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https://doi.org/10.5194/hess-2019-540
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-540
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 22 Jan 2020

Submitted as: research article | 22 Jan 2020

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This preprint is currently under review for the journal HESS.

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

Marieke Paepen1, Daan Hanssens2, Philippe De Smedt2, Kristine Walraevens1, and Thomas Hermans1 Marieke Paepen et al.
  • 1Laboratoy of Applied Geology and Hydrogeology, Department of Geology, Ghent University, Krijgslaan 281-S8, Ghent, 9000, Belgium
  • 2Research Group Soil Spatial Inventory Techniques, Department of Soil Management, Ghent University, Coupure links 653, Ghent, 9000, Belgium

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.

Marieke Paepen et al.

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Latest update: 19 Feb 2020
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Short summary
Fresh groundwater can flow to oceans and seas, possibly adding nutrients/pollutants to coastal ecosystems. For the first time, three complementary geophysical methods (sensitive to salinity) are combined to delineate this outflow in a highly dynamic coastal environment. Providing temporal and spatial information on the salt and freshwater distribution on land, in the intertidal zone, and offshore. Allowing to visualize groundwater discharge around the low water line at De Westhoek (Belgium).
Fresh groundwater can flow to oceans and seas, possibly adding nutrients/pollutants to coastal...
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