Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge (SGD) in the littoral zone

Abstract

Abstract. 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.

The two salinity peaks mode of marine salt supply to coastal underground brine during a single tidal cycle

“…The depth-of-investigation (DOI) index is used to analyze the influence of inversion parameters on the model and evaluate the reliability of the inverted resistivity data (Oldenburg and Li, 1999;Paepen et al, 2020;Zhang Y. et al, 2021). The DOI index is calculated using Equation 1.…”

“…It is calculated based on two additional inversions (R inv,1 and R inv,2 , which are inverted resistivities) using two reference models (R app,1 and R app,2 ) which are 0.1 and 10 times the average observed apparent resistivity of the datasets (Oldenburg and Li, 1999;Paepen et al, 2020;Zhang Y. et al, 2021).…”

“…A low DOI index is obtained when the resistivity structures in the model are driven by the data and not by the inversion process, which is influenced by the reference model (Paepen et al, 2020;Zhang et al, 2023). All cells with a DOI index greater than 0.2 were considered as…”