2003). The piezometric gradient varies between 1 and 4‰.

Sampling and analytical methods Forty-eight groundwater samples from the Turonian (Table 1) and CI (Table 2) aquifers (Fig. 1) were collected during January–April 2006 from pumping wells. Several analyses such as the water temperature, pH, electrical conductivity (EC), and total alkalinity as HCO3− were carried out on-site. All samples for chemical analyses were collected in low density polyethylene bottles and filtered in the laboratory through 0.45-μm membrane filters. Samples revealing relatively high salinity (exceeding 3 g/l) were diluted before analysis. DOI 10.1007/s10040-010-0668-z

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207 219 325

150 375 50 410 515 670 140 550 350 70 109

4,790 4,830 4,490 3,560 3,480 3,480 3,200 1,150 2,610 4,690 3,450 3,740 3,750 8,940 3,900 4,060 1,680 9,650 1,730 4,810 4,350 4,830 500 2,300 5,600 3,290 3,486 6,400 970 3,120 2,200

24.7 25.3 29.5 26.7 23.3 20.5 21.6 27.2 22.7 20.2 23.7 25.0 24.8 24.4 27.0 31.0 27.6 20.0 22.0 16.1 22.4 22.8 21.6 22.7 28.0 22.0 22.0 22.0 20.5 22.0 22.0

7.6 7.25 7.86 7.4 7.23 8.11 7.62 7.45 7.21 7.48 7.18 7.55 7.53 7.81 7.6 7.54 7.75 8.0 7.8 7.6 8.27 8.28 7.2 8.09 7.7 7.52 7.15 7.9 8.36 7.83 7.8

516.0 545.9 588.3 387.8 370.0 352.6 216.3 234.9 243.1 550.4 268.1 394.3 408.3 1165.4 357.7 397.1 240.7 884.7 253.0 550.8 417.8 414.5 141.4 640.1 740.3 328.0 375.0 542.6 75.8 369.6 251.7

36 37 63 199 101 27 55 361 220 125 210 113 135 85 68 107 295 225 224 380 350 330 360 271 190 – 250 – – 252 331

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

75 44 342 355 233

Na+ (mg/l)

Well Water Elevation EC T pH No. level (m) (m a.s.l.) (μS/cm) (°C) 406.0 438.2 332.4 284.7 274.5 229.4 421.4 99.4 257.2 382.6 382.6 302.9 300.5 601.1 301.7 388.2 222.5 842.6 54.0 521.5 233.2 303.6 100.7 221.8 296.6 256.0 288.0 669.3 70.2 256.7 128.0

136.5 144.6 82.4 111.9 102.6 175.8 100.5 44.1 110.1 157.8 119.6 111.1 105.1 254.7 85.2 87.4 66.0 353.0 66.0 208.0 131.2 101.8 57.7 110.6 139.8 205.0 95.0 243.2 23.9 98.0 69.3

<0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 10.7 16.0 0.0 89.5 9.0 81.0 45.3 65.5 15.5 <0.05 24.6 9.0 0.0 24.6 3.2 53.6 32.8

741.0 829.5 859.6 559.2 500.2 321.2 204.1 204.0 400.5 939.9 376.6 567.2 584.3 2057 484.2 547.1 271.9 1996 298.0 920.8 1066 655.1 144.2 639.6 1443 568.0 461.0 815.5 119.1 528.8 437.1

1,596 1,704 1,132 1,153 1,129 1,651 1,557 402.2 891.5 1,402 1,386 1,263 1,154 2,330 1,024 1,131 684.2 2,455 365.0 2,153 541.8 1,386 407.9 1,108 745.1 1,276 1,066 2,304 108.4 882.4 472.5

134.2 146.4 36.6 134.2 115.9 18.3 109.8 225.7 183 122 176.9 122 122 24.4 36.6 134.2 164.7 152.5 66 115.9 24.4 12.2 213.5 170.8 158.6 75 146 244 213.5 128.1 152.5

