Lastly:
The chief spokesperson for the Maria Lenk pool in Rio says they were caught off-guard by the change in colour, adding: ''Chemistry is not an exact science.''
No, no it isn't.

I was being sarcastic.

Well, unless you actually study chemistry, in which case one would know that the exact same experiment, performed correctly, would yield the same result each time. How silly of me to think that the Olympics would have this under control. There are 100 million pools. This is not the frontier of space.
Duchess of Verona's post is supported by at least these two references:
How is hydrogen peroxide dosed?
Most hydrogen peroxide applications consist of hydrogen peroxide injection into flowing water. No other chemicals or equipment are required. This application is used to control biological growth, to add oxygen,
to remove chlorine residues and to oxidize sulphides, sulphites, metals and other easily oxidized materials. The suitability of hydrogen peroxide for these applications is influenced by pH, temperature and reaction time.
Ref:
www.lenntech.com/processes/disinfection/chemical/disinfectants-hydrogen-peroxide.htmLiveScience
Did Hydrogen Peroxide Really Turn Olympic Pools Green? Not So Fast
By Laura Geggel, Senior Writer
August 16, 2016 07:14am ET
Olympic officials in Rio de Janeiro announced this past weekend that they had found out why the water in two pools turned bright green, but their explanation has at least one chemist scratching her head.
The confusion comes down to an understanding of basic chemistry and swimming pool maintenance, said Susan Richardson, a professor of chemistry at the University of South Carolina and a former pool owner.
...''Someone, the officials said, mistakenly added 160 liters [42 gallons] of hydrogen peroxide on Aug. 5, neutralizing the chlorine and allowing the growth of 'organic compounds' that might have included algae,'' the Times said.
...If enough hydrogen peroxide were poured into a chlorinated pool, it would react with all of the bleach, until
there wouldn't be any chlorine left to kill algae and other organisms that thrive in warm pools, she said.
...However, when Olympic officials monitored the pools, they found the proper amount of chlorine in the pools, but didn't know that the chlorine wasn't working, Gustavo Nascimento, director of venue management for the Rio Olympics, told the Times.
This doesn't make sense, Richardson said. ''Even if they had 'accidently' added hydrogen peroxide, they should be checking the chlorine level regularly,'' Richardson said. ''They would have known immediately that the chlorine was gone.''
Ref:
www.livescience.com/55770-hydrogen-peroxide-in-olympic-pools.htmlSo everyone swimming in green water pools was swimming with algae bloom and ''other organisms'' in the pool and the Oly maintenance crew wasn't in a hurry to fix it. I truly hope no one has any after-effects from the poorly maintained pools.