Application of electrical resistivity tomography and ground penetrating radar to assess salinity in coastal aquifers with tidally-driven saline recirculation cells

Seawater intrusion into coastal aquifers is a major issue worldwide, as these aquifers are the primary source of drinking water to more than one billion people. With climate change and higher population densities in coastal areas, this problem is likely to get worse in the coming years. To evaluate and reduce the environmental risks of this phenomenon it is necessary to characterize coastal aquifers and seawater intrusion. Here, we present the application of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques to characterize a coastal aquifer at Magilligan (Northern Ireland, United Kingdom) where tides control coastal groundwater dynamics driving an upper saline recirculation cell beneath the intertidal zone. The coastal aquifer of Magilligan is an unconfined, homogeneous sand aquifer. The sand is generally about 20 m thick and rests on practically impermeable mudstones.

MALÅ ground penetrating radar systems with several frequencies (50, 100 and 500 MHz) were used to collect 2D GPR profiles at Magilligan beach. The upper saline recirculation cell was enough to attenuate the radar signal in all cases. However, GPR profiles were crucially important to demarcate the interfaces between freshwater and saltwater near the surface. GPR profiles obtained using higher frequencies (500 MHz) were the most informative.

2D ERT profiles were conducted at Magilligan beach using the SYSCAL Pro 72 ERI system (Iris Instruments). Two different array configurations (Wenner-Schlumberger and dipole-dipole arrays) were used to achieve both good horizontal and vertical resolutions. Because of the homogeneity of the medium, the ERT profiles made it possible to identify very clearly the upper saline recirculation cell and the fresh groundwater discharging “tube”. The presence of the tidally driven circulation cell causes fresh groundwater to flow below the saline recirculation cell (“discharge tube”) and discharges around the low tide mark. According to the ERT profiles, the upper saline recirculation cell at Magilligan has a resistivity of approximately 1 Ωm and a thickness of 8 m. Under the saline recirculation cell, the resistivity is always higher than that of the saline cell but decreases slightly as it moves towards the low tide mark, suggesting a possible mix between saline water that flows landward and the freshwater discharge. As a method of verifying the accuracy of the resistivity values measured in the ERT profiles, water samples were collected at various distances along a perpendicular transect of Magilligan beach; from the high-water mark to the low tide mark. The electrical conductivities of the water samples were measured and compared with the resistivities obtained in the ERT profiles using Archie's law. Similar values were obtained in both cases.

The research work carried out at Magilligan allows us to conclude that the application of ERT and GPR techniques is effective in delineating seawater intrusion in aquifers where tides create an upper saline recirculation cell beneath the intertidal zone. In addition, ERT profiles very clearly identified the upper saline recirculation cell through field measurements (which in most cases is studied through numerical models and laboratory tests).