Authorities in Washington and Arlington, Virginia, on Wednesday advised all 920,000 residents to boil their drinking water after they discovered that algae blooms in the Potomac river had raised pollution levels in a reservoir.

The boil water advisory, which extended to the Pentagon, Arlington National Cemetery and Reagan National Airport, will remain in place until authorities confirm that it is safe to drink, the District of Columbia Water and Sewer Authority said in a statement.

It is unusual for a boil water advisory to cover the entire city, said John Lisle, a spokesperson for the Washington water utility, who added that he had not seen such an expansive warning in the 11 years that he worked there.

Cloudy or hazy water could be a sign of lower water quality, Arlington County said in a statement.

The water may contain bacteria, viruses and parasites that cause nausea, cramps, diarrhoea and headaches, authorities said. Infants, children, older people and people with compromised immune systems can be more vulnerable to them, they said.

Customers should discard any drinks or ice made after 9pm (local time) on Wednesday, authorities said.

They should also bring water to a rolling boil for at least one minute and let it cool before using it for drinking, brushing teeth, washing food, preparing baby food, making ice or giving to pets, they said.

After observing the increase in algae and turbidity, the US Army Corps of Engineers, which runs the Washington Aqueduct, said on Wednesday that it had increased its filtering capacity by washing existing filters and purchasing additional ones.

The water authority in Washington said the turbidity was found at the Dalecarlia Water Treatment Plant.

The Washington Aqueduct reduced the amount of water it received from there, initially shifting all of the water treatment to the area’s other facility, the McMillan plant.

But concerns about not having enough supply, especially for possible fires, prompted aqueduct officials to resume pumping from the Dalecarlia plant, the water authority said.

Lisle added that the decision to use the water from the Dalecarlia plant was also made in part because the Fourth of July holiday typically sees an increase in water use.

It was unclear when the advisory would be lifted, he said.

Fairfax Water, a water utility company that serves another suburb in Northern Virginia, said that there were no boil water notices because it had stopped receiving water from the Washington Aqueduct earlier on Wednesday.

New York Times News Service