Review of groundwater age of a transboundary aquifer system using 14C and 36Cl, case of the Continental Intercalaire aquifer in Algerian Sahara and Southern Tunisia

The Continental Intercalaire aquifer (CI) (North Africa) is one of the largest transboundary aquifer systems in the world. It contains around 20 000 × 10⁹ m³ of groundwater. The recharge comes mainly from the occidental part of the aquifer (southern foothills of the Saharan Atlas, Algeria) going east to the outlet area in South Tunisia. Several studies have showed, using ¹⁴C, that the principal recharge has taken place in Late Pleistocene and Holocene. However, doubts exist about the results of some of these studies as it was demonstrated that samples containing less than 10pmc could have been contaminated by modern air during the sampling procedure.

Based on a new field survey and the combination of new ¹⁴C and ³⁶Cl data, the data of the previous studies are reviewed and new insights on the groundwater age are given. Close to recharge area, ¹⁴C age showed a significant amount of modern recharge (less than 2Ky B.P.). Any sample was found to have an age between 2 to 11Ky P.B. This means that during this period, Northern Sahara was dominated by a hyperarid climate. From 11 to 30Ky, it is observed that there is cyclicity of approximately 4Ky between given ages. This indicates that this period is dominated by an arid climate. Those ¹⁴C ages are calculated using F&G model.

Beyond 30 Ky P.B., the use ¹⁴C is critical because of its limited half-time. ³⁶Cl is more relevant in that case (half-time equal to 301Ky). Samples close to recharge area, where ¹⁴C activities are sufficiently high and halite dissolution is not significant, are explored in order to define initial values of ³⁶Cl. There, the initial ³⁶Cl/Cl ratio is around 145 × 10^-15 at/at and the initial chlorine concentration is around 175mg/L. Based on these values, the age of CI groundwater in the Great Oriental Erg basin and South Tunisia has to be 300 to 700Ky old instead of 40 to 50Ky calculated by ¹⁴C. This is supported by the ages calculated by ⁸¹Kr performed in the South Tunisia. However, the applicability limit of each technic has to be investigated.