High risk facilities like oil refinery plant represent a source of hazard on the surrounding environment due to its production from gaseous pollutants that emit to air or liquid pollutants that release to surface water and groundwater. All local legislations and environmental management systems recommend that the surrounding environment of such facilities should be covered with a network of monitoring systems among them such as a system for monitoring groundwater to determine the compliance of the facility with legal requirements. In this work, six vertical electrical soundings were used to define the hydrogeological units of the subsurface in the area. The interpretation of VES's proved that the geo-electrical cross sections consist of two main geo-electrical layers. A-layer represents the surface layer. B- Layer consists of three main sub-layers and its characterized by the presence of clay lenses. This layer represents the water-bearing aquifer. The surface layer (A) and sub-layers B1 and B2 belong the Recent and Quaternary eras. While sub-layer B3 belong the Miocene. Based on this result, six well selected locations have been proposed for monitoring groundwater pollution inside the fence of the oil refinery plant.