Southeastern Tunisia) Using Isotope Tracers
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Environ Earth Sci (2016) 75:636 DOI 10.1007/s12665-016-5445-4 ORIGINAL ARTICLE Determination of the origins and recharge rates of the Sfax aquifer system (southeastern Tunisia) using isotope tracers 1,2 1 2 1 3 Rahma Ayadi • Kamel Zouari • Hakim Saibi • Rim Trabelsi • Hafedh Khanfir • Ryuichi Itoi2 Received: 15 July 2015 / Accepted: 8 February 2016 / Published online: 11 April 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract The origin of groundwater in the Sfax aquifer -5.70 % for d18O and from -42.40 to -38.89 % for d2H. system was studied using environmental isotopic tracers The mixing proportions inferred from stable isotopic mass (d18O, d2H, and 3H). In total, 164 water samples were balance calculations suggest that the deep aquifer con- analyzed for stable isotopes: 73 collected from the Sfax tributes significantly to the middle aquifer through geologic shallow aquifer, 63 from the Sfax middle aquifer, and 28 structures and may reach 100 % in the Menzel Chaker from the Sfax deep aquifer. Recent recharge of the region. The isotopic mass balance model also indicates that groundwater in the Sfax aquifer was identified using tri- the middle groundwater aquifer may contribute up to tium concentrations in 82 groundwater samples from dif- 100 % of the shallow Plio-Quaternary aquifer, particularly ferent depths. The isotopic ratios of the shallow aquifer in the western and northeastern parts of the study area, range from -5.55 to -1.59 % for d18O and from -38.38 between Bir Ali ben Khalifa and Djebeniana. The tritium to -14.19 % for d2H, and the isotopic ratios in the middle data support the existence of recent recharge. The tritium aquifer range from -6.86 to -2.97 % for d18O and from and stable isotope data clearly indicate the presence of -44.18 to -22.38 % for d2H. The deep aquifer exhibited mixing processes, especially in the northwestern and markedly lower isotopic values, ranging from -6.70 to coastal portions of the study area. A conceptual model is established, explaining the pressure differences that gen- erate vertical leakage, which is a reasonable mechanism for & Rahma Ayadi flow between the aquifers. [email protected] Kamel Zouari Keywords Isotopes Á Aquifer Á Leakage Á Recharge Á [email protected] Balance Á Sfax Hakim Saibi [email protected] Rim Trabelsi Introduction [email protected] Hafedh Khanfir Stable isotopes in water have long been used as ground- khanfi[email protected] water tracers (Clark and Fritz 1997) to identify possible Ryuichi Itoi recharge areas and mixing within aquifer systems. The [email protected] identification of recharge sources in semi-arid areas pro- vides insights into recharge processes that are required to 1 Laboratory of Radio-Analysis and Environment, National School of Engineers of Sfax, Sfax University, BP 1173, develop sustainable water resource management plans 3038 Sfax, Tunisia within the context of climate variability (Scanlon et al. 2 Department of Earth Resources Engineering, Faculty of 2006). Stable and radiogenic isotopes combined with Engineering, Kyushu University, Fukuoka, Japan groundwater chemistry can provide helpful tracers for 3 Water Resources Division of Sfax, Agriculture Ministry, identifying the recharge sources of a groundwater system Sfax, Tunisia on both local and regional scales (Edmunds and Tyler 123 636 Page 2 of 21 Environ Earth Sci (2016) 75:636 2002). The study of isotopes is an important complemen- and shallow aquifers (Maliki et al. 2000) has been con- tary tool in the evaluation of hydrogeological and hydro- firmed by several previous isotopic studies, and the rate of chemical processes that affect water masses, such as leakage has been computed. However, for the first time, evaporation and mixing in any groundwater system (Tijani this paper computes the rates between the deep, shallow et al. 1996). In this way, isotopic tracers have been used to and recently identified exploited middle aquifer. The pre- highlight the origins of water and residence times (Geyh vious quantifications will be updated with respect to the 2000). In Tunisia, particularly in the south of the country, addition of the middle aquifer. groundwater is the main water resource and is used mainly In the present study, the Mio-Plio-Quaternary aquifer by agricultural and domestic sectors. The Sfax region, system of the Sfax basin was the subject of an isotopic located on the east coast of the country, has a groundwater study, which uses a set of stable isotope tracers (d18O, d2H) aquifer system featuring a deep confined aquifer, a middle and tritium isotope (3H) to identify the origins of the aquifer and shallow aquifers that are delimited by their groundwaters in the Sfax basin. The objectives were to (1) respective catchment areas. Recently, water management identify the origin of groundwaters of the