The River Seine is a key element in the Paris 2024 Olympics. A boat parade will take place there, as well as two events: the triathlon swimming leg and the marathon swimming.

The pollution levels of the Seine has raised concerns about athletes' health.

Where are we now?

Weekly results are published by the monitoring group (State and City of Paris) based on Eau de Paris analysis. The latest data, published on 12 July, showed that water quality at Olympic swimming sites would be good for swimming six days out of seven.

WHAT ARE THE CONDITIONS FOR THE RIVER TO BE SUITABLE FOR SWIMMING?

The test measures the levels of two bacteria – E. coli and Enterococcus faecalis.

According to European standards the limit of E. coli for 100ml is 900 colony forming units (cfu), while Enterococcus has a threshold of 330cfu/100ml.

WHAT FACTORS AFFECT WATER QUALITY

Benjamin Raigneau is the Director of Water Quality for Paris City Hall. He says that four factors are responsible for water pollution.

- Rainfall. More rain more pollution runoff.

UV index Raigneau said that the higher the UV index is, the quicker bacteria will die.

- Temperature. The bacteria will die faster if the temperature is higher. Raigneau said that the difference between the temperature of water at 16 degrees Celsius and 22 Celsius, which is the usual summer temperature, is significant.

- The speed of the flow. The pollution increases with the speed of the river.

What has been done to improve the quality of the water?

To reduce pollution risks on the Seine, the city of Paris constructed a basin that can hold 46,000 cubic meters of waste water.

After the rainwater is collected in Austerlitz basin, a 30-metre-deep structure that's the size of 12 Olympic swimming pools, it will be pumped through a tunnel and into a treatment facility. Once the water has met the health standards, it is poured into Seine.

HOW IS THE DECISION TO ALLOW SWIMMING MADE?

A technical committee is formed every afternoon, before each session of familiarisation (athletes swimming) and each competition. It brings together all of the parties involved in the water quality issue in the Seine, including the City of Paris, Paris2024, the International Federation of Swimming, regional authorities and Meteo France.

After the meeting of the technical committee, a preliminary analysis of weather conditions is done.

A final meeting with all players is held at 3.30am the day before the competition.

Who makes the decision?

The International Federation of the sport.

WHAT IF THE WATER IS NOT SUITABLE FOR SWIMMING ON D-DAY?

Organisers said that contingency days for both sports are planned for summer 2024, giving us some flexibility.

The Eau de Paris lab will analyse the water twice daily, allowing for a more precise and refined decision-making.

In the event that the competition is not able to take place despite the postponements the marathon swimming will be held at Vaires-sur-Marne reserve, where canoeing and rowing events take place. The triathlon will then become a duathlon. (Reporting and editing by Michael Perry; Reporting by Julien Pretot)