Mapping and Increasing Freshwater Availability Flemish Coastal Area

Published: 21 April 2020

Droughts and the stricter protection of groundwater resources forced the management authorities of the West Flanders coastal area in Belgium to look for sustainable alternatives to the current water sources, in particular for agriculture. These alternatives are actually located just below us in the shallow subsurface, where excess precipitation can be stored in the winter, creating a natural freshwater buffer for the summer.

Survey campaign

A survey campaign was conducted for the Flanders Environment Agency and as part of the EU TOPSOIL project. It mapped out the fresh-salt distribution in the subsurface from a helicopter using Skytem’s airborne electromagnetic (EM) technology. The monitored area included the west coast as well as a section of the Meetjesland and the left bank of the Scheldt, and approximately 2,400 line kilometres were measured. In close collaboration with other knowledge institutes, the next step was to use advanced computer models to transform the measurements into information about the distribution of fresh-salt groundwater.  This FRESHEM (FREsh Salt groundwater distribution by Helicopter ElectroMagnetic) survey campaign produced a wealth of data that is now here available online.

Salt or fresh

The survey results – the fresh-salt charts – show that salinity is quite high in the subsurface of the coastal area of West Flanders. Particularly in the low-lying polder areas (consisting of reclaimed salt marshes), brackish or saline groundwater is found no lower than two to three metres below the surface. There are freshwater lenses below the elevated sandy areas such as creek ridges and the dune area. Here, water can infiltrate more easily and water tables are higher than in the reclaimed salt marshes. As a result, fresh infiltration water has, over time, displaced the salt water and created freshwater lenses.

Groundwater salinity at a depth of 5 m-TAW (approx. 7.3 m-NAP) based on the FRESHEM results. The bleu areas indicate the freshwater lenses.

Creek ridge infiltration system

The potential of different measures to improve water availability for farming was determined and mapped in the second part of the project: GO-FRESH (Geohydrological Opportunities FRESH water supply). This phase drew on the FRESHEM measurements and other data such as elevation, soil type and the composition of the subsoil. Workshops with farmers and water managers resulted in the identification of potential areas and the right conditions for a sustainable Creek Ridge Infiltration System. Freshwater buffers can be effectively increased in creek ridges in a relatively simple way. For example, raising the water table by ten centimetres can result in an increase in the thickness of the freshwater lens by four metres.

The conceptual design of the Creek Ridge Infiltration System. The freshwater lens grows by infiltrating fresh water during the winter.

Identifying social costs and benefits

A cost-price analysis showed that a Creek Ridge Infiltration System is cost-effective for water needs in excess of 3,500 m3 per year compared to tap water and larger than 5,500 m3 per year compared to the capture and storage of rain water. Particularly when more water is required, the price can come close that of other water sources, but this depends very much on the local circumstances. However, the temporary storage of water in creek ridges is also promising when less water is needed because it can provide an almost guaranteed supply of fresh water of consistent quality in dry periods. Follow-up studies will be conducted to identify logical applications for adequate supplies of fresh water and, ultimately, sufficient food production in the Flemish coastal area.

 

More information about our international FRESHEM projects, please contact gualbert.oudeessink@deltares.nl

 

Click here for the research reports (Dutch only)

TOPSOIL Airborne EM kartering van zoet en zout grondwater in Vlaanderen (FRESHEM Vlaanderen)

TOPSOIL – GO-FRESH Vlaanderen: Potenties om de zoetwaterbeschikbaarheid te verbeteren