ORIGINAL ARTICLE Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia) Badia Chulli • Aysen Davraz • Jalila Makni • Mourad Bedir • Hamed Ben Dhia Received: 26 July 2010 / Accepted: 2 July 2011 / Published online: 9 August 2011 Ó Springer-Verlag 2011 Abstract The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 lS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO 4 –Cl waters. They issue from Miocene units which are made up sands and sand- stones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gra- dient which are determined downhole temperature mea- surements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to d 18 O and d 2 H values, all thermal and cold groundwater is of meteoric origin. Keywords Geothermal water Á Geothermometers Á Sfax Basin Á Tunisia Introduction Tunisia has an important thermal potential and is well- known since the Roman period. More than 80 thermal well located in the north and south of the Tunisia and also include 50 hot water springs. The investigated area (Sahel Sfax Basin) is located in eastern Tunisia. Sfax is the second largest city of Tunisia (Fig. 1). It covers the geographical surface ranging between the parallels 34° and 35°30 0 and meridian the 9°30 0 and 11°30 0 . The Sfax Basin which is located in eastern Tunisia is bounded to the east by the Mediterranean Sea, the N–S Axis moun- tain chain to the west, the Korj, Bouthadi, Chorbane, Zeramdine and Djemmel hills to the north and Mezzouna Mountain to the south. A number of Sebkhas (salt plains) are distributed. The study area is characterized by an arid climate with mean annual precipitation of about 250 mm. The Sahel Sfax Basin has shown in recent decades a significant development of agricultural and industrial activities, associated with a high population growth. To meet these needs, the Sfax deep aquifer system, con- taining quite important groundwater reserves, is now in a continuously growing exploitation (Agoubi et al. 2010). Furthermore, the Sfax Basin has also been revealed as a promising target for the thermal waters by means of oil wells and hydrogeological researches (Kouas and Ben Dhia 1990; Ben Dhia et al. 1992; Chulli 2000; Chulli et al. 2002). To assess hydrogeothermal resources of the Sfax Basin, hydrogeothermal study comprising geological, hydrogeo- logical, hydrogeochemical and geothermometric B. Chulli Á M. Bedir Water Researches and Technologies Center, Borj-Cedria Technopark, Route Touristique Soliman, BP 273, 8020 Soliman, Tunisia e-mail: bchoulli@yahoo.fr A. Davraz (&) Department of Geological Engineering, Suleyman Demirel University, Isparta, Turkey e-mail: adavraz@mmf.sdu.edu.tr J. Makni Á H. Ben Dhia Laboratory 3E, National School of Engineering, Sfax, BP W 3038, Sfax, Tunisia 123 Environ Earth Sci (2012) 66:1–16 DOI 10.1007/s12665-011-1281-8