Updated geochemical and isotopic data from the Continental Intercalaire aquifer in the Great Occidental Erg sub-basin (south-western Algeria)

Abstract

Due to its size and to its strategic importance as a huge transboundary aquifer, the Continental Intercalaire (C.I.) has been widely studied, making use of the most classical groundwater research approaches as well as the latest techniques. Investigations focused more on the shared parts and neglected the Great Occidental Erg sub-basin occurring exclusively in Algeria. The present research deals with recent hydrochemical and isotope results obtained for that sub-basin. The latter has seen major developments and an increase in water demand during the last two decades. The Occidental sub-basin occurs in an arid region limited by the Atlas Mountains (north), Tademaït plateau (south), Saoura valley (west) and the M’Zab uplift (east). Rainfall is relatively high only in the mountainous recharge area (Saharan Atlas). Rainfall decreases southward with decreasing altitude, from ∼200 mm to 20 mm yearly in the discharge area. The sub-basin is characterized by a dense run-off network (wadis flowing southward from the southern Atlas foothills). The C.I. reservoir is contained in the continental formations of the lower Cretaceous. The aquifer is mostly unconfined, unlike the eastern part. Its depth increases from north to south. It has a shape of a depression that disappears further south where it becomes confined. TDS increases southward along the recognized flowpath. Sample points align parallel to the halite dissolution line on a Cl–Na plot, confirming the origin of those elements. Major elements increase with the increase of TDS. Moreover, a good correlation between calcium and sulfates suggests gypsum and anhydrite leaching as water flows underground. Isotopically speaking, groundwater are enriched and exhibit large variations in stable isotopes (18O and 2H). They are distributed along an evaporation line corresponding to a mixing of two end-members: the Erg aquifer that feeds the C.I. in the north, and unaffected C.I. groundwater in the south. In the west, Saoura valley alluvial aquifer samples align in the continuation of the evaporated C.I. line, mirroring the intermingling of two aquifers during wadi Saoura flooding. Radiocarbon data showed that appreciable amount of 14C in the C.I. is traceable only up to few distances from the aquifer exposure zones up north (Atlas escarpments). The confined part of the reservoir shows depleted stable isotopes data and low 14C content, reflecting the absence of modern water at the discharge zone.