The plan is for the aquatic phase of the triathlon and the 10km open water marathon for Paris 2024 to take place in the Seine from 30 July, with familiarisation tests scheduled for the morning of Sunday 28 July.

Bacteria levels rise with rainfall, despite government efforts to clean up the river, which has been polluted for decades and has been closed to swimming almost every day for the past 100 years.

Official measurements show that on some days the levels of E. coli and Enterococcus bacteria have exceeded the limits set by the International Triathlon Union. This is significant because, until now, measurements have been carried out by French government officials and the organising committee, something they have no control over during the competition.

The first event, the men's individual triathlon, is scheduled for 30 July, and the final event, the men's open water swimming, will take place on 9 August in the same natural setting where the opening ceremony was held, with around 8,000 athletes on boats parading for 6 km and 300,000 people cheering in the heavy rain that lashed Paris.

Although the Seine is more than 770 kilometres long and runs through almost the whole of France, the Olympic phases will take place specifically in the city centre, on the Pont Alexandre III, less than two kilometres from the Eiffel Tower. At this point, the organisers will carry out daily tests to ensure that the water quality is suitable for sporting activities.

The International Triathlon Union sets the limit for E. coli (a bacterium found in faeces) in inland waters such as the Seine at 500 colony-forming units per 100 millilitres of water. In the first few weeks of July, the level was well exceeded, reaching 2,000 on 21 July, according to official readings updated daily. This is four times the permitted limit.

This figure caused alarm in Paris, although optimism prevailed in the face of a favourable weather forecast, until it changed drastically and what appeared to be drizzle turned into intense and heavy rain over the French capital for more than 30 hours. When it rains heavily, these pipe networks become overloaded and wastewater is discharged into the nearest body of water, which in Paris is the River Seine, which runs through the city.

The equation is simple: the more rainfall, the greater the likelihood of pollution in the Seine. Although a mega-project costing 1.4 billion euros and nine years of work has improved the water treatment system, it is still not enough.

A similar problem arises with the second bacterium that the International Triathlon Union requires to be measured: Enterococcus. The organisation's limit is 200 CFU/100 ml, and on 10 July alone at the Pont Alexandre III, according to the organising committee, there were more than 1,400 units, and on 21 July there were 800.

The organising committee has repeatedly stated that there is no plan B and that the events will not be moved. In such cases, the World Triathlon Committee's guidelines allow for competitions to go ahead despite evidence of water contamination: "If the water quality test shows values outside the above tolerance limits, swimming will be cancelled unless the World Triathlon Medical Committee allows the Good Water Quality category".

What exactly does this mean? It means a two event triathlon instead of three if E.Coli and Enterococcus levels remain high.

What does it depend on? On Mother Nature taking a hand and stopping the rain, and on the weather forecast showing no rain between Saturday 27 and Wednesday 31, although everything could change as it did for the Opening Ceremony and the first day of the main competitions.

It is likely that the rain that has fallen on Paris over the past 24 hours will prevent the athletes from taking part in the familiarisation (warm-up) session scheduled for Sunday 28 July at 20:00 (CEST). The decision on whether the session will take place will be made at 16:00 (CEST) on Sunday.

Despite this and the likely cancellation of the tests, organisers remain confident that the events will go ahead as planned from 30 July. "The weather forecast for the next 48 hours is encouraging and the water will return to acceptable levels".

The men's individual triathlon is scheduled for Tuesday 30 July and the women's for Wednesday 31 July, both at 20:00 (CEST), while the mixed relay is scheduled for Monday 5 August at the same time.

The same problem could affect the open water swimming events, with the women's 10km on Thursday 8 August and the men's 10km on Friday 9 August at 19:30 (CEST).