NEW ORLEANS (AP) — A small balloon striking a utility line near a New Orleans drinking water plant caused only a brief power outage but was followed by a drop in water pressure, a serious injury to a worker trying to restart water pumps, and a boil water advisory for most of the city that is expected to last until Thursday afternoon.

The outage Tuesday night and the unusual circumstances that followed pointed to both the vulnerability of New Orleans’ infrastructure — including the system that provides drinking water and street drainage — and the recurring problem of Mylar balloons striking power lines.

Entergy New Orleans, which provides electricity in the city, said in a statement that a Mylar balloon caused a “flicker” of low voltage Tuesday night at the water treatment plant that serves most of New Orleans. But it caused four pumps at the station to trip off, according to the executive director of the city’s Sewerage and Water Board.

Water flowing from huge tanks at the plant provided pressure for a time, Ghassan Korban, told reporters at a news conference Wednesday afternoon. And under normal circumstances, the pumps would have been restored to service in plenty of time to avoid a pressure drop.

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But one of the Sewerage and Water Board employees tasked with restoring the pumps suffered an unspecified, “significant” injury, Korban said. He gave no details, but said fellow employees had to tend to their injured coworker while summoning medical help. That led to a delay in restoring the pumps, and a drop in water pressure.

Low pressure can result in bacteria entering the water system, officials said. So, as a precaution, water system customers are advised to boil water before consuming or cooking with it until tests can be completed. The advisory covered most of the city, which has a population of nearly 370,000.

Korban said completion of the tanks at the water plant has helped alleviate the need for frequent boil water advisories that plagued the city several years ago. And he said his agency is working on a power complex for the city’s street drainage system to reduce the dependence on the Entergy system, and is seeking funding to tie that complex into the drinking water system.