With three weeks to go until the Paris Olympics, organizers are making contingency plans as questions remain about the safety of the River Seine that is due to host open-water swimming events during the summer Games.

The latest testing results from monitoring group Eau de Paris released on Thursday raised some hopes, showing that water pollution levels in Paris’s River Seine have improved over the past week.

The concentrations of E.coli and enterococci bacteria were below legal thresholds six out of nine days between June 24-July 2, according to the data published by the city of Paris.

“Despite a flow rate that remains high, the water quality of the Seine has improved over the period observed, with water quality in line with the thresholds defined by the European directive over six days,” the city said in a statement along with the results.

Just a week ago, test results revealed that enterococci exceeded a concentration of 1000 colony-forming units (cfu)/100 ml, more than double the 400 cfu/100ml limit set by European law and the E.coli concentration was almost four times higher than permitted.

If the pollution levels remain high, Paris organizers will likely have to turn to a Plan B to avoid health risks to athletes, said Marc Habash, an associate professor at the University of Guelph’s school of environmental sciences.

“If there are high levels of E.coli and enterococci suggesting the water is unsafe, the water is considered of poor quality, for the most part, they will have to make a decision on whether they’ll hold the swimming event or not,” he told Global News in an interview.

“If the water quality is deemed not safe, they won’t allow the swimmers to swim.”

The summer Games are set to kick off on July 26 with the opening ceremony which will see more than 10,000 athletes cruising on boats through the River Seine.

Marathon swimming events from August 8 to 9 and the triathlon on July 30, July 31 and August 5 are also scheduled to take place in the Seine.

On Friday, organizers told Reuters that they are looking at other options, such as a different venue for marathon swimming and scrapping the swimming leg from the triathlon altogether.

“The rules of World Triathlon allow, as a final resort, for the competition to be held in a duathlon format,” a Paris 2024 spokesperson told Reuters in French.

“On the other hand, in order to guarantee that marathon swimming events could still be held if all other contingency plans were exhausted, we have initiated a fallback plan based on the Vaires-sur-Marne Nautical Stadium,” the spokesperson said.

“The competition site, already used for rowing and canoeing events, has all the necessary features to host these events if required.”

Organizers also told the Associated Press on Friday that the events could be postponed by a few days if conditions at the Seine are unsafe.

Global News did not a get a response from Paris 2024 about the Seine’s safety and the backup plan by the time of publication.

Habash said it’s not unusual for sporting bodies to test the water quality ahead of a competition to make sure it’s deemed safe for swimming.

The Canadian Olympic Committee told Global News that its medical team is in frequent contact with World Aquatics and Paris 2024 organizers, adding that Team Canada’s health and safety was the top priority.

“We are confident that the events can be safely held in the Seine,” the COC said in an emailed statement.

 The French capital has been working on cleaning up the Seine so people can swim in it again, as was the case during the 1900 Paris Olympics.

But a sewer problem last summer led to the cancellation of a pre-Olympics swimming event.

Water pollution levels depend on rainfall and water temperature among other factors, so Paris 2024 organizers will be hoping that the weather cooperates during the Games.

Recent heavy rain, which was more than usual for this time of the year, has been a big cause of the elevated numbers of E.coli and enterococci, Habash said.

“If they don’t get a lot more rain in the coming weeks just before the Olympics the anticipation is that those levels will decrease over time,” he said.

“And that’s really the unknown at this point is, will they get more rain? And if they do, that may cause the levels to remain high.”

E.coli and enterococcus are used as measures of water quality to check for any sewage contamination or fecal pollution, experts say.

Although E.coli infection in water can happen in certain cases, it’s typically not the biggest concern, said Ted Steiner, an infectious disease physician at Vancouver General Hospital.

E.coli is a marker that there may be other parasites, bacteria or viruses that could potentially be more contagious, he told Global News in an interview.

“The concern for swimmers is that as they’re breathing, they can certainly get some water in their mouth and swallow enough potentially to get a large enough dose to get sick,” he said.

Gastrointestinal infections could cause an upset stomach, vomiting and diarrhea, Habash said. There is also a risk of infecting the ears and respiratory tract, he added.

Steiner said skin infections in rivers are relatively rare and usually happen if there is a significant break, like a cut or abrasion.

The risks of getting sick from contaminated water are higher for people with underlying immune system disorders, he said.

If someone does get ill, there are some medications to treat, but most of the infections resolve on their own, Steiner said.

“Young, healthy athletes with a normal immune system, if they do get sick, it would typically be a self-limited illness, typically, less than a week, sometimes a bit longer,” he said.

— With files from Reuters and The Associated Press.