Hydrochemical and isotopic characteristics of groundwater in the Continental Intercalaire aquifer system: Insights from Mzab Ridge and surrounding regions, North of the Algerian Sahara

Study region: North of Sahara, Algeria. Study focus: The effects of water/rock interactions and hydrodynamic conditions on the characteristics of groundwater in the Continental Intercalaire aquifer (CI) are demonstrated using hydrochemical and isotopic data from the Mzab Ridge. New hydrological insights: The results confirm that CI groundwater in the oriental basin is highly mineralized (1010 > EC > 3500 mu S/cm) and is of the SO4-Cl-Na type. However, it is poorly mineralized (340 > EC > 960) in the occidental basin and predominantly belongs to the HCO3-Ca type. The important contribution of evaporite minerals has been deduced using saturation indices and minor element ratios of Sr/Ca and Br/Cl. This is explained by lithological heterogeneities within the CI aquifer and corroborates the observations from regional piezometric features which suggest a groundwater divide located between 31.55 degrees and 31.57 degrees N latitude. delta O-18-NO3 and delta N-15-NO3 analyses show that NO3 has a predominantly natural origin, which is soil nitrification even for the high concentrations exceeding 50 mg/L. CI groundwaters are highly depleted in both delta O-18 and delta N-15 compared to modern rainfall isotopic signatures. The most depleted ones are those situated in the Great Oriental Erg sub-basin. Isotopic values as low as delta O-18= 6 parts per thousand can be taken as the limit for groundwater that is typically old. In the El Golea region, isotopic enrichment seems to indicate mixing between old groundwater and evaporation-affected groundwater infiltrating through the dunes of the Great Occidental Erg.

Automated summary

This study focuses on the effects of water/rock interactions and hydrodynamic conditions on groundwater characteristics in the Continental Intercalaire aquifer in North of Sahara, Algeria. Results show that groundwater in the oriental basin is highly mineralized, while in the occidental basin it is poorly mineralized, with differences attributed to evaporite minerals. Isotopic analyses suggest a predominantly natural origin of NO3 and the presence of old groundwater mixing with evaporated groundwater in certain regions.