"One of the wellheads where we store the hydrocarbons was under some scheduled maintenance when the leak occurred," company spokesman Jonathan Moser said Thursday.

The leaking wellhead was spraying ethylene, a chemical taken from natural gas that is used to make polyethylene plastic pellets at Dow's nearby plant in Fort Saskatchewan. The colourless, slightly sweet-smelling gas was escaping at the rate of between five and 10 kilograms per minute, said Moser.

He didn't expect any environmental impacts from the release, which began shortly after 11 a.m. and was sealed shortly before 4 p.m.

"There isn't any impact to the surrounding environment or the public at large," he said. "(The gas) is just rising up.

Health guidelines suggest a maximum exposure of 200 parts per million over an eight-hour period.

"All the downwind monitoring is indicating we're below this level," Moser said.

There were no injuries. Dow's chemical plants remained in operation during the leak.

The well was soaked with water and kept wet to prevent it from catching fire.

The RCMP had closed two nearby roads, including the highway leading into the plant.

"Right now, that's just as a precaution," said Const. Renee Bates of the Fort Saskatchewan RCMP. "There's been no outside threat to either citizens of Fort Saskatchewan (or) the environment."

The area is home to a number of petrochemical plants and is often referred to as "Upgrader Alley."