Environ Earth Sci DOI 10.1007/s12665-010-0771-4

ORIGINAL ARTICLE

Recharge mode and mineralization of groundwater in a semi-arid region: plain (central )

Hichem Yangui • Kamel Zouari • Rim Trabelsi • Kazimierz Rozanski

Received: 18 May 2010 / Accepted: 26 September 2010 Ó Springer-Verlag 2010

Abstract Groundwater is the most important source of Introduction water supply in Sidi Bouzid plain located in central Tuni- sia. Proper understanding of the geochemical evolution of In arid and semi-arid countries, issues related to water groundwater is important for sustainable development of resources are of growing concern due to different envi- water resources in this region. A hydrogeochemical survey ronmental, economic and social factors. Continuously was conducted on the Mio–Plio–Quaternary aquifer system increasing abstraction of groundwater resources to meet using stable isotopes, radiocarbon, tritium and major ele- rising industrial, agricultural and domestic needs, coupled ments, in order to evaluate the groundwater chemistry with severe drought periods during the past decades leads patterns and the main mineralization processes occurring in to growing deficit of water. The drawdown of piezometric this system. The chemical data indicate that dissolution of levels and progressing degradation of water quality are the evaporate minerals and evaporation are the main processes main consequences of such intensive exploitation. controlling groundwater mineralization. The isotopic data The plain of Sidi Bouzid (SBZ) which was the subject show that groundwater in the study area is a mixture of of this study is located in central Tunisia (Fig. 1). SBZ is recent shallow waters located upstream and along Wadi Al a typical example of a semi-arid climate region where Fakka bed and paleowaters located towards plain limits and groundwater resources are intensively exploited for human discharge areas. Low 3H and 14C contents are observed in needs as a result of agricultural and demographic devel- major part of the plain indicating that recharge of the opment. In 2005, the renewable water resources in the aquifer occurs mainly through direct infiltration at Wadi Al region were estimated to 19.5 Mm3/year with annual Fakka while there is no evidence of significant recharge in exploitation in the order of 27.1 Mm3 (DGRE 2005). major part of the plain and mountains piedmonts. Rainfall occurs on an irregular basis in brief, high-intensity events causing strong floods mainly in Wadi Al Fakka. Keywords Semi-arid region Á Geochemistry Á Stable Thus, significant part of precipitation and the resulting isotopes Á 3H Á 14C Á Groundwater recharge Á Plain of Sidi surface runoff are lost via evaporation and cannot con- Bouzid Á Central Tunisia tribute to groundwater recharge. Therefore, there is a need to explore possibilities to store those surface waters and utilize them for artificial recharge schemes of the aquifer system. Different structures to control surface water flows H. Yangui (&) Á K. Zouari Á R. Trabelsi were constructed between 1983 and 1994 in the upstream Laboratoire de Radio-Analyses et Environnement, bed of the main watercourse on SBZ plain. The objective Ecole Nationale d’Inge´nieurs de , BP 1173, 3038 Sfax, Tunisia of these structures was to apply an artificial spreading of e-mail: [email protected] Wadi Al Fakka floods for irrigation and then for an artifi- cial recharge of the aquifer. K. Rozanski Although several modelling studies focussing on artifi- Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, cial groundwater recharge on SBZ plain have been con- al. Mickiewicza 30, 30059 Krakow, Poland ducted in 1980s and 1990s (Bouzaiane and Lafforgue 1981; 123 Environ Earth Sci

Fig. 1 Geographic location, simplified geology and piezometric map (status as of 2005) of Mio–Plio–Quaternary aquifer system located in the plain of Sidi Bouzid, central Tunisia. AA’ cross-section as shown in Fig. 2