3,770 3,880 3,240 2,730 2,640 2,940 2,910 700 2,040 3,750 2,820 2,870 2,890 7,020 2,622 3,134 1,200 8,029 1,080 3,739 3,134 3,677 967 2,900 4,222 2,600 2,440 5,452 542 2,452 1,500

−2% −3% 1% −2% −2% −4% −2% 3% 1% −2% −3% −4% −2% −3% 2% 4% 5% 1% 6% −2% 0% −5% 1% 4% 0% 0% −2% 2% −4% 3% −2%

−6.72 −6.80 −8.31 −7.24 −6.97 −7.50 −5.25 −6.70 −5.80 −6.92 −6.05 −7.01 −7.34 −7.74 −6.55 −7.34 −6.73 −6.03 −5.49 −8.13 −8.04 −7.85 −6.23 −6.81

−48.5 −47.5 −57.8 −54.0 −49.3 −56.0 −25.2 −39.1 −31.9 −49.7 −32.5 −51.6 −52.2 −53.7 −46.2 −52.2 −42.6 −36.6 −32.8 −59.0 −60.1 −55.1 −37.5 −46.9

11.4 37.0 11.4 43.0 9.5 22.7 11.0 18.1

−8.1 −6.6 −5.2 −7.8 −6.5

−5.5 −7.0

−6.7

16.9

30.2 12.5

−7.3 −6.2

−8.8

11.2

−4.5

14 δ2H δ13C C Ca2+ Mg2+ K+ Cl– SO42− HCO3− TDS Ion δ18O (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) balance (%) (‰ VSMOW) (‰ VSMOW) (‰ VPDB) (pmc)

Table 1 Isotopic and geochemical data for the Turonian aquifer

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421 Table 2 Isotopic and selected geochemical data for the Continental Intercalaire aquifer Well Nos.

Water level (m)

Elevation (m a.s.l.)

EC (μS/cm)

T (°C)

pH

Cl− (mg/l)

δ18O (‰ VSMOW)

δ2H (‰ VSMOW)

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

86 620

390

2,520 4,710 6,870 4,360 6,360 5,660 4,800 2,570 7,550 8,570 6,042 2,650 3,460 3,510 3,100 5,250 5,900

25.2 32.9 34.8 20.0 34.0 20.6 33.6 51.0 26.8 31.5 25.0 23.6 65.0 69.0 15.0 50.0 51.6

7.0 8.0 7.5 7.5 7.6 8.1 7.8 8.1 7.4 7.4 7.2 7.6 7.2 8.2 8.0

396.7 1,140 1,566 1,044 1,418 1,237 1,492 554.0 2,047 2,516 1,708 414.8 676.0 294.4 431.9 974.8 1,624

−6.91 −7.70 −8.78 −8.10 −7.01 −7.02 −8.92 −7.94 −8.56 −8.43 −8.34 −7.04 −8.10 −6.99 −7.23 −7.87 −8.16

−45.7 −57.0 −66.8 −62.7 −49.7 −46.9 −68.2 −61.1 −66.1 −64.3 −64.1 −47.2 −59.0 −48.5 −49.6 −59.0 −62.6

