A semi-trailer hauling gasoline rolled over on Highway 2 north of Calgary early Tuesday morning, spilling thousands of litres of fuel into wetlands.

The single-vehicle crash happened at the Balzac overpass near CrossIron Mills at about 4 a.m. MT, RCMP said.

The route was closed in both directions for most of the day as hazardous materials response teams cleaned up the spill.

Most of the highway lanes have reopened since the spill, and the truck has been removed, but police cautioned drivers to expect delays.

Alberta Environment says the vehicle was carrying about 35,000 litres of gasoline and 4,500 litres of diesel. It is estimated 25,000 litres of fuel spilled into the ditch and wetlands.

Local officials had earlier estimated that 2,000 litres of fuel had spilled into the ditch and 13,000 litres into the wetlands.​

Vacuum trucks at the scene have been concentrating efforts in the wetlands. Free-flowing product is being removed and all contaminated snow and dirt will be removed.

Alberta Environment and Alberta Emergency Management Agency crews were on the scene to monitor the effects on the wetlands.

"Fortunately, it's gasoline," said Grant Kaiser with Rocky View County, "not a heavier oil … so we are hoping the cleanup with be fairly quick and there won't [be] any long-term consequences."

RCMP Cpl. Darrin Turnbull said driver fatigue is being looked at as a possible cause for the accident.

“We’re still investigating to determine … what exactly happened here, however the driver of the tanker truck did make comments to the emergency personnel arriving on scene initially that he may have fallen asleep,” he said.

The night-shift driver was on a routine run from Edmonton to Calgary. The company said the driver blacked out and he's seeking medical attention to find out why.

"He said he was feeling fine," said Greg Keffer, owner of G&B Fuels Inc. "First day back after four days off, he was feeling really good and he doesn't remember."

The company said it is keen to co-operate with officials to figure out what happened, because these types of crashes of the potential to be disastrous for drivers, the public and the environment..