Electrical resistivity tomography (ERT) is particularly suited for the characterization of saltwater intrusion and submarine freshwater discharge, given its sensitivity to the electrical conductivity of pore water. A comprehensive survey might combine land ERT collected during low tide and marine ERT acquired with lower sensitivity due to the presence of a seawater layer, most often, inverted separately. In this contribution, we propose a new methodology to jointly invert land and marine ERT acquired along a same profile, bringing them to a common situation at high tide. We first estimate the electrical resistivity distribution under the land profile. Then, we compute a corrected data set for land ERT, simulating a data set collected with the same level of water as the marine ERT. Finally, both land and marine ERT data sets are inverted jointly. We show that the approach allows to better image the freshwater/saltwater distribution across the shoreline.

Submarine groundwater discharge (SGD) is an important gateway for nutrients and pollutants from land to sea. While understanding SGD is crucial for managing nearshore ecosystems and coastal freshwater reserves, studying this discharge is complicated by its occurrence at the limit between land and sea, a dynamic environment. This practical difficulty is exacerbated by the significant spatial and temporal variability. Therefore, to capture the magnitude of SGD, a variety of techniques and measurements, applied over multiple periods, is needed. Here, we combine several geophysical methods to detect zones of fresh submarine groundwater discharge (FSGD) in the intertidal zone, upper beach, dunes, and shallow coastal area. Both terrestrial electrical-resistivity tomography (ERT; roll-along) and marine continuous resistivity profiling (CRP) are used from the shallow continental shelf up to the dunes and combined with frequency domain electromagnetic (FDEM) mapping in the intertidal zone. In particular, we apply an estimation of robust apparent electrical conductivity (rECa) from FDEM data to provide reliable lateral and vertical discrimination of FSGD zones. The study area is a very dynamic environment along the North Sea, characterized by semi-diurnal tides between 3 and 5 m. CRP is usually applied in calmer conditions, but we prove that such surveys are possible and provide additional information to primarily land-bound ERT surveying. The 2D inversion models created from ERT and CRP data clearly indicate the presence of FSGD on the lower beach or below the low-water line. This discharge originates from a potable freshwater lens below the dunes and flows underneath a thick saltwater lens, present from the dunes to the lower sandy beach, which is fully observed with ERT. Freshwater outflow intensity has increased since 1980, due to a decrease of groundwater pumping in the dunes. FDEM mapping at two different times reveals discharge at the same locations, clearly displays the lateral variation of the zone of discharge, and suggests that FSGD is stronger at the end of winter compared to the beginning of autumn. ERT, CRP, and FDEM are complementary tools in the investigation of SGD. They provide a high-resolution 3D image of the saltwater and freshwater distribution in the phreatic coastal aquifer over a relatively large area, both off- and onshore.

With the growing population and the adverse effects of climate change, the pressure on coastal aquifers is increasing, leading to a larger risk of saltwater intrusion (SI). SI is often complex and difficult to characterize from well data only. In this context, electrical resistivity tomography (ERT) can provide high-resolution qualitative information on the lateral and vertical distribution of salinity. However, the quantitative interpretation of ERT remains difficult because of the uncertainty of petrophysical relationships, the limitations of inversion, and the heterogeneity of aquifers. In this contribution, we propose a methodology for the semiquantitative interpretation of ERT when colocated well data are not available. We first use existing wells to identify freshwater zones and characterize the resistivity response of clayey deposits. Then, we approximate the formation factor from water samples collected in the vicinity of ERT data to derive a resistivity threshold to interpret the saline boundary. We applied the methodology in the shallow aquifers of the Luy River in the Binh Thuan province, Vietnam, where water resources are under pressure due to agricultural, aquacultural, and industrial production. Twenty-one ERT profiles were collected and revealed a much larger intrusion zone, compared to the previous study. Saltwater is present in lowland areas of the left bank over almost the whole thickness of the aquifer, while the right bank is constituted of sand dunes that are filled with freshwater. At a larger distance from the sea, a complex distribution between fresh and saltwater is observed. Our methodology could be applied to other heterogeneous aquifers in the absence of a dense monitoring network.

