Concerns over the water quality of Paris’ Seine River have cast a shadow over the 2024 Olympics, leading to the cancellation of a pre-race triathlon swimming event. Despite a $1.5 billion investment to clean the river and assurances from officials that it would be safe, recent tests revealed unsafe levels of E. coli, particularly after heavy rains. This has reignited doubts about whether the Seine is ready for Olympic swimming events, even as Parisian Mayor Anne Hidalgo recently took a dip to demonstrate its safety. The stakes are high as the Games approach, with athletes and organizers hoping for a last-minute improvement in water quality.

Due in large part to the low quality of the water, swimming in the Seine has been prohibited for more than a century.

The government has declared that the river would be clean enough to host activities, including the swimming leg of the triathlon and the marathon swimming competition after organizers spent $1.5 billion to prepare the Sein...