625

405 80 1,268 1,368 94 1,188 1726

245 385 285 362 271 353 272 402 300 393 53 75 369 83 60

7.6

Nineteen groundwater samples were selected for 14C measurements. Dissolved inorganic carbon was precipitated in the form of BaCO3 in the field from 160 l of water. In 4-l samples the stable isotope composition (δ18O, δ2H) was determined. All samples for stable isotope analyses were collected in 30 ml polyethylene bottles with poly-seal caps. Chemical and radiocarbon analyses were performed at the Laboratory of Radio-Analyses and Environment of the National Engineering School of Sfax (ENIS: Tunisia). Stable isotope composition of water samples was determined at the laboratory of the International Atomic Energy Agency (IAEA) in Vienna using the CO2 equilibration method for δ18O (Epstein and Mayeda 1953) and isotopic exchange with H2 gas for δ2H (Colpen et al. 1991), followed by mass spectrometry measurements. Results of analyses are reported in ‰ deviations from VSMOW standard (Vienna-Standard Mean Oceanic Water). Stable carbon isotope composition (δ13C) was measured by mass spectrometry (at the isotope hydrology laboratory of the IAEA). The 14C content in BaCO3 samples was determined using benzene synthesis and liquid scintillation spectrometry (Fontes 1971). Results of 13C analyses are reported in ‰ versus the VPDB (Vienna Pee Dee Belemnite) standard and 14C abundances are expressed as percent modern carbon (pmc). Analytical uncertainties are in the range of ±0.1‰ for the δ18O, ±1‰ for the δ2H, ±0.3‰ for δ13C and between 1 and 1.5 pmc for 14C.

δ13C (‰ VPDB)

14

C (pmc)

−9.9 −10.0

37 5 17

−11.3 −10.5 −10.7

3.8 5.1

−7.6 −10.5

23

−7.8

11

16.1 to 31°C. The lower values were recorded in the vicinity of the Cretaceous and Miocene outcrops (recharge areas), and the higher values were measured near the basin outlet. The spatial distribution of TDS over the basin is somewhat similar. Generally, TDS increases from the mountainous regions towards the discharge area. In the Turonian aquifer, the δ18O values range from −5.2 to −8.3‰, with a mean of −6.9‰. Values of δ2H cover the range from −25.1 to −60‰, with a mean of −46.6‰. These ranges and mean values of the isotopic composition of Turonian groundwater are lower than those observed in rainwater of Sfax, the meteorological station located nearest the investigated area and representing the same climatic conditions (Henia 1993; Melki 1996). The weighted average isotopic composition of precipitation at Sfax is equal to −4.4‰ for δ18O and −24.9‰ for δ2H. The dissimilarity between the isotopic composition of the Turonian groundwater and that of the present rainfall can reflect recharge during colder climatic conditions and/or the altitude effect (Abid et al. 2008; Abid et al. 2009). Based on available data, the altitude effect in precipitation in the region can be estimated at the level of −0.5‰/100 m for δ18O (Abid et al. 2008, 2009; Abid 2010). The δ18O values of the Continental Intercalaire groundwater samples are between −6.9 and −8.9‰ with a mean of −7.9‰, and δ2H values vary from −45.7 to −68.2‰ with a mean of −58.4‰ (Table 2). In contrast to the values measured in Turonian groundwater, samples from the CI aquifer are more negative and homogeneous.

Results Discussion Most groundwater samples originating from the Turonian aquifer are characterized by a high total mineralization, with EC ranging from 500 to 9,650 μS cm−1, although at the piedmont zone of the Dahar upland the lower EC values (EC less than 1,800 μScm−1) are observed (samples Nos. 8, 17, 19, 23, 29). The temperature of most of the groundwater samples falls in the range from Hydrogeology Journal (2011) 19: 415–427

Chemistry of groundwater The proportions between main cations and anions are presented in the form of a Piper diagram in Fig. 5. Major cations reveal similar proportions in the Turonian aquifer, intermediate between Na+ and Ca2+ content. Results of chemical analyses indicate enrichment in SO42− relative to DOI 10.1007/s10040-010-0668-z

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Fig. 5 Piper diagram of the Turonian groundwater samples

Fig. 6 Relationships between major elements in the analysed groundwater samples: a Cl−/Na+ and b SO42−/Ca2+ Hydrogeology Journal (2011) 19: 415–427

