The Lincoln Memorial Reflecting Pool turned green within weeks of being refilled. President Donald Trump had its grey bottom repainted “American flag blue,” and the pool was refilled earlier this month. NPR reported that the multimillion-dollar project produced subtle results to many observers, even as Trump and Interior Secretary Doug Burgum, whose agency managed the work, touted its success.
Then came algae. Rosalina Stancheva Christova, a professor of aquatic ecology at George Mason University, took water samples on Tuesday and confirmed the algae belongs to the genus Desmodesmus. It is “growing in excessive amounts,” she said, but is not toxic or harmful. It is a common green algae found around the region, especially now, and the pool offers “excellent conditions” for it: shallow, stagnant water, strong sunlight and no shade.
Why it happened
“It could happen every single summer,” Christova said. “But it seems that the disturbance of the pond during the renovations [is] accelerating this process.” She said the work may have thrown off the pool’s nutrient balance.
Steve Goodale, a Canadian swimming pool specialist known online as “Swimming Pool Steve,” calls it “New Pond Syndrome.” Start up a clear body of water in open air, he said, and green water often follows almost immediately. The pool’s size, 2,030 feet long with about 338,000 square feet of surface, stretched the process to days. He also pointed to the darker interior. “The new, darker interior surface is going to absorb more sunlight,” Goodale said, which means warmer water and more algae growth.
The administration gave a different account. It said the algae came from residual material in supply lines that sat dormant for weeks, made worse by extreme heat in Washington last week, with heat index values of 95 degrees and above.
The cleanup
Crews are pouring hydrogen peroxide into the water. The Interior Department calls it a “milder treatment than chlorine” with no harmful side effects to marine life. Workers are also using nanobubble ozone technology, which the department says is validated by several universities and the National Oceanic and Atmospheric Administration. On Wednesday the department said on X that the technology had “very effectively killed the algae,” and National Park Service crews would spend several days vacuuming the dead algae from the bottom.
On Thursday morning, much of the pool, especially the center, was still bright green. Crews worked at both ends with trash pumps and long-handled vacuums, and some paint had visibly peeled off in places.
Loay Hidmi, a civil engineer who specializes in water treatment, has watched the progress all week and thinks it is about 80% of the way there. He worries the algae could return. “In water systems, when you fix something, you need to look at the step before it and the step after,” he said. Goodale added that pipes stay full when a system is taken offline and need flushing before refilling, and that killing algae does not replace fixing filtration problems. Christova wants the water monitored weekly.
Cost and history
The project’s cost grew from Trump’s initial $2 million figure to at least $14 million, and federal contract records show $1.7 million going to an Ohio-based company for the nanobubble technology alone. Algae has surfaced in the pool before, including right after the 2012 renovation, and in 2019 crews drained four million gallons to fix a broken water line. An Interior spokesperson said contaminants have “long plagued the Reflecting Pool since 1922.” White House spokesperson Taylor Rogers said new lining and industrial-grade materials “will permanently seal” the pool, which she said previously leaked 16 million gallons a year.
