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

Abstract

Abstract. 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.

Geophysical Delineation of Freshwater–Saline Water Interfaces in Coastal Area of Southwest Bangladesh

“…This is particularly more important in saline grounds. The electrical current flow tends to concentrate in highly conductive zones which will significantly decrease the DOI [58]. The DOI was calculated following the Equation ( 3) by Oldenburg and Li [56].…”

“…In contrast, the DOI value around 1 indicates significant discrepancy of the inverted model. The DOI values between 0.1 and 0.2 are often used to delimit the reliable part of the inverted model [56][57][58][59][60]. In this study, we used the subjective value 0.1 to define the DOI of the inverted model.…”

Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography

“…Understanding the dynamics of salt-and freshwater is thus essential but challenging for the management of the coastal aquifer [8]. Previous investigations mapped the transition of saline and freshwater into two different areas based on shallow boreholes in the downstream part of the Luy River.…”

“…The decrease of sensitivity and resolution with depth is well documented [51] and must be accounted for to avoid the overinterpretation of resistivity structures. This is even more crucial in saline grounds, as electrical flow lines tend to concentrate in high conductivity zones, which can significantly reduce the depth of investigation [8].…”

“…Geophysical data can be used as a characterization tool but can also be more systematically integrated into the development of groundwater conceptual models and the calibration of numerical models [6]. Electrical and electromagnetic methods are particularly well suited for investigating SI because of their sensitivity to the bulk electrical conductivity of the ground, which is directly related to the salinity of pore water [8]. If airborne methods are becoming more popular to control large areas [9,10], they generally require an investment of local authorities to cover the large mobilization costs [11].…”