Groundwater flowpaths and residence times inferred by 14C, 36Cl and 4He isotopes in the Continental Intercalaire aquifer (North-Western Africa)

• Published in: Journal of hydrology (Amsterdam) 2018-05, Vol.560, p.11-23
• Main Authors: Petersen, J.O., Deschamps, P., Hamelin, B., Fourré, E., Gonçalvès, J., Zouari, K., Guendouz, A., Michelot, J.-L., Massault, M., Dapoigny, A., Team, Aster
• Format: Article
• Language: English
• Subjects: Continental-scale aquifer; Groundwater budget; Radioactive isotopes; Recharge; Stables isotopes; Tunisia and Algeria
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2018.03.003

•We demonstrate a general agreement between hydrogeological and multi-isotope models.•Our combination of tracers successfully characterises a complex system.•We identify different recharge rates across the CI using mass balance and 14C.•We close the mass balance for stable isotopes and chloride across the CI.•We show low 14C activities obtained with improved methodology can be trusted. In a semi-arid to arid climate context, dependency on groundwater resources may lead to overexploitation and deterioration of water quality. The Continental Intercalaire (CI) aquifer is one such continental-scale aquifer (more than a million of km2), which is mainly confined, poorly recharged but intensely abstracted. To date, the management of this resource relies on hydrogeological modelling and key parameters such as recharge/discharge rate and groundwater dynamics. We use a combination of residence time indicators (14C, 36Cl, 4He) and stable isotopes of water (2H and 18O) to give greater constraint on the groundwater residence time in the CI. In previous studies, 14C measurements and steady state modelling indicate a residence time of less than 100 ka whereas in others, 36Cl measurements and transient scenarios modelling suggest a longer residence time (>500 ka). In this study, most of the 14C measurements are below the limit of detection, establishing residence times greater than 40 ka and confirming the necessity of strict sampling protocols to exclude all air and AMS measurements when low 14C concentrations are expected. In the Tunisian recharge area, detectable 14C indicate sporadic recharge episodes (3–7 ka and 29–43 ka), whereas 4He and 36Cl concentrations in central areas suggest very old (