Identification and characterisation of hydrogeological relays of continental intercalaire aquifer of southern Tunisia

Abstract

In southern Tunisia, the major aquifer system is characterised by the superposition of two principal levels; the “complex terminal” (CT) which forms the surface aquifer and the “continental intercalaire” (CI) which constitutes the deep aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics, lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills. A study of the CI potentiometric map shows three principal flow directions. These flow directions converge at the Sill Zone. The total mineralisation evolution shows an increase from the periphery of the basin to the discharge area. Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The dissolution of these evaporitic rocks is confirmed by mineral saturation indices, which show an under-saturation of water samples with respect to the mentioned minerals. Moreover, the insulation of certain compartments of the reservoir and intercommunications between the aquifer levels seem to play a principal role in the deterioration of chemical quality of water. In this study, the isotopic tools have been applied to understand the hydrodynamic functioning in the region. These techniques have confirmed the results of the hydrogeologic and hydrochemical studies concerning the interconnection between the different aquifers. They indicate also the recent local recharge of the CI reservoir throughout Cretaceous outcrops, and the older origin of all groundwater in the aquifer system.