Vol.:(0123456789) 1 3 Environmental Earth Sciences (2019) 78:346 https://doi.org/10.1007/s12665-019-8341-x ORIGINAL ARTICLE Hydrogeologic and geochemical survey of aquifers based on chemical and isotopic characterisation of groundwater and rain waters: a case study in the Sisseb el Alem Basin (central‑east Tunisia) E. Gagliano Candela 1  · G. Pecoraino 1  · R. Favara 1  · S. Morici 1 Received: 11 February 2019 / Accepted: 21 May 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Chemical and isotopic composition of groundwaters and rain waters in the Sisseb el Alem basin (Tunisia) was monitored to obtain a hydrologic and geochemical characterization of the aquifers. The studied aquifers are the only water resource for domestic and irrigation use in the basin. Two aquifers have been identifed by hydrogeologic data, one, shallower, fowing in Plio-Quaternary sediments, the other, deeper, in Miocenic–Oligocenic sediments. Although this area is intensively farmed with widespread fertiliser use and settlements are unsewered, groundwater in the area is relatively uncontaminated by land- use activities. The chemical and isotopic compositions of the sampled waters, which have a meteoric origin, are largely controlled by both the geological signatures of their drainage area and by evaporative processes and water–rock interaction (reverse cationic exchange) phenomena. Geochemical data suggest, on the basis of the Na/Cl, Cl/SO 4 ratios and concentra- tions of Li, Sr, and Mn, that a hydraulic connection exists between two distinctive aquifers. One strongly infuenced by Lake Saadine water characterised by Na/Cl > 1, high Cl/SO 4 and Li, Sr, and Mn > 0.1 meq/l, the other, deeper, characterised by Na/Cl < 1, low and near constant Cl/SO 4 and Li, Sr, and Mn < 0.1 meq/l. Despite relatively high salinity and SAR values, all sampled waters are suitable for irrigation in well-draining soils. The isotopic composition of the rain waters highlights the importance of evaporation in controlling the composition of the groundwater. Keywords Hydrogeochemistry · Stable isotopes · Sisseb el Alem Basin (Tunisia) · Water budget · Groundwater · Rain water Introduction The Sisseb el Alem basin is one of the most important aquifers in Tunisia. However, the recent large considerable increase in water requirements due to demographic and agriculture development has caused an overexploitation of the groundwater resource. Consequently, the elevation of the water table in the region is declining at a rate of about 1.5 m/a (DGRE 2004) which has led to the drying up of many wells in the region. The overexploitation has caused the quality of the groundwater to deteriorate, and has been exacerbated by the building of the great Nebhana dam and many other small hill-dams and by a large increase in the number of wells (more than 800) in northern Tunisia (Hamed and Dhahri 2013). As groundwater is the only water source for potable and agricultural use for local communities in the region, it is important to understand how the local groundwater fow systems function. Geochemical methods are particularly useful tools for understanding groundwater fow processes and can be used to identify processes that alter the quality of the groundwaters during their fow, such as the physi- cal and chemical characteristics of the recharge waters (i.e., meteoric, from artifcial reservoirs, and irrigation), the inter- actions between groundwater and aquifer minerals and pol- lution from human activities. The aim of this study is, therefore, to use geochemical techniques to investigate the hydrogeological, chemical, and isotopic characteristics of the groundwaters in the north- western sector of the Sisseb el Alem basin and to model the circulation and recharge of the groundwater that feed * G. Pecoraino giovannella.pecoraino@ingv.it 1 Istituto Nazionale di Geofsica e Vulcanologia, Sezione di Palermo. Via Ugo La Malfa 153, 90144 Palermo, Italy