The hydrogen peroxide will have no harmful effect on marine animals or the environment, the department said.

Explosions of algae are a regular problem the agency has confronted at the shallow pool. But the growth has dealt a blow to the White House’s efforts to clean up the famous memorial — and at a bargain rate — before July 4.

Trump announced the plan to renovate the pool with a commercial swimming pool liner in April, promising to cheaply fix what previous presidents had burned money on.

“The Biden administration and the Obama administration spent hundreds of millions of dollars trying to get it to work, and they failed,” Trump said at the time, referring to a $34 million renovation of the monument in 2012 completed during the Obama administration. That work sought to address algae issues, stagnant water and leaks, although once it was done both the leaks and algae resurfaced.

The National Park Service had received bids to try again with a total renovation of the granite and grouting. But that could have cost hundreds of millions, Trump said earlier this spring.

His solution was to use swimming pool contractors to coat the granite with a spray-on liner, saying the total cost would be between $1.5 million and $2 million. That cost later ballooned to $14.2 million, according to federal records.

Questioned on whether the NPS’ more extensive renovation plan would be a superior project to the pool liner, Trump said in April he had disagreed.

“No, this is much better … for much less money,” he recalled saying. “It will look far more beautiful, more beautiful than it did in 1922 when they built it.”

Kym Hall, the former regional director for NPS’ capitol region, said Trump’s fix ignored the larger, more expensive, problems that the pool faced. She noted that the filtration system and piping need renovations.

“I’m not sure how this administration thought they were going to somehow overcome a long-standing challenge of keeping [the pool] clear (not to mention wildlife contributing waste to the water) by painting it,” she said.

Last week Interior said the algae came from water in the pipes that had been left to stagnate while the pool was being renovated. When the pool was refilled after the pool liner was applied, it carried those contaminants into the pool.

Hall said that explanation was possible. The pipes and filtration system hold about 500,000 gallons of water of the roughly 4-million-gallon system, which is fed from the water in the Tidal Basin. NPS treats the water with ozone, which helps kill bacteria and depress algae growth.

“Trying to keep the algae at bay is a huge battle. So, they clean it each year and keep trying to keep it clean,” she said. “If this problem could have been easily solved or cheaply solved, somebody would have freaking done it.”