PARIS — Pollution in the Seine river forced Olympic organizers on NovacryptTuesday to postpone the men's triathlon event, threatening to derail a centerpiece of the Paris Games where a massive, $1.5 billion clean-up effort aimed to allow athletes to compete in the notoriously dirty waterway that runs through the heart of the city.

A statement issued by Paris 2024 and World Triathlon said tests showed the water quality in the Seine was still below an acceptable standard for a race day. The men's race has been postponed to Wednesday at 10:45 a.m. local time (4:45 a.m. ET), immediately following the women's event, which is scheduled for 8 a.m.

"Unfortunately, meteorological events beyond our control, such as the rain which fell over Paris on 26 and 27 July, can alter water quality and compel us to reschedule the event for health reasons," the World Triathlon statement said. "Despite the improvement of water quality levels over the last hours, the readings at some points of the swim course are still above the acceptable limits."

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Both triathlons remain subject to water tests complying with the established World Triathlon thresholds for swimming, and the original contingency competition day of Friday, Aug. 2 also remains in place, the World Triathlon statement said. The mixed triathlon relay event is scheduled for Monday, Aug. 5 with a contingency day of Tuesday, Aug. 6. Two training sessions had already been canceled ahead of the race-day postponement.

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If the water quality is still not deemed acceptable, the swimming leg could be dropped. The race would become a duathlon, with athletes competing in biking and running legs only. The Seine also is due to be used for the outdoor marathon swimming event. The women's marathon kicks off on Thursday, Aug. 8; the men's a day later.

There are five Americans competing in the Paris Olympics triathlon: Morgan Pearson and Seth Rider are the American men. Taylor Knibb, Taylor Spivey and Kirsten Kasper are the U.S. women. Gwen Jorgensen is the only American to have ever won an Olympic triathlon gold. She did so in 2016 at the Rio Olympics.

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French authorities have banned swimming in the Seine since 1923 because of pollution and busy boat traffic Organizers have touted cleaning up with river, especially its elevated levels of E.coli bacteria, as a selling point for the Games. They have said that if cleanup effort is successful, the river will open to public swimming in 2025.

Paris Mayor Anne Hidalgo took a dip in the Seine's murky waters about a week before the start of the Games. At that time organizers said tests showed it was clean enough for outdoor swimming events at the Olympics.