The level of faecal bacteria in the Seine was above the World Aquatics standards of 400 colony-forming units of enterococci per 100 millilitres. The women's marathon swim competition is set for Thursday, while the men are scheduled to race Friday.
ADVERTISEMENTA test run meant to allow athletes to familiarise themselves with the marathon swimming course in the Seine River was cancelled on Tuesday due to concerns about the quality of the Paris waterway.
Fluctuating bacteria levels have been a constant concern throughout the Games due to the swimming portion of the triathlon and the marathon swimming events, both planned in the river.
The cancellation of Tuesday's marathon swimming test event comes a day after the triathlon mixed relay event was held in the river that runs through the centre of the French capital.
Most strains of E. coli and enterococci are harmless, and some live in the intestines of healthy people and animals.
But others are dangerous, and even a mouthful of contaminated water can cause infections in the urinary tract or intestines. Several factors determine whether a person falls ill after exposure, chief among them a person’s age and general health.
Four triathletes — out of the more than 100 who competed in the men’s and women’s individual races last week — became sick in the following days, though it’s unclear whether the water was to blame.
Monday's enterococci levels were available by midmorning Tuesday, and while they showed an improvement in the river's water quality, one of four tests still fell short of World Aquatics standards.
Under both World Aquatics and World Triathlon guidelines, “good” water quality can include up to 1,000 colony-forming units of E. coli per 100 millilitres and up to 400 colony-forming units of enterococci per 100 millilitres.
Data released Tuesday show that a sample taken early Monday at Port du Gros Caillou, which is on the marathon swimming course but beyond the point where triathletes turned around Monday, showed a level of 436 units of enterococci.
Paris Mayor confident events will go ahead
Organisers said they “remain confident” that the marathon swimming events will happen in the Seine as planned based on “a favourable weather forecast and forward-looking analysis.”
Paris Mayor Anne Hidalgo, who took a highly publicised swim in the Seine last month to allay fears about water quality ahead of the Olympics, echoed that confidence.
“We will of course wait to get the results of the water quality but the event will take place because there’s been a clear improvement of the weather these past few days,” she said.
"So I’m really proud and happy and to all those who want to continue saying it’s impossible to depollute a river, I tell them, ’Yes it’s possible, we did it.'”
Seine swimming has been banned for over 100 years
With a few exceptions, swimming in the Seine has been prohibited since 1923 because the water has been too toxic.
Paris undertook an ambitious plan, including €1.4bn in infrastructure improvements to ensure that some swimming events could be held in the river.
ADVERTISEMENTThat included building a giant basin to capture excess rainwater and prevent wastewater from flowing into the river, renovating sewer infrastructure, and upgrading wastewater treatment plants.
Water quality in the Seine is closely linked to the weather. Heavy rains can cause wastewater and runoff to flow into the river, resulting in elevated bacteria levels, while warm temperatures and the sun's ultraviolet rays can kill the germs and lower levels.