Reference application: Protecting fresh groundwater resources against sea-level rise in Perkpolder (Netherlands)

Description

To restore tidal salt marshes along the Westerschelde (estuary of the North Sea) agricultural land (75 ha) was given back to the sea. The average water level for this new tidal area changed from surface water level of 1.0 m below sea level to average sea level and consequently will affect the groundwater system in the adjacent agricultural area. This can be seen as a local sea level rise of about 1.5 m. Since 2010, hydraulic heads and the fresh-saline interface are being monitored to capture the reference situation and to determine possible effects resulting from the development of the new tidal area.

Below the adjacent agricultural area, a freshwater lens of about 10 to 15 meters provides farmers with fresh groundwater for irrigating their crops. In order to protect this freshwater lens from shrinking, a self-flowing seepage system called SeepCat was designed and installed to compensate the effects resulting from this local sea level rise.

From the measurements it was concluded that SeepCat was functioning well enough to compensate the effects of the new tidal area. Moreover, the results showed that the seepage system is able to increase the freshwater lens by extra lowering the hydraulic head during times of precipitation surplus. Since the system is technically and geohydrologically working well, it could also be applied in other coastal areas, like small ocean islands. Their vulnerable freshwater lenses are seriously threatened by the expected future sea level rise and over-exploitation and SeepCat could help to protect these precious freshwater resources. The system is easily combined with an artificial recharge system to create larger volumes of freshwater.

Keywords

Protecting fresh water lenses, Mitigating sea level rise, Salinization, Over-exploitation

URL

https://www.deltares.nl/en/news/seepcat/ , https://www.youtube.com/watch?v=K3RbSjucxIk , https://www.youtube.com/watch?v=RNpNydgqQqY

Key lessons

• The pilot and monitoring results showed that SeepCat is working well in the case of Perkpolder. However, the working largely depends on the local hydrogeology and this should always be assessed when planning implementing such a system. • The design and construction of the system is relatively simple: the system consist of vertical pipes or wells (e.g. PVC), installed at a depth of 10 – 20 meters, connected to horizontal collector pipes or ditches which is connected to a pump. The system could easily be adjusted to the local situation. Maintenance and operational costs are low.

Organisation

Freshwater production

Freshwater storage volume

Duration of storage/recovery cycle