Central Hoggar groundwaters and the role of shear zones: 87Sr/86Sr, δ18O, δ2H and 14C isotopes, geochemistry and water-rock interactions

Abstract

In an environment of volcanic and plutonic rocks such as the Central Hoggar, the knowledge of groundwater flows and their residence time is of prime importance. This is especially the case in the arid zone of Central Sahara where the main town of southernmost Algeria, Tamanrasset, with its rapidly growing population (>200,000 inhabitants extreme South of Algeria), put an important pressure on the local aquifers. Central Hoggar belongs to the LATEA metacraton, which is a Precambrian basement largely fractured and invaded by Cenozoic mostly basaltic volcanic rocks. In addition to superficial alluvial reservoirs, these two contrasted lithologies and large and deep faults determine the geometry of the aquifers and the nature and composition of water. We show here that distinct aquifers exist along faults oriented from NW-SE to NE-SW with no lateral communications. Some waters are in equilibrium with the Cenozoic volcanic system and others with the Precambrian basement although the latter bear often a Cenozoic volcanic signature attributed to volcanic coating along faults at depth. Alluvial, volcanic and Precambrian aquifers display contrasted measured pCO2 and specific conductivity (SC) values, stable (δ18O, δ2H), 87Sr/86Sr, 3H, δ13C and 14C isotopes. A major role is given to the Pan-African Adriane shear zone (ASZ), running just east of Tamanrasset, along which waters with high residence time (0.2<A14C < 45 pMC) are aligned in a 160–180 km long and 15 km wide corridor, including a natural soda spring (Tabahort). The latter have contrasted geochemical and isotopic signatures with respect to the other groundwaters originated from local evaporated precipitations. 87Sr/86Sr ratios vary from 0.70409 in volcanic aquifers to 0.72281 in Precambrian aquifers while C isotopes exhibit value from 0 to 117% pMC for 14C activity and δ13Cvalue from −12.5 to −2.12‰ vs VPDB. Trace elements are lower than the Maximum Admissible Concentration (MAC) except in the ASZ where Rb, Cs and U are found in higher concentrations elsewhere in Hoggar. This study enhances the peculiar status of the Central Hoggar groundwater aligned along the ASZ, close to the Tamanrasset City.