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The Midyan Basin is characterized by arid climate with rare surface water flow. Groundwater is the main water resources in this region. In order to evaluate their suitability for drinking, domestic and irrigation purposes, chemical characteristics of groundwater in the Midyan Basin (northwestern Saudi Arabia) have been investigated and evaluated. A total of 72 water samples were collected from different wells and analyzed for hydrogen ion concentration, total dissolved solids, electrical conductivity, total hardness, Ca2+, Na+, Mg2+, K+, Cl-, HCO3-, NO3- and SO42-. To understand quality of groundwater and their suitability, chemical indices like sodium percentage, sodium adsorption ratio and salinity values have been calculated using analytical techniques. From traditional Piper diagram for water classification, the water quality is placed at Na+-Ca2+-SO42--C1- type. According to the results of electrical conductivity and sodium adsorption ratio, 37% of the studied samples are regarded as highly saline, while 63% of them are classified as very highly saline. As per the Arab Gulf and international standards, such waters are not suitable for irrigation under normal condition and further action is required to remediate such problem by salinity control. Sodium content in 24% of the studied samples is regarded as high that can’t be used for irrigation purposes for any soils. Thus high level of salinity, sodium adsorption ratio and sodium percentage in most water samples have made them unsuitable