A nonprofit organization said Monday that it has found "alarming" levels of bacteria in portions of the Seine River in recent months, raising concerns about the river's suitability as an open-water swimming venue for the 2024 Paris Olympics later this year.

The Surfrider Foundation, which focuses on the preservation of waterways and oceans, wrote in an open letter to Olympic stakeholders that it has collected bi-monthly water samples from the Seine River for more than six months − more specifically, from the water near a pair of bridges on the Seine River that are slated to serve as part of the triathlon swimming course.

The foundation said it has consistently found levels of E. coli and enterococci bacteria that exceed recommended threshholds − in some cases by significant amounts.

"It is therefore clear that the athletes who will be taking part in the Olympic and Paralympic events planned for the Seine will be swimming in polluted water and taking significant risks to their health," the Surfrider Foundation said in a news release.

French officials dismissed the foundation's findings as flawed, in large part because of the months in which they were collected (September to March).

Officials from Île-de-France precinct, which includes Paris, wrote in a letter that they were not actively working to treat portions of the river during the winter months, which also featured heavy rains that contributed to the elevated bacteria levels. They also said several key aspects of their plan to clean up the river, including a massive tank that will store stormwater runoff, won't be implemented until later this month or next.

"It’s well known that the (winter) period, which is a time with a lot of rain, is not suited for swimming," Pierre Rabadan, Paris' deputy mayor for sports and the Olympics and Paralympics, told The Washington Post on Tuesday.

The Seine River is slated to play a starring role in the upcoming Olympics, which begin July 26. In addition to hosting open-water swimming events, it will effectively serve as the venue for the opening ceremony, with athletes floating down the river on barges rather than marching into a stadium.

For French officials, cleaning up the river represents one of the Games' great challenges but also a potential part of their legacy. For more than a century, the Seine River has been unsafe to swim in because it collects runoff from Paris' outdated sewer system, in which rainwater flows through the same pipes as the city's sewage. Heavy or sudden storms lead to overflows, sending both rainwater and sewage into the Seine.

Government officials are hoping to address this issue by, among other things, building a massive runoff tank to collect and treat sewage system runoff before it gets to the river. Île-de-France officials wrote in their letter that the tank, which can store 13.2 million gallons of runoff, is expected to be put into service by next month.

If all goes according to plan, officials say portions of the Seine will not only be suitable for Olympic competition but also become accessible for public bathing in 2025 and beyond. Emmanuel Macron, France's president, and Anne Hidalgo, the mayor of Paris, are among the high-profile government officials who have promised to swim in the river to show that it is safe.

Despite the French government's efforts, however, some athletes still have concerns.

"We know what the Pont Alexandre-III (bridge) and the Eiffel Tower represent but I think that the health of the athletes must come first," Ana Marcela Cunha, who won a gold medal for Brazil in open-water swimming at the 2021 Olympics, told Agence France-Presse last month.

"The organisers must accept that perhaps it will, unfortunately, not be possible to hold the events where they want to."

Organizers have said they are prepared to delay triathlon and open-water swimming events, in the event of heavy storms or unsafe water conditions, but they do not have plans to move the events from the Seine River.

Contact Tom Schad at [email protected] or on social media @Tom_Schad.