Saline intrusion
Saline intrusion is the process by which (PDF) freshwater (ground- and surface water) bodies become increasingly salinised. In coastal areas and estuaries, the transition zone where both surface water and groundwater systems interact with the sea, saline intrusion is a natural process. Because of its geographical location next to the sea, salinisation has occurred for decennia in the coastal regions of the Netherlands.
Broadly speaking, saline intrusion into Dutch water bodies occurs through two main pathways: via groundwater and via surface water, through estuaries, locks, and sluices. For more information on soil salinisation, see the page on soil degradation.
Saline intrusion via groundwater
In the Dutch coastal zone, groundwater may be saline at shallow depths, as a result of past marine incursions. Saline or brackish groundwater may now flow upwards to enter the soil or flow out into surface water. This upward flow is a consequence of centuries of dewatering, land reclamation and land subsidence. For more information on the latter, see the page on subsidence.
Saline intrusion via estuaries, locks and sluices
Estuaries, the zones where rivers meet the sea, are areas where saline intrusion naturally occurs (PDF). Here, the saltwater moves upstream along the riverbed. This is a natural process. The degree of saline intrusion in estuaries depends on the tidal cycle, river discharge and sea water levels, which can be influenced by wind direction and intensity.
Saline intrusion also happens in sea locks (PDF) through various forms. One example is during the levelling process (PDF), when opening the lock gates causes exchange of water columns. Saline intrusion also occurs through leakage loss, for example due to leaks in the sealing of the doors. Here, the leakage rate varies with the outside water level. Lastly, saltwater can also enter freshwater bodies when the sluices are temporarily opened during the rising or falling tides, allowing fish to pass through.
Both types of salinisation are strongly connected to changes in sea level and periods of drought. More information on how sea level rise and droughts are projected to influence saline intrusion can be found on the trends & projections page.
Impact of saline intrusion
Saline intrusion can impact (PDF) sectors that rely on freshwater, such as agriculture, drinking water, industry, and nature. In any case, the impact of saline intrusion depends on the geographical features of the area, the extent and duration of the intrusion, and the vulnerability of the local water systems.
Agriculture
Most crops require freshwater to grow. When groundwater or irrigation water becomes too saline, crop yields can decline, although sensitivity to salinity varies by crop. During the droughts of 2018 and 2022 (PDF), increased salinisation contributed to lower farm incomes. To counteract this, waterboards often divert fresh river water into salinised areas to flush the system and keep surface waters fresh. In regions where saline intrusion is a natural and recurring process, parts of the agricultural sector have gradually adapted by shifting to salt-tolerant and salt-resistant crops.
Drinking water
Saline intrusion may affect drinking water production (PDF). If sources become too saline, they may no longer be suitable for use, which can necessitate the relocation of intake points or the temporary suspension of abstraction. In the western part of the Netherlands, the coastal dunes are being used for drinking water production. To mitigate the risk of salinisation, since the 1950s these dune water extractions have been transferred into managed aquifer recharge (MAR) systems. Pre-treated surface water is infiltrated into the dunes, to maintain a balance between groundwater extraction and recharge. Some of the inland groundwater extractions are vulnerable to salinisation, and therefor closely monitored and managed.
Drinking water production in the western Netherlands predominantly relies on surface water from Lake IJssel and the major rivers Rhine and Meuse, either for direct treatment or to supply the coastal MAR systems. The surface water intake points are increasingly vulnerable due to saline intrusion via estuaries, locks and sluices, especially during low flows of the river systems. During the drought of 2018, drinking water intake from Lake IJssel was temporarily suspended (PDF) due to deteriorating water quality. Drinking water utilities together with authorities are developing response strategies, for example to alter lock regimes during periods of low river flow.
Industry
Industry also depends on freshwater (PDF) for production processes and cooling. For example, the Brielse Meer in the Netherlands supplies water to the Port of Rotterdam and supports industrial water use in the surrounding area. The presence of salt and other unwanted minerals in industrial process water can lead to additional costs, either through the need for extra treatment to meet quality standards or through increased maintenance costs resulting from corrosion.
Nature
Natural ecosystems are vulnerable to salinisation, as many plants and animals depend on freshwater conditions. While some species can tolerate a limited range of salinity, others are more sensitive. When chloride concentrations become too high, freshwater-dependent plant and animal species may decline or disappear. At the same time, in areas that were historically exposed to saline influences, increasing salinity can also create opportunities for the return of salt-tolerant habitats and species, potentially leading to the development of unique ecological communities.
