Together with blue-green algae and bacteria, many Dutch water bodies increasingly contain an amount of chemical substances over the allowed levels. According to Natuur & Milieu and the Netherlands Institute of Ecology, people in the Netherlands are not sufficiently warned about those before going for a swim. One of the substances that was found particularly often, for example, is imidacloprid, usually used against fleas on dog collars.

The research, which ran from 2015 to 2024, found that designated swimming water bodies are not cleaner than other surface water sources in the country. It examined the quality of hundreds of swimming locations, checking for 122 chemical substances. Of those, 35 were found to be above the desired levels.

The substances found vary per spot. “In rural areas, agricultural pesticides turn up more often,” Koenraad Backers from Natuur & Milieu said to NOS. In cities, on the other hand, more of the substances found in the water come from cars and industrial emissions.

Backers says that some swimming sites are approved for use despite having high levels of chemicals present. That is because the waters are usually only checked for blue-green algae and bacteria, such as E. coli, not chemicals.

Beckers, however, added that the presence of chemicals does not necessarily mean a person will immediately feel their negative impacts on their health. The health risk “depends in part on how much [water] a person takes in and how long the exposure lasts,” said Backers.

“The government does measure whether chemical substances are in the water; it just is not required to report that.” Backers told NOS. “But under the European Water Framework Directive, it is required to keep those substances below the set standard. Right now, they are in violation, and the Netherlands will be held accountable for that in 2027.”