Multiscale geoelectrical characteristics of seawater intrusion along the eastern coast of the Gulf of Aqaba, Jordan

Abstract

Jordan's eastern coast of the Aqaba gulf is an example of seawater intrusion in extremely arid conditions and escalating urban development. The presented research is the first to use a large dataset of ground–based transient electromagnetic, which consists of 160 TEM soundings of various loop dimensions (100 m × 100 m, 50 m × 50 m, and 25 m × 25 m) that were conducted in a quasi–grid layout (35 km2 surveyed area), and also with dense TEM sites combined along three ERT profiles. To obtain valid and reliable resistivity interpretation, geological and hydrogeological analyses of existing boreholes were used in the interpretation of multiscale dimensions of geophysical data, such as 1D–quasi 3D–inversion of TEM data and 2D inversion of ERT data.1D TEM resistivity models (at depths ranging from 40 to 360 m) show potential seawater intrusion of several kilometers from the gulf of Aqaba into a terrestrial unconfined sandy aquifer of resistivity <3 Ω·m. A resistivity >3 Ω·m and <70 Ω·m, on the other hand, best describes brackish seawater–bearing sediments with a wide range of total dissolved solids (1400–7000 mg/l). Resistivity models suggest upwelling along faults where the deep terrestrial semi–confined Kurnub sandstone aquifer is uplifted and salinized from direct seawater intrusion and/or sub–Cretaceous aquifers. The combined interpretation of resistivity models along profiles (ERT and TEM data) near coast areas demonstrated that the clastic sequence is a heterogeneous medium at the survey scale, with saline water–bearing (0.6–1 Ω·m) sediments having a very sharp interface with brackish water–bearing (10–30 Ω·m) sediments. Several geoelectrical signatures appear in the intrusion zone as an irregular pattern like alluvial sediments thickening and abrupt lateral resistivity changes. These patterns have been associated to lithological or structural control factors, which allowed the explanation of seawater intrusion characteristics.