Olympic athletes dive into the Seine — days after it was deemed too contaminated with poop for safe swimming

Rain showered down on athletes, onlookers, can-can dancers and opera singers at the 2024 Paris Olympics opening ceremony on July 26, and more rain over the weekend reportedly drove an uptick in disease-causing bacteria in the Seine river.

This decline in water quality prompted officials to cancel the swimming portion of a training session for the triathlon events, and the men's race, scheduled for Tuesday (July 30), was postponed. Finally, on Wednesday (July 31), both the women's and men's triathlons were allowed to proceed, with athletes plunging headfirst into the Seine for a 0.9-mile (1.5 kilometers) swim before jumping onto bikes for the next leg of the race.

France and Great Britain snagged gold in the women's and men's races, respectively. But even as the athletes take the podium, there's more river swimming to come, as the mixed relay and marathon swimming events are scheduled for next week.

Are there any lingering concerns about the water quality in the Seine? And what risks could subpar water pose to the comPetitors? Live Science spoke with experts to find out.

Related: Will Olympic athletes ever stop breaking records?

Poop pollution

Paris invested the equivalent of $1.5 billion in making the Seine swimmable in time for the Olympics. The renovation involved connecting thousands of residences to municipal sewer systems and constructing a huge underground cistern. This massive tank, which holds 20 Olympic swimming pools' worth of water, is intended to prevent untreated sewage from flowing into the river.

Paris has a combined sewer network that captures stormwater and wastewater, funneling both to treatment plants. But in times of heavy rain, some untreated water is instead diverted into the Seine and its tributaries to prevent disastrous backups. The new basin beneath Paris helps curb such outflows by capturing excess water and then slowly releasing it back into the sewers so it can go through the normal treatment process.