Paris Olympic organizers have ambitious plans to hold the swimming sections of the triathlons and the marathon swimming events in the iconic River Seine, despite fears over the water quality.

The plan follows on from a string of controversies over water pollution that have hit sports which need open water rather than a chlorinated swimming pool, such as rowing, triathlon, sailing, surfing and marathon swimming.

The French capital region is spending €1.4 billion ($1.5 billion) to try to clean up the river after complaints from swimmers in warm-up events over potential sewage levels. Some test events were even canceled.

Persistent heavy rainfall causes locals canals to overflow, meaning waste water enters the river and increases the concentration of bacteria. Money is being spent on special pumps and so-called catch basins to prevent sewage reaching the Seine.

But a leading expert at Oxford University told DW that even stopping sewage getting into the river is not necessairly enough to make the water safe for swimming.

"It is widely advised not to swim in...inland waters for several days after (heavy) rain. This is due to the potential for sewer overflows, but also increased risk from animal feces being washed into the water. This might include dog and cat poo," said Dr. Katrina Charles, Associate Professor and Senior Research Fellow at Oxford's School of Geography and the Environment. 

"Even without sewer overflows you would have an increase in contamination and health risk after heavy rain," she said.

Revert to being a duathlon

A big deluge generally happens only four or five times a year in Paris and Olympic organizers hope the summer weather during the July and August Games prevents any issues.

But if a deluge did happen, the marathon (also called open water) swimming and triathlon competitions would have to be postponed or canceled. Paris 2024 president Tony Estanguet said the triathlon could even revert to being a duathlon, with just cycling and running, if the worst happens.

"We can postpone for rainy conditions. Because it's programmed at the beginning of the Games, we can wait for better conditions. So we're confident it will be possible to use the Seine," he told reporters recently. "And there is a final decision where we could not swim. It is part of the rules of the international federation. It's what we want to avoid of course."

E-coli, a bacteria found in human waste, is the big problem in the Seine according to water pollution charity Surfrider Europe.

It is not a new problem for the Olympic sports which need open water. Rowing and canoeing often use purpose-built courses, where pollution can be more easily managed, at Games  but triathlon, marathon swimming and to a lesser degree sailing and surfing are a challenge. The surfing in "Paris" will take place 15,700 kilometers (9,755 miles) away in Tahiti and the sailing in Marseille.

At the Rio 2016 Games, a long-term study by Associated Press found waterways such as those off Copacabana beach were contaminated with raw human sewage including dangerous viruses and bacteria. Swimmers took antibiotics to avoid illness and evidence of actual sickness after the Games was scant, although one Belgian sailor did report ill health.

But as Paris is proving, the issue is not confined to the likes of Brazil, but mature developed Western nations. 

High rainfall

Britain's illustrious University Boat Races, where Oxford and Cambridge universities battle it out in rowing on London's River Thames rather than a man-made course, were marred this year by research revealing high levels of E.coli in the river.

Lenny Jenkins said he and several of the losing Oxford crew were ill on the morning of the event following training sessions on the Thames.

"This is in no way to take away from Cambridge — we've had a few guys go down pretty badly with the E.coli strain," he told reporters. "I was throwing up and I wasn't sure if there was going to be a chance of me being in the boat but I ultimately kept that quiet and that's on my shoulders."

Cambridge were advised not to throw any team mamber into Thames water, as traditionally happens with the cox, who guides the boat from the front. The local water company, Thames Water, was pilloried in the British media amid reports the firm has allowed around 72 billion litres of sewage into the river since 2020.

The company, which is in financial difficulty, blamed high rainfall for the state of London's storied river.

Rowers and sailors can limit the health risk by wearing special gloves to avoid contaminated splashes from the water, but swimming through dirty water with your mouth open is a whole different battle.

England's biggest lake, Windermere, is a popular spot for sports and an annual triathlon takes place there in June. Windermere also has a problem with untreated sewage and Hollywood actor Steve Coogan joined a recent protest against local water firm United Utilities.

Chris Matthews, from United Utilities, told Britain's ITV: "We are taking this problem extremely seriously as part of a huge investment program."    

Paris has similarly invested huge amounts to make its open water swimming events safe, but it might not be enough.

"There will always be an increase of pathogens in river water after rainfall," Oxford's Dr. Charles said.

Edited by: Matt Pearson