Hydrochemical and geoelectrical investigation of Marine Jeffara Aquifer, southeastern Tunisia

Belgacem Agoubi 0 1 2 Adel Kharroubi 0 1 2 Tarek Abichou 0 1 2 Habib Abida 0 1 2 0 T. Abichou Department of Civil and Environmental Engineering, Florida State University , 2525 Pottsdamer street, Tallahassee, FL 32311, USA 1 B. Agoubi (&) A. Kharroubi UR: Applied Hydro-Sciencces, Higher Institute of Water Sciences and Techniques, University of Gabes , Gabes, Tunisia 2 H. Abida Department of Earth Sciences, Faculty of Sciences, University of Sfax , Sfax 4000, Tunisia Hydrogeochemical and geophysical investigation were carried out to assess the geochemical water composition and processes governing groundwater hydrochemistry in Marine Jeffara Aquifer, southeastern Tunisia. A total of 74 groundwater samples were collected and analyzed for various parameters such as pH, electrical conductivity and major ions. Statistical and geochemical modeling were performed to understand the processes controlling groundwater geochemistry. According to their dominance, major ions are classified as follows Cl [ Na [ SO4 [ Ca [ Mg [ HCO3. Hydrochemical, Na-Ca-ClSO4, Ca-Na-Mg-Cl-SO4 and Mg-Ca-Cl-SO4 are the dominant forms in groundwater, mainly as a result of rockwater interaction and saltwater intrusion. Ten vertical electrical resistivity tests were performed in three profiles. The measured low values of electrical resistivity in the coastal areas indicate saltwater mixing with groundwater, as a result of saltwater intrusion. - Introduction Fig. 1 Geological map and location of study area (Afer, Jedoui 2000) Well nb. Mesure in situ CE ms/cm 25 C TDS (g/l) Major ions (Meq/l) Table 1 continued Well nb. Fig. 3 Piper diagram describing groundwater quality Mesure in situ CE ms/cm 25 C TDS (g/l) Major ions (Meq/l) Chemical facies (%) Chlorinated calcium and magnesium sulfate to sodium CaNaMgClSO4 Magnesium sulfate to calcium MgCaClSO4 Chlorinated and sulfated sodium calcium and magnesium with dominance of NaCaClSO4 Table 2 Chemical facies of groundwater from marine Jeffara Water percentage Water type CaMgClSO4 CaMgNaSO4Cl CaNaClSO4 CaNaMgClSO4 CaNaSO4Cl MgCaSO4 MgNaCaSO4Cl MgNaClSO4 MgNaSO4Cl HCO3 Agoubi et al. 2012). Hydrogeochemical modeling, as well as statistical and variographic analysis were most used for aquifer investigation and saltwater intrusion. The directcurrent resistivity method was also widely used in groundwater investigation and saltwater intrusion due to a rapid advance in microprocessors and numerical modeling techniques (Ahmad et al 1988, Mazac et al 1990; Yang and Lee 2002; Bonomo et al 2002; Kumar et al. 2007; Al-saigh 2010). In this study, statistical and geostatistical analysis, geochemical modeling and geoelectrical methods were used to investigate groundwater parameters and the main processes controlling groundwater salinity and potential saltwater intrusion in coastal areas of the Marine Jeffara aquifer. Study area varies between 43 and 352 mm, with an annual average of 200 mm (OSS, 2010). Humidity ranges from 43 to 84 % and evaporation rates exceeds 1,700 mm/year. The region is characterized by an intermittent flow regime because of long drought periods and the aforementioned limited surface water resources. Groundwater constitutes the main water resource required to satisfy the different consumption needs of the area (Zouari et al. 2010; Agoubi et al. 2012). The specific study area is a low-altitude plate (435 m) covered by Mio-Plio-quaternary deposits. The overall structure is characterized by faults with small discharges, organized in parallel bands of northwest-southeast orientation (Bouaziz 1995; Jedoui 2000) (Fig. 2). This MioPliocene series fills the Marine Jeffara, whose thickness increases towards the northeast, eventually reaching several hundred meters. The outcrops of these deposits are usually found at the foot of cliffs that border depressions (Agoubi et al. 2012). These deposits come in different facies: red clays, sometimes silty; gypsum crystals that often form the top of the series; fine yellow sand; and conglomerates or sandstone outcrops that often overcome the red clay gypsum. The Marine Jeffara aquifer is lenticular. Groundwater is housed in the alternations of Mio-Plio-quaternary sand and sandy clays. Its spatial configuration reflects the appearance of horst and graben (Fig. 2). The thickness of the layer increases from west to east and plunges beneath the graben of Ajim, where it exceeds 50 m. In 1985, the total number of shallow wells was 637, distributed throughout the Marine Jeffara. This number has continued to increase, with 1,365 shallow wells present in 2010 (ODS, 2010). This evolution is mainly due to the expansion of irrigated areas and easy access to these water resources. Materials and methods Fig. 4 Relationship between TDS and major ions a anions, b cations, and c Na vs Cl Fig. 5 Relation between (Ca ? Mg ? Na ? K) and Cl ? SO4 understand the mechanisms of fault contribution to saltwater intrusion in the shallow aquifer. Water sampling and chemical analysis Hydrochemistry Statistical and variographic analysis ch 2N1h XNih1 Zxi Zxi h Where c(h) is the estimated value of the semivariance for lag h; N(h) is the number of experimental pairs separated by vector h, and Z(xi) and Z(xi?h) are values of the variable Z at xi and xi?h, respectively. The experimental semivariograms were fitted to various theoretical models generally used in earth sciences, such as spherical, exponential, and Gaussian models (Mehrjardi et al. 2009). The theoretical model yielding the minimum standard error was subsequently chosen for further analysis. Nugget/sill ratio was used to express the extent of spatial autocorrelations of environmental factors. If the ratio is low (\25 %), the variable has strong spatial autocorrelations at a regional scale. A high ratio of nugget effect ([75 %) plays an important role in spatial heterogeneity (Hani and Karimineja 2010). Direct current method Direct current method utilizes four electrodes arranged in a straight line on the ground surface. The Schlumberger array is one of the most commonly used for vertical electrical sounding (VES). Four electrodes (AMNB) are placed along a straight line on the ground surface and the apparent resistivity is computed using Eq. 2. with K IPI2Win version 3.0.1a developed by Moscow State University (Bobachev 2003). Results and discussion Groundwater hydrochemistry A Piper diagram was used to represent several water samples simultaneously, consisting of two triangles to represent different water facieses. Groundwater samples analyzed data were plotted on the Piper diagram (Fig. 3), using the DIAGRAMS software to identify chemical trends in groundwater (Simler 2009). These results show a different chemical composition from one area to another, indicating varying sources of mineralization. The abundance of Na and Cl in the Jorf region indicates that potential seawater intrusion may be the regions origin of mineralization. Sodium, chloride, magnesium and sulfate (...truncated)