Substances in tap water: minerals, hardness, pollutants

Tap water contains dissolved minerals: calcium, magnesium, sodium, potassium, bicarbonate, sulphate and chloride. The amounts vary greatly from region to region because they depend on the subsoil. In addition, the water can contain substances that do not belong there. These include nitrate from agriculture, heavy metals from domestic plumbing and residues for which there is no legal limit.

Tap water contains seven relevant minerals: calcium, magnesium, sodium, potassium, bicarbonate, sulphate and chloride. In addition, there are trace elements such as iron and manganese. Calcium and magnesium also determine the water hardness.

What matters is the difference between “contained” and “relevant for nutrient supply”. The concentrations are low. They cover only a small part of daily requirements. With hard water, calcium makes a noticeable contribution; with soft water, it remains small. Tap water supplements your intake. However, it cannot replace a good diet.

Chemically, tap water consists of water molecules, i.e. hydrogen and oxygen, as well as dissolved salts and gases. In terms of quantity, the dissolved components account for very little. However, they determine taste, hardness and suitability for household appliances.

The components can be divided into three groups:

• Desirable minerals: calcium, magnesium, sodium, potassium, bicarbonate, sulphate, chloride and trace elements such as iron and manganese.
• Process-related substances: residues from treatment at the waterworks, such as chlorine for disinfection. Added to this is whatever the water picks up in the pipe network and the domestic plumbing.
• Undesirable inputs: nitrate, heavy metals, residues of pesticides and pharmaceuticals, microplastics and per- and polyfluoroalkyl substances (PFAS).

Bicarbonate (chemically HCO3-) is the salt of carbonic acid. It is found in almost all tap water. As a buffer, it neutralises acids and stabilises the pH value. In the body, it helps to maintain the acid-base balance.

A common assumption is that bicarbonate only occurs in mineral water. That is not true. As with calcium, the content depends on the geological subsoil. In limestone regions it is high; with acidic rock it is low. A high bicarbonate content usually goes hand in hand with high water hardness.

Water hardness describes how much calcium and magnesium is dissolved in the water. Section 9 of the German Detergents and Cleaning Products Act (Wasch- und Reinigungsmittelgesetz, WRMG) defines three hardness ranges, measured in millimoles of calcium carbonate per litre:

Under § 9(1) WRMG, water suppliers must communicate the hardness range at least once a year. The same applies whenever there is a lasting change. The value is also shown on the drinking water analysis.

Hardness has practical consequences. Hard water leads to limescale deposits in kettles, coffee machines and shower heads. The amount of detergent needed increases. Hard water is harmless to health. It even supplies more calcium and magnesium than soft water.

A common misconception: limescale in water is neither a quality defect nor a sign of contamination. It is dissolved calcium carbonate, in other words a mineral.

A drinking water analysis lists three pieces of information for each parameter: the measured value, the unit and usually the permissible limit. These values help you put the results into context:

A value below the limit means that the water meets the requirements of the Drinking Water Ordinance.

A stricter rule is coming for lead. Annex 2 of the Drinking Water Ordinance lowers the limit from 0.010 to 0.005 mg/l as of 12 January 2028. For buildings with old pipes, replacing them therefore becomes more urgent. Note also: the limit applies at the point of delivery. If the lead comes from pipes inside the building, it does not show up in the supplier's analysis.

For nitrate, there is also a sum rule. Nitrate and nitrite are assessed together: the nitrate concentration divided by 50 plus the nitrite concentration divided by 3 must not exceed a value of 1.

Besides minerals, tap water can contain substances that do not belong there. They come from three sources: the raw water, the public mains network and the domestic plumbing.

There are three ways. They answer different questions.

The supplier's drinking water analysis. It shows the values at the point of delivery. Free of charge, usually as a PDF on the supplier's website. It includes the hardness range, mineral content, nitrate and the limit-value parameters tested. What it doesn't cover: anything that enters the water from your domestic plumbing.

Laboratory analysis of your own sample. It shows what actually comes out of your tap, including anything that enters the water from the pipes in the building. Useful if you suspect lead or copper pipes, in older buildings and if the taste is unusual. Make sure the laboratory is accredited and check which parameters are included in the package.

Rapid tests for home use. They give a rough indication, for example of hardness. They are of little use for reliable statements about pollutants.

Find out more in the guide How to measure water quality.

A filter is no substitute for a functioning water supply. If the values are flawless, you can do without one. It makes sense in three situations:

• Issues arising from the domestic plumbing. Old lead pipes, copper pipes or elevated metal levels in the analysis are an objective reason.
• Taste and odour. Chlorine and similar substances can be significantly reduced with activated carbon.
• Substances without a limit value. Public water treatment has no dedicated stage for pharmaceutical residues and microplastics.

A note on choosing: an activated carbon filter reduces chlorine, organic substances and anything that spoils the taste. It is not suitable for retaining bacteria and germs – for that you need an appropriately certified germ filter. With both methods, the dissolved minerals remain in the water.