Study of the saltwater-freshwater interface with EM-31 and direct measurements: implications for the discharge pattern to Ringkøbing Fjord (Denmark)

Publikation: KonferencebidragPaperForskning

  • Carlos Duque Calvache
  • Johnathan Kinnear
  • Peter Knudegaard Engesgaard
  • Andrew Binley
  • Kinza Haider
  • Torben O. Sonnenborg
Ringkøbing Fjord, a brackish coastal lagoon system, in the western part of Denmark is an EU ecological habitat, where the salinity and water stage is controlled primarily by the natural inflow from the Skjern River and by the management of a sluice between the fjord and the open sea. So far nobody has provided any estimates of how much groundwater is discharging to the fjord and its effect on the water and salinity budget. The first task is therefore to develop a conceptual model of how groundwater discharges from the adjacent
sandy aquifer to the fjord. Previous studies using SkyTEM hydrogeophysics indicated the existence of two distinct groundwater discharge paths to the fjord; one at the near-shore zone and another further off-shore. Here we report on a more local hydrogeophysical investigation using EM-31 surveys in combination with direct measurements of salinity distributions and groundwater discharge near the shore-line. It was possible with the EM-31 to map the relative
salinity distribution in the 24 ha study area during 4 days with more than 10 km of resistivity measurements of the first meters below the lagoon bottom. Mapping of the saltwaterfreshwater zone was complemented by direct measurements; (a) ~2 m deep Electrical Conductivity (EC) profiles, (b) differential head measurements between lagoon stage and groundwater level (direction and magnitude), and (c) seepage meter measurements. The EC
profiles confirm the EM-31 results and both differential head and seepage measurements also indicate much higher freshwater discharge near the saline wedge. The analysis shows that the freshwater discharge in the study section is restricted to the near-shore area (maximum 15-20 m from shore line). It is hypothesized that a clay layer observed inland could extend off-shore and divide the sandy regional aquifer into a partly isolated upper unconfined section and a much thicker deeper confined section. This would explain the short distance near the shore, where groundwater discharges to the fjord and, at the same time, the off-shore discharge at distant areas from the shore.
OriginalsprogEngelsk
Publikationsdato2012
Antal sider5
StatusUdgivet - 2012
Begivenhed22nd Salt Water Intrusion Meeting - Armacao dos Buzios-RJ, Brasilien
Varighed: 17 jun. 201222 jun. 2012
Konferencens nummer: 22

Konference

Konference22nd Salt Water Intrusion Meeting
Nummer22
LandBrasilien
ByArmacao dos Buzios-RJ
Periode17/06/201222/06/2012

ID: 47321267