What’s up with the green pool at the Rio 2016 Olympics?

The Olympic diving pool was closed Friday morning, after the water turned green. According to a Rio 2016 spokesperson, diving training was cancelled in order to accommodate cleaning efforts to return the pool to its normal blue color. The pool water first turned green on Tuesday, during a synchronized swimming event.

Many theories have circulated as to why the pool suddenly changed color. A spokesperson for Rio 2016 initially announced that algae growth was to blame. However, officials later concluded that a fall in alkalinity caused the change in color. Another theory proposes that green ink seeped into the pool from surrounding signage.

So why did the Rio 2016 Olympic diving pool actually turn green?

Our Senior Electrochemist, Alex, says that while any explanation is speculation at this point, algae growth is probably not the culprit. The Olympic diving pool changed color from blue to green, but maintained a relatively clear, unclouded appearance. If algae had bloomed in pool, Alex believes that the water would have become cloudy.

If it wasn’t algae, then what turned the pool green?

Olympic officials have indicated that a decrease in alkalinity is to blame. This means that the pH level in the pool dropped, causing the water to become slightly more acidic. Alex explains why this might in fact be the reason:

The slightly acidic water may have caused corrosion of the pipes and infrastructure in the pool. As the water began to attack and dissolve the metals, new chemicals were introduced into the water. Transition metals, like manganese and copper, are among the chemicals that may have leeched into the water due to corrosion. Once in the pool, these metals combined with other chemicals, producing compounds that gave the water its green color.

For example, Alex explains that you can make a green solution with copper chloride (CuCl) in water. Copper chloride is one of the compounds that could have formed as a result of the alkalinity drop and subsequent corrosion.

How can the pool be fixed?

Olympic officials initially responded to the green swimming pool issue by adding more chlorine to the water. However, the high chlorine levels in the pool caused a whole new problem. Aquatic athletes, including water polo players, experienced eye irritation that distracted them from competition.

Will adding chlorine make the green pool blue again?

If the pool turned green due to a decrease in alkalinity, then chlorine was probably not the answer. In fact, chlorine addition may have even exacerbated the corrosion, making the green water problem even worse!

Alex explains that sodium hydroxide or sodium bicarbonate could be added to increase the alkalinity of the pool. Once the alkalinity of the pool water increased, the green compounds would hopefully begin to precipitate out, causing the water to become cloudy or chalky. The precipitate would then have to be filtered out of the water.

If you have ever cleaned a home aquarium, you might be familiar with this process. Of course, filtering an entire Olympic swimming pool would take a lot longer than filtering a home aquarium.

If Alex is right and the pool did turn green due to alkalinity shifts, then this issue may have been prevented with proper pool chemistry control. If you want to learn more about pool chemistry control, check out our application note here.

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