IAHS2022-56, updated on 22 Sep 2022
https://doi.org/10.5194/iahs2022-56
IAHS-AISH Scientific Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Youcef Hakimi1, Philippe Orban1, Pierre Deschamps2, and Serge Brouyère1
Youcef Hakimi et al.
  • 1University of Liège, ArGEnCo, Belgium (y.hakimi@uliege.be)
  • 2Aix Marseille Univ, CEREGE, Aix-en-Provence, France

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.

How to cite: Hakimi, Y., Orban, P., Deschamps, P., and Brouyère, S.: 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, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-56, https://doi.org/10.5194/iahs2022-56, 2022.