The owner of a Wentworth Street North industrial site has discovered "hundreds" of barrels of waste hidden behind a concrete wall.

The owners of 350 Wentworth St. N. called in the Ministry of Environment on Tuesday to test "unknown waste" found concealed in the basement.

Investigators have only seen the barrels through a small portal in the wall, but estimate there are "hundreds" of them, said district manager Geoffrey Knapper.

They are still determining the contents of the barrels, which had been there "quite a while," Knapper said.

"Today was our first day on site, so we can't really make a determination."

The ministry closed the site, which is a former coal tar plant, on Tuesday afternoon. Inspectors called in the fire department and public health officials to deal with any potential public safety concerns for nearby residents.

"At this time, the barrels are secured in an area and obviously behind a wall," so "we don't have any concerns about that," he said.

The property, which has had several owners, has gained ministry attention in previous years. In 2010, there was a large mandatory and voluntary clean up of toxic materials on the lot.

Matt Jelly, a lower-city activist who has followed environmental efforts at the site, said the latest news concerns him.

"This is definitely an environmental problem. Barrels corrode over time," he said.

"I'm always been concerned about these kinds of properties left by industry."