The groundwater quality of Wadi Al Arab aquifer, northwest Jordan, was assessed for its high molybdenum concentrations, which reach 1.62 mg/L. This value of Mo-concentration was far beyond the Jordan Institution for Standards and Metrology (JISM) and World Health Organization (WHO) guidelines. Hydro-geochemical studies were carried out in order to assess the chemistry of the groundwater quality focusing on identifying the hydrogeochemical processes and mechanisms that are involved in the high Mo concentration. The water quality data of Wadi Al Arab boreholes (1986-2018) was collected, and the dataset analyzed. Interpretation of hydrochemical data showed that the anions are dominated by with abundance order of , while the main cation is Ca 2+ , with abundance order of Ca 2+ > Mg 2+ > Na + > K + ; water type is . The reason is the passage of groundwater through carbonate rocks. The distribution of the sampling points in the Gibbs diagram showed that the majority of samples are plotted within the field of rock weathering, which indicates that the main factor controlling dissolved hydrochemical components of the water samples is the dissolution of the rock-forming minerals. The scattered plot of the Mo with other critical heavy metals shows two different trend lines suggesting initiation of molybdenum to groundwater which is probably derived from oil shale and phosphate. The effects of pH and redox potential on Mo mobility in water are reviewed using the paurbaix diagram. The Mo species of the sampled wells are confined to the field of : anoxic/non-sulfidic condition (Fe reduction) which is located in the unconfined part of the aquifer, and which is sulfidic condition (low and high) in the confined aquifer.
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