Human activities, such as managed aquifer recharge (MAR) and groundwater pumping, are influencing the natural groundwater flow in coastal areas. This might induce saltwater intrusion and impact fresh submarine groundwater discharge (FSGD). The use of resistivity methods (electrical resistivity tomography and continuous resistivity profiling) in coastal studies is very effective to investigate the salt–freshwater distribution, but it can be difficult to interpret quantitatively. In this study, the Western Belgian coast is investigated with resistivity methods, and image appraisal tools are systematically used to quantitatively interpret inversion models. Synthetic resistivity models, which reflect the existing situation at the Western Belgian coast, are first created and assessed quantitatively by means of the model resolution matrix, cumulative sensitivity matrix, and depth of investigation index. They reveal that no quantitative interpretation is possible of the FSGD, although lateral qualitative changes can be deduced from the inversion models. The field data show freshwater outflow from the lower beach to below the low water line, and they indicate that MAR has a positive impact on FSGD, while groundwater extraction reduces the outflow of freshwater to the North Sea. A comparison with existing groundwater models indicates their inability to reproduce the actual FSGD footprints.

The presence of fresh groundwater is not limited to land; it also extends offshore and discharges as submarine groundwater discharge (SGD). The freshwater component of SGD (fresh submarine groundwater discharge [FSGD]) can be detected using geophysical techniques that are sensitive to salinity such as resistivity measurements. However, these measurements are often limited to either the land or marine realm, neglecting the land-marine interface. In this study, we focus on this gap by combining onshore and offshore techniques to assess variability in the FSGD footprint near the Belgian coastline through electrical resistivity tomography and continuous resistivity profiling. The difficult working conditions of the highly dynamic North Sea make this offshore survey one of the first of its kind. The footprint varies from limited outflow on the upper beach (e.g., Wenduine) to discharge around and below the low water line (e.g., De Panne, Oostduinkerke, and Knokke-Heist) in the studied areas. The occurrence, footprint, and quantity of SGD seem to be controlled by the presence and size of dune formations that constitute freshwater resources along the shore. Heterogeneity can also play a determining factor in FSGD location.

Submarine groundwater discharge (SGD) is an important gateway for nutrients and pollutants from land to sea. The study of SGD is thus important for nearshore ecosystems and the management of coastal freshwater reserves. The discharge occurs at the limit between land and sea, a dynamic environment, making the assessment difficult. And more important, SGD is characterized by a significant spatial and temporal variability. Therefore, a variety of techniques and measurements in multiple periods is needed to capture the magnitude of SGD in one particular site. To detect zones of discharge, we combine several geophysical methods in the intertidal zone, as well as on the upper beach, in the dunes, and shallow coastal area. Electrical resistivity tomography (ERT) – on land, roll-along ERT and on sea, marine continuous resistivity profiling (CRP) – is used from the shallow continental shelf up to the dunes, combined to frequency domain electromagnetics (FDEM) mapping in the intertidal zone. The highly saline environment we work in causes FDEM instruments operated under low induction number (LIN) to underestimate the apparent electrical conductivity (ECa). Here, we apply a quadrature-phase algorithm to obtain a robust ECa (rECa), valid at low and high induction numbers. De Westhoek (Belgium) was chosen as a test site, which is bordered by the North Sea. It is a strong dynamic environment, with semi-diurnal tides between 3 and 5 m. Providing approximately 6 hours between low and high tide, this leaves little time to work close to the low water line. CRP is usually applied in calmer conditions, but we prove that a survey is possible on the North Sea, providing additional information to the ERT survey which is mainly restricted to the land. The 2D inversion models created from ERT and CRP data clearly indicate the presence of SGD on the lower beach (in the East) or below the low water line (in the West of the study area). The discharge originates from a potable freshwater lens underneath the dunes, which delimit the sandy beach. The fresh groundwater flows underneath a thick saltwater lens, present from the dunes to the lower beach, which is fully observed with ERT. FDEM mapping reveals discharge at the same locations and clearly displays the lateral variation of the zone of discharge. ERT, CRP, and FDEM are complementary tools in the investigation of SGD. They provide a high-resolution 3D image of the salt and freshwater distribution in phreatic coastal aquifer over a relatively large area, both off- and onshore.