Bouraoui 1984; Amouri 1994; Simonot 1996), the effi- plain from west to east, to discharge then into a large ciency of these management schemes remains largely evaporation area in the north-east (Naggada). The baseflow unknown because of the climate aridity and the lack of of Wadi Al Fakka is in the order of 8.8 Mm3/year (Laf- quantitative data with regard to effective infiltration rates forgue 1981). The major part of this flow infiltrates before and exploitation of the system. As it will be demonstrated reaching SBZ plain. Annualy, there are 12–15 floods into below, the use of hydrochemical and particularly isotopic Wadi Al Fakka bed totalling to approximately 36.3 Mm3 tools for characterization of groundwater resources in the (Bouzaiane and Lafforgue 1981; Lafforgue 1981). study area resulted in a better understanding of the recharge mode and mineralization processes occurring in the system. Geological and hydrogeological setting This in turn can guide future groundwater management schemes in the area. The plain of SBZ is located in the oriental part of central Atlas of Tunisia. The latter is characterized by northeast- southwest fold belts bordered by fault corridor systems and Study area separated by basins filled with Mio–Plio–Quaternary deposits (Burollet 1956; Khessibi 1978; Ben Ayed 1986; The SBZ plain is located in central Tunisia, on the eleva- Boukadi 1994;Be´dir 1995; Chekhma 1996; Ben youssef tion of ca. 350 m.a.s.l (meters above sea level). It is bor- 1999; Zouaghi 2008). On SBZ plain, the sedimentary series dered by mountains (Dj) of Hamra to the west and the consist of deposits going from Lower Cretaceous to Qua- north-west, Al Hfay to the south-west, Kebar to the south ternary, affected by important tectonic features occasion- and Lessouda to the north (Fig. 1). The plain is charac- ally associated with salt Triassic activities (Figs. 1, 2). The terized by a semi-arid climate with low and irregular deposits are characterized by several hiatuses and uncon- rainfall of about 250 mm/year and an intense potential formities; the most important are represented by the middle evaporation rate estimated to ca. 1,600 mm/year (Amouri Turonian–Maastrichtian and Paleogene series (Fig. 2). 1994). Indeed, since middle Turonian, a large part of central The Wadi Al Fakka and its tributary constitute the Tunisia (including the study area) corresponds to an principal watercourse in the study area. It crosses the SBZ emerged area known as ‘‘ islets’’ (Burollet 1956)

123 Environ Earth Sci

Fig. 2 Hydrogeological cross- section of Mio–Plio–Quaternary aquifer located in the plain of Sidi Bouzid, central Tunisia. For the position of the cross- section, see Fig. 1

and, as consequence, it was devoid of deposits character- heterogeneity of the aquifer formation (Amouri 1994). The istic of this period. exploitation of the aquifer is carried out through boreholes After a local transgressive phase (Lower Miocene) the with depths ranging from 100 to 400 m and by dug wells central Tunisia emerged in totality (Middle and upper for the shallow levels (40–100 m). Miocene) and was subject of extension–compression The piezometric map shows that groundwater flows regimes (Atlas phase) which led to the formation of fold mainly from SW to the NE, along the major axis of Wadi ranges oriented NE-SW and separated by many Mio–Plio– Al Fakka bed (Fig. 1). Two natural discharge areas of the Quaternary microbasins as a result of active erosion aquifer are shown on the map: the evaporation area of affecting uplifts (Mannaı¨-Tayech 2006; Zouaghi et al. Naggada at the north-east (outlet of surface waters), and 2008). In central Tunisia, these thick continental infilled the depression of Garet Al Akarisch to the east of the plain. basins represent important groundwater reserves held by multi-layer aquifer systems. The aquifer system of SBZ plain is located in 800 m Sampling and analytical procedures Mio–Plio–Quaternary sediment-filled basin bordered by fold belts (Fig. 1) whose main outcrops are formed by During this study, deep and shallow levels of the SBZ upper cretaceous deposits (Zebbag formation) (Fig. 2) The aquifer system were sampled at 38 locations in June 2005. aquifer is composed essentially of sand (with varying grain Water samples for chemical and isotope analyses were sizes), inter-bedded with layers of clay and sandy clay as collected from 14 boreholes and 24 dug wells (Fig. 3). The well as gravels, pebbles and gypsum. The deposits are chemical analyses (major ions) were conducted by liquid- characterized by significant vertical and lateral heteroge- ion chromatography (HPLC) in the laboratory of radio- neity. Thus, the SBZ plain aquifer system is identified as a analysis and environment (LRAE) at the National School single multi-layer aquifer, represented by three major of Engineering in Sfax (Tunisia). Stable isotope composi- lithologic sections (Fig. 2) (Koschel 1980; Bouraoui 1984; tion of water samples (d18O, d2H) was determined by Amouri 1994; Simonot 1996): (1) 0 to 100 m (clay and isotope ratio mass spectrometry at the laboratory of Inter- sand); (2) 100 to 400 m (sand and clayey sand), and (3) 400 national Atomic Energy Agency (IAEA) in Vienna. The to 800 m (clay and sandy clay). Transmissivity values results are expressed as relative deviations d (in per mil) obtained by pumping tests vary from 0.73 9 10-3 m2 s-1 from the Vienna standard mean ocean water (VSMOW). to 1.14 9 10-2 m2 s-1 reflecting substantial lithological The analytical precision of stable isotope analyses (one

