RIO DE JANEIRO — Olympics officials have given what they described was a definitive explanation why the water in two competition pools had turned green last week. 

Someone, the officials said, mistakenly added 160 litres of hydrogen peroxide to them on Aug 5, neutralising the chlorine and allowing for the growth of “organic compounds” that might have included algae.

On Saturday (Aug 13), the water in the diving pool was still a murky green, while the water in the second pool, used for water polo and synchronised swimming, had begun to look a bit lighter and bluer. 

Athletes have been complaining that the water or the chemicals have been hurting their eyes and that murkiness has made underwater visibility impossible.

Mario Andrada, a spokesman for the Rio Games, said that to ensure that the competition could go ahead, officials would drain the water from the pool — all 3,725,000 litres — and replace it with clean water from a nearby practice pool.

The procedure was expected to take 10 hours: Six to drain the pool, and four to refill it, said Gustavo Nascimento, the director of venue management for the Rio Olympics.

Andrada and Nascimento held a news conference on Saturday at the Maria Lenk Aquatic Center, the site of the pools, where they said that while they did not want to assign blame for the mishap, it appeared that the errant hydrogen peroxide had been added by a local pool-maintenance worker who had thought he was doing the right thing.

Andrada said he was sorry to have promised at several points that the problem would be cleared up right away. “Of course it’s an embarrassment,” he said. “We are hosting the Olympic Games and athletes are here, so water is going to be an issue. We should have been better in fixing it quickly. We learned painful lessons the hard way.” THE NEW YORK TIMES