By Yiming Woo

PARIS (Reuters) - Olympic triathletes tested swimming in the Seine in Paris on Wednesday, 10 days after heavy rainfall caused the water quality to dip below minimum health standards and forced the cancellation of another open water competition.

The Seine is in the spotlight as the river - where swimming has long been banned - is due to be the venue for marathon swimming at next year's Olympic Games.

"I always dive with open mouth. It's not going to be funny if I wake up tomorrow morning with ... whatever," triathlete Kristian Blummenfelt said, rubbing his stomach, just before he dived into the water.

Blummenfelt and others were jumping in to get used to the course and the currents before an official Paris 2024 test event scheduled for Thursday.

Bathing in the Seine has been banned since 1923, with promises to restore water quality going back to 1990, when Paris mayor Jacques Chirac - later French president - vowed to make the Seine safe for swimming again.

Risks come from heavy rain, which can cause the Parisian sewage system to overflow and be discharged into the river, polluting it with faecal bacteria E.coli and Enterococcus.

World Triathlon has said the Seine's water is tested multiple times per day and that if water quality does not meet the requirement of World Triathlon and public health authorities, the race would be shifted to a duathlon format.

(Reporting by Yiming Woo; Writing by Ingrid Melander, editing by Pritha Sarkar)