123 Environ Earth Sci

Fig. 3 Distribution map of TDS values (mg l-1) showing location of sampling sites

sigma) was in the order of 0.1% for d18O and 1% for d2H. Fakka bed, especially in the Saddouguia area where the The tritium content was measured at the IAEA laboratory TDS values are the lowest (W.17, W.18, W.19 and W.21). using electrolytic enrichment and liquid scintillation The increase in TDS values upstream of the plain spectrometry (Taylor 1977). The results are expressed in (TDS \ 3,000 mg l-1, W.1, W.2, W.3 and W.4) might be Tritium Units (TU) (one TU is equivalent to one 3H atom related to infiltration of concentrated base flow of Wadi Al per 1018 atoms of hydrogen) (Clark and Fritz 1997) with Fakka and a possible dissolution of gypsum, limestone and the analytical uncertainty in the order of 0.3 TU. The dolomite (Zebbag Formation) at the south-east piedmonts radiocarbon concentration in the total dissolved inorganic of Hamra and locally, El Hfay mountains (B.11). carbon pool (TDIC) was measured at the LRAE Laboratory The mineralization of groundwater increases also through benzene synthesis and liquid scintillation spec- downstream of the plain towards the discharge areas of the trometry (Fontes 1971). The measured 14C concentrations aquifer, reaching values between 3,000 and 5,000 mg l-1. are expressed as percent modern carbon (pmc) by com- This can be attributed to a contamination of shallow aquifer paring carbon-14 activities with the radioactivity of the levels (W.14, W.22, W.23, and W.24) by the evaporated modern radiocarbon standard of Oxalic Acid I (C2H2O4) surface water in the Naggada depression (outlet of surface (Taylor 1987). The results of chemical and isotope analyses waters). The water table is close to the surface in this of the investigated groundwater samples are presented in area. High TDS values in the deep boreholes (B.8, B.9, Table 1. B.11, B.13, and B.14) may result from long residence time of groundwater and associated enhanced water–rock interaction. Results and discussion Geochemical characteristics and origin of mineralization Hydrochemistry Concentrations of major elements in the analysed ground- Total dissolved solids (TDS) water samples are listed in Table 1. When plotted in Piper diagram (Piper 1944), they indicate that the analysed

The TDS values of groundwater samples range from 1,500 groundwater represent SO4–Ca–Na–Mg water type (Fig. 4). to 5,000 mg l-1 (Table. 1). Spatial distribution map The correlation diagrams of different major elements ver- (Fig. 3) shows relatively homogenous, low TDS values sus TDS values (Fig. 5) show that groundwater minerali- -1 (varying from 1,500 to 2,500 mg l ) located in the central zation is mainly dominated by SO4, Ca and Mg contents for part of the plain. This may reflect a dilution effect at shallow levels and by SO4, Cl, Na, and Ca for the deep shallow aquifer levels by a local recharge through Wadi Al aquifer levels.

123 nio at Sci Earth Environ Table 1 Geochemical and isotopic data of sampled groundwater in the SBZ plain (central Tunisia) 18 2 3 14 13 Location Code Depth TDS Ca Mg Na K HCO3 Cl SO4 NO3 d O d H H C C (m) (mg l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) (mEq l-1 ) % vs. UT pmC % vs. PDB SMOW

