The Continental Intercalaire groundwaters of the Tidikelt (In-Salah region, Algeria). Hydrochemical and isotopic features.

The Continental Intercalaire (CI) groundwaters of the Tidikelt (In-Salah region) are characterized by a high salinity and ion concentrations higher than the maximum standards for drinkability. The total dissolved solids range from 850 to 3390 mg L-1 for conductivities ranging from 1470 to 6780 μS cm-1. Their chemical facies is dominated by Cl-, SO42- and Na+, respectively. Alkali ions, Cl- and SO42- are acquired through the dissolution of Halite (NaCl) and Gypsum (CaSO4, 2H2O). CI waters have depleted δ values for 18O and 2H, corresponding to a cold end-member. This is an indication of a very homogeneous aquifer which is similar to what was observed for the CI in the eastern sub-basin (Great Oriental Erg) and for palaeowaters elsewhere in the Middle-East and Libya. Tritium analyses show that these waters are all very weakly tritiated, which is a testimony of the non-renewed character for these waters. Both δ13C and 14C measured on more than a dozen of samples also show that CI groundwaters are old, with ages comprised between 19,000 and 35,000 years with an average δ13C of -10 ‰. This means that these waters are derived from old precipitation whose features were totally different from the very scarce prevailing ones.