PARIS (AP) — Olympic triathletes plunged into the Seine River Monday morning as the mixed relay event got underway after organizers said the bacteria levels in the long-polluted Paris waterway were at acceptable levels.

The plan to hold the swimming portion of the triathlons and the marathon swimming events in the Seine was an ambitious one. Swimming in the river has, with some exceptions, been off-limits since 1923 because it has been too toxic.

Representatives from World Triathlon and the International Olympic Committee along with Paris Games organizers and regional and weather authorities met Sunday night to review water tests. The results indicated the water quality at the triathlon site had improved over the preceding hours and would be within the limits mandated by World Triathlon by Monday morning, they said in a statement.

The decision to allow the event to go forward with swims in the Seine came after Belgium’s Olympic committee announced Sunday that it would withdraw its team from the mixed relay triathlon after one of its competitors who swam in the river last week fell ill. It was not clear whether her illness had anything to do with her swim in the Seine.

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Paris spent 1.4 billion euros ($1.5 billion) on infrastructure improvements to clean up the river that flows through its center. That included the construction of a giant basin to capture excess rainwater and keep wastewater from flowing into the river, renovating sewer infrastructure and upgrading wastewater treatment plants.

Heavy rains that have fallen off and on during the Games have caused headaches for organizers as they result in elevated levels of fecal bacteria, including E. coli and enterococci, flowing into the river. Drenching rains fell Thursday night, but conditions had otherwise been dry except for light rain Saturday evening. The sun shone brightly as the race unfolded Monday and athletes said organizers reassured them that there were no concerns with water quality.

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Organizers have continued to express confidence that warm temperatures and the sun’s ultraviolet rays would combine to kill enough of the germs ahead of each event set to include a swim in the Seine.

Athletes swam in the river for the men’s and women’s individual triathlons Wednesday, though the men’s race had been delayed by a day because of the water quality. Elevated bacteria levels in the waterway have caused cancellations of the swimming portion of training sessions for the relay event.

Daily water quality tests measure levels of the fecal bacteria, including E. coli. World Triathlon’s water safety guidelines and a 2006 European Union directive assign qualitative values to a range of E. coli levels.

Under World Triathlon’s guidelines, E. coli levels up to 1,000 colony-forming units per 100 milliliters can be considered “good” and can allow competitions to go forward.

The triathlon mixed relay involves four-person teams made up of two men and two women, with each athlete swimming for 300 meters (yards), cycling for 6.8 kilometers (4.2 miles) and running for 2 kilometers (1.2 miles).

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