Algae clouds Reflecting Pool after Trump’s blue repaint

The Lincoln Memorial reflecting pool has turned green again after being refilled following President Trump’s decision to repaint its neutral grey bottom “American flag blue.” Scientists and pool experts say algae blooms like the current one are expected under

By Tuesday on the National Mall, the Lincoln Memorial’s reflecting pool had gone green again—an unsettling flash of color on one of Washington’s most carefully maintained backdrops.

The pool had been refilled earlier this month. after President Donald Trump directed its neutral grey bottom to be repainted “American flag blue.” The project was presented as a success by Trump and by Secretary of the Interior Doug Burgum. whose agency managed the renovation. But in recent days the water has taken on a verdant hue. tied to algae blooms that experts say are expected in these conditions.

Steve Goodale. a Canadian swimming pool specialist known online as “Swimming Pool Steve. ” described what’s happening as “New Pond Syndrome.” He said it can occur when a natural. clear body of water sits open to the elements and is “start[ed] up. ” often resulting in green water “almost immediately.” In this case. Goodale said the algae took longer to fully show itself—days rather than hours—likely because of the reflecting pool’s sheer size. The pool measures 2,030 feet long, with a surface area of approximately 338,000 square feet.

Rosalina Stancheva Christova. a professor of aquatic ecology at George Mason University in Virginia. took water samples from the pool on Tuesday and confirmed the algae belongs to the genus Desmodesmus. She said it is growing in excessive amounts but is “not toxic or harmful.” Christova said this common green algae is found across the region. especially this time of year. and that the reflecting pool offers “excellent conditions” for algae growth: shallow. stagnant water. strong sunlight. and no shade. “It could happen every single summer,” she said. She added that the renovations—described as a disturbance of the pond during the work—appear to have accelerated the process.

Christova said the renovation last month may have affected the balance of nutrients in the pool. potentially pushing the bloom faster. Goodale pointed to another mechanism: the new, darker interior surface. He said it will absorb more sunlight, warm the water, and ultimately lead to “more prolific algae growth.”.

The Trump administration has offered its own explanation for the source of the bloom, saying the algae came from residual material in supply lines that had lain dormant for weeks. Officials also cited extreme heat in Washington last week, with heat index values of 95 degrees and above.

Algae has resurfaced in the reflecting pool periodically over the years. After it reopened from its last major renovation in 2012. algae showed up immediately. forcing the National Park Service to drain the pool. refill it. and recalibrate its ozone level. In 2019, crews had to drain four million gallons to fix a broken water line that had algae growing in it.

A spokesperson for the Interior Department told NPR by email that algae and other contaminants have “long plagued the Reflecting Pool since 1922. ” pointing to the Obama-era renovation as an example. The spokesperson added that “unlike under Obama and Biden. the National Park Service is actually maintaining the beautifully completed Reflecting Pool.”.

To address the current bloom, the Trump administration has been using mitigation strategies meant to kill algae and neutralize pathogens. Workers have been pouring hydrogen peroxide into the water. The Interior Department said hydrogen peroxide is a “milder treatment than chlorine” and is used in spas and specialty pools such as natural swimming pools. It also said there are “no harmful side effects to marine life or to the environment.”.

At the same time. crews are deploying what the department calls “high-tech nanobubble ozone technology” to neutralize algae and other pathogens. The Interior Department said the approach is validated by several universities and the National Oceanic & Atmospheric Administration. Goodale described the nanobubble system as producing bubbles so tiny the human eye can’t even see them. He said the bubbles are “neutrally buoyant,” meaning they won’t simply rise and vanish at the surface. They can persist “for weeks, if not months,” doing oxidizing work that keeps algae at bay.

On Wednesday. the Interior Department posted on X that the nanobubble technology had “very effectively killed the algae. ” and that National Park Service crews would spend several days vacuuming up dead algae from the bottom of the pool. But as of Thursday morning, much of the pool—especially in the center—was still bright green.

Work continued on both ends of the reflecting pool. Nanobubble machines lowered tubes into the water, while mobile vacuuming systems known as “trash pumps” hummed from the shore. Workers stood either in the pool or along the edge, moving long-handled vacuums. The contents—described visually as pistachio-colored water—poured from hoses laid in nearby grass. Orange cones marked the work zones, but passersby along the pool appeared relatively unfazed. Some leaned in to take pictures of the green water itself, including sections of paint that had visibly peeled off. Others focused on getting photos of the Washington Monument in the background.

Loay Hidmi, a civil engineer who specializes in water treatment, has been walking the pool’s edges throughout the week. The relatively new D.C. resident said he estimates the water is about 80% improved. “I’m taking pictures of it … for the last week and I can see the gradual change,” he said. “So I’m hopeful. But we’ll have to see if it gets sustained.”.

Hidmi is not just worried about appearance. He said the conditions that feed algae—sunny. shallow water—could allow it to return. and he sees that as a question about the process that led to the repainting and refilling. “In water systems. when you fix something. you need to look at the step before it and the step after. ” he said.

Goodale agreed that algae treatment alone is not the finish line. He said it takes more than simply shutting off a system and restarting it: when a water system is taken offline. the pipes still retain water and “need to be flushed out before any refilling.” He also said eliminating algae doesn’t substitute for dealing with filtration and nutrient issues underneath. “That’s like the equivalent of mowing the lawn when perhaps it needs to be something else that addresses the source nitrates and phosphorus. so that it’s more like pulling the weeds out by the root. ” he said.

In the meantime, Christova said she would like weekly monitoring of the water. “If we don’t have any control over algal growth, we don’t know what is growing,” she said, adding that not all types of algae are as harmless as the current bloom.

When asked about plans for maintenance and algae prevention. White House spokesperson Taylor Rogers said: “Thanks to President Trump. new lining and industrial grade materials will permanently seal the Reflecting Pool. which previously leaked 16 million gallons per year and wasted countless taxpayer dollars.”.

Even with the Obama-era renovation. the reflecting pool experienced broken pipes and water leaks that required costly refills. according to a Department of the Interior report from fiscal year 2023. That report called for new expansion joints. supply and return lines with thicker walls. and said an “improved distribution system” would ensure water could be circulated through the treatment plant. filtered. and treated with ozone.

This latest renovation, however, does not appear to have addressed the pipe problems, even though it involved replacing failing expansion joints, resealing the pool, removing truckloads of garbage, and “fixing the water system, drainage and so much more,” as Trump wrote on Truth Social in May.

The project’s price tag also climbed. Trump’s initial estimate was $2 million, but the cost grew to at least $14 million. Federal contract records show the government is paying $1.7 million to an Ohio-based company for the nanobubble technology alone.

Trump tied the scope expansion to upcoming events. writing that the “scope of the Project has been greatly enlarged” because leaders realized how important the site was to Washington. D.C. and because of the record number of visitors coming for the city’s now very safe Capital for events tied to the 250th Anniversary.

The National Mall is hosting semiquincentennial events on and around July 4, including a weekslong state fair that kicks off Wednesday evening.

Lincoln Memorial reflecting pool algae bloom Desmodesmus hydrogen peroxide nanobubble ozone Trump Doug Burgum National Park Service National Mall