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)

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 identified by hydrogeologic data, one, shallower, flowing 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/SO4 ratios and concentrations of Li, Sr, and Mn, that a hydraulic connection exists between two distinctive aquifers. One strongly influenced by Lake Saadine water characterised by Na/Cl > 1, high Cl/SO4 and Li, Sr, and Mn > 0.1 meq/l, the other, deeper, characterised by Na/Cl < 1, low and near constant Cl/SO4 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.