One of the key infrastructure projects designed to help tackle pollution has been a new rainwater storage basin – called the Austerlitz basin – which can hold 20 Olympic swimming pools worth of water, according to a statement from Paris 2024.

Following rainy weather on June 17 and 18, the basin filled to 80% capacity, successfully preventing 40,000 cubic meters (40 million liters) of wastewater and rainwater from being discharged into the Seine, according to the city of Paris.

But the water quality can change fast. Official data shows one-off rainfall events can quickly cause E. coli levels to spike again, and as recently as June 30 – following rain the previous day – E. coli levels increased to around 2000 CFU/100mL at Alexandra III Bridge, where the race is set to begin.

That’s double the level needed for “good” water quality, as per World Triathlon standards.

For much of the month before the games, E. coli levels were above acceptable levels, which could expose athletes to serious health concerns. If E. Coli levels are above 1000 CFU/100mL, the swimming leg of the triathlon will have to be cancelled, according to World Triathlon competition rules, unless the organization’s medical committee decides the race can go ahead.

The individual triathlon races took place with a 1.5-kilometer swim, followed by a 40K bike race – passing by the Le Grand Palais and the Champs-Élysées – before finishing with a 10K run through the heart of the city.

The mixed team relay race involving four athletes – two men and two women – saw each team member completing a 300-meter swim, followed by a 5.8K bike ride and 1.8K run.