Staff are using hydrogen peroxide and a nanobubble filtration system to try to curb an algal bloom that has turned the Lincoln Memorial Reflecting Pool green.
The discoloration appeared after the pool was refilled following a temporary closure for lining and repair work that began on April 10, 2026, and was scheduled to run through June 10, 2026. The restoration was part of a rushed renovation effort ahead of the nation’s 250th anniversary celebrations.
A number of explanations have been suggested for why the bloom developed, including claims that the pool’s new “American flag blue” finish may have contributed. Officials, however, have described the green tint as “residual” algae left behind in water pumps and pipes after the construction work.
Efforts are now underway to reduce the algae, with National Park Service workers adding hydrogen peroxide to the water as part of the cleanup.
The Department of the Interior (DOI) says the agency is also relying on an advanced filtration setup.
Officials say the system uses ozone-injected nanobubbles to break down contaminants and toxins, and that both this process and the hydrogen peroxide have “no harmful side effects to marine life or to the environment”.
So how is the hydrogen peroxide helping?
Pool maintenance specialist Steve Goodale explained the treatment’s role in comments to CNN.
“It is definitely a legitimate process for dealing with an open-water environment like this, where you can’t use something that would be more harmful like chlorine, which you would find in a residential swimming pool,” he told the outlet.
“But that wouldn’t be good for the local wildlife, so that’s not the kind of system you would use here.”
Goodale said hydrogen peroxide acts as a boost for oxidation in the water, helping suppress the algae.
“This is some six-and-a-half million gallons of water we’re talking about here, so that’s a lot of bottles of anything that you’d have to add,” he said.
“But the hydrogen peroxide is an oxidizer itself, in the same way that the nanobubbler system is, so the two systems are essentially the same thing, just a different approach to it.”
At this stage, the treatment has not cleared the entire pool, and visible improvement has been limited mainly to the edges near the Lincoln Memorial. Other sections have remained bright green, while crews continue to skim, strain and treat the water.
The unusual split appearance has left the site looking half-blue, half-green, with dark blue water around part of the perimeter and algae still concentrated farther out toward the World War II Memorial side.
The renovation project reportedly cost about $14 million. Donald Trump later pushed back on criticism that the work amounted to little more than a “paint job” in a post on Truth Social.
“This was not a paint job,” he wrote. “This was highly sophisticated material, industrial strength, that could last for 100 years … The material is thick, strong, flexible, and has a natural, beautiful color, the dark blue of the American Flag!”