Habib Nsiri B.1 340 2,441 17.60 11.52 11.95 0.14 3.80 7.18 27.85 nd -5.68 -39.2 3.3 30.4 -7.22 Nouail B.2 102 2,253 21.24 14.66 10.33 0.21 3.10 5.29 37.01 nd -5.67 -39.4 6.3 51.0 -9.0 Mnassri Med B.3 155 1,962 11.65 9.31 13.05 0.15 4.40 6.70 21.26 nd -5.9 -38.3 \0.3 Khlifi Laroussi B.4 160 1,879 9.99 11.44 10.19 0.18 5.30 7.98 17.19 0.56 -6.00 -42.0 22.1 -8.18 Ferme 23 B.5 271 2,208 15.78 4.24 14.16 0.27 4.80 7.80 21.52 nd -6.21 -40.4 \0.3 25.4 -10.16 Ferme 21 B.6 310 2,383 16.96 4.39 14.77 0.23 4.70 8.16 23.95 nd -6.01 -38.6 \0.3 28.8 -9.45 Ferme 19 B.7 401 2,229 11.60 9.55 15.24 0.40 4.90 7.95 21.71 0.14 -6.13 -40.0 Usine jellali B.8 220 3,079 23.09 13.67 20.06 0.31 5.75 15.06 31.54 nd -6.17 -44.2 \0.3 8.0 -8.34 Ferme 26 B.9 156 2,928 13.27 12.64 18.49 0.33 4.40 12.71 25.94 nd -6.11 -40.3 \0.3 14.3 -8.07 Fre`res Mnasri B.10 186 2,114 11.54 9.40 13.58 0.21 6.90 6.84 21.96 nd -6.00 -40.0 0.50 34.0 -9.70 Akrimi Abderazak B.11 200 4,120 24.65 16.36 22.60 0.22 4.00 19.05 41.99 0.78 -6.16 -42.9 \0.3 27.3 -8.18 Rchid Akrimi B.12 160 1,710 10.30 9.21 11.29 0.14 5.30 5.58 17.77 0.35 -6.03 -40.1 Ferme 13 B.13 327 3,530 16.36 17.09 26.11 0.37 2.80 22.73 29.08 0.65 -6.39 -43.1 \0.3 8.4 -6.35 Om Ladham 2 B.14 180 2,674 11.92 10.97 18.95 0.36 4.80 15.28 18.20 0.43 -6.18 -42.3 \0.3 14.0 -6.20 Bouazizi Mahmoud W.1 40 2,496 16.65 11.12 11.70 0.13 3.20 7.53 28.11 nd -5.62 -37.3 2.4 Nayli Mongi W.2 53 2,259 17.57 11.29 7.93 0.09 4.60 5.04 28.12 nd -5.30 -35.5 9.5 Nayli Hedi W.3 48 2,986 23.50 15.53 10.47 0.16 2.80 7.32 36.50 nd -5.54 -37.3 Nayli Ezzedine W.4 45 2,206 17.37 12.62 8.42 0.09 2.80 5.29 27.62 nd -5.82 -38.0 Nayli Taher W.5 46 1,960 16.44 10.55 6.31 0.28 2.80 3.11 27.11 nd -5.80 -37.5 2.7 Mnasri Abdelkader W.6 49 1,979 10.95 8.96 11.52 0.14 3.24 6.07 19.79 nd -5.96 -39.2 0.5 Mnasri Mongi W.7 55 2,134 12.96 10.24 12.61 0.13 5.20 6.59 22.93 nd -5.87 -37.7 \0.3 Ncibi Jamel W.8 37 2,068 18.02 9.52 5.03 0.11 2.80 2.14 28.97 nd -6.04 -43.6 Ncibi Med 1 W.9 41 2,350 20.06 13.33 6.29 0.21 3.20 2.75 33.54 nd -5.78 -38.8 11.1 Bouazizi khlifa W.10 42 2,008 13.87 12.17 8.41 0.11 3.20 4.26 25.14 0.18 -5.93 -40.10 Bouazizi Naceur W.11 40 2,508 17.90 14.90 10.65 0.19 5.20 6.58 28.67 0.36 -5.86 -40.10 61.30 -8.27 Bouazizi Taher W.12 43 2,230 12.29 9.28 13.63 0.33 4.00 5.88 25.05 nd -5.80 -36.30 Nayli Ammar W.13 42 2,097 13.12 10.80 15.73 0.19 4.40 7.05 26.64 0.20 -5.75 -39.00 Hamdi Neji W.14 16 3,309 19.01 20.67 20.66 0.28 4.60 13.20 39.61 0.64 -4.98 -37.0 Ncibi Med 2 W.15 60 2,325 13.92 11.47 13.10 0.19 5.20 5.54 26.10 nd -5.88 -35.50 Saoudi Abdelhamid W.16 68 2,268 14.07 9.79 11.92 0.14 3.20 6.34 26.49 nd -5.76 -37.40 \0.3 29.00 -7.63 Issaoui Med.B. Ahd W.17 27 1,645 9.99 6.84 10.78 0.20 4.00 2.76 21.99 nd -6.06 -37.80 0.30 42.10 -6.86 123 Daly Mahmoud B Aouni W.18 41 1,480 7.23 13.05 8.69 0.00 5.25 1.89 19.68 nd -5.85 -38.05 \0.3 Nassri Abdallah W.19 54 1,642 12.86 10.30 7.25 0.25 2.80 2.14 25.20 0.14 -6.10 -39.10 \0.3 52.3 -7.43 Boufoufi Md B Hassen W.20 60 2,260 16.44 13.53 5.17 0.00 2.50 3.00 31.33 0.82 -5.58 -35.20 Environ Earth Sci vs. PDB 8.28 9.36 C - - 13 % C 14 0.3 76.20 0.3 32.40 H 3 UT pmC \ \ H 38.10 39.30 37.50 37.90 1.28 2 d - - - - vs. O 6.18 5.69 5.59 5.55 18 d % SMOW - - - - ) 1 - 3 NO ) (mEq l 1 - 4 ) (mEq l 1 - Fig. 4 Piper diagram of analysed groundwater samples Cl SO

