PARIS — On Tuesday morning, athletes should have been thrashing down the River Seine in Paris, battling to come out on top of the Olympic triathlon's first leg: 1,500 meters of open water swimming.

Instead, not a single swimmer was in sight.

The reason? Once again, the Seine was too polluted to swim in.

In a press release published at 5 a.m., just three hours before the race, World Triathlon announced the men's trial was postponed to Wednesday for "health reasons", after water testing "revealed water quality levels that did not provide sufficient guarantees to allow the event to be held".

The news, which followed the cancellation of two training sessions in the river for the same motive, prompted furious reactions from triathletes, who had gotten up before the crack of dawn in hope of competing.

"Changing the day like that in the middle of the night is disrespectful to the years of preparation of the athletes and to all (y)our fans that were going to watch live or on TV," Belgium's Marten Van Riel, who ranked fourth in the men's triathlon at the Tokyo Olympics, wrote on Instagram.

The resurrection of public swimming in the Seine, which has been banned for 100 years because its waters are filled with hazardous bacteria, is a decades-long dream of French politicians. It is meant to be one of the Games' great legacies for Parisians, who will supposedly be able to swim freely in the river starting next year.

To clean up the river, the French authorities have invested €1.4 billion to build a giant pool designed to store wastewater and prevent the sewage system from spilling into the river, which can happen during heavy rainfall.

But French politicians knew it would take more to reassure the public. That's why Paris Mayor Anne Hidalgo, who has bet her political legacy on making the French capital a greener city, recently braved the river's murky waters in front of the world's cameras, in a carefully orchestrated media stunt that stood in sharp contrast with outgoing Sports Minister Amélie Oudéa-Castéra's solo dip, a few days prior.

While Hidalgo had convened international media and was escorted by several officials when jumping in the river, Oudéa-Castéra was accompanied by one Paralympic athlete and one French TV channel. She almost slipped upon going in, and then frantically screamed while floating on her back, which resulted in a clip that has since become viral.

Still, athletes are far from convinced. Some have resorted to gobbling probiotics, while others have stopped washing their hands after going to the toilet to build up their immune system, Le Monde reported.

According to data from the city of Paris analyzed by POLITICO, concentrations of the E.Coli bacteria exceeded European safety standards on more than half of the days between June 3 and July 23 — including the day Hidalgo swam in the river.

Aurélie Merle, the Games' sports director, said during a press conference on Tuesday morning that the bacteria levels measured in the river on Sunday "were between 980 and 1553" colony-forming units (cfu) per 100 milliliters of water. Three of the four measurement points in the Seine were above the threshold for international swimming competitions, set at 1000 cfu/100 ml.

With heavy rain forecasted to fall on Paris tonight, there's a risk that both the men's and women's triathlons will be postponed again.

The Games' organizers have a contingency plan: both trials could be held on August 2, if the weather allows it.

If not, the triathlon will be turned into a duathlon, scrapping the swimming trial — a prospect deemed "unthinkable" by the French triathlon team's technical director on Tuesday morning, as reported by RMC radio.

Giovanna Coi contributed reporting.