The hydrochemistry of major ions and environmental isotope compositions (¹⁸O, ²H) of water samples have been used to identify the chemical characteristics and the origin of groundwater in the Souss upstream basin. The total dissolved solids (TDS) did not exceed 1000 mg.L^-1 with an average of 635 mg.L^-1. Two chemical water types were observed: A calcium and magnesium bicarbonate type (Ca²⁺-Mg²⁺-HCO₃^-), forming the dominant water type generally observed in the plain along the Souss river and towards the piedmont of the Anti-Atlas Mountains; and a calcium sulphate type (Ca²⁺-SO₄^2-), from the northwest of the study area, linked to the presence of phosphatic gypseous marls and limestone marls (Upper Cretaceous). The calculations of saturation indexes versus the main mineral phases have been carried out, using the PHREEQC program. The groundwater is saturated and slightly oversaturated with respect to carbonate minerals and under saturated with respect to evaporite minerals; surface waters show an oversaturation with respect to carbonate mineral phases, mainly dolomite. The groundwater composition is largely controlled by the dissolution of carbonate rocks known in this part of the basin. Stable isotope contents of groundwaters ranged from -7.96 to -6.26‰ for δ¹⁸O and from -49.47 to -39.28‰ for δD. The hydrogen (δD) and oxygen (δ¹⁸O) isotope signatures indicate a low evaporation of precipitations during infiltration and that the aquifer is highly influenced by the contribution of recharge water recharge from the High Atlas Mountains.

Key words: Groundwater, hydrochemistry, stable isotopes, arid climate, Souss, Morocco.