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Netherlands

Salt intrusion

Definitions

Only some 2.5% of all water on the earth is fresh. At present, one third of fresh water used is groundwater. This fraction is growing because the demand of fresh groundwater increases due to the rise of world population and economic growth, the loss of surface water due to contamination, the availability of huge quantities, and the high quality relative to surface water. Disadvantages of groundwater are the high mineral content, the potential of land subsidence, high extraction costs, and finally, the threat of salt water intrusion (6).

Groundwater is classified according to chloride concentration into fresh water (< 300 mg/l), brackish water (300 – 10,000 mg/l) and saline water (>10,000 mg/l) (7). The drinking water standard in the European Community is a chloride concentration of 250 mg/l (17). A chloride concentration equal to approximately 100 mg/l indicates the taste limit of human beings (8), whereas ocean water has a chloride concentration of 19,000 mg/l.

Causes of salt intrusion

A number of processes affects salt water intrusion in coastal aquifers:

  • sea level rise (13,14);
  • over-exploitation of drinking water reservoirs to keep up with the increasing demand for domestic water (13,15);
  • land subsidence (14);
  • change in river discharge (10);
  • change in precipitation / evaporation ratio (16);
  • environmental conditions in the Holocene (13,14);
  • disruption of the natural hydrological system like the upstream intake of water from rivers, which reduces their discharge. This can lead to upstream migration of seawater in the river mouth, and shoreline retreat due to the reduction of the sediment load to the coastal zone (13);
  • the number and depth of ditches and the amount of water pumped from ditches, since the seepage flux is proportional to the difference between sea level and ditch water level (12).

Climate change versus human activities

Worldwide, excessive over-pumping of especially coastal aquifers is the most important anthropogenic cause of salt water intrusion. Furthermore, reducing recharge areas to develop touristic centres causes a decrease of outflow of fresh groundwater, inducing an inland shift of the salt water wedge. In some areas, land reclamation (e.g. in the Netherlands from about the 17th century on) caused a lowering of piezometric heads, and subsequently, sea water has rapidly intruded the coastal aquifer ever since (6).


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Vulnerabilities The Netherlands - Current, autonomous salinization of groundwater

Seepage of salt water into inland water systems of the Netherlands takes place as a result of the reclamation of polder areas during the period 1550–1900 A.D. This process is called autonomous salinization. Salt loads into the surface water system of the polders are determined by seepage flux and groundwater salinity. Large salt loads (>1000 kg Cl−/ha/y) already occur in the present situation in the deep polders with surface levels lower than −5 m msl, like the Zuidplas polder, Haarlemmermeer polder and Groot‐Mijdrecht polder and in the southwestern islands (18).


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Vulnerabilities The Netherlands - Future projections of groundwater salinization

The autonomous salinization is only caused by increasing salt concentrations, whereas seepage fluxes remain relatively constant. However, seepage fluxes are significantly changed by climate change, land subsidence and sea level rise, because these processes cause hydraulic heads and phreatic water levels to change. Model results show that the impact of sea level rise is limited to areas within 10 km of the coastline and main rivers (18).


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Vulnerabilities The Netherlands - Future projections of salinization lakes IJsselmeer and Markermeer

Lake IJsselmeer and Lake Markermeer form the largest artificial freshwater system in both the Netherlands and northwestern Europe and are fed primarily by the Rhine River. Both lakes are an important source of freshwater for drinking water production, agriculture and industry in the Netherlands.

The main inflows of chloride into Lake IJsselmeer include the IJssel River (11% of the Rhine discharge), drainage from the surrounding polders and seepage from the Wadden Sea through the tidal closure dam. The main inputs of water and chloride to Lake Markermeer are exchange with Lake Ijsselmeer (predominantly Rhine River water), rainfall and polder drainage (19).


