Substances that occur in groundwater in concentrations of several milligrams per litre, such as chloride and sulfate, are known as major constituents. They generally enter groundwater naturally from soil and rock. In settlement areas and especially along roads, chloride can also originate from the use of de-icing salt, which consists of sodium or potassium chloride. Around 4% of monitoring sites in Switzerland exceed the chloride limit value of 40mg/l.
Major constituents include the cations calcium and magnesium, the anions hydrogen carbonate, sulfate and chloride, and silicium. Groundwater has characteristic values of individual major constituents, depending on the type of aquifer and its interactions with soil and rock. For groundwater that is used or intended for use as drinking water, the Waters Protection Ordinance (WPO) sets a limit value ('numerical requirement') of 40 milligrams per litre (mg/l) for chloride and sulfate. This does not apply when elevated values stem from natural conditions, i.e. when chloride or sulfate originate from soil and rock.
All 550 sites of the NAQUA National Groundwater Monitoring were sampled and analysed for major constituents as part of a pilot study conducted in 2018 and 2019.
Chloride
In Switzerland, only small quantities of chloride are naturally present in shallow groundwater aquifers. Concentrations above 40mg/l in groundwater appear exclusively at monitoring sites near roads or in settlement areas. These elevated concentrations are primarily due to the infiltration of road runoff contaminated with de-icing salt and are therefore not of natural origin. In settlement areas, more than 15% of the monitoring sites exceed the limit value of 40mg/l.
Sulfate
Elevated sulfate concentrations in groundwater are predominantly of natural origin. The natural (geogenic) sources of sulfate are mainly the dissolution of evaporite rock (gypsum or anhydrite) and, more rarely, the oxidation of sulphide minerals (e.g. pyrite). Thus, concentrations are naturally above 40mg/l in various karst aquifers in the eastern Jura and the northern edge of the Alps, as well as in the unconsolidated aquifers in the upper Rhone and upper Rhine valleys and the Geneva region, due to the local rock. These regions contain evaporite rocks consisting of gypsum or anhydrite.
Hintergrundwerte in oberflächennahen Grundwasservorkommen der Schweiz (PDF, 83 MB, 26.10.2023)Im Auftrag des BAFU
Taking into account the regional natural background values for sulfate, the sulfate concentrations are more than 40 mg/l above the natural background value at only three monitoring sites. At these monitoring sites, the exceedances are most likely due to sulfate entering the groundwater from anthropogenic sources. Anthropogenic sources in this case are primarily landfills containing construction waste. Sulfate can also enter the groundwater through certain fertilisers or the use of sulphur as a plant protection product.
Hydrogen carbonate
The concentration of hydrogen carbonate in groundwater is controlled by the lime-carbonic acid equilibrium and depends primarily on the concentration of carbon dioxide dissolved in the leachate and the carbonates present in the soil and aquifer. Rocks that contain carbonates, such as limestone and dolomite, are therefore the largest natural sources of hydrogen carbonate. Non-natural sources include the construction industry and the use of lime fertilisers in agriculture.
The highest concentrations of hydrogen carbonate in groundwater (over 300mg/l) occur naturally in the carbonate-rich unconsolidated aquifers of the Swiss plateau and in the molasse sandstones of the clastic fissured aquifers. The karst areas in the Jura and in the foothills of the Alps also have high concentrations of hydrogen carbonate in the groundwater. Low concentrations of less than 100mg/l are found in the crystalline fissured aquifers of the Alps, where no carbonate-bearing rock occurs.
In addition to its high concentrations of hydrogen carbonate, groundwater in unconsolidated and clastic fissured aquifers and in karst aquifers contains naturally high concentrations of calcium and magnesium.
Calcium concentrations are generally between 10 and 200mg/l. Magnesium, sodium, sulfate and chloride concentrations are between 0.1 and 100mg/l, while those of potassium and silicium are less than 10mg/l.
Further information
Last modification 18.03.2025
