Characterization of groundwater in the arid Zenaga plain: Hydrochemical and environmental isotopes approaches

The Zenaga inlier, located in an arid area in southeastern Morocco, is mostly characterized by igneous and metamorphic fractured rocks. The surface water is quasi-absent and local populations (95% rural) are entirely dependent on groundwater resources for drinking and irrigation purposes. To evaluate the main factors controlling the origin and salinization of groundwater resources, we have investigated the chemical and stable isotope compositions of 23 groundwater samples. In addition, water quality assessment parameters including sodium absorption ratio (SAR) and water quality index (WQI) were also calculated. The results show that the majority of wells and boreholes are characterized calcium bicarbonate water type (78% of samples), with the following ionic sequence Ca²⁺>Na⁺ >Mg²⁺ >K⁺, and HCO₃⁻> Cl⁻>SO₄²⁻>NO₃⁻ for cations and anions, respectively. A significant enrichment on major ions is steadily rising with depth as the water type changes toward calcium chloride. The interpretation of hydrochemical data suggests that the silicate weathering, cation exchange and evaporites dissolution are responsible for the groundwater chemistry. The majority of water samples are good to permissible for irrigation and poor to good for drinking purposes. The mineral salts found at greater depths degrade groundwater quality. In some areas, nitrate contents imply anthropogenic contamination derived especially from traditional septic tanks. Stable isotopes δ¹⁸O and δ²H reveal a distinction between the shallow aquifer and deep circulations. The uncommon depleted isotope signature found in deep circulations was associated with cold altitudes recharge, whereas the heaviest groundwater observed in shallow aquifer was interpreted as the contribution of surface water from Taghdout Dam. The Anti-Atlas Mountains and Siroua Massif's high altitude served as a source of recharge. The mixing at considerable depths and along the groundwater path appears to decrease the isotopic variability in the shallow and deep groundwater.