National Park Service workers were seen emptying jugs labelled "12% hydrogen peroxide" into the green water of the nearly century-old basin that stretches between the Lincoln Memorial and the Washington Monument on the National Mall.
A spokesperson for the Interior Department said the pool is being treated with "high-tech nanobubble ozone technology" that kills algae, pathogens including E coli, and other contaminants. The department said hydrogen peroxide is also being used, saying it's "a milder treatment than chlorine" that is used in spas and natural swimming pools.
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 is a common short-term treatment for algae that is not likely 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 can create low oxygen conditions that can be lethal to fish.
It might not be a long-term fix, though, he warned.
"There's always going to be a rebound of the algae," Gobler added. "Hopefully it works as a quick fix, but these algae do tend to rebound."
The reflecting pool was refilled with water starting on June 5 as part of a renovation project directed by Trump that involved draining, cleaning and resurfacing it.
The bottom of the pool was repainted in a colour Trump calls "American flag blue".
