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

Marieke Paepen (UGent) , Daan Hanssens (UGent) , Philippe De Smedt (UGent) , Kristine Walraevens (UGent) and Thomas Hermans (UGent)
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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.
Keywords
SALT-WATER INFILTRATION, SEAWATER INTRUSION, COASTAL AQUIFERS, FRESH-WATER, SALINITY, DYNAMICS, TIME, SEA, 2D, SALINIZATION

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MLA
Paepen, Marieke, et al. “Combining Resistivity and Frequency Domain Electromagnetic Methods to Investigate Submarine Groundwater Discharge in the Littoral Zone.” HYDROLOGY AND EARTH SYSTEM SCIENCES, vol. 24, no. 7, 2020, pp. 3539–55, doi:10.5194/hess-24-3539-2020.
APA
Paepen, M., Hanssens, D., De Smedt, P., Walraevens, K., & Hermans, T. (2020). Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone. HYDROLOGY AND EARTH SYSTEM SCIENCES, 24(7), 3539–3555. https://doi.org/10.5194/hess-24-3539-2020
Chicago author-date
Paepen, Marieke, Daan Hanssens, Philippe De Smedt, Kristine Walraevens, and Thomas Hermans. 2020. “Combining Resistivity and Frequency Domain Electromagnetic Methods to Investigate Submarine Groundwater Discharge in the Littoral Zone.” HYDROLOGY AND EARTH SYSTEM SCIENCES 24 (7): 3539–55. https://doi.org/10.5194/hess-24-3539-2020.
Chicago author-date (all authors)
Paepen, Marieke, Daan Hanssens, Philippe De Smedt, Kristine Walraevens, and Thomas Hermans. 2020. “Combining Resistivity and Frequency Domain Electromagnetic Methods to Investigate Submarine Groundwater Discharge in the Littoral Zone.” HYDROLOGY AND EARTH SYSTEM SCIENCES 24 (7): 3539–3555. doi:10.5194/hess-24-3539-2020.
Vancouver
1.
Paepen M, Hanssens D, De Smedt P, Walraevens K, Hermans T. Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone. HYDROLOGY AND EARTH SYSTEM SCIENCES. 2020;24(7):3539–55.
IEEE
[1]
M. Paepen, D. Hanssens, P. De Smedt, K. Walraevens, and T. Hermans, “Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone,” HYDROLOGY AND EARTH SYSTEM SCIENCES, vol. 24, no. 7, pp. 3539–3555, 2020.
@article{8675821,
  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.},
  author       = {Paepen, Marieke and Hanssens, Daan and De Smedt, Philippe and Walraevens, Kristine and Hermans, Thomas},
  issn         = {1027-5606},
  journal      = {HYDROLOGY AND EARTH SYSTEM SCIENCES},
  keywords     = {SALT-WATER INFILTRATION,SEAWATER INTRUSION,COASTAL AQUIFERS,FRESH-WATER,SALINITY,DYNAMICS,TIME,SEA,2D,SALINIZATION},
  language     = {eng},
  number       = {7},
  pages        = {3539--3555},
  title        = {Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone},
  url          = {http://dx.doi.org/10.5194/hess-24-3539-2020},
  volume       = {24},
  year         = {2020},
}

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