) (mEq l In order to decipher the origin of groundwater salinity, 1 -

3 scatter diagrams of major chemical constituents in the analysed groundwater samples were established (Fig. 6a– d). The Ca and Mg are well correlated with SO (R2 = 0.76

) (mEq l 4 1

- and 0.53, respectively) indicating that they most likely derive from the same evaporate dissolution. However, somes water samples are placed below line 1:1 indicating a ) (mEq l 1

- deficit of Ca and Mg versus SO4 (Fig. 6a, b). This deple- tion on Ca and Mg indicates the contribution of another geochemical process that affects cation contents. ) (mEq l

1 Groundwater sampled from shallow aquifer levels, - especially in the region located upstream of the plain (W.1,

W.2, W.3, W.4, W.5), reveals high Ca and SO4 contents -1

) (mEq l ([40 mEq l ). This can be attributed to a local contami- 1 - nation from Wadi base flow by dissolution of gypsum and not detected calcimagnesic rocks characterizing soil texture of the wadi nd (mEq l Ca Mg Na K HCO bed (Koschel 1980). The computation of saturation indices ) 1

- (SI) by Wateq F program (Plummer et al. 1976) shows a progressive saturation in gypsum (average SI of -0.4) with TDS (mg l an increase in SO4 concentrations (Fig. 6e). The positive correlation between Na and Cl contents (m) (R2 = 0.8), indicates the contribution of halite (NaCl) total dissolved solids, dissolution to groundwater mineralization (Fig. 6c). The -1 TDS highest concentrations ([15 mEq l ) are observed down- stream of the plain at the discharge areas of the aquifer and surface waters. They can be explained by leaching of sa-

dug well, liferous soil in the evaporation area of Naggada. This can be W proved by the under-saturation state of groundwater with continued respect to the halite (SI =-5.9) (Fig. 6f). Water samples are placed under line 1:1 indicating excess of Na versus Cl. borehole, ? Guedri Majid et Fr W.21 53 1,878 16.02 9.18 4.87 0.12 2.00 2.68 23.53 0.23 Zinoubi Abdelkader W.22 43 2,156 12.15 9.50 13.12 0.17 7.00 9.20 18.03 0.71 Garbi B. Hedi B. Rchid W.23 42 2,470 12.79 10.22 15.40 0.00 5.62 10.75 20.67 1.04 Omor B Ali B Aicha W.24 28 5,000 29.36 23.30 21.95 0.00 5.00 25.36 37.47 0.96 B Table 1 Location Code Depth This Na excess is balanced by a deficit or depletion of 123 Environ Earth Sci

Fig. 5 Correlation of Ca, Mg, Na, HCO3, Cl and SO4 versus TDS

Ca2? and Mg2? contents indicating cation-exchange pro- 11.1 TU are measured in groundwater samples collected in cess occurring on clay minerals. the upstream part of Wadi Al Fakka bed. Tritium con- The contribution of carbonate minerals to groundwater centration in present-day rainfall in central Tunisia ranges mineralization in the study area (calcite and dolomite) is between 0 and 5 TU (Maliki 2000; Celle et al. 2001; Jribi not evident despite the existence of carbonate deposits on 2004; Ben Ammar et al. 2006; Kamel et al. 2006). High the anticline outcrops (mountains) around the plain. This is tritium contents observed in the western part of the plain suggested by the absence of correlation between Ca and (cf. Fig. 9) directly indicate modern recharge by surface - 2 HCO3 contents (R = 0.06; Fig. 6d). waters probably from Wadi AL Fakka floods. On the other hand, lack of tritium in the remaining parts of the plain, Environmental isotopes even in shallow wells, suggest a negligible contribution of surface waters to the recharge of the aquifer outside of Stable isotope composition of the analysed groundwater western parts of the Wadi Al Fakka bed. samples range from -6.39 to -4.98% (d18O) and from The measured deuterium and oxygen-18 isotope com- -44.2 to -35.2% (d2H). The average values are position of the analysed groundwater samples is plotted in -5.86 ± 0.2% and -39.05 ± 1%, respectively. Individ- the d2H–d18O space (Fig. 7). All analysed samples are ual data points are listed in Table 1 and shown in Fig. 7.In situated below the regional meteoric water line (RMWL) Fig. 8 the d18O values are plotted as a function of chloride defined on the basis of monthly precipitation samples content. Tritium concentration has been measured in 24 collected at Sfax station located on the Mediterannean samples. More than 50% of the analysed samples reveal no coast ca. 150 km east of the study area (RMWL: detectable tritium (measured tritium content\0.3 TU). The d2H = 8d18O ? 13.5—Maliki et al. 2000). Isotopic com- highest measured tritium content was in dug well No. 9 position of rainfall on the SBZ plain has not been measured (11.1 TU). The radiocarbon concentration and 13C content in this study. As the recharge of SBZ aquifer occurs pre- of TDIC pool were analysed in 17 samples. The obtained dominantly during flash floods, one may expect that iso- values range from 8.0 to 76.2 pmc and from -10.2 to topic composition of newly formed groundwater in this -6.2%, respectively. Radiocarbon and tritium concentra- system will be heavily biased towards most negative d18O tions are listed in Table 1 and their spatial distribution is and d2H values of the local rainfall. The weighted mean shown in Fig. 9. and the range of d18O and d2H in monthly precipitation at In the major parts of the plain tritium concentration is Sfax are about of -4.6 and -23.3%, respectively (Fig. 7). below or close to the detection limit (range between 0 and It is apparent that groundwater in the study area is depleted 0.5 TU). Higher values ranging between 2.4 TU and in heavy isotopes (by ca. 1.3 and 15% in 18O and 2H,