Cl−, except for samples Nos. 21 and 25 which are depleted in SO42− with respect to Cl− ions. Consequently, most of the Turonian groundwater samples reveal similar water types as in the CI aquifer (Abid et al. 2009): Ca2+–SO42−, mixed, to Na+–Cl− type. In order to specify the likely origin of each major element contributing to groundwater mineralization, plots of Na+ versus Cl− and Ca2+ versus SO42− content (Fig. 6) are shown. The Na+–Cl− relationship has often been used to identify mechanisms responsible for the origin of water salinity in arid and semi-arid regions (Magaritz et al. 1981; Dixon and Chiswell 1992; Guendouz et al. 2002). The relationship between these two ions shows that the majority of points cluster along the halite dissolution line (line of slope 1). The dissolution of this mineral has already been described in the mineralogy of the study area (Edmunds et al. 2003; Trabelsi et al. 2008; Abid et al. 2009). Observed depletion in Cl− content relative to Na+ concentration in some samples probably reflects the cation exchange reactions leading to adsorption of Ca2+ on clay minerals and simultaneous releasing of Na+ ions. Those samples in which Na+/Cl− molar ratios are higher than one (Fig. 7) also show a deficit in Ca2+ with respect to SO42− ions (samples located above the line of slope 1, Fig. 6b). This remains in good agreement with a Ca2+–Na+ cation exchange theory leading to a softening of water (Hidalgo et al. 1995). On the other hand, the SO42− versus Ca2+ plot (Fig. 6b) shows a more pronounced loss of Ca2+ with respect to SO42−. This may be the result of calcite precipitation controlled by gypsum dissolution which tends to maintain saturation or oversaturation with respect to calcium bearing minerals. Bicarbonate formed by CO2 dissolution is balanced by Na+ and Ca2+ released from clay minerals (Andrews et al. 1994). This is shown by the evolution of the total alkalinity (TAC) and Ca2+ contents which are inversely correlated due to calcite precipitation as explained by Valles et al. (1989) (Fig. 8).

Fig. 7 Variation of Na+/Cl− molar ratio versus Cl− content in investigated groundwaters DOI 10.1007/s10040-010-0668-z

423 3.5

state close to saturation with respect to calcite towards an oversaturation. Cardenal et al. (1994) ascribed oversaturation in calcite to incongruent dolomite dissolution (Eq. 1) and dedolomitization (dolomite dissolution (Eq. 2) and calcite precipitation driven by gypsum dissolution), both of which cause precipitation of calcite. Once the system is saturated with respect to calcite, the hydrochemical evolution is affected by dissolution of gypsum, which will be an influencing factor in the process of dolomite dissolution.

Log molarity

3.0 2.5 2.0 1.5 1.0 0.5 0.0 2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

Log (TDS) Log (Ca)

Log (TAC)

Fig. 8 Variation of Ca and total alkalinity (TAC) versus total dissolved salts (TDS) suggesting the precipitation of calcite forced by gypsum dissolution

Figure 9 shows the saturation indices of calcite and dolomite, calculated using WATEQ-F computer code (Plummer et al. 1976), as a function of (Ca2+ + HCO3−) and (Ca2+ + HCO3− + SO42−), respectively. Results of these calculations indicate that the water evolves from a

– Incongruent dolomite dissolution (Edmunds et al. 1982) CaMgðCO3 Þ2ðsÞ þ H2 CO3 ! CaCO3ðsÞ þ Mg 2þ þ 2HCO3 

ð1Þ – Congruent dolomite dissolution (Edmunds et al. 1982) CaMgðCO3 Þ2ðsÞ þ 2H2 CO3 ! Ca2þ þ Mg 2þ þ 4HCO3 

ð2Þ

For some samples, a relative deficiency of Na+ with respect to Cl− is observed (Fig. 6a). This probably reflects the cation exchange process where Ca2+ is removed from the aquifer’s exchangers and replaced by Na+ (Abid et al. 2009; Abid 2010). The referred cation exchange is confirmed through the relation characterized by a slope of −1 (Fig. 10) traced by the position of the samples (Garcia et al. 2001). In the absence of this exchange all

