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

Tuesday night’s power outage and the unusual circumstances that followed highlighted both the vulnerability of New Orleans’ infrastructure – including the system that provides drinking water and street drainage – and the recurring problem of Mylar balloons Strike against power lines.

Entergy New Orleans, the city’s electricity provider, said in a statement that a Mylar balloon caused a low-voltage “flicker” Tuesday night at the water treatment plant that serves most of New Orleans. However, according to the general manager of the city’s sewer and water utility, it caused four of the plant’s pumps to fail.

Water from the power plant’s huge tanks provided pressure for a while, Ghassan Korban told reporters at a press conference on Wednesday afternoon. And under normal circumstances, the pumps would have been restarted in time to avoid a drop in pressure.

But one of the Sewerage and Water Authority employees tasked with repairing the pumps suffered an unspecified “serious” injury, Korban said. He did not provide details but said his colleagues had to tend to their injured colleague while calling for medical help. This led to a delay in repairing the pumps and a drop in water pressure.

Low pressure can cause bacteria to enter the water system, officials said, so as a precaution, water system customers are being advised to boil water before consumption or cooking until testing is complete. The warning applies to most of the city, which has a population of nearly 370,000.

Korban said the completion of the tanks at the water plant has helped reduce the need for the 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 dependence on the Entergy system and is looking for funding to tie that complex into the drinking water system.