A test event on the Seine was cancelled due to heavy rainfall. GETTY IMAGES

An open water swimming test event scheduled for next week in the Seine has been cancelled due to heavy rainfall, the French capital's authorities said on Friday. During this year's Olympic Games, the opening ceremony as well as triathlon and open water competitions will take place in the river.

Heavy rainfall can lead to increased pollution in the Seine with officials warning it could overwhelm Paris' sewerage system. The water level in the Seine before Monday's now scrapped event is at 349 cubic square metres, with the level desired for this summer's events at 100 cubic square metres. 

A rehearsal for the opening ceremony in May was postponed until June 17, five weeks before the start of the Games on 26 July.

Although a new water treatment plant was inaugurated upstream of Paris at the end of April, another major Olympics-related water infrastructure project has yet to enter service. 

The giant new underground storm water facility close to the Austerlitz train station in eastern Paris, which will stock water to prevent discharges into the river, is scheduled to come on stream in early June.

Cleaning up the Seine is seen as one of the key legacy achievements of the Games, with Mayor Anne Hidalgo promising three public bathing spots for locals next year. 

Around €1.4 billion has been spent by French authorities upgrading sewage treatment and storm water facilities around Paris to reduce the amount of untreated faecal matter flowing into the river and its main tributary, the Marne.