Fig. 9 Saturation indices of a calcite and b dolomite versus (Ca2+ + HCO3−) and (Ca2+ + HCO3− + Mg2+), respectively Hydrogeology Journal (2011) 19: 415–427

Fig. 10 Relationship between [(Ca2+ + Mg2+)–(HCO3− + SO42−)] and (Na+ + K+–Cl−) reflecting the cation exchange processes. The deficit or excess of (Na+ + K+) over Cl− ions is compensated by the corresponding changes of (Ca2+ + Mg2+) over the (HCO3− + SO42−) ions DOI 10.1007/s10040-010-0668-z

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Fig. 11

Stable isotope composition of CI and Turonian groundwaters

analytical points should lie close to the origin (McLean et al. 2000).

Isotopic study

The δ18O and δ2H values from the Turonian and CI aquifers are shown in Fig. 11 where the global meteoric water line (GMWL) and the local Sfax meteoric water line (SMWL) are also presented. The local meteoric water line is defined by the equation: δ2H=8 × δ18O+13 (Abid et al. 2009). The δ2H–δ18O diagram shows that almost all points representing CI groundwaters are situated below the GMWL, except three points (wells 32, 37 and 43) which lie on this line. Water samples from these three wells were collected in CI carbonates in the Dahar region. Their stable isotope composition suggests the presence of an admixture of recently recharged water (Fig. 11). Position of other samples representing CI indicates their recharge during the late Pleistocene and the early Holocene (Fontes et al. 1983, Zouari et al. 2003). The groundwater of the Turonian aquifer formation in Dahar upland (in particular that of Matmata region) reveals more positive values of δ18O and δ2H. In Fig. 11 they are marked by a dashed-line ellipse. Their location on the δ2H–δ18O diagram suggests an admixture of young local component which probably inflows via faults and/or fractures. The other points representing waters of this aquifer are situated almost parallel to the GMWL towards the paleoclimatic waters of the CI aquifer suggesting an existence of mixing of two different end-members. The δ18O and δ2H values of this cluster decrease gradually towards the major fault where they become indistinguishable from some observed isotopic composition of the CI groundwater. This strongly supports the hypothesis about a vertical leakage of CI groundwater to the Turonian aquifer through Hydrogeology Journal (2011) 19: 415–427

Fig. 12 δ2H–Cl− relationship for the Turonian and CI aquifer groundwaters

deep faults. Samples of this group reveal the lowest 14C concentrations (well No. 12: 9.5 pmc; well No. 16: 11 pmc). Further evidence that supports the role of mixing and recent water infiltration as the main processes responsible for the variation in salinity of groundwater in the Turonian aquifer is presented in Fig. 12. It is noteworthy that the isotopic compositions of both the dilute and the most saline groundwaters are different. The dilute groundwaters are found at shallow depths, in the vicinity of Miocene and Cretaceous outcrops. These have the mean concentration of Cl− at the level of 288 mg/l and mean δ2H value of about −34.5‰. Data representing paleowaters reveal the most negative δ2H values (−68.2 to −62.6‰) and the highest chloride content (1,492 to 2,515 mg/l) being in agreement with their long residence time in the aquifer. The other samples reflect a rather complicated image of mixing effects and show variable Cl− and δ2H content (Fig. 12). The radiocarbon data show a linear trend, from 43 to 4 pmc, from the recharge area towards the basin outlet. According to Guendouz et al. (2002) the range between 4 and 12.5 pmc in the deeper groundwaters should correspond to the recharge in the late Pleistocene (Fig. 13). The range between 12.5 and 16.5 pmc has been interpreted as corresponding to the gap in the recharge conditions during the last glacial maximum when hyperarid but cool climate prevailed over the Sahara. Values of 14 C exceeding 16.5 pmc represent recharge during the Holocene (Guendouz et al. 2002). It is apparent from the 14 C data that waters of Holocene age are found in the Dahar upland and the Djeffara basin. Most of these groundwaters are enriched in both 18O and 2H isotopes. In contrast, most waters of the late Pleistocene age are found in the Chotts region and Saharan platform. They are characterized by lower δ18O and δ2H values. The available chemical and isotopic data allow construction of the conceptual model of groundwater migraDOI 10.1007/s10040-010-0668-z

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Turonian groundwater is mixed with the CI water ascending through the fault system.

