Smart use of subsurface for a sustainable water provision and water management

COASTAR knowledge programme researches implementability in four practical cases

The COASTAR 2019-2020 knowledge programme makes it clear: the subsurface can, as a supplement to surface options, offer an important source and storage space for a sustainable provision of freshwater. And this can be done by simultaneously tackling freshwater shortages, pluvial flooding and salinisation. Water users, water managers, drinking water producers, businesses, government and research institutes are pooling their strengths in the programme, so that sufficient freshwater will also be available in 20 years, at all times, for all users, and for all purposes. COASTAR represents another way of thinking and acting with regard to freshwater supply and water management, in line with the possibilities presented by the surrounding environment as well as the subsurface. The objective, following positive programme results and decision-making, is to launch four field trials in 2021.

In the densely populated Dutch delta, water is the source of daily contradictions. There is too much, or too little of it. More than 3.5 million people live and work in the Province of South Holland alone, and their number is growing. Residents and businesses want to keep their feet dry, and also expect to have access, always and everywhere, to sufficient freshwater. How can we, for example, in instances of winter pluvial flooding, prevent the waste of valuable freshwater through its discharge to the sea, while this same water is in fact badly needed in the summer to flush the polders? And how do we prevent the salinisation of groundwater and surface water, while this old brackish groundwater could actually be a good source of water.

Solutions in the subsurface

The new COASTAR knowledge programme focuses on large-scale freshwater supply through the smart use of the subsurface. By storing water in the subsurface, one can even out the discrepancies between supply and demand; both in terms of time, but also of location, in cases where there might be too much or too little water available. At the same time, COASTAR addresses the increasing salinisation by producing freshwater from brackish groundwater. Working with partners in South Holland, the programme is developing solution ideas towards pilots and applications in practice. Concern and respect for the subsurface and the environment are a constant in these efforts.

Field trials

COASTAR is working towards field trials to be run at four sites in South Holland in 2021. In the knowledge programme, conducted by the Deltares and KWR research institutes and Arcadis consultancy, the COASTAR measures will be further elaborated in collaboration with businesses and governments in the province.


COASTAR aims to contribute to subsurface solutions for a robust freshwater provision with the following cases:

  • Brackish water extraction in polders: the extraction of brackish groundwater in the Middelburg and Tempel polders to stop its upward seepage. This controls salinisation and reduces the amount of freshwater needed to flush the polder ditches. The brackish water is used to produce drinking water.
  • Brackish water extraction in dunes: extraction of brackish groundwater below the dunes as an additional source for the production of drinking water. Moreover, by removing the brackish water, the freshwater lens below the dunes can grow, thereby enlarging the strategic groundwater supply of the Dunea drinking water utility.
  • Westland: dry feet, sufficient irrigation water. In this project, horticulturalists infiltrate surplus precipitation into the subsurface, thereby establishing a so-called ‘water bank’. This has three benefits: a better balance between water supply and demand, containment of groundwater salinisation, and space is made available in the precipitation water basins to capture heavy rainfalls.
  • Rotterdam: Cities2Recharge. Instead of letting the precipitation surplus flow away unused, overburdening the sewer system, the water is infiltrated underground. It can then be pumped up later when it is needed, for instance, for use in green spaces or to sprinkle sports pitches.

The implementability of COASTAR is not limited to South Holland. The knowledge programme is relevant elsewhere in the Netherlands and worldwide, whenever COASTAR solutions can contribute to a good freshwater provision and water management.


The COASTAR knowledge programme supports the national Delta Programme on Freshwater Supply by developing concrete measures and securing the freshwater provision in low-lying areas of the Netherlands. COASTAR is co-financed by the Water and Maritime Top Sector.