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

TY - CONF

T1 - Study of the saltwater-freshwater interface with EM-31 and direct measurements

T2 - 22nd Salt Water Intrusion Meeting

AU - Calvache, Carlos Duque

AU - Kinnear, Johnathan

AU - Engesgaard, Peter Knudegaard

AU - Binley, Andrew

AU - Haider, Kinza

AU - Sonnenborg, Torben O.

N1 - Conference code: 22

PY - 2012

Y1 - 2012

N2 - 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 adjacentsandy 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 relativesalinity 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 ECprofiles 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.

AB - 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 adjacentsandy 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 relativesalinity 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 ECprofiles 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.

M3 - Paper

Y2 - 17 June 2012 through 22 June 2012

ER -