Assessment of shallow aquifer salinity in the Aqaba Coastal plain using ERT method: a case study of Maqnah region, northwestern Saudi Arabia

An electrical resistivity tomography survey has been conducted along the eastern coast of the Gulf of Aqaba in order to assess the salinity of the coastal shallow aquifer. The study area is extended from Ra’s Shikh Humayd to the town of Maqnah, northwest of Saudi Arabia. Eight electrical profiles were conducted using the Wenner–Schlumberger configuration. The total length of each profile was 360 m providing an expected maximum depth of penetration of about 50 m. All electrical profiles were conducted perpendicular to the coast and directed in the west–east direction. The inversion of the electrical data provided a clear two-dimensional image of the subsurface in the area that reflected the variations in the resistivity of the shallow aquifer system. The subsurface is composed of three layers: the upper layer corresponds to the dry alluvial and gravel deposits that have resistivity values in the range between 40 and 1400 Ω m. The second layer is the saturated zone of the alluvial sediments which represents the coastal aquifer in the study area. The resistivity value of the saturated zone is in the range of 0.15 to 25 Ω m. The lower layer in the subsurface stratification is the resistive bedrock which has resistivity values of more than 100 Ω m. It is composed of Pleistocene–Miocene sediments. The extension of saline water body has been mapped along all electrical profiles. The interpretation of the resistivity models shows that the salinity of the groundwater in the area is strongly related to three main factors: (1) the closeness to the coast; (2) closeness to wadis and low areas which allow saline surface water to infiltrate into the shallow aquifer and; (3) the abundance of gypsum outcrop in the vicinity. This last factor emphasizes the important effect of the local geological setting on the salinity of groundwater; the abundant supply of leached gypsum contributes to the salinity of the shallow aquifer.