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^–, SO₄^2– and Na⁺, respectively. Alkali ions, Cl^– and SO₄^2– are acquired through the dissolution of Halite (NaCl) and Gypsum (CaSO₄, 2H₂O). CI waters have depleted <i>δ</i> 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 <i>δ</i>¹³C 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 <i>δ</i>¹³C of –10 ‰. This means that these waters are derived from old precipitation whose features were totally different from the very scarce prevailing ones.