The annual Big Jump events in Leuven and at Lake Genval-Overijse have been cancelled this weekend because of poor water quality.
Big Jump is a mass dive held each year to raise public awareness about the cleanliness of rivers and other waterways.
In Leuven, permission for the event was withdrawn after tests in the River Dyle found excessively high levels of E. coli bacteria. Enterococcus levels were also high.
Contact with contaminated water can cause health problems, particularly for children, pregnant women, older people and those in fragile health.
Instead of entering the water, participants in Leuven will take part in a symbolic 'Dry Jump' and sign a water declaration.
"It is ironic that an action in support of better water quality is being cancelled because of poor water quality, but that only reinforces the message," said Thomas Van Oppens, Leuven councillor for Climate and Sustainable Development for Groen.
He added that the city's wastewater treatment rate has now reached about 98%.
The planned dive at Lake Genval-Overijse has also been called off, even though more than 200 people had registered.
Tests carried out by an accredited laboratory found that the risk of contamination was too high if water was swallowed. Sailing and boating on the lake are still allowed.
Organised by GoodPlanet Belgium since 2005, the Big Jump highlights the European goal of cleaning up all surface water and groundwater by 2027.
