Hydrochemical Diversity of a Large Alluvial Aquifer in an Arid Zone (Draa River, S Morocco)

In arid areas, with rivers functioning episodically, alluvial resources are the main source of water. Considering the intensified regulation of discharge in montane catchments, supplying the intermittent rivers, in the nearest future alluvial aquifers will gain key importance for the functioning of people in arid zones. The research aimed to investigate the diversified chemistry of alluvial waters typical of large intermittent river valleys in hot arid zones as well as to analyse processes determining the water chemistry and affecting its diversity. The detailed study, carried out in October 2015, covered the Draa river valley (1100 km total length) in the region of the Mhamid Oasis. The examined water was sampled from all wells found in the study area. Concentrations of the main cations: Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, and Li⁺, anions: Cl⁻, SO₄²⁻, HCO₃⁻, and NO₃⁻, as well as trace elements: Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sr, and Zn, were identified. Results were analysed with statistical, hydrochemical, and geochemical modelling methods. Alluvial waters of the eastern and western part of the oasis differed in concentrations of numerous components, what resulted from the regulation of irrigation. Specific electrical conductivity showed a 3.5-fold increase, from 3800 to 13800 μS/cm, consistent with the direction of water flow in the oasis, from east to west. Even a greater rise was observed for ions: Cl⁻ (6x), Na⁺ (5.5x), Mg²⁺ (5.0x), Ca²⁺, and SO₄²⁻ (3.5x). Such a composition indicated multiionic hydrochemical type of waters dominated by Na⁺ and Cl⁻. Additionally, high Pearson correlation coefficients were recorded for Na⁺ and Cl⁻ (0.98) as well as Mg²⁺ and Cl⁻ (0.97). The saturation index suggested that the main water components originated from dissolving of minerals such as halite, anhydrite, sylvite, and gypsum. Groundwater chemistry in the Mhamid Oasis was determined mainly by geogenic processes, such as dissolving of evaporates, precipitation of carbonate minerals, and ion exchange.