Using Environmental Isotopes and Krypton-81 to Characterize and Date Continental Intercalaire Paleogroundwater (Southern Tunisia)

In a semi-arid to arid climate region, socio-economic development is mainly dependent on deep groundwater resources. This is the case of the Continental Intercalaire (CI) groundwater which is one of the most important aquifers in the North Western Sahara in Africa. This aquifer system, extending over more than a million of km2, is mainly confined, poorly recharged but intensely abstracted in Southern Tunisia. Efficient management of this resource relies on accurate data such as recharge/discharge rate and groundwater dynamics. In this study, environmental isotopes (2H, 18O, 13C, and 14C) were combined with long time lived radio-nuclide (81Kr) to give greater constraint on the groundwater residence time in the CI. Stable isotope signature is depleted compared to the modern rainwater of Sfax station with very low deuterium excess suggesting a paleoclimatic effect. This finding is strongly supported by 14C measurements where most of the analyzed samples are below the detection limit. The used carbon-14 correction models indicate residence times greater than 35 ka. However, the estimated ages range using 81Kr contents are from 150 to 600 kyr, and are clearly much older than the 14C ages, confirming that this method is not suitable for dating CI groundwater.