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One of the swimming pools at Rio 2016 Olympics has turned green:

enter image description here

While proliferation of algae is a likely culprit, there have been some alternative explanations offered by local organizers. CNN has the following quote from a Rio 2016 official:

A change in alkalinity

Mario Andrada, the communication director for the Rio 2016 local organizing committee, says a sudden change in alkalinity is the culprit.

"We expect the color to be back to blue soon," Andrada said, adding there is "absolutely no risk to the athletes or anybody."

  • How could a change in alkalinity occur on such a rapid time scale (a few hours)? And why would it directly change the color of the water? I can imagine that a more alkaline water could lead to precipitation of green metal hydroxide compound, but you would need a massive concentration of e.g. nickel to start with — does not seem very likely.

  • This article (http://www.independent.co.uk/sport/olympics/rio-2016-green-pool-drained-as-olympic-organisers-reveal-hydrogen-peroxide-is-cause-of-colour-change-a7189616.html) claims, that the treatment of the water via hydrogen-peroxide is what has caused the rapid change in the chlorine levels (which in turn made it possible for the algae to pollulate coloring the pool). Is this chemically possible? If so, how (or if not, why not)?

  • Also, the organizers claimed, that if was safe to enter the pool even after it had turned green. Was it really safe, or were the organizers lying about the hazards to cover up what they had done?

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    $\begingroup$ This article (independent.co.uk/sport/olympics/…) claims, that the coror change was caused by algae, after the chlorine in the water was neutralized by hydogen-peroxide. Could anybody explain how's that possible and was it really safe, to enter the pool after it has turned green? $\endgroup$ – szentsas Dec 14 '16 at 19:03
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Although not strictly alkalinity, this article quotes a changed pH level and an expert gives two possible explanations

One: A low or high pH can bring out minerals. "Calcium would show itself as cloudy water, copper would show up as a blue-green," he says.

Two: A too-high pH would render the chlorine ineffective, which could then allow for the proliferation of algae. (Hernandez says it's doubtful, though, that the green color is mainly from algae. It typically doesn't bloom that fast in such large pools.)

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    $\begingroup$ I can see an indirect effect (allowing algae to bloom), but the direct effect I'm skeptical… what concentration of metals would it require to begin with? $\endgroup$ – F'x Aug 11 '16 at 8:14
  • $\begingroup$ @F'x Very low. Remember, water is mostly transparent. It doesn't take a lot to change the colour of something that is already mostly colour-less (or white). And different countries (or cities, or even districts in the same cities) have different water supplies, with different kinds of impurities (either straight from the source, or from the water infrastructure itself - say, using copper pipes to transfer water). $\endgroup$ – Luaan Apr 24 '17 at 12:58

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