123 Environ Earth Sci

Fig. 6 Hydrochemical relationships a (Ca/SO4), b (Mg/SO4), c (Na/Cl), d (Ca/HCO3), e (IS gypsum/SO4), f (IS Halite/Cl)

respectively) when compared with the mean isotopic sig- strongly affected by evaporation process and thus they are nature of Sfax rainfall. This depletion may stem from enriched in heavy stable isotopes. These rainfalls did not continental and/or altitude effect, but in major part it is contribute significantly to the groundwater recharge. most probably induced by selection of heavy, isotopically Although the exact position of the local meteoric water depleted rainfalls in the infiltration process. Indeed, rain- line in the study area is not known, it is apparent from falls occurring in small amount in dry and hot seasons are Fig. 7 that most of the analysed groundwater samples show

123 Environ Earth Sci

Fig. 8 The relationship between d18O and Cl content in the analysed groundwater samples. See text for details

change of stable isotope signature. This provides a proof that elevated Cl contents in these waters are not induced by evaporation process but rather stem from prolonged water– Fig. 7 d2H–d18O diagram of the analysed groundwater samples. See rock interaction in particular evaporate dissolution as text for details mentioned in the chemical discussion. As seen in Fig. 9 the increase in salinity is broadly linked to longer residence signs of evaporation. The highest degree of evaporation time of groundwater. Highest concentrations of Cl are reveals well number W.14 located in the discharge area of recorded in deep boreholes situated in the eastern part of the aquifer. the study area. There is also clear trend in radiocarbon Judging from the position of the data points on d2H– concentration, with lowest concentrations occurring in d18O diagram, three groups of samples can be distin- northern and eastern parts of the plain (boreholes B.14, guished. Group A consists predominantly of shallow wells, B.13, B.8). The lack of tritium and relatively low con- with the data points clustering along the global meteoric centrations of radiocarbon, combined with elevated salinity water line (GMWL: d2H = 8d18O ? 10 (Craig 1961) with levels, point to significant age of waters belonging to this some points located above and some below the line and group. Distinct stable isotope compositions suggest that some points showing clear signs of evaporation. The they may stem from the last humid Pleistocene or Holocene evaporation signature of several wells from this group is period in central Tunisia (Ouda 2000; Zouari et al. 2003). further confirmed by their position on the evaporation Numerous isotopic studies of Mio–Plio–Quaternary aqui- trajectory (Fig. 8) describing evolution of d18O and Cl fers in central Tunisia such as Garat Al Hamra, Sfax, content in water parcel as it undergoes evaporation. In Maknessy, and (El Amri 1992; Maliki several wells of this group significant concentrations of 2000; Ouda 2000; Zouari et al. 2003; Dassi 2004; Jribi tritium have been detected (e.g. well W.2, 9.5 TU; W.1, 2004) confirmed the existence of a groundwater recharged 2.4 TU; W.24, 1.3 TU). High tritium concentrations are under colder climatic conditions than at present. accompanied by relatively high radiocarbon contents, Excepting the recharge area in upstream of Wadi Al suggesting that waters of this group represent recent Fakka bed, the tritium and 14C contents are homogeneous recharge. As seen in Fig. 9, this recent recharge occurs in the major part of the plain suggesting low groundwater predominantly in the Wadi Al Fakka region. flows with minimal surface contamination. The recharge of The second group of samples (group B in Fig. 7)is the aquifer occurs principally by progressive infiltration of represented mainly by boreholes (B.8, B.11, B.13, and surface flows within Wadi Al Fakka bed which can con- B.14). It is characterized by distinctly lower d2H and d18O tribute to a relatively dilution of groundwater salinity on values when compared with group A. The data points form the shallow levels. However, underground flow from Wadi a tight cluster below the GMWL. Water samples belonging Al Fakka bed is the main recharge process of shallow and to this cluster do not contain detectable tritium and have deep levels in the rest part of the aquifer. The intensive relatively low radiocarbon concentrations (between ca. 8 exploitation of water table leads to drawdown of piezo- and 27 pmc). As seen in Fig. 8, the samples show a clear metric level and the use of deep and more mineralized increasing trend in chloride content without noticeable groundwater, especially towards the discharge areas of the