Sfax aquifer authorities have been facing problems of declining water system, (2) estimate the mixing proportions between dif- quality and increasing water requirements due to a rapidly ferent aquifers using stable isotopes and (3) verify the increasing population and the expansion of agricultural occurrence of recent recharge using tritium isotopes in activity. The arid climate coupled with intensive exploita- terms of recent versus old waters. tion of groundwater resources is leading to water resource deficits and groundwater quality degradation (Bouchaou et al. 2008; Yangui et al. 2010). The groundwater of the Study area shallow aquifer is over-exploited, with approximately 54.45 Mm3 of water pumped from this aquifer in 2014, The Sfax region, located on the eastern coast of Tunisia, is corresponding to a deficit of 21.32 Mm3 and resulting in the second largest urban area after Tunis (1.2 million seawater intrusion. Most of the water needs in the Sfax inhabitants). The Sfax basin is bordered to the east by the basin are supplied by the Miocene deep aquifer, from Mediterranean Sea, the N–S axis (J. Gouleb 736 m, J. which approximately 10 Mm3 of water has been pumped Zebbouz 541 m, J. Boudinar 716 m, J. Goubrar 622 m, J. annually from this aquifer between 1978 and 1986. Krechem el Artsouma 655 m) to the west, the J. Korj, J. Bou Between 1987 and 2000, the annual amount of extracted Thadi, and J. Chorbane to the north and Skhira in the south water has increased to 26 Mm3. The exploitation of the (Fig. 1). The present study area is characterized by semi-arid deep aquifer reached 112.55 % in 2014. Previous hydro- climatic conditions with an average annual precipitation of geological studies of this region (El Batti and Andrieux 239 mm (I.N.M. Sfax 2011) and an evapotranspiration of 1977; Beni Akhy 1994; Maliki 2000; Fedrigoni et al. 2001; approximately 1829 mm/year in 2011 (I.N.M. Sfax 2011). Trabelsi et al. 2005) have shown that the Sfax plain con- tains two main aquifers: a shallow aquifer (Plio-Quater- nary) overlying a deep aquifer (Miocene). However, recent Geology and hydrogeology studies (Gassara and Ben Marzouk 2009; Hchaichi 2008; Hchaichi et al. 2013) have proved the existence of a middle The geology of the area was described by Castany (1953), aquifer in the detrital Mio–Pliocene deposits. The renew- Burollet (1956) and Zebidi (1989). The Sahel in the Sfax able water resources of this middle aquifer were estimated area is characterized by a repetitive topography of small to be 11.3 Mm3 using the Darcy equation accented hills, separated by wide basins occupied by sab- (Q = L 9 T 9 i)(T = transmissivity of 1.23 9 10-3 m2/ khas. The geology of the study area is dominated by out- s; L = length of the groundwater flow front; i = hydraulic crops of Mio-Pliocene and Quaternary deposits. Most of gradient). The Sfax aquifer system constitutes the main the outcrops are affected by the major tectonic phases that water resource in southeastern Tunisia. Intensive have occurred in the region (Ben Akacha 2001). The study exploitation of the aquifer in recent years, due to the area contains long-wavelength anticlines with relief of less continuous population and economic growth in Sfax than 200 m (Belgacem et al. 2010). The lithology includes region, has induced declining water levels and progressive the Souar Formation, considered to be of Eocene age and degradation of groundwater quality due to salinization. To composed of marine sediments (Bouaziz 1994). The Oli- implement efficient management of these groundwater gocene sediments feature a lower marine unit and an upper resources under heavy anthropogenic stress, quantitative continental sandy unit. The Miocene deposits are thick and information on the dynamics, origin and regional mixing feature alternating units of clay, sand and sandstone. These patterns of the groundwater in the Sfax aquifer are needed. deposits are divided into three units (Tayech 1984): the Ain The mixing of waters via upward leakage between deep Ghrab Formation (Burdigalian), consisting primarily of 123 Environ Earth Sci (2016) 75:636 Page 3 of 21 636 Fig. 1 Location and geological map of the study area (from the geological map of Tunisia at 1/500,000) limestone interbedded with gypsum in the lower part; the Shallow aquifer Oum Douil Formation (Langian to Tortonian), consisting of a variable proportion of silt and clay; and the Segui The shallow aquifer is a phreatic superficial unit located in Formation (Messinian), consisting of alternating conti- the Quaternary and Mio–Pliocene deposits, comprising nental sand, silt and clay. Pliocene marl deposits discor- sand and silty clay layers that are separated by sandy-clays dantly overlie the older formations. The Pleistocene is (Hajjem 1980; Maliki 2000). The aquifer‘s thickness varies divided into two units: a lower one, characterized by cal- from 8 to 60 m, with an average of 30 m.