Investigation of seawater intrusion in the Dibdibba Aquifer using 2D resistivity imaging in the area between Al-Zubair and Umm Qasr, southern Iraq

Electrical resistivity surveying for delineating seawater intrusion was performed in the Dibdibba aquifer in the area between the cities of Al-Zubair–Safwan and Al-Zubair–Umm Qasr in the vicinity of Khor AL-Zubair Channel, Basrah governorate, southern Iraq. Fourteen 2D resistivity profiles with a total length of 14 km were collected in the study area. The resistivity sections were compared with lithological data extracted from 11 boreholes. Thirty-nine groundwater samples were collected within the area and analyzed for chemical constituents; internal hydrogeological reports and unpublished studies were also evaluated. Results reveal the existence of three major resistivity layers, ranging from 0.1 to 130 Ωm at various depths and locations. The first layer has very low electrical resistivity (0.1–5 Ωm) representing a layer saturated with saltwater intruded from Khor AL-Zubair Channel. The second layer shows resistivity in the range of 5–130 Ωm, attributed to a transition zone and an unaffected zone saturated with brackish groundwater. The last resistivity layer (< 3 Ωm) represents coarse-grain sediments saturated with saline groundwater. Furthermore, a hard clay bed (Jojab) appears with a resistivity of 3–7 Ωm in all 2D imaging lines within a depth of 20–28 m. Electrical conductivity (EC) measurements from seven wells collected in 2014 and 2016 show a positive EC difference increasing landward with an average increase of 1927 µS/cm. In addition, six chemical relationships (Na/Cl, [Ca + Mg]/[HCO₃ + SO₄], SO₄/HCO₃, SO₄/Cl, Mg/Ca and Cl/[HCO₃ + CO₃]) are used to detect the source of salinity in groundwater. This study proves that extensive use of high-resolution 2D imaging sections, alongside lithological and hydrogeological data, can serve as a useful tool to delineate the boundaries between aquifers, identify hydraulic boundaries between groundwater with different salinities and allocate hard clay layers between the upper and lower Dibdibba aquifer. In general, the combination of 2D imaging and hydrochemistry enables conceptualization of the hydrogeological situation in the subsurface and characterization of the salinity source, here seawater intrusion, in the study area. There have been no studies published so far on the characteristics of saltwater intrusion in the study area, and this study is considered to be important for monitoring and studying the intrusion and regression of seawater.