Salt content

The salt content of a water system determines which aquatic flora and fauna it will feature. If the volume of salt in surface water increases, salinisation will occur. Salinisation may pose problems for nature, farming, industry and drinking water companies. In some cases, farmers or nature management bodies will even need to convert to other crops or pursue other types of natural systems. The paragraphs below explain more about the various control variables that affect the salt content of a water system. The table below shows how climate change can affect these control variables.

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Screenshot from the knowledge document. Downloading the  knowledge document (pdf, 13 MB) gives you access to the interactive pages.

Especially in coastal areas, climate change is leading to increased salinisation. Contributing factors are, e.g., the rising sea level, soil subsidence and changes in river discharge volumes. As a result, more and more saltwater seepage is entering freshwater systems, whilst river counterpressure is changing. Drought and heat can also cause salinisation of surface water. For example, in periods of drought, foreign water from the major rivers is occasionally let in, whilst such water may have quite high salt contents. In addition, more water tends to evaporate during warm periods, thus raising the salt content of the water even more.

Impact of control variables on salt content

The most important control variables that can affect salt content are:

Saltwater wedge

In the coastal area of the Netherlands, seawater intrusion via the major rivers can cause salinisation of groundwater and surface water. This will create a saltwater wedge, whereby salty water slides underneath the freshwater. Whether a saltwater wedge can develop somewhere depends on the river discharge and the seawater level. In times of low river discharges, saline water will encroach further inland. An increase in the salinity of surface water will have a direct impact on the living conditions of flora and fauna. A water system in which freshwater is available up to 0–5m below the surface is highly vulnerable to the development of a saltwater wedge. If freshwater is available up to 5–10m below the surface, the water system will be moderately vulnerable to this effect. A water system is not vulnerable to the development of saltwater wedges if freshwater is available at more than 10m below the surface.

Inlet of surface water

The inlet of fresh surface water will dilute the saline water in saltwater wedges or brackish seepage. Freshwater inlet thus counteracts salinisation. Consequently, the inlet of freshwater has a positive effect on the salt content of a water system.

Seepage

The lower-lying parts of the Netherlands often see brackish groundwater coming up to the surface. This is referred to as brackish seepage. Via the soil, the seepage ends up in surface water. An increase in the salinity of surface water will have a direct impact on the living conditions of flora and fauna. A water system with an inflow of brackish seepage into surface water of less than 0.5mm per day is not vulnerable.

In part, the vulnerability of a water system to these different control variables depends on its use. Is the water used for swimming, as cooling water, as drinking water or for irrigation? The limiting values differ for each functional use and also differ per water system. The Urban Water Quality, Climate and Adaptation Tool indicates approximate limiting values for the control variables outlined above. These limiting values are not geared to a water system and can, therefore, only be used on an indicative basis. With respect to salinisation issues, it should be noted that large parts of our coastal zone used to be saline. Therefore, artificial systems are developing, featuring a saline waterbed and fresh supply water. Are you interested in finding out more about how to deal with this freshwater-saltwater dynamic? Helpful information is provided in the Freshwater-Saltwater Dynamic Assessment Framework on the website of the Foundation for Applied Water Research (STOWA).

Effect of climate change on the control variables

The table below shows how climate change impacts the various control variables that together determine the nutrient content of a water system. For example, the table reflects that increasing waterlogging causes more sewer overflows, which may have a negative effect. At the same time, sewer overflows are decreasing as a result of the increasing drought, which is a positive effect. More information can be found in the Excel file on Functional Uses of Water Types  (xlsx, 499 kB). The table below has been derived from a table in this file, which is contained in sheet “Effects of climate change”, part C.

EFFECT OF CLIMATE CHANGE ON THE CONTROL VARIABLES
Control variable Heat Waterlogging Drought Water safety
Saltwater wedge Increase, negative Increase, negative
Inlet of surface water Increase, negative Decrease, negative Decrease, negative
Seepage Increase, negative Decrease, positive Increase, negative

Are you interested in the quantitative effects per water system? These can be determined using the calculation tool in the  Guide to Water Quality Stress Tests. Or use this guide to draw up a detailed water and substances balance based on climate prognoses. Are you interested in finding out more about how to deal with this freshwater-saltwater dynamic? Helpful information is provided in the Freshwater-Saltwater Dynamic Assessment Framework on the website of the Foundation for Applied Water Research (STOWA).

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