The city of Houston has ordered more than 2 million residents to boil water after a power outage affecting a purification plant.

Houston officials issued the advisory Monday morning after the outage Sunday caused water pressure at the East Water Purification Plant to drop below the minimum allowed by the Texas Commission on Environmental Quality (TCEQ).

Under Texas law, a citywide boil-water advisory cannot end until 24 hours after the TCEQ says the samples are at acceptable levels. Mayor Sylvester Turner’s office said in a statement that the earliest the city could receive an all-clear is Monday night or the early hours of Tuesday morning.

Turner (D) denied there was any hazard of contamination on Twitter, telling NBC News’s Mike Hixenbaugh “When the water pressure falls before 20 [pounds per square inch] generally there is a regulatory requirement to issue a boil water notice even if we believe the water is safe to drink. It remains in place until water samples are tested.”

Texas’s largest city found itself in a similar situation last February, when the state’s self-contained power grid failed during extreme winter weather. Houston issued another boil-water advisory in response to that crisis, which also led to widespread freezing of pipes.

At the other extreme, heat and drought conditions also contributed to issues with the water supply this summer, when the city’s department of public works fielded four times the typical number of water leak reports. The city blamed Houston’s clay-derived soil, which becomes brittle in dry conditions and can increase shifts in the ground, causing pipes to crack or disconnect.

Meanwhile, Austin saw a boil-water advisory this February after officials in the state’s capital warned of unspecified “errors” at a treatment facility.

In 2021, the city reached a consent decree with the Environmental Protection Agency requiring the city to upgrade its sewage pipes to the tune of $2 billion.