Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone
<p>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 la...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2020-07-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | https://www.hydrol-earth-syst-sci.net/24/3539/2020/hess-24-3539-2020.pdf |
| _version_ | 1811341012306493440 |
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| author | M. Paepen D. Hanssens P. De Smedt K. Walraevens T. Hermans |
| author_facet | M. Paepen D. Hanssens P. De Smedt K. Walraevens T. Hermans |
| author_sort | M. Paepen |
| collection | DOAJ |
| description | <p>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.</p> |
| first_indexed | 2024-04-13T18:51:22Z |
| format | Article |
| id | doaj.art-e65b4a009a3c40d699dea5da711dc118 |
| institution | Directory Open Access Journal |
| issn | 1027-5606 1607-7938 |
| language | English |
| last_indexed | 2024-04-13T18:51:22Z |
| publishDate | 2020-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Hydrology and Earth System Sciences |
| spelling | doaj.art-e65b4a009a3c40d699dea5da711dc1182022-12-22T02:34:26ZengCopernicus PublicationsHydrology and Earth System Sciences1027-56061607-79382020-07-01243539355510.5194/hess-24-3539-2020Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zoneM. Paepen0D. Hanssens1P. De Smedt2K. Walraevens3T. Hermans4Laboratory for Applied Geology and Hydrogeology, Department of Geology, Ghent University, Krijgslaan 281-S8, Ghent, 9000, BelgiumResearch Group Soil Spatial Inventory Techniques, Department of Environment, Ghent University, Coupure links 653, Ghent, 9000, BelgiumResearch Group Soil Spatial Inventory Techniques, Department of Environment, Ghent University, Coupure links 653, Ghent, 9000, BelgiumLaboratory for Applied Geology and Hydrogeology, Department of Geology, Ghent University, Krijgslaan 281-S8, Ghent, 9000, BelgiumLaboratory for Applied Geology and Hydrogeology, Department of Geology, Ghent University, Krijgslaan 281-S8, Ghent, 9000, Belgium<p>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.</p>https://www.hydrol-earth-syst-sci.net/24/3539/2020/hess-24-3539-2020.pdf |
| spellingShingle | M. Paepen D. Hanssens P. De Smedt K. Walraevens T. Hermans Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone Hydrology and Earth System Sciences |
| title | Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| title_full | Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| title_fullStr | Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| title_full_unstemmed | Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| title_short | Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| title_sort | combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone |
| url | https://www.hydrol-earth-syst-sci.net/24/3539/2020/hess-24-3539-2020.pdf |
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