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During the first few games of diving I had not noticed this color, but on Tuesday, August 9, 2016 the water of the Olympic diving pool in Rio 2016 was green.

What would be the reason for this color?

Here you can see the difference between two close pools:

Greenwater_rio2016

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This was unintentional, and caused by the pH of the pool being "outside the usual range". Quoting from BBC News:

The International Swimming Federation (Fina) explained it was caused by water tanks running out of certain chemicals.

But Fina stressed that there was "no risk" to athletes.

It explained that the discoloration was caused by the pH level - or acidity level - of the water being "outside the usual range".

  • Could it have been a cleaning substance or something of the sort? – Noir Antares Aug 10 '16 at 9:59
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    Chloramine, a common sanitizing agent and a family of sanitizing by-products, will turn the water green. For residential pools, shock helps to reduce those chloramines by changing them into more volatile chemicals that evaporate out of the water (creating that "chlorine smell"). I'm not sure what a pool this size would be using. – KeithS Aug 10 '16 at 15:36
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A third-party contractor used hydrogen peroxide to clean the pools. Hydrogen peroxide is a de-chlorination agent, it neutralizes the chlorine which is added to the water to prevent the growth of algae and other micro-organisms.

With the chlorine levels lowered without anybody knowing, algae grew in the water and turned it green and reportedly slightly smelly. Now that the problem has been corrected, the chlorine is killing off the algae, and the pools are slowly returning to normal.

The pool used for synchronized swimming had to be completely drained and re-filled (using water from a training and warm-up pool for water polo, which is no longer needed because after the qualifying phase, the water polo competition was scheduled to move into the big stadium anyway) because neither the athletes nor the judges would have been able to clearly see what goes on under water, which is where most of synchronized swimming happens.

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