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The Olympics are all about the unexpected. And for marathon swimmers and triathletes, that includes a notoriously unpredictable partner—the venue, which is typically an ocean, a lake, or, in the case of the Paris Olympics, the city’s historic river Seine.

As iconic as the Seine is, with its picturesque vantage points of city landmarks such as the Eiffel Tower and Musée d’Orsay, it’s also a contaminated body of water. There are enough risks to human health that the city has banned swimming in the Seine for more than 100 years.

But Olympic organizers wanted to change that, even temporarily, by staging the marathon swimming and swimming portion of the triathlon in the river, and poured $1.5 billion into building a huge tank under the Seine to hold stormwater during heavy rains and storms. The water in the tank is then funneled, gradually, into a wastewater facility where it is treated and then the clean water is flowed back into the Seine. Engineers also upgraded sewage pipes from boats and docks along the Seine to limit the amount of contaminated sewage seeping into the river.

Read More: Inside the Billion-Dollar Effort to Clean Up the World’s Most Romantic River

But despite these efforts, the Seine has continued to pose a challenge all the way up to the planned events themselves. Following early morning water quality tests on July 30, officials postponed the men’s triathlon event that was scheduled for that day to July 31 because water quality readings were “above acceptable limits.”

It’s the third day in a row that bacteria levels haven’t been safe enough for swimming after heavy rains deluged the first two days of the Games, causing officials to cancel training sessions in the river. It’s a pattern that’s plagued Paris 2024 officials since spring. At the end of July, testing continued to reveal unsafe levels of bacteria—particularly E. coli and enterococcus—in the water. The levels vary, depending on a number of factors, from the amount of rain to the number of sunny days and the speed of the current in the river. From June 24 to July 2, for example, the river was acceptable for swimming, by European standards, on six out of nine days, according to weekly reports published by Paris officials.

In an effort to demonstrate to the world their confidence in the cleanliness and safety of the Seine, Paris Mayor Anne Hidalgo and Paris 2024 organizing committee president Tony Estanguet, dove in for a few minutes on July 17, even dunking their heads underwater for a few freestyle strokes. “After the Games we will have a swimming pool in the river for all the people,” Hidalgo said after her dip.

Whether that happens still remains uncertain. The ultimate decision on whether to hold the marathon swimming and swimming portion of the triathlon in the river will be made by the respective sports’ governing bodies, depending on data gathered by Paris officials from the day before and morning of the scheduled events. According to Reuters, a meeting with the stakeholders—Paris officials, Paris 2024 Olympic authorities, the sport’s federation, regional authorities, and Meteo France (the country’s meteorological organizations)—will occur at 3:30 a.m. Paris time on each event day to make a final decision on whether the athletes will compete in the Seine. Another assessment is scheduled for Wednesday to see if the rescheduled men’s event and the originally scheduled women’s event can go ahead as planned.

That’s assuming, of course, that the lab testing for E.coli can be done quickly enough to provide reliable measurements, says Natalie Exum, assistant professor of environmental health and engineering at Johns Hopkins Bloomberg School of Public Health. “It takes about 24 hours to culture E. coli bacteria,” she says. “There may be some other proxy indicator for bacteria that they are using, but it’s not like you can stick something in the water and know how many E. coli are present in that instant.”

If the river continues to have fluctuating levels of pollutants, there are health risks athletes might face if officials decide the river is safe to swim in during the scheduled event. The most common organic contaminants—E. coli and enterococcus bacteria—are responsible for gastrointestinal illnesses, including diarrhea, abdominal cramps, vomiting and nausea—not something anyone welcomes, not the least athletes who need to be in peak physical condition for competition. “There are certain pathogenic strains of E. coli that cause what we call traveler’s diarrhea,” says Dr. Susan Kline, professor of medicine at the University of Minnesota Medical School. Kline says anyone exposed to untreated water might expect risks similar to those that campers might face in rivers or lakes, which include parasites like giardia.

Then there are the possible chemical pollutants that come from runoff from industrial waste, which could pose serious health risks, as well as skin irritation. And having cuts or open wounds on the skin could also increase the risk of any infection from whatever is in the water, says Dr. Timothy Brewer, professor of medicine at epidemiology at University of California Los Angeles.

Read More: Watch How Paris Plans to Make This Polluted and Iconic River Swimmable Again

It’s not as if the sewage system in Paris and the river are intentionally connected. During heavy rainfalls, the sewage flowing into water treatment plants gets mixed with overflow water from the rains, and the facilities’ normal treatment systems become overwhelmed by the sheer volume of water that needs to be treated. Some of that untreated water then flows into the Seine, contaminating the river. “We call that first flush,” says Exum of the flood of water that gushes through wastewater treatment plants in the aftermath of a rainfall. “I would not advise anybody anywhere in the world to swim in a river 24 hours after a major flush,” she says. “The whole sewer system is designed to manage the low flow capacity of toilets, not gushing water. These systems are not designed for the intensity of rain events that we are now seeing as a result of climate change.”

Even with a heavy rain, ordinarily, a certain amount of sewage-derived bacteria could also be killed off by the ultraviolet rays of the sun and higher temperatures. But a rainy and cloudy spring in Paris has left conditions ripe for the bacteria to thrive in the river. And the forecast for the first week of the Games is cloudy. Even with the overflow tank officials built to catch sewage during downpours, the river continues to host unsafe levels of bacteria.

The tank is an important but not sufficient step to truly getting the Seine clean, says Exum. “To decouple the rainwater system from the sewer system all throughout Paris is what essentially needs to happen but that takes decades, and is an iterative process.” Cities like Paris that don’t host too many green spaces to soak up rainwater end up sending most of it into the sewer system underground, which just builds up the pollution in the river.

Read More: Paris Mayor Anne Hidalgo Has a Lot Riding on the 2024 Olympics

Organizers, who were banking on two sunny days leading up to the triathlon, told Reuters that the Seine’s water would be tested twice daily as the Games approach, and that there is an alternative venue for marathon swimming at Vaires-sur-Marne, and a contingency plan to delay the triathlon or drop the swimming portion altogether if the river is deemed unsafe on competition days. But no official announcement to change course has yet been made, and for now, the two Seine events remain scheduled for the city’s river.

The uncertainty is adding additional stress to an already fraught situation for athletes, who would welcome some decision on where, and if, they will be competing—and reassurance that they will be safe while doing so.

—Chad de Guzman contributed reporting.

Correction, July 30
The original version of this story misattributed a quotation about the need for a backup plan. It was said by Team USA swimmer Ivan Puskovitch, not Team USA open water coach Ron Aitken. The quotation no longer appears in the updated version of this story.

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