CALGARY — Canadian Natural Resources Ltd. says a mechanical failure at an old well is behind an bitumen leak at its Primrose project on the Cold Lake Air Weapons Range, but says the damage has been contained and cleanup is ongoing.“Canadian Natural is confident the cause