The Lincoln Memorial Reflecting Pool in Washington, D.C., has become the site of an unexpected battle against algae, commonly known as pond scum, hindering the Trump administration's effort to have the pool appear "American Flag blue" for the nation's upcoming 250th birthday celebration. The 6.5-million-gallon pool underwent a renovation costing over $16 million, which included draining, lining with a blue coating, and refilling.
Shortly after the refilling process, microscopic green algae began to proliferate, turning the water a murky green. The blue liner also started to peel. Federal workers attempted to combat the bloom by adding hydrogen peroxide, but the algae persisted. Scientists consulted on the issue expressed skepticism about the possibility of completely eradicating the algae and suggested that even if the water temporarily clears, the issue is likely to recur.
Algae are plant-like organisms that multiply rapidly under favorable conditions. Microscopic varieties, such as Desmodesmus or Scenedesmus, can divide multiple times daily, leading to exponential population growth and a rapid change in water clarity. For such blooms to occur, specific environmental factors are necessary, including sunlight, carbon dioxide, warmth, and crucially, nutrients like phosphorus and nitrogen.
The Reflecting Pool's design and location present an ideal environment for algae. Its large, shallow surface area maximizes sun exposure, and summer temperatures in Washington, D.C., contribute to warmth. Furthermore, the water used to refill the pool, often sourced from the Tidal Basin or the city's water supply, can contain elevated nutrient levels. DC Water, for instance, adds phosphorus to its supply to prevent lead from leaching from pipes, which can inadvertently feed algae.
The pool's urban surroundings also contribute to nutrient load. Runoff from fertilized park areas and the droppings of local wildlife, such as geese and ducks, can introduce additional phosphorus and nitrogen into the water. The pool's position lower than the surrounding ground facilitates the accumulation of this runoff.
While the pool is equipped with a system designed to filter and treat incoming water to control algae, it has proven imperfect and does not appear to adequately remove the nutrients fueling the current bloom. The administration's approach of trying to kill the algae with hydrogen peroxide and nanobubbles, followed by vacuuming, addresses the symptom rather than the root cause, according to experts.
To effectively manage the algae problem and maintain a clear pool, experts recommend removing nutrients from the water before it enters the pool and continuing nutrient removal treatments within the pool itself. Methods such as using aluminum compounds to bind with phosphate before filtration could be employed. Some suggest that chemical treatments are temporary solutions and propose introducing algae-eating organisms like zooplankton as a slower, more sustainable method.
As of June 23, the Reflecting Pool still exhibited a greenish hue, though it appeared to be fading. Some observers suggest that a natural appearance with algae might not detract from the pool's aesthetic, as reflections of the Washington Monument can still be achieved in greenish water.