WASHINGTON — The Lincoln Memorial reflecting pool is once again making headlines, this week for turning green.
The Washington, D.C. landmark was refilled with water earlier this month after President Trump had its neutral grey bottom repainted "American flag blue." The multi-million dollar project produced subtle results in the eyes of many observers, even as Trump and Secretary of the Interior Doug Burgum — whose agency managed the renovation — touted its success.
In recent days, however, the pool has taken on a verdant hue — the result of algae blooms that experts say are to be expected in these conditions.
"It's called 'New Pond Syndrome,'" says Steve Goodale, a Canadian swimming pool specialist known online as "Swimming Pool Steve." "It's a known thing that happens when you take a natural, clear body of water like this that sits in an open air environment and you try to start it up, very often you end up with green water almost immediately."
Goodale says the process took longer — a matter of days — to unfold in this case likely due to the sheer size of the pool, which measures 2,030 feet long and has a surface area of approximately 338,000 square feet.
"Excellent conditions" for algae growth
Rosalina Stancheva Christova, a professor of aquatic ecology at George Mason University in Virginia, took water samples from the pool on Tuesday. She confirmed the algae belongs to the genus Desmodesmus, which she said is "growing in excessive amounts" but is not toxic or harmful.
Christova says this kind of common green algae is found all over the region, especially this time of year. The reflecting pool in particular provides "excellent conditions" for algae growth, she said: shallow, stagnant water, strong sunlight and no shade.
"It could happen every single summer," she added. "But it seems that the disturbance of the pond during the renovations [is] accelerating this process."
Christova said last month's renovations may have affected the balance of nutrients in the pool, potentially accelerating the algae blooms. Goodale similarly views the resurfacing as one of several contributing factors.
"The new, darker interior surface is going to absorb more sunlight," Goodale says. "It is going to result in water that's warmer, and that ultimately is going to lead to more prolific algae growth."
The Trump administration has said the algae came from residual material in supply lines that had lain dormant for weeks. Their growth was likely exacerbated by the extreme temperatures that hit D.C. last week, bringing heat index values to 95 degrees and above.
Algae has resurfaced in the reflecting pool periodically over the years — including immediately after it reopened from its last major renovation in 2012, forcing the National Park Service to drain it, refill it and recalibrate its ozone level. And in 2019, crews had to drain four million gallons from the pool to fix a broken water line that had algae growing in it.
An Interior Department spokesperson told NPR over email that algae and other contaminants have "long plagued the Reflecting Pool since 1922," pointing to the Obama-era renovation as an example.
"Unlike under Obama and Biden, the National Park Service is actually maintaining the beautifully completed Reflecting Pool," they added.
Responding with tiny bubbles and big vacuums
The Trump administration is using a mix of mitigation strategies, including pouring hydrogen peroxide into the water to kill the algae.
The Interior Department says hydrogen peroxide is a "milder treatment than chlorine and is used in spas and specialty pools like natural swimming pools," adding "there are no harmful side effects to marine life or to the environment."
Workers are also deploying what the department calls "high-tech nanobubble ozone technology" to neutralize algae and other pathogens in the pool. The department says that approach is validated by several universities and the National Oceanic & Atmospheric Administration.
Those ozone bubbles are so tiny the human eye can't even see them, Goodale says.
"The best way to describe it is that the bubbles are neutrally buoyant, so they won't just rise to the surface and disappear readily," he explains. "They can last for weeks, if not months in the water, doing their oxidizing thing and keeping the algae at bay."
'A monumental effort'
Goodale says it's more complicated than treating the average backyard swimming pool, since the reflecting pool — despite its name — is actually more akin to a "manmade shallow lake." He says it's hard to predict just how long it will take to completely solve the algae problem, calling it "a monumental effort, literally."
The Interior Department posted on X Wednesday that the nanobubble technology had "very effectively killed the algae," and National Park Service crews would spend several days vacuuming up the 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 pool. Nanobubble machines deposited their tubes into the water, as mobile vacuuming systems known as "trash pumps" hummed loudly from the shore. Handfuls of workers stood either in the pool or on the edge maneuvering long-handled vacuums back and forth. Their contents, including pistachio-colored water, poured out of hoses laying in the nearby grass.
The work zones were marked off by orange cones, but passersby walking the length of the pool appeared relatively unfazed. Some stopped to peer down and snap pictures of the water itself — including sections of paint that had visibly peeled off — while others were more focused on getting a photo of the Washington Monument in the background.
Loay Hidmi was walking deliberately along the edge of the pool closest to the Lincoln Memorial, hands clasped behind his back, looking over the ledge. The relatively new D.C. resident is a civil engineer who specializes in water treatment, and has been coming by the pool all week to see the progress for himself. He estimates it's about 80% of the way there.
"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."
What happens next?
Hidmi worries that the algae could come back, given the favorable conditions posed by the sunny, shallow pool.
He acknowledges that's mostly an aesthetic concern, given how much the administration has just spent on repainting the pool, but says it also raises questions about their process.
"In water systems, when you fix something, you need to look at the step before it and the step after," he said.
Goodale agrees. He says that when a water system is taken offline, the pipes still remain full of water — "they don't just gravity-drain away" — and need to be flushed out before any refilling. And he says eliminating algae is no substitute for dealing with underlying filtration issues.
"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 says.
In the meantime, Christova, the algae expert, would like to see the water monitored weekly.
"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 one currently blooming in the pool.
When asked about plans for maintenance and algae prevention, White House spokesperson Taylor Rogers told NPR: "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 after the Obama-era renovation, the reflecting pool suffered from broken pipes and water leaks requiring costly refills, according to a Department of the Interior report from fiscal year 2023. It called for new expansion joints, supply and return lines with thicker walls, saying "an improved distribution system will ensure the water can be circulated through the treatment plant, filtered, and treated with ozone."
This latest renovation does not appear to have addressed the pipe problems, even though it did involve 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.
Along the way, the cost of the project grew from Trump's initial $2 million price tag 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.
"The scope of the Project has been greatly enlarged as we became involved because we realized how important it would be to Washington, D.C., and the record number of visitors coming to our now very safe Capital for all of the upcoming events in celebration of our 250th Anniversary," Trump wrote.
The National Mall is hosting a number of semiquincentennial events on and around July 4, including a weekslong state fair that kicks off Wednesday evening.
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