123 Environ Earth Sci

Fig. 9 Distribution map of tritium (3H) and Radiocarbon (14C) content on SBZ plain

aquifer. Thus evaporation process is added and contributes mixing between old groundwater possibly originating to increase of groundwater salinity. from the pluvial phase(s) in Holocene and being a part of regional groundwater flow, with recent recharge occurring locally on the plain. Concluding remarks Isotope and chemical data gathered in the study provide strong evidence that artificial recharge schemes applied in The geological setting of Mio–Plio–Quaternary aquifer the basin by spreading flood waters are limited to the system of SBZ plain and the Wadi Al Fakka flows plays an western part of Wadi Al Fakka bed. As current exploitation important role in the geochemical and hydrodynamic of groundwater resources of SBZ aquifer system has a non- characteristics of deep and shallow levels of the studied sustainable character, other management schemes should aquifer. The combination of hydrochemical and isotopic be worked out with the main aim of reducing wadi flood tools applied in this study allowed a better understanding of losses and increasing the aquifer recharge. the functioning of the aquifer under present climatic regime. While tritium and radiocarbon content yield Acknowledgments The authors would like to thank IAEA and important clues on the timescales of groundwater flow in LRAE teams for their help in performing of isotopic and geochemical analyses. the studied aquifer, stable isotope composition of water allowed identification of different types of water occurring in the system. References Analysis of chemical characteristics of water in the studied groundwater revealed that the observed minerali- Amouri M (1994) Etude hydroge´ologique du syste`me aquife`re de Sidi zation is primarily linked to dissolution of evaporates, with Bouzid, DGRE, Tunisia significant contribution of secondary processes such as Be´dir M (1995) Me´canismes ge´odynamiques des bassins associe´s aux evaporation and/or exchange with clay minerals. couloirs de coulissement de la marge atlasique de la Tunisie, ` Environmental isotope data obtained in the framework sismostratigraphie, sismo-tectonique et implications petrolieres. D.Sc. Thesis, Univ. II, p 315 of this study provide a strong evidence that current Ben Ammar S, Zouari K, Leduc C, M’barek J (2006) Caracte´risation recharge of the studied aquifer system occurs principally isotopique de la relation barrage-nappe dans le bassin de through heavy floods on Wadi Al Fakka bed. The area of Merguellil (Plaine de Kairouan, Tunisie centrale). HSJ 51(2): active recharge is relatively small, limited to western part 272–284 Ben Ayed N (1986) Evolution tectonique de l’Avant—pays de la of the Wadi Al Fakka bed. Stable isotope signatures of the chaıˆne alpine de Tunisie du Me´sozoı¨que a‘ l’Actuel. D.Sc. analysed water samples provide evidence for large-scale Thesis, Univ. Paris Sud Orsay, p 327 123 Environ Earth Sci