Conclusions

Fig. 13 Plot of δ18O against 14C (pmc) for Turonian and CI aquifers

tion within the Turonian aquifer. Such a model is presented in Fig. 14. The Dahar upland is the main recharge area. In the Gulf of Gabes region, stable isotope composition and 14C content in some water samples suggests the admixture of present infiltration water. Thus, this area also participates (although to a limited extent) in recharge of this part of the Turonian aquifer. Mineralization of the Turonian groundwater is built up by the dissolution/precipitation processes and cation exchange reactions along the flow path from the Dahar upland towards the Chotts region. In the El Hamma region the

The Turonian aquifer is one of the most important groundwater resources in Tunisia. It is the main water source for the local population and agriculture. Therefore, deciphering of mechanisms responsible for observed mineralization and reduced water quality in this system, pursued in the framework of this study, is of great practical importance. Simultaneous analysis of both hydrochemical and isotopic data allowed better understanding of the hydrodynamic functioning of the Turonian aquifer (Fig. 14). The potentiometric map (Fig. 4) and the stable isotope data (Tables 1 and 2) point to the significant role of the Dahar upland in local recharge of this aquifer. Stratigraphic, tectonic and hydraulic conditions seem to be important factors controlling the geochemical evolution of groundwater in the study area. The principal changes in chemical composition of Turonian groundwater result from mixing with CI water of deeper circulation. Closer analysis of available chemical data reveals the importance of dissolution/precipitation processes as well as cation exchange reactions in evolution of groundwater chemistry. In general, mineralization of groundwater increases along the flow path from the Dahar upland towards the Chotts region. The Na+/Cl− molar ratio suggests that the dissolution of halite in evaporates within the Mio-Pliocene and Cretaceous sequences are the main source of these ions in groundwater. The values of this ratio, higher than 1, observed in some of the wells may indicate the

Fig. 14 Conceptual model of the Turonian aquifer in southern Tunisia Hydrogeology Journal (2011) 19: 415–427

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occurrence of cation exchange reaction releasing Na+ into the water with simultaneous removal of Ca2+. The existence of such exchange is confirmed by the balance between the excess of (Na+ + K+) over the Cl− ions and excess of (Ca2+ + Mg2+) over the (HCO3− +SO42−) anions (Fig. 10). Chemical data indicate also the importance of gypsum dissolution as a factor controlling groundwater chemistry. The occurrence of low-saline waters at the outcrops suggests local recharge of the Turonian aquifer by recent rainwater infiltration through faults and/or fractures (Fig. 14). Piezometric levels, as well as chemical and isotopic composition of groundwaters, confirm hydraulic connection between the Turonian and Continental Intercalaire aquifers (Fig. 14). Overlap of some major characteristics (δ18O, δ2H, Cl−) in these two aquifers suggests that mixing processes considerably influence the hydrochemical evolution of water. With limited supply by the recent infiltration waters, the increased exploitation of wells can lead to further deterioration of the quality of groundwater resources available for the population in this area. Acknowledgements The authors gratefully acknowledge the contributions of the staff members of Gabes, Kébili, Tataouine and Medenine Water Resources Division/Agriculture Ministry, for their help during field work. We also thank the technical staff at the Laboratory of the International Agency of Atomic Energy (IAEA) and the Laboratory of Radio-Analyses and Environment of the National Engineering School of Sfax (ENIS) for their help and assistance during laboratory analyses. Contribution of M.D. was supported by statutory funds of the AGH University of Science and Technology (project No. 11.11.220.01)

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