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By Alice Gibbs
Senior Reporter
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The Lincoln Memorial Reflecting Pool has turned an unexpected shade of green just days after reopening, with footage showing workers pouring what appears to be hydrogen peroxide into the water to tackle a fast-spreading algal bloom.
The high-profile cleanup effort comes after a multimillion-dollar renovation aimed at giving the century-old pool a deeper “American flag blue” finish—raising fresh questions about what went wrong and whether the chemical treatment now being used is safe.
Officials have confirmed hydrogen peroxide is being deployed alongside filtration systems to try to restore the pool’s clarity.
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Why the Lincoln Memorial Reflecting Pool Turned Green
Algae blooms in the reflecting pool are not new. The shallow, slow-moving body of water—fed in part by the nearby Tidal Basin—has long created ideal conditions for growth.
But the speed of this latest bloom has drawn attention.
The pool was drained, repainted and refilled within a compressed timeframe, potentially leaving residual algae in pipes and systems while introducing freshly warmed water during early summer heat.
“Given that a single cell can reproduce into millions within days, this is not viable to fully prevent,” Christopher Lowe, a marine biologist at Swansea University, told Newsweek, noting that algae can re-enter water systems through multiple routes—including being carried in by birds.
For a bloom to take hold, Lowe said, the conditions are relatively simple: “You need to have a sufficient supply of light and nutrients.” While sunlight cannot be controlled in an outdoor site like the National Mall, nutrient levels can.
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Why Hydrogen Peroxide Is Being Used
Hydrogen peroxide is commonly used in water treatment as an oxidizing agent that can break down algae and other organic contaminants.
Officials described it as a “milder treatment than chlorine,” often used in natural swimming pools and similar environments where wildlife is present.
The chemical is being used alongside so-called “nanobubble” ozone systems designed to kill algae and pathogens throughout the water column.
Pool maintenance expert Steve Goodale—known online as “Swimming Pool Steve”—said hydrogen peroxide works in tandem with other systems already in place.
“Adding peroxide is, in effect, doing the same thing that the nanobubbler system does—oxidation of the organic debris. So they do not need to add enough peroxide to combat the algae entirely, just enough to help support the ozone nanobubblers,” he told Newsweek.
However, he noted that effectiveness depends heavily on conditions inside the pool, including algae density and debris levels.
But in a pool holding roughly 6.5 million gallons of water, treating the bloom at scale presents a significant logistical challenge.
"At this stage we are moving away from natural cycles and into pool maintenance, which is essentially a way of trying to control those cycles. The use of hydrogen peroxide is very common in this situation,” Lowe said. "The hydrogen peroxide will have the advantage of both killing the algae and making it colorless (bleaching), which will help the appearance."
Making D.C. safe and beautiful again one cleanup at a time. 🇺🇸 pic.twitter.com/lRG6HZLtMx
— The White House (@WhiteHouse) June 15, 2026
A spokesperson for the United States Department of the Interior told Newsweek: “Unlike under Obama and Biden, the National Park Service is actually maintaining the beautifully completed Reflecting Pool. To keep the water in the Reflecting Pool not only crystal clear but also clean, we are deploying high-tech nanobubble ozone technology.
“This technology is actively killing algae, pathogens (e.g., E. coli), and contaminants that have long plagued the Reflecting Pool since 1922, most infamously, the Obama pool reopening that resulted in massive algae clumps taking over the pool’s surface following years of construction that cost taxpayers millions upon millions only to be broken and disgusting days later.
“Additionally, hydrogen peroxide, which is a milder treatment than chlorine and is used in spas and specialty pools like natural swimming pools, is also treating the pool. There are no harmful side effects to marine life or to the environment. The nanobubble technology has been tested and validated by the National Oceanic & Atmospheric Administration (NOAA), The Ohio State University and The University of Florida.”
Problems With Using Hydrogen Peroxide
Workers pouring hydrogen peroxide into the Lincoln Memorial Reflecting Pool to tackle a sudden algal bloom have sparked fresh questions—not just about what’s being used, but whether the approach will work at all.
