Athletes swam in the Seine on August 18 for the first time since 2012 only for World Triathlon to cancel the swimming segment of both the Para triathlon test event and mixed relay competition and instead opt for a duathlon format.

"At this stage, the preferred hypothesis is that of the malfunction of a valve of the sewerage network located at the Tolbiac bridge," said the City of Paris, as reported by Le Monde.

"Investigations are continuing to understand the sequence of events and determine measures to be taken to ensure the perfect quality of water for the holding of the events in 2024."

World Triathlon ruled that a "significant discrepancy" in the analysis of the quality of the water led to the decision, before an investigation by Paris 2024 and public authorities.

A reading went beyond the threshold of 1,000 E. coli per 100 per 100 millilitres despite the absence of rain which was blamed for previous cancellations.

"It's regrettable, but it's a situation that we will be able to better understand in the future," said Paris' Deputy Mayor Pierre Rabadan, as reported by Agence France Presse.

In a previous Paris 2024 test event this month, World Aquatics was forced to cancel its Open Water Swimming World Cup event in the French capital due to pollution concerns in the Seine. 

This was put down to thunderstorms and exceptional levels of rain.

The still-under-construction Austerlitz basin is hoped to prevent heavy rain pushing wastewater into the river. 

It is due to be able to store rainwater up to 50,000 cubic metres, the equivalent of 20 Olympic-sized swimming pools, and will contribute to Parisian Mayor Anne Hidalgo's aim to have three sites of the Seine open for public swimming in 2025.