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This study shows that hydrological systems in arid areas where water level data are incomplete can be better understood and constrained by using chemical and isotopic data. These constraints significantly alter the conceptual hydrological model of the Azraq area. Previous studies, based on water level data, suggest that water in the Azraq oasis is derived from two major piezometric highs; the western basalt aquifer to the northwest and the Tulul el Ashaquif highlands to the northeast of the oasis. Water from the Tulul el Ashaquif aquifer is also considered to drain to a piezometric low at Ruwaishid to the east, and then into deeper aquifers. Chemical data from this study show that the northwest basalt aquifer provides most of the water (80‐85 %) to the Azraq oasis. Water from the Tulul el Ashaquif supplies a relatively small proportion, but due to its high solute content, imparts its chemical signature on the oasis water. This study has shown that water from Tulul el Ashaquif are not supplied to the Ruwaishid area, as previously thought. Stable isotopic data show that each piezometric high contains water which originated in a distinct climatic setting. Both water types are poor in tritium. Traces of tritium at the Azraq oasis and at Ruwaishid indicate some recent local recharge.

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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--Cl- 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.

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