Deep and fossil aquifers in Morocco: A multidisciplinary assessment of groundwater dynamics and salinization

In Morocco, groundwater serves as a strategic water resource for drinking, domestic, agriculture, and industrial purposes due to the scarcity of surface water and high vulnerability to climate change. This dependency stems from the arid to semi-arid climate conditions where ecosystems depend on groundwater. Successive droughts, population growth, and economic development, particularly in agriculture, lead to the overexploitation of groundwater, especially shallow aquifers during the three last decades. This situation necessitates an investigation of these deep aquifers to manage this essential resource effectively. Hence, the aim of this study is to provide insights into deep aquifers, including their geometry, hydrodynamic, hydrochemical characteristics, recharge origins, geothermal potential, and residence times to determine which aquifers take longer to recharge and which should remain unexploited. The investigated deep aquifers exhibit a wide range of salinity, with electrical conductivity values ranging from 231 to 49040 μs/cm, whereas pH values range from 5.64 to 10.76. The high salinity is primarily attributed to Cl - , SO 4 2- , Na + and HCO 3 - with average concentrations of 829, 475, 463, and 316 mg/L, respectively. The recharge of the majority studied deep aquifers, particularly the Cretaceous ones (Souss, Essaouira, Ouarzazate, Tadla, Haouz, and Errachidia), is mainly from the High Atlas Mountains, except for the deep aquifers of Sahara where recharge is carried out during a specific context. The deep Jurassic aquifers, including the South Rifain Corridor, High and Middle Moulouya, High Plateaus, Horst Chain, Taourirt-Oujda Corridor, Beni-Bouyahi, and Bni-Znassen aquifers, are recharged from the Middle Atlas and North-Eastern Moroccan Mountains. Most investigated deep aquifers containing old waters are considered fossil or less renewable, with residence times exceeding 20 kiloannum before present (ka BP). The deep Sahara aquifers are the most significant groundwater potential reserves due to their lithologic composition, and their extensive area of approximately 90 000 km 2 , but with very low groundwater replenishment rates dating back over 46.5 ka BP according to radiocarbon dating. Because of their variable mineralisation related to their variable geological contexts and human activities, all the investigated systems show a high vulnerability mainly within their outcrops, which are mainly karstic or fissured areas. These deep aquifers require more stringent management and protection due to arid climate and very low precipitation through all the investigated regions. The results provide indicators about the functioning of the Moroccan deep aquifers and could guide decision-makers and policies for highlighting the need for careful management and conservation. • The deep Liassic and Turonian aquifers are the most important reservoirs in Morocco. • Many deep Moroccan aquifers are primarily recharged from the Atlas Mountains. • Most deep aquifers are fossil aquifers, with residence times exceeding 20,000 years. • Several deep aquifers are predominantly characterized by a Ca-Mg-HCO 3 and Na-Cl chemical composition.