Hydrogeochemical characterization of the Complexe Terminal aquifer system in hyper-arid zones: the case of wadi Mya Basin, Algeria

Abstract

The groundwater resources management of the wadi Mya basin, located in the North East of the Algerian Sahara and witnesses a hyper-arid climate, poses salinity problems. In order to identify the mechanisms involved in causing these problems, physicochemical analyses were performed on 114 water samples. The hydrogeological system of Complexe Terminal of the wadi Mya basin is very weakly recharged and contains two highly mineralized aquifer levels. These are the Mio-Pliocene sands aquifer and the underlying Senonian carbonates aquifer. They are separated to a great extent by an intermediate evaporitic Eocene layer. Both aquifers show analogous hydrochemical facies indicating, thus, intercommunication between them. The overall mineralization is dominated by chloride sulfate, sodium and magnesium facies and is explained by the dissolution of the evaporitic components of the intercalations in the aquifer matrix, but also by the old character (fossil) of water and cationic exchange reactions resulting from water-rock interactions over time. The salinity variation exhibit a trend that conform to the groundwater flow direction. Vertical transfer of water, sometimes saltier downward from the phreatic aquifer, sometimes softer upward from the deep Albian sandstone aquifer, was noticed. Areas of low mineralization were also identified in the western edge of the basin, where wadi’s floods, which drain the eastern side of the M’zab dorsal, are wide spread.