The substance is the inky black colour of oil, and the treetops are brown. Across a broad expanse of northern Alberta muskeg, the landscape is dead. It has been poisoned by a huge spill of 9.5 million litres of toxic waste from an oil and gas operation in northern Alberta, the third major leak in a region whose residents are now questioning whether enough is being done to maintain aging energy infrastructure.

The spill was first spotted on June 1. But not until Wednesday did Houston-based Apache Corp. release estimates of its size, which exceeds all of the major recent spills in North America. It comes amid heightened sensitivity about pipeline safety, as the industry faces broad public opposition to plans for a series of major new oil export pipelines to the U.S., British Columbia and eastern Canada.

In northern Alberta, not far from the town of Zama City, the leak of so-called "produced water" has affected some 42 hectares, the size of 52 CFL fields, in an area less than 100 kilometres south of the Northwest Territories border.

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"Every plant and tree died" in the area touched by the spill, said James Ahnassay, chief of the Dene Tha First Nation, whose members run traplines in an area that has seen oil and gas development since the 1950s.

Apache spokesman Paul Wyke called the spill "salty water," with "trace amounts" of oil. The Energy Resources Conservation Board, Alberta's energy regulator, said it contained roughly 200 parts per million of oil, or about 2,000 litres in total. But information compiled by the Dene Tha suggests the toxic substance contains hydrocarbons, high levels of salt, sulphurous compounds, metals and naturally occurring radioactive materials, along with chemical solvents and additives used by the oil industry.

Produced-water leaks are considered easier to clean up than oil spills. But the Dene Tha suspect this is a long-standing spill that may have gone undetected for months, given the widespread damage it has done. Apache and the Alberta government say its duration is under investigation.

The leak follows a pair of other major spills in the region, including 800,000 litres of an oil-water mixture from Pace Oil and Gas Ltd., and nearly 3.5 million litres of oil from a pipeline run by Plains Midstream Canada.

After those accidents, the Dene Tha had asked the Energy Resources Conservation Board, Alberta's energy regulator, to require installation of pressure and volume monitors, as well as emergency shutoff devices, on aging oil and gas infrastructure. The Apache spill has renewed calls for change.

"We don't believe that the government is doing enough to ensure upgrades and maintenance of the lines," Mr. Ahnassay said.

The Apache spill took place in an area rich with wetlands. Though the Dene Tha suspect waterfowl have died, the company said it has seen no wildlife impacts. The spill has not reached the Zama River, although the Alberta government said it has affected tributaries. Water monitoring is ongoing.

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Neither Apache nor Alberta initially disclosed the spill, which was only made public after someone reported it to a TV station late last week. The National Energy Board, by comparison, sent out a news release Tuesday after a spill of five to seven barrels of oil at an Imperial Oil Ltd. refinery in Sarnia, Ont.

Bob Curran, a spokesman for the ERCB, defended the late release of information, saying it took 10 days to determine the size of the spill.

"The second we knew the volumes, we put out a news release," he said. Asked how it could take so long to determine the severity of a large spill, he said Wednesday: "We didn't know it was over 42 hectares. We found that out last night."

Environmental groups have long criticized the government for being slow to notify the public when things go wrong with the oil industry, the province's financial lifeblood. "This latest spill should call into question the provincial government's decision to hide the pipeline safety report they received last year and the failure to follow through on the public pipeline safety review the Minister of Energy promised last July," said Greenpeace campaigner Mike Hudema in a statement.

A spokesman for Energy Minister Ken Hughes said the province trusts its energy regulator to decide when to release information based "on a process of established science and protocol."

Apache said in a statement that it has halted the leak and "taken steps to contain the release as the company continues to map, sample and monitor the impacted areas."