For visitors arriving at the Lincoln Memorial in mid-June, the Washington D.C. Reflecting Pool was green with algae.
Just days after the 6.75-milliongallon Reflecting Pool was refilled following a $14.7 million resurfacing project, portions of the basin were coated with green slime. Workers could be seen vacuuming the pool, manually scraping algae from surfaces, repeatedly applying hydrogen peroxide, and installing temporary water-treatment equipment in an effort to restore the pool's famous mirror-like appearance.
How could a newly renovated reflecting pool turn green almost immediately?
And if this was a state-of-the-art water treatment system, why didn't it prevent the bloom?
The available evidence suggests the June 2026 algae bloom developed during startup rather than reflecting the long-term performance of the upgraded treatment system. The central question isn’t whether the upgraded system could ultimately control algae, but whether it was fully operational during the first days after the pool was refilled.
Despite its name, the Lincoln Memorial Reflecting Pool operates very differently from a swimming pool. Its modern water-treatment system dates to a major 2012 rehabilitation, which transformed what had essentially been a large basin filled with municipal drinking water into a sophisticated recirculating system supplied primarily by untreated river water.
Under the current design, most makeup water is pumped from the nearby Tidal Basin, a 107-acre inlet connected to the Potomac River, with additional water supplied by the nearby National World War II Memorial fountain system. Rather than periodically draining and refilling the basin, water is continuously circulated through pumps, sand filters, and ozone treatment before returning to the pool.
Ozone rapidly decomposes back into oxygen after reacting with contaminants, making it compatible with an ornamental water feature supplied with untreated river water and frequented by wildlife.
The 2012 redesign repaired the leaking historic structure and dramatically reduced potable water use, but it also transformed the water-treatment challenge. Instead of maintaining treated municipal water, operators now continuously treat river water — making algae control during startup far more demanding.
When the Reflecting Pool reopened after its 2012 rehabilitation, algae also appeared within days of refilling. National Park Service officials described the bloom as part of the system's 'break-in period' while the new treatment system was being adjusted. After recalibrating the system and increasing ozone treatment, the pool stabilized.
That history provides important context for 2026. In both cases, the pool was drained, refilled with untreated Tidal Basin water, and developed an algae bloom within days. The pattern suggests the greatest challenge may not be maintaining water quality during normal operation, but establishing biological control immediately after millions of gallons of untreated river water enter the system.
Unlike the 2012 rehabilitation, which created the Reflecting Pool's recirculating treatment system, the 2026 project enhanced that existing infrastructure by resurfacing the basin and adding nanobubble ozone technology supplied by Greenwater Services of Brookfield, Ohio.
Federal contract records show Greenwater received a $1.7 million contract in April 2026. Temporary trailer-mounted treatment units reportedly arrived around June 6, only days after refilling began, while the permanent installation was not reported to be fully operational until June 25.
Visible algae appeared around June 10 or 11 — roughly five days after refilling and before the permanent Greenwater installation was reported to be fully operational. Workers responded with repeated hydrogen peroxide applications, manual algae removal, vacuuming and continuous operation of the temporary treatment equipment while the permanent installation continued. Hydrogen peroxide breaks down into water and oxygen, leaving no chemical residual.
Based on Greenwater's published technical information and public statements, the process appears to operate as follows: 1. Water is continuously withdrawn from the Reflecting Pool through the existing circulation system.
2. It passes through the existing sand filters, where suspended solids are removed.
3. Ozone gas is generated and injected into the treatment stream.
4. Greenwater's proprietary equipment shears the ozone into microscopic nanobubbles approximately 70 to 200 nanometers in diameter.
5. Because the bubbles are extremely small, they remain suspended far longer than conventional ozone bubbles, increasing ozone transfer efficiency and dissolved oxygen.
6. The treated water is returned to the pool, where the process repeats continuously.
Unlike chlorine, ozone leaves virtually no lasting sanitizer residual. As it reacts with algae and other organic material, it decomposes back into oxygen.
According to Greenwater, the nanobubble process improves ozone utilization while increasing dissolved oxygen, allowing algae to be controlled without maintaining a chlorine residual.
Although the overall treatment concept is reasonably we l l understood, the engineering details of the Reflecting Pool installation have never been released.
Among the missing information are:
• Design flow rate.
• Ozone production capacity.
• System treatment capacity.
• Hydraulic contact time.
• Design turnover rate.
• Startup operating procedures.
• Performance expectations during startup.
Those aren't merely technical specifications. They are the information needed to determine whether the system was operating at its intended capacity during the first days after refilling, whether the temporary equipment was designed to treat the entire 6.75-million-gallon basin, and whether startup unfolded as engineers intended.
Without those records, it is impossible to calculate how quickly the pool could be treated, how much ozone was being delivered, or whether the system was capable of staying ahead of algae growth during the critical startup period.
The timeline, however, provides one important clue. Visible algae appeared roughly five days after refilling began — before the permanent Greenwater installation was reported to be fully operational. That timing suggests the bloom became established during startup rather than during routine operation.
Whether hydrogen peroxide was part of the original startup strategy or introduced only after the bloom became established remains unknown because startup procedures have not been released.
Another possibility for the algae bloom involves the plumbing. National Park Service officials said algae remained inside portions of the Reflecting Pool's piping during the approximately eight-week resurfacing project.
Published research indicates algae embedded within biofilms can survive inside piping and rapidly seed new blooms once warm water, sunlight and nutrients become available.
Without the missing engineering and startup records, it is impossible to determine how much of the bloom originated from incoming Tidal Basin water, residual biological growth within the plumbing, startup limitations, or some combination of those factors Viewed in isolation, the June 2026 algae bloom looks like an embarrassing maintenance problem. Viewed in the context of the Reflecting Pool's history, it looks considerably different.
The evidence does not prove the Greenwater upgrade failed. Instead, it suggests that the bloom became established during startup, before the permanent Greenwater installation was reported to be fully operational.
The similar bloom that followed the 2012 rehabilitation points to startup as a recurring challenge whenever the Reflecting Pool is drained, refilled with untreated Tidal Basin water, and returned to service.
Whether the June 2026 startup unfolded as engineers intended cannot be determined from the available documentation. Key engineering records have not been released.
Until those records become available, the June 2026 bloom is best understood not as evidence that the completed treatment system failed, but as an event that occurred during the biological restart of one of the world's largest ornamental recirculating water systems.
