Chemical and Isotopes Indicators of Mixing Between Multilayered Aquifer Systems of Tadla Plain, Morocco

The Tadla plain located in the Oum Er-Rbia River Basin constitutes one of the principal agricultural production areas in Morocco. The groundwater resources are derived from the karst aquifer of the Atlas Mountains and the multilayered system (superficial and deep aquifers) of the Tadla plain. The Turonian constitutes the main productive aquifer in the area. The isotopic composition and concentration of strontium in 43 groundwater samples, combined with solute concentration data, provide important details regarding groundwater geochemical evolution, flow pathways and mixing processes in the Tadla multilayered aquifers. Shallow aquifers are characterized by significantly higher salinity, particularly in irrigated perimeter areas (Beni Amir and Beni Moussa). The relationship between chloride and EC shows a similar correlation for all water groups, except for some Atlas springs with conspicuously lower chloride/EC ratios. Stable isotopes suggest that the waters originated from meteoric water that was infiltrated into the different aquifers through permeable formations without secondary surface evaporation. The strontium isotope ratios of low saline water from the Turonian aquifer and the High Atlas spring waters overlap and show mixing (0.7078–0.7092) between Sr derived from the Turonian age limestone aquifer (0.703) and a higher 87Sr/86Sr source. Two types of chloride saline water were found in all aquifers: (i) low 87Sr/86Sr (0.7078) and Br/Cl (1.7 × 10–4) source that affects groundwater in the Turonian and Senonian aquifers. This type of water is originated from the dissolution of gypsum and halite deposits. (ii) High 87Sr/86Sr (0.7110) and marine Br/Cl (~ 1.5 × 10–3) source that occurs mostly in the Eocene aquifer.