Ben Youssef M (1999) Stratigraphie ge´ne´tique du Cre´tace´ de Tunisie, Koschel R (1980) Etude hydroge´ologique de la nappe de Hajeb Micropale´ontologie, stratigraphie se´quentielle et ge´odynamique Layoun——Ouled Askar. Projet de coope´ration technique des bassins de la marge sud et pe´ri-te´thysienne. D.Sc. Thesis. Tuniso-Allemande No 6520/7, DRES, Agricultral ministry 01- Univ. de Tunis II, p 420 245 Boukadi N (1994) Structuration de l’Atlas de Tunisie: signification Lafforgue A (1981) Etude hydrologique de l’oued Negada a` la station des noeuds et des zones d’interfe´rences structurales au contact de Bled Lassoued. DRES, OSTROM, Tunis, Tunisia 01-145 des grands couloirs tectoniques. D.Sc. Thesis. Univ. Tunis II, Maliki MA (2000) Etude hyroge´ologique, hydrochimique et isotop- p 249 ique du syste`me aquife`re de Sfax (Tunisie). Doc. Thesis, Tunis II Bouraoui S (1984) Impact des eaux des rejets de l’usine de cellulose University, Tunis, Tunisia de Kasserine (Tunisie) sur l’environnement. PhD Thesis, Uni- Maliki MA, Krimissa M, Michelot JL, Zouari K (2000) Relationship versity of Pierre et Marie Curie, Paris 6, France between shallow and deep aquifers in the Sfax bassin. Earth Bouzaiane S, Lafforgue A (1981) Etude ge´ne´rale de la propagation Planet Sci 331:1–6 des crues sur la branche sud de l’oued Ze´roud; effet de la plaine Mannaı¨-Tayech B (2006) Miocene silicoclastic series from North- de Sidi Bouzid. DRES, OSTROM. Tunisia 01-45 Eastern to South-Western of Tunisia: standpoint. Geobios Burollet PF (1956) Contribution dans l’e´tude stratigraphique de la 39:71–84 Tunisie centrale. Ann Min Geo 18, Tunisia Ouda B (2000) Paleohydrologie isotopique du bassin de Maknassy. Celle H, Zouari K, Travi Y, Daoud A (2001) Caracte´risation (Tunisie Centrale) pendant le quaternaire re´cent, PhD Thesis, isotopique des pluies en Tunisie. Essaie de typologie dans la University of Sfax, Tunisia re´gion de Sfax. CR Acad Sci Paris 6:625–631 Piper AM (1944) A graphic procedure in the geochemical interpre- Chekhma H (1996) Etudes stratigraphiques, se´dimentologiques et tation of water analyses. Trans Am Geophys Union 25:914–923 tectoniques de la re´gion de Bir El Hafey— Plummer LN, Jones BF, Truesdell AH (1976) WATEQF, a Fortan IV (Tunisie centrale). The`se Univ. Tunis II, p 247 version of WATEQ, a computer program for calculating Clark ID, Fritz P (1997) Environmental isotopes in hydrogeology, chemical equilibrium of natural waters. US Geol surv water Lewis Publishers, New York, p 328 Resour Invest 76:61 Craig H (1961) Isotopic variation in meteoric waters. Sciences RE DG (2005) Annuaire d’exploitation des nappes phre´atiques. 133:1702–1703 Tunis, Tunisia Dassi L (2004) Etude hydroge´ologique, hydrochimique et isotopique Simonot M (1996) Plaine de Sidi Bouzid : Etude de l’impact de du syste`me aquife`re du bassin de Sbeitla (Tunisie centrale). PhD l’e´pandage des crues sur la recharge de la nappe phre´atique. Thesis, Sfax University, Tunisia Final rapport, DGRE, Tunisia, FIDA, Rome, Italy 01-40 El Amri R (1992) Contribution a` l’e´tude hydroge´ologique du bassin Taylor CB (1977) Tritium enrichment of environmental waters by de l’oued El Hchim-Garaa Hamra. (Re´gion de Kasserine. electrolysis: development of cathodes exhibiting high isotopic Tunisie). PhD Thesis, Hydrogeology, University of paris du separation and precise measurements and applications. High sud, France Tatras, Czechoslovakia, October 1975, Bratislava Fontes JC (1971) Un ensemble destine´ a` la mesure de l’activite´ du Taylor RE (1987) Radiocarbon dating. An archaeological perspective. radiocarbone naturel par scintillation liquide. Rev Geog Phys Academic Press, Orlando Ge´ol Dyn 13(1):67–86 Zouaghi T (2008) Distribution des sequences de depot du Cre´tace´ Jribi L (2004) Caracte´risation hydrochimique et isotopique des eaux (Aptien—Maastrichtien) en subsurface: role de la deformation du syste`me aquife`re du bassin de Ze´roud (Plaine de Kairouan, tectonique, l’halocine`se et evolution ge´odynamique (Atlas Central Tunisie centrale). PhD Thesis, University of Tunis El Manar, Tunisien), PhD Thesis, University Tunis El Manar, Tunisi Tunis Zouaghi T, Be´dir M, Abdallah H, He´di Inoubli M (2008) seismic Kamel S, Dassi L, Zouari K (2006) Approche hydroge´ologique et sequence stratigraphy, basin structuring, and hydrocarbon impli- hydrochimique des e´changes hydrodynamiques entre aquife`res cations of Cretaceous deposits (Albian—Maastrichtian) in profond et superficiel dans le bassin de Dje´rid, Tunisie. HSJ central Tunisia. Cretaceous Research 2008.02.05, pp 1–21 51(4):713–729 Zouari K, Chkir N, Ouda B (2003) Paleoclimatic variation in Khessibi M (1978) Etude ge´ologique du secteur Meknessy-Mezouna Maknassi basin (central Tunisia) during Holocene period using et du Jebel El Kebar (Tunisie centrale). MSc Thesis, University pluridisciplinary approaches. Tech Report 28–80. International of Claude Bernard, Lyon Atomic Energy Agency (IAEA), Vienna, Austria

123