By Karolos Grohmann

RIO DE JANEIRO (Reuters) - The Rio Olympics diving pool will be back to its blue colour later on Wednesday, Games organisers assured, blaming a drop in alkalinity levels for an uninviting green hue during competition on Tuesday.

The water in the pool was bright green on Tuesday, baffling competitors in the women's 10 metre synchronised event, who said they could not see their partner underwater.

The mysterious shade of the water, which contrasted sharply against the blue of the water polo pool beside it, was also clearly visible on television to millions of viewers around the world, many of whom joked about algae and dye on social media.

The international swimming federation (FINA) blamed a lack of chemicals in the system.

"FINA can confirm that the reason for the unusual water colour... is that the water tanks ran out some of the chemicals used in the water treatment process," it said in a statement.

"As a result the pH level of the water was outside the usual range, causing the discolouration."

Organisers said tests at the Maria Lenk Aquatic Centre's diving pool were conducted regularly and the quality of the water was no threat to the divers' health.

"There was a sudden decrease of alkalinity," said Games spokesman Mario Andrada. "We probably failed to note that with more athletes the water could be affected."

He assured divers the water posed no health risk and despite the rain slowing down the process of turning the water blue, this would happen some time later on Wednesday.

"There is absolutely no risk to anybody. The independent group (charged with checking the water quality) confirmed the measures to bring the pool back to its normal colour," Andrada said.

"The rain affected a bit the changes in the state of the water. It should go back to classic blue colour during the day."

Athletes were unaffected by the change in colour but Mitch Geller, chief technical officer of Diving Canada said the team had brought in a pool expert to help.

"He arrived this morning from Canada, he's assisting with the hosting group to try to get it back under control," Geller told reporters.

   "We don't think it is dangerous or we wouldn't have our athletes in the water. But we are going to monitor to see if anyone is getting red eyes here, if their ears start hurting, we'll be dumping a whole lot of antibiotics into them."

The problem comes on top of worries over dangerous levels of pollution in Rio's Guanabara Bay and concerns that floating garbage could damage or slow sailors.

Organisers are also struggling with major transportation, ticketing and security issues in the first five days of the competitions.

(Additional reporting by Brenda Goh, editing by Susanna Twidale and Toby Davis)