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We present here the first detailed hydrogeochemical study about Lake Qarun. It is a closed, saline, and alkaline lake located in the North African Sahara Desert. It has no outflow except by evaporation. This lake is the deepest area in the Fayoum Depression with elevation 43 m below sea level. In this area, Nile River is the main source of water and Lake Qarun acts as the main reservoir of all drainage waters. Along the flow path of water, the salinity of water increases with increasing proximity to Lake Qarun and the water chemistry has developed from Ca-Mg-HCO3 at head waters to Na-Cl-SO4 in low lands and in Lake Qarun. The main processes that control the water chemistry in the studied area are dissolution of soluble salts along with continuous evapoconcentration. The progressive evaporation of drainage water inflow has increased the concentrations of Na, Mg, Cl, and SO4 in Lake Qarun water, while Ca and HCO3 have been depleted through CaCO3 precipitation. This is confirmed by the application of Hardie and Eugster’s model parallel with a PHREEQC simulated evaporation model. Both models demonstrated that the evolution of lake water during evaporation should reach the final composition of "Na-Mg-SO4-Cl." Oxygen isotope (delta O-18) values of the studied water samples showed a strong positive correlation with electrical conductivity values supporting the effect of evapoconcentration process on the evolution of the lake brine. This study presented an integrated geochemical approach that can help in understanding similar cases studies in arid environments.