The green pool mystery is kind of solved, thanks to a statement from the Olympic aquatic commission.

On Wednesday afternoon, FINA — Fédération Internationale de Natation, the organization that oversees all the Olympics aquatic competitions — released the awaited reasoning for the sudden green color of the diving pool on Tuesday. It was because the water tanks ran out of some of the water treatment chemicals and this allowed the pH of the pool water to shift out of its normal range, causing the weird color.

“There was no risk to the health and safety of the athletes, and no reason for the competition to be affected,” assures FINA in its statement.

This is a little contradictory to earlier statements, but it is the most official statement about what happened. It would also explain why the water polo pool has also been getting green throughout the day, which didn’t fit into the statements about algae made Tuesday to the Associated Press.

This is the entire official statement from FINA, which is the organization in charge of Olympic water sports.
This is the entire official statement from FINA, which is the organization in charge of Olympic water sports. 

Although the statement from FINA doesn’t say which chemicals ran out, a good guess is something that controls pool alkalinity, like sodium bicarbonate. A pool that’s low in alkalinity can become green, and the alkalinity can drop quickly if it’s not treated. Once the alkalinity is funky, it takes time to get a pool back to normal, which is probably why the pool is still green.

When pools are chemically imbalanced, the pH can bounce, irritating the eyes and skin of swimmers and can cause damage to the pool walls by becoming mildly corrosive. And since alkalinity prevents the pH from changing, it’s one of the most important things to control. So, although FINA says that the pool is safe for swimmers, it’s probably not the most comfortable experience for the Olympians.

Both the diving pool and water polo pool Wednesday morning -- the color of the diving pool is unchanged, and water polo is significantly greener than it was yesterday.
Both the diving pool and water polo pool Wednesday morning -- the color of the diving pool is unchanged, and water polo is significantly greener than it was yesterday.

It is also a little worrying that Olympic organizers ran out of some of the necessary chemicals to treat the pool, but at least it’s not as bad as the conditions for the open water athletes. Let’s hope that they figure it out before the water polo pool is taken over too.

Photos via FINA, Getty Images / Tom Pennington, Getty Images / Adam Pretty

Dyani Sabin is a science writer from small-town Ohio transplanted to New York City. Former biology researcher and library supervisor, you can also find her writing at Scienceline.