Why Did the Lincoln Memorial Reflecting Pool Turn Green?

The Lincoln Memorial Reflecting Pool is one of the most famous landmarks in the United States. Earlier this month, workers refilled the pool with water after repainting its bottom a bright blue color, following orders from President Trump. But just days later, the pool turned green — thanks to a natural takeover by tiny living things called algae. Scientists and pool experts say this kind of bloom was almost guaranteed to happen, and fixing it is proving to be a big job.

President Trump had the pool's bottom repainted 'American flag blue,' replacing its old neutral grey color. The project cost at least $14 million — much more than the $2 million Trump first announced. Many people who saw the finished pool said the color change was hard to notice. Trump and Interior Secretary Doug Burgum still called the project a success.

The green color came from algae, which are tiny plant-like organisms that grow in water. Rosalina Stancheva Christova, a professor at George Mason University in Virginia, took water samples from the pool. She found the algae belong to a group called Desmodesmus. She said they are growing in very large amounts but are not toxic or harmful to people.

Experts say the pool is a perfect place for algae to grow. It has shallow, still water, lots of strong sunlight, and no shade. Christova said the recent renovation stirred up nutrients in the water, giving the algae an extra boost. She warned that this kind of bloom could happen every single summer.

Steve Goodale, a swimming pool expert from Canada, said this is a well-known problem called 'New Pond Syndrome.' He explained that when you refill a large, open body of water, green water can appear almost right away. He also noted that the new dark blue paint absorbs more sunlight, making the water warmer — which helps algae grow even faster.

Workers are now using several methods to fight the algae. One method involves pouring hydrogen peroxide into the water, which the Interior Department says is mild and does not harm wildlife. Workers are also using a special technology that pumps tiny bubbles of ozone gas into the water to kill algae and other harmful organisms. The government is paying $1.7 million to an Ohio company just for this bubble technology.

These ozone bubbles are so small that you cannot see them with the naked eye. Goodale explained that the bubbles float in the water for weeks or even months, slowly destroying algae the whole time. On Wednesday, the Interior Department said the technology had 'very effectively killed the algae.' However, as of Thursday morning, large parts of the pool were still bright green.

Workers continued vacuuming dead algae off the pool's bottom using long-handled tools and machines called 'trash pumps.' Pistachio-colored water poured out of hoses into the nearby grass. Some visitors stopped to take pictures of the green water and patches of paint that had peeled off the bottom. Others were more interested in photographing the Washington Monument in the background.

Loay Hidmi, a civil engineer who works in water treatment, has been watching the pool's progress all week. He estimated the cleanup is about 80% done but worries the algae could come back because the pool's sunny, shallow conditions are still perfect for new growth. He said that fixing a water system means looking at every step — not just treating the problem once and moving on.

Goodale agreed, comparing the current treatment to mowing a lawn instead of pulling weeds out by the roots. Christova said the pool should be tested every week so scientists know what is growing in it. She warned that not all types of algae are as harmless as the kind blooming right now. With big July 4th events planned on the National Mall, the pressure to fix the pool quickly is very high.