Flooding and Drinking Water

During heavy rainfall or flooding, local drinking water quality can be affected, and it may be advisable to avoid using tap water. In extreme cases, overflowing sewage systems may release various pollutants.

In August / September 2020, for example, heavy rain in Avegno (Ticino) prompted Alertswiss to issue a warning: do not use the tap water for drinking, cooking, or washing. (SRF, 30.08.2020)

How Contaminants Enter Drinking Water

Bacteria, nitrates (e.g. from manure), and other substances may be washed from the surface into rivers and groundwater. These pollutants can persist and only appear in the drinking water supply weeks or months later. Often, elevated pathogen levels are detected in rivers about a month after heavy rain—and in groundwater after up to two months.

Infant Health and Nitrates

Nitrates can convert into nitrosamines in the bodies of infants and small children—substances that may be carcinogenic. For older children and adults, dietary nitrate intake is generally not considered a health risk. (Swiss Association for Gas and Water)

Other Contaminants

It's not only pathogens or organic matter that get flushed into the water supply. Pesticides and herbicides are also often present in increased concentrations after heavy rainfall or floods.

Scientific Evidence

A global study (1910–2010) identified heavy rainfall and flooding as the most common triggers of waterborne disease outbreaks. Over half of these outbreaks were linked to contaminated drinking water.

Cann KF, Thomas DR, Salmon RL, et al. Extreme water-related weather events and waterborne disease. Epidemiology and Infection. (2013). 131: 671–686

Health Risks After Flooding

Most commonly, gastrointestinal illnesses are reported. However, Legionella outbreaks may also occur. In some regions, floods can also increase the concentration of heavy metals in drinking water.

You can find professional water analyses for Legionella, bacteria, and heavy metals here.