Interconnected multi-layer aquifer with evaporitic fossil waters in Chott-El-Gharbi endorheic basin (Western high plateaus, Algeria): Hydrochemistry, environmental and strontium isotopes

Abstract

Wetland are of paramount importance for the entire province of Nâama as well as for the entire Algerian Western High Plateaus. One of them, Chott El Gharbi, is a major but poorly-known hydrogeological unit shared between Algeria (80%) and Morocco (20%). Chott El-Gharbi is a multi-layered aquifer system whose most productive layers are the Middle Jurassic (Bajocian-Bathonian dolomite and limestone) and the Cenozoic continental sediments (lacustrine intercalation of clays and limestones, marls and sands). Groundwaters exhibit depleted δ18O (−8.98 to −5.53‰) and δ2H (−69.4 to −48.0‰) isotope ratios linked to an evaporative process. Groundwaters are old, with 14C activities from 2 pmC to 58 pmC and residence times of 23 ky to 9 ky. These old recharges occurred during a more humid and colder epoch. 87Sr/86Sr ratios, coupled with major element abundances, determine that Gharbi waters resulted from the mixing of waters having been in contact with the Triassic evaporite-rich sequence (0.70810, rich in sulfates) and waters in contact with Lago Mare (late Miocene) sequence (0.70875, less rich in sulfates). Water with Triassic signature is present in the Jurassic aquifer (Bathonian-Bajocian) while water with Lago Mare or mixed signature is present in the Miocene, locally along faults in the Jurassic aquifer. 14C ages indicate that the mixing process occurred at c. 10,000 years along faults probably during an active tectonic period. Waters with modern signature are only known outside the endorheic Gharbi basin in shallow Jurassic aquifers. The Gharbi aquifer system is separated from the large Chergui aquifer system to the east.