Why is the diving pool green? Strange water colour at Rio 2016 explained

It’s probably a lack of chlorine, which though not worrying in itself might present big problems

For once, the most important colour at the Olympics isn’t gold. Instead, it’s green – and more specifically the mystery of why the diving pool suddenly turned verdant.

The diving pool at the Rio aquatics centre has changed from the lush, sparkly blue that it had been since the games began and instead turned green. And it’s not entirely clear why.

Soon after the controversy erupted, Olympic officials said that they didn’t know what had happened. But they did say that the diving – which continues today – would still go ahead, and that those participating would be safe.

Experts, who aren’t so sure that the green water is safe, have suggested that the most likely cause of the colouring is a lack of chlorine. That may have allowed algae to grow quickly, and turned the water from its blue to green.

“It’s almost certainly lack of chlorine in the water,” said Howard Gosling, an independent pool and spa adviser and former chairman of the Pool Water Treatment Advisory Group. “With very warm temperatures, chlorine dissipates rapidly.”

That chlorine should usually be topped back up, to keep levels high and the water clean. But something appears to have gone wrong that is stopping it from being replenished.

“If it is algae, I would question the operation of the pool,” said Ralph Riley, the vice chairman of Pwtag. “I think there’s been some breakdown in the operation of the pool.”

That could happen for a number of reasons, said Mr Gosling. But it probably means that the dosing system that usually automatically adds chlorine to the pool has broken, and so no chlorine is being pumped in.

But there are other possibilities.

“If the water is a low pH, and there’s copper in the heating system for the pool, that’s another thing,” said Mr Riley. “The low pH could affect the copper so you get some sort of corrosion taking place.”

And some others have suggested that the pool might have gone green because the blue is mixing with the yellow of urine, or fake tan. But even though the water stands mostly still to avoid any interfering with the diving itself, the pool is just too big for either of those things to make a meaningful impact on the colour of the water.

Both experts agreed that fixing the problem would be relatively simple, if indeed chlorine is to blame. Checking would be simple too: it just means taking a sample of the water and seeing how much is there.

If that was the problem, then the chlorine can just be re-added to the pool, said Mr Gosling. That might take a short while – especially in a diving pool, where the deep water tends not to move around as much – but chlorine diffuses fairly rapidly.

Notably, the other pool in the aquatics centre – which is used for water polo and synchronised swimming – has retained the sparkly blue colour that the diving pool once had.

That’s probably because the systems are run entirely independently. But it also adds to the mystery of what went wrong, said Mr Riley, since the diving pool would usually be expected to be the less likely one of the two to go green.

“Although they work on independent tanks, they should operate the same way,” he said. “The water should be recirculated – that’s the same in both pools. Unless something has gone wrong in the system in the diving pool, it should be the same colour.

“Almost the opposite: the diving pool contains a huge body of water and very few people actually use it, so you’re not going to any sort of demands imposed upon it whatsoever. It should easily cope.”