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The Nubian sandstone aquifer is one of the major aquifers in Egypt, as it is a very important water resource for different purposes, in addition to its wide distribution through the western desert. The groundwater of Nubian sandstone aquifer in Siwa Oasis suffers from water quality deterioration mainly due to excessive extraction from production wells. This study aims to evaluate the geochemical conditions of Nubian sandstone aquifer in Siwa Oasis. Depths to groundwater surface and some Physiochemical parameters, e. g., water temperature (T degrees C), pH value, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO) and Hydrogen sulfide -H2S were measured in the field. Groundwater samples were collated for chemical analyses of major anions (carbonate - CO32-, bicarbonate - HCO3-, chloride -Cl-, and sulfate - SO42-), major cations (calcium - Ca2+, magnesium - Mg2+, sodium - Na+, and potassium - K+) and iron (Fe). The geochemical model (NETPATH) was applied to estimate the saturation states of the relevant minerals and tendency of the groundwater to dissolve or to precipitate minerals from solution. The results of this study indicated that the groundwater of Nubian sandstone aquifer has a high tendency to precipitate black insoluble iron sulfide, siderite and silica, which adversely affect the public health, and well screens.