A plant near Edson that is now the focus of an advisory from Alberta Health Services leaked unsafe levels of sulfolane six years ago, CBC News has learned.

On March 12, Alberta Health Services advised residents near the South Rosevear Gas Plant not to drink their well water after a monitoring well tested high for sulfolane, a chemical used to remove compounds from sour gas.

Provincial government officials now confirm that the same plant leaked sulfolane in 2008, but Alberta Environment spokeswoman Katrina Bluetchen says her department wasn’t told about it until 2012.

However, no sulfolane was detected in tests in 2010, 2011 and 2012.

“So at that point there was no report to follow-up on,” Bluetchen said. A report from 2009 could not be found, even though companies are supposed to submit one each year.

Alberta Chief Medical Officer of Health Dr. James Talbot says the 2008 leak is why the advisory included all homes within a five kilometre radius of the plant, because it may have had time to spread further.

"We were sure that the exposure had been at least weeks to months,” he said. “And that there was the potential that it could have been years."

The earlier leak was not mentioned on last month’s health advisory. Wildrose health critic Heather Forsyth says that’s unacceptable.

"It's incumbent upon the government to let people know how long they knew about it, what have they been doing, and how are they going to fix it," she said.

Bonavista Energy bought the South Rosevear Gas Plant from Suncor in 2010.

Bonavista says the current leak — which has 125 times the interim Health Canada guidelines for safe sulfolane levels —started before they took over.  

Investigators from the Alberta Energy Regulator are now trying to determine the cause and the size of the leak, a process that will take months.

Alberta Health will keep the advisory in effect until they receive more information.

The advisory will remain in effect while the investigation is underway.

With files from the CBC's Marion Warnica