Identification of hydrogeochemical and stable isotopic groundwater processes in the Laayoune-Dakhla region (Southern Sahara, Morocco)

Abstract

The assessment of major elements and stable isotopes of groundwater in the Laayoune-Dakhla region was carried out in order to determine the various geochemical processes that contribute to the mineralization of these waters. A total of 30 groundwater samples were collected from wells in the Laayoune-Dakhla region (southern Sahara, Morocco). Using a hydrogeochemical assessment technique, this study will look at the general conditions of groundwater for drinking reasons. The statistical results of the major cations (Na+, Ca2+, Mg2+, K+) of the major anions (Cl−, SO42−, HCO3−, NO3−), show that the abundant cations and anions are of the order of Na+ > Ca2+ > Mg2+ > K+ and Cl− > SO42− > HCO3− > NO3−, respectively. The EC value of the water in the study area ranges from 1290 to 6895 µS/cm with an average of 3341.53 µS/cm. The pH value of the water samples is between 6.88 and 7.75. The waters of the study area were determined to be hydrogeochemical facies Na-Cl (86.66%) and Ca-SO4 (13.33). Their chemistry seems to be mainly controlled by sulphate, calcium, chloride and sodium and is explained by the dissolution of the evaporative formations characteristic of the Saharan regions. Ionic reports show that rock weathering and mineral dissolution and evaporation control the chemical evolution of groundwater. In addition, a good correlation between calcium and sulphate suggests leaching of gypsum and anhydrite. The values of δ18O vary from − 6.96 to -8.93‰ while those of δ2H vary between − 51.5 and − 65.56‰. The presence of evaporation was confirmed by stable isotope levels (δ18O, δ2H). The variation in oxygene-18 levels can be interpreted in terms of the difference in altitude between the recharge altitudes.