Dutch research institute Deltares has designed a unique seepage system to preserve fresh groundwater resources from sea level rise.

The system protects groundwater reserves from salinisation due to sea-level rise and over-exploitation. Deltares is now looking for a suitable oceanic island to install and test the seepage system. An operational seepage system has already been tested in the Netherlands for a year, and the essential mechanisms of the innovation work.

Calculations indicate that the seepage system can not only protect highly vulnerable freshwater lenses on small oceanic islands, but even enlarge them.

Deltares’ groundwater experts Perry de Louw and Gualbert Oude Essink will present their findings today at the International Salt Water Intrusion Meeting in Cairns, Australia.

The seepage system consists of a series of vertical pipes that are drilled some 15-25 metres into the saline subsoil of the coastal belt on a small oceanic island. The main purpose of the system is to capture the excess saline groundwater - the amount of which will increase as sea levels rise - that flows towards the freshwater lenses. Once captured, the saline groundwater is collected in small watercourses and returned to the sea. 

Field tests in the Netherlands

The seepage system was tested last year in a rural area on the south-western coast of the Netherlands. This region underwent an instantaneous sea-level rise of no less than 1.5 metres when former agricultural land was turned into a ‘new’ tidal area. 

Despite their tradition of protecting land from the waters, the Dutch are now opening up dikes to restore natural tidal marshlands. Farmers near the restored tidal area wanted to protect their fresh groundwater resources, which are vital for crop irrigation and which are expected to be reduced by the local sea-level rise. The seepage system was installed to stop the underground encroachment of saline groundwater and the resulting shrinkage of the freshwater lenses.

By the end of 2015, test results had shown that the seepage system was functioning well enough to compensate for the effects of the local sea-level rise induced by the new tidal inlet. The seepage system can actually enlarge the freshwater lens by reducing groundwater pressure further when there is a rainfall surplus.

Preliminary calculations made by Deltares indicate that the Dutch seepage system will also protect freshwater lenses on small oceanic islands which are extremely vulnerable to sea-level rise. Most of them are situated no more than a few metres above the sea mean level, which means that fresh groundwater resources will be seriously threatened in the coming decades.

Looking to the future

Any long-term solution for the protection of freshwater lenses on these islands from salinisation will probably involve a combination of tools to safeguard a sustainable freshwater supply in the future. The Deltares Groundwater Management expert Perry de Louw thinks that the seepage system can make a valuable contribution to protecting fresh groundwater resources in the coming decades.

Perry and his colleague Gualbert Oude Essink will continue to study the feasibility of the seepage system and they will focus on economic and infrastructure considerations. However, testing the innovation on an oceanic island threatened by sea-level rise and adapting the seepage system to the local situation is a first and important step. “We are ready to work with one of the smaller islands in the Pacific or Caribbean as soon as possible. Now it is the time to act.”