US President Donald Trump’s US$16-million ($20-million) attempt to turn Washington’s Lincoln Memorial reflecting pool “American flag blue” has failed.

Trump embarked on the expensive renovation of the almost 100-year-old landmark after condemning it as a “filthy” and “dirty” site and promising a “beautiful” monument in time for the country’s 250th birthday celebrations.

Now, a little over a week after the work was completed, the pool is green rather than blue as it is overtaken by an algae bloom.

US National Park Service ‌workers were seen on Wednesday emptying jugs labelled “12 per cent hydrogen peroxide” into the green water of the near century-old basin that stretches between the Lincoln Memorial and the Washington Monument on the National Mall in Washington DC.

A spokesperson for the US Interior Department said the pool ‌was being treated ‌with “high-tech nanobubble ⁠ozone technology” that killed algae, pathogens including E coli, and ​other contaminants.

The department said hydrogen peroxide was also being used, saying it was “a milder treatment than chlorine” that is used in spas and natural swimming pools.

The reflecting pool is one of Washington DC’s most historically symbolic attractions and was the setting of Martin Luther King’s 1963 “I have a dream” speech.

Its renovation is one element of Trump’s plans to remodel the US  capital during his second presidency.

The Trump administration claimed works to waterproof and repaint the site, would clear “residual” algae, but the problem has been exasperated by warm weather.

The pool was the scene of Martin Luther King jr’s historic speech. Photo: Wikipedia

Christopher Gobler, a professor at Stony Brook University in New York who has studied harmful algal blooms ⁠for more than 30 years, said hydrogen ‌peroxide ​was a common short-term treatment for algae that was unlikely to have major ​health effects ‌on the ducks and geese that frequent the pool.

“It’s most harmful to animals ​that are 100 per cent in the water,” Gobler said, adding the chemical could create low oxygen conditions that could be lethal to fish.

Nor would it be a long-term fix, he said.

“There’s always going to be a ‌rebound of the algae,” Gobler said. “Hopefully it works as a quick fix, but these algae do tend to rebound.”

As of late Tuesday afternoon local time, the water closest to the Lincoln Memorial had reportedly turned a blue-grey colour.

The east side of the Reflecting Pool near the World War II Memorial remained bright green as crews poured chemicals and strained the water by hand.

The pool was built in 1922 and 1923 with an asphalt and tile bottom. That was replaced with concrete in an extensive restoration between 2009 and 2012.

The pool was completely drained in 2017 to control an outbreak of the parasite schistosome, which infected its snail population and ultimately killed more than 80 ducks and ducklings.

-With AAP