Availability of fresh groundwater, salinisation

The availability of fresh ground and surface water is important for agriculture, industry, drinking water and nature. Salinisation of ground and surface water takes place in the coastal area of the Netherlands by intrusion of seawater through the large rivers and salty torment (the flow of salt groundwater up to the surface). It is expected that due to predicted climate change and future sea level rises, salty torment and salt penetration from the sea will increase and the availability of fresh ground and surface water will decrease. This factsheet deals with the availability of fresh groundwater. Groundwater The current availability of fresh groundwater in the Dutch coastal area is limited by the shallow occurrence of salt groundwater. This salty groundwater is “old” seawater that was withdrawn from the subsurface during the Holocene floods of the sea. Since the laying of polders by man, this salty groundwater has flowed back to the surface (salt torment). Due to these processes, the spatial variation of the occurrence of fresh groundwater is large. In the southwestern delta, and the northern coastal areas, the salty groundwater is very shallow (< 5 mv) while in the western Netherlands the salty groundwater is found between 25 and 50 mv. Under the dunes, thick freshwater bubbles (up to 100 m) are found that are used for drinking water. Under higher sandy creek ridges come freshwater bubbles of 5 to 25 m thickness from which agriculture is extracted for irrigation. The map with the interface of 1 000 mg/l chloride indicates on a national scale to what depth fresh groundwater is found and thus gives a good indication of where fresh groundwater is available. The map is based on a large number of measurements but the spatial variation is so large that the map is not suitable for use on a local scale. The fresh-breaking groundwater interface does not vary much over time because flow processes in the subsoil are slow. However, when extracting fresh groundwater in coastal areas, too large abstraction flow rates can attract deeper salt groundwater in a short period of time. This process is called saltwater cone and limits freshwater availability. The figure below shows that salinisation of abstractions is a serious problem. Definitions: Chloride is a conservative substance that is relatively high compared to other substances and is the dominant representative of the salt content of water. Definitions of sweet, brackish and salt water are therefore based on chloride concentration. Class: Chloride concentration (mg Cl/l) Sweet: < 1000 Brackish: 1000-3000 Salt: > 3000