Paleoclimatic reconstitution from geochemical investigations of the recharge conditions of a deep confined aquifer in an arid region (case of the Continental Intercalaire in Southern Tunisia)

International audienceThe stable isotope contents in the Continental Intercalaire (CI) aquifer throughout the Southern Tunisia (30°10’ – 34°10’ N; 7°30’ – 10°30’ E) reflect differences in paleoclimatic conditions of the recharge scenario from the late Pleistocene to the present. δ18O and δ2H signatures shown in the deeper groundwater of the CI aquifer (Nefzaoua area), typical of old water, have ranges of -8,6 to -7,3‰ vs V-SMOW for δ18O and -67,01 to -50,5‰ vs V-SMOW for δ2H. These depleted δ18O and δ2H signatures are related to humid periods of the Late Pleistocene and Early Holocene. This water age is consistent with the negligible radiocarbon contents. However, the measured stable isotope contents in the shallower groundwater of the CI (Eastern piedmont of Dahar uplands) range from -7,7 to -4,8 ‰ vs V-SMOW for δ18O and from -59.9 to -32.9 ‰ vs V-SMOW for δ2H. Compared with the current annual isotopic precipitation signatures of the nearest stations such as Sfax (-4.60 ‰ vs V-SMOW for δ18O for the period 2012 – 2013) and Kairouan (-4.52 ‰ for δ18O vs V-SMOW from 2008 to 2012), the most enriched samples of the aquifer system could present a process of recent recharge. Tritium activities of these samples showed values of more than 1 TU indicating therefore that the origin of this component comes from recent precipitations on the CI outcrops in the eastern piedmont of Dahar. Also, results from chemical analysis support the evolution of the CI groundwaters from the piedmont of Dahar up to the Chott region. The temperature increases progressively with the deepening of the aquifer which highlights the thermal gradient in accordance to the SE-NW flow path of the aquifer. Groundwater well head temperatures range around 21°C in the Dahar area and 60°C in the Nefzaoua area. The total dissolved solids show values less than 1g/l in the piedmont of Dahar which reflects the infiltration of rain water in the CI outcrops. However, in the region of Nefzaoua they increase up to 5g/l which could be mainly explained by an interaction from the hosting rock enhanced by a long residence time of water in the reservoir