The first training session for triathletes at the Paris Olympics was scrapped on Sunday due to pollution in the River Seine, organisers said.
Following tests and a meeting to discuss water quality, "a joint decision was taken to cancel the swimming part of the triathlon orientation", a Paris 2024 and World Triathlon statement said.
The River Seine was declared clean enough for swimming in tests from mid-July, but doubts remained as to whether the waters would be fit for competition.
"Paris 2024 and World Triathlon reiterate that the priority is the health of the athletes. The analysis carried out yesterday in the Seine showed water quality levels that... do not present sufficient guarantees to allow the event to take place," the statement said.
Seine fit for swimming most of past 12 days, Paris city hall says ahead of Olympics
Raw sewage
The quality of the Seine – which is set to be used for triathlon and marathon swimming – is dependent on the amount of rain that falls in and around Paris.
Heavy downpours of the sort seen on Friday during the sodden opening ceremony overwhelm the city's sewage system, leading to discharges of untreated effluent into the waterway.
"We're not particularly worried," chief organiser Tony Estanguet told reporters on Sunday.
"We know that for 24 to 48 hours after heavy rain that we might need to delay [events]. The weather forecast shows that in theory the rain shouldn't return."
The triathlons are set to take place on 30-31 July and 5 August, followed by the open-water swimming on 8-9 August.
Due to an exceptionally wet spring and start to summer, the Seine had been consistently failing water tests until the start of July.
Levels of the E.coli bacteria – an indicator of faecal matter – were sometimes 10 times higher than authorised limits.
Organisers can delay the outdoor swimming events by several days in the event of rain.
As a last resort, they have said they will cancel the swimming leg of the triathlon and move the marathon swimming to Vaires-sur-Marne, on the Marne river east of Paris.
Clean-up operation
French authorities have invested 1.4 billion euros over the last decade to clean up the Seine.
French authorities have invested in new water treatment and storage facilities in and around Paris, as well as ensuring that thousands of homes and canal boats without waste-water connections are linked up to the sewerage system.
Paris Mayor Anne Hidalgo, Paris 2024 chief organiser Tony Estanguet and French Sports Minister Amelie Oudea-Castera are among the dignitaries who have taken a dip in the Seine in recent weeks in a bid to prove the river is swimmable.
Hidalgo plans to create three public bathing areas in the Seine for the city's residents next year – a century after swimming in it was banned.
(with AFP)