Hydrogeochemical and isotope evidence of groundwater evolution in El Guettar oasis area, Southwest Tunisia

Abstract

In southern Tunisia, the major aquifer system is characterized by the superposition of three principal levels; the upper unconfined Quaternary alluvial aquifer; the intermediate semi-confined/Confined Complex Terminal (Segui, Beglia and Zebbag Formations) and the Continental Intercalaire (Bou Dinar and Sidi Aïch Formations) which constitutes the deeper confined aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics (Gafsa and Sehib faults), lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills. A study of the potentiometric maps shows four principal flow directions. These flow directions converge at the Sill Zones (Chott El Guettar and Chott Djerid depressions). The total mineralization evolution shows an increase from the periphery of the basin to the discharge area (1.75 ≤ TDS ≤ 14.4 g/l). Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted δ18O and δ2H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters and the influence of the water dam. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. According to radiocarbon residence time estimates, the deep groundwater is approximately 40 ka, and was recharged during a period when the climate was wetter and colder. Based on radiocarbon content, the shallow groundwater is older than 0.5 ka, and represents modern waters mixed with a limited quality of palaeo-recharge. Moreover, the insulation of certain compartments of the reservoir and inter-communications between the aquifer levels seem to play a principal role in the degradation of quality of groundwater.