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Vulnerabilities - Agriculture

Sea level rise will cause an increase of the salt water seepage in the coastal zones of the Netherlands, and an increase of the salt water intrusion in the main rivers in combination with lower river discharges in summer. Both types of salinization can harm salt sensitive crops in agriculture and horticulture. However, recent findings show that damage to crops due to salinization, in areas with a freshwater lens and well drained fields, are relatively low (4).


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Vulnerabilities - Drinking water supply

The expected rise in salt intrusion will increasingly result in unusable inlet points for agriculture and drinking water (5). Climate change impacts on salinization and the long-term availability of drinking water resources along the river Lek, a tidal branch of the Rhine delta, have been assessed for 2050. It was found that all current locations for fresh water abstraction are increasingly vulnerable to salt intrusion caused by the combination of sea level rise and decreasing river discharges. It was also found that diverting a higher fresh water discharge from one of the other Rhine branches to the Lek of several tens of cubic meters per second reduces the risk of salinization at the upstream inlet locations. However, the increased influence of seawater intrusion on the drinking water inlets cannot be fully compensated for by this measure (23). 

Adaptation strategy - Technical countermeasures

Countermeasures to compensate the salinization process should be taken in time, since the time lag is considerable (several decades to centuries) before these measures result in effective changes in the salinity distribution of the aquifer. The following technical countermeasures to prevent or retard the salinization process can be considered (6):


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Adaptation strategy - Agriculture

Currently available or conceivable adaptation strategies to the salinization of agricultural land are:

  • improving the efficiency of freshwater use in areas subject to salinization, including the retention and storage of rain water, a better separation of fresh and salt water, and the increase of surface water levels to suppress salt water seepage (2);
  • the growing of halophyte cultures (2);
  • the growing of macro- and micro-algae (2);
  • the growing of bait for fish in saltwater basins on the land (2;
  • the conversion of salted arable land to grassland, nature or sea culture parks (2);
  • irrigation using brackish water: a recent study into the possibility to use brackish rainwater for irrigation in Zeeland and west Brabant showed that, in contrast to the established knowledge, irrigation using brackish water is profitable for flower bulb growing, vegetable growing and salt-sensitive arable crops like onion and carrot (3);
  • use or design of salt tolerant crops;
  • changing land use.

The surface water system in low-lying areas probably has to be flushed more frequently in the future to dispose the excess of salt, which originates from the deeper groundwater system, affecting (10).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for the Netherlands.

  1. MNP (2005), in: Nillesen and Van Ierland (2006)
  2. Langeveld et al. (2005), in: Nillesen and Van Ierland (2006)
  3. Clevering (2005b), in: Nillesen and Van Ierland (2006)
  4. Clevering et al. (2005c), in: Nillesen and Van Ierland (2006)
  5. Bresser (2006)
  6. Oude Essink (2001)
  7. Stuyfzand (1986), in: Oude Essink (2001)
  8. Todd (1980), in: Oude Essink (2001)
  9. Van Dijk et al. (2009)
  10. Oude Essink (2008), in: Van Dijk et al. (2009)
  11. Oude Essink, 2003, in: Van Dijk et al. (2009)
  12. Maas (2007), in: Van Dijk et al. (2009)
  13. Post (2005), in: Van Dijk et al. (2009)
  14. Oude Essink (2001b), in: Van Dijk et al. (2009)
  15. Abd-Elhamid and Javadi (2008), in: Van Dijk et al. (2009)
  16. Oude Essink (2007), in: Van Dijk et al. (2009)
  17. Council European Union (1998)
  18. Oude Essink et al. (2010)
  19. Bonte and Zwolsman (2010)
  20. Van Den Hurk et al. (2006); Van Den Hurk et al. (2007), both in: Bonte and Zwolsman (2010)
  21. Webb and Howard (2011), in: IPCC (2014)
  22. Ferguson and Gleeson (2012); Loaiciga et al. (2012), both in: IPCC (2014)
  23. Van den Brink et al. (2019)

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