The treatment is part of a broader effort to restore the pool’s appearance after it turned green days following a multimillion-dollar renovation. Officials say hydrogen peroxide is being used as a “milder” alternative to chlorine, alongside filtration systems designed to kill algae and contaminants.
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1. The Sheer Scale Problem
The Lincoln Memorial Reflecting Pool holds millions of gallons of water—around 6.5 million by most estimates.
Goodale said estimating the required dose is inherently difficult without knowing the extent of contamination.
“The amount of peroxide needed depends not just on the strength concentration… and the size of the body of water, but also importantly the amount of algae and physical debris in the water. So I cannot give a truly accurate estimate here because the amount of algae is unknown,” he said.
Still, he offered a rough benchmark for scale. "As a rough number—something in the order of 2,000 gallons of 12 percent peroxide would be needed to effectively treat the 6.5 million gallons."
That scale has led to skepticism over how effective manual dosing can be. Videos show workers manually pouring containers of 12 percent hydrogen peroxide into the pool from the edge, but treating a body of water that size would typically require far larger quantities to significantly reduce algae levels.
“Dosing for a system this large would normally happen in the filtration equipment area. It would definitely not be standard protocol to manually treat a body of water this large,” Goodale said. “Using 1-gallon jugs certainly looks like a half-measure approach to such an immense body of water," Goodale said.
Experts say dosing in open systems must be carefully calibrated to concentration rather than volume—something that becomes increasingly complex at scale.
2. Cost vs. a Short-Term Fix
Hydrogen peroxide is widely used as an algicide, but it is not a permanent solution.
“The hydrogen peroxide only works in the very short term,” Lowe said.
Because it breaks down rapidly into oxygen and water, it does not remain in the system long enough to prevent future growth.
That raises questions about cost-effectiveness. Treating a pool of this size at sufficient concentrations—and repeating that process as algae returns—could potentially run into significant expense, particularly when compared with longer-term fixes focused on nutrient control and circulation.
“Every bit of peroxide will help,” Goodale said, “but [whether it is enough] remains to be seen.”
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3. A Design Trade-Off: Still Water vs. Circulation
At the heart of the issue is a long-standing tension in the reflecting pool’s design.
Algae thrives in warm, slow-moving water—conditions that are effectively required for the mirror-like surface the landmark is known for.
While increased circulation or aeration can help reduce algal growth, it can also disrupt reflections.
Experts say managing the pool therefore becomes a balancing act between aesthetics and water quality—one that has challenged multiple renovation efforts over decades.
The recent overhaul, which included resurfacing and changes to the basin, may also have altered how heat and nutrients behave in the system, potentially accelerating bloom conditions.
Goodale said recent weather and design changes may also be contributing factors.
“This is definitely a reactionary approach to trying to prevent the algae from getting worse—which it will quickly with warm weather, lots of rain, and the new darker interior surface absorbing more heat from the sun,” he said.
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4. Wildlife and the Wider Environment
The reflecting pool is not an isolated system. Birds, debris and runoff all interact with the water daily. While the chemicals being added to the water will remove the algae in the pool, because of the ratio used it will not pose a risk to wildlife or humans nearby.
Goodale said dilution plays a key role in limiting risk. “The peroxide itself is being diffused into such a large body of water that the concentrations should not be harmful,” he said. “Hydrogen peroxide is a good choice here, leaving only water and oxygen, neither of which should be harmful to local wildlife.”
Though the wildlife does play a part in continued algae growth. Lowe noted that even if the pool were sterilized, algae would quickly return.
“Algae will enter through a variety of routes such as being washed in, or come in attached to birds’ legs,” he said.
There are also ecological trade-offs to repeated chemical control. While hydrogen peroxide is generally considered less harmful than chlorine and breaks down into non-toxic components, maintaining the pool through continuous dosing could limit what plants and other matter can survive in the water.
“The hydrogen peroxide should have minimal impact on the wider environment, but his will essentially mean that from the standpoint of nature the pool will be an ecological desert with nothing being able to live in the water,” Lowe said.
Update 06/17/26 08:10 a.m. ET: This story was updated with comment from Steve Goodale and the United States Department of the Interior.
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