Water quality was acceptable when Olympic athletes swam in the Seine, data shows

By Kate Brumback, Associated Press

PARIS (AP) — Water quality data used by Paris Olympic organizers show that bacteria levels in the Seine were within acceptable limits on days athletes swam in the famed waterway, according to their tests and established thresholds.

The Associated Press obtained results from daily tests of fecal bacteria on water samples taken at four points along the river two or three times a day. The data covers a period from July 19, a week before the opening ceremony, through Thursday, the day of the women’s marathon swim and the day before the men’s marathon swim.

Officials shared some water quality data in briefings during the Games, but the new information is a more comprehensive set of numbers, including results from at least two tests per day for all four sampling sites.

The Seine, the river that runs through the heart of the French capital, is so polluted that swimming has been banned for a century, with few exceptions. Paris has launched a massive renovation project costing 1.4 billion euros ($1.5 billion) to allow the Olympic triathlon and marathon swimming events to be held in the Seine.

Officials said the competitions were successful because they ran largely as planned.

But the data shows that levels of E. coli and enterococci were well above those considered acceptable for competition on many days during the three-week period, typically settling at “bad” for three or four days after heavy rain. That raises questions about Paris’ long-term plans to allow the public to swim in the river from next summer.

Peruvian Maria Alejandra Bramont-Arias competes in the women’s 10km marathon swimming event at the 2024 Summer Olympics, Thursday, Aug. 8, 2024, in Paris, France. (AP Photo/David Goldman)

The weather has a huge impact on the cleanliness of the Seine. Rain can wash sewage and bacteria into the river. Conversely, high temperatures and the sun’s ultraviolet rays can kill germs. The warm, sunny weather during the Games sometimes gave way to torrential rain.

High bacteria levels following torrential rains led to the cancellation of some test races that were to allow athletes to familiarise themselves with the course ahead of the triathlon and marathon swim events. It also led to the men’s individual triathlon being postponed by one day, but the other Olympic events went ahead as planned in the river.

Several athletes who swam in the Seine reported gastrointestinal illnesses after their events, but it was unclear whether bacteria in the water were to blame.

Torrential rains were observed during the opening ceremony of the Games on July 26 and on the night of July 31, after the individual triathlon events that had taken place earlier in the day. On each occasion, bacteria levels in the river increased dramatically and it took several days to return below the level considered acceptable, the data shows.

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The figures reveal how levels can vary from one testing site to another, even though all areas are relatively close to each other.

On Monday, during the mixed triathlon relay, all four sites had acceptable levels of E. coli, while the enterococci level was too high at one site. But that site, Port du Gros Caillou, was outside the relatively short course used for the relay.

According to guidelines set by World Triathlon and World Aquatics, the two sports governing bodies that decided whether the Olympic events would take place each day, “good” quality water can include up to 1,000 colony-forming units of E. coli and up to 400 colony-forming units of enterococci per 100 milliliters.

Bacteria levels can change dramatically in a short period of time. The Port du Gros Caillou site recorded a “low” level of 436 enterococci units at 5:45 a.m. Monday. By 12:25 p.m., it was at a “very good” level of 99 units.

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 sip of contaminated water can cause urinary tract or intestinal infections. Several factors determine whether a person becomes sick after exposure to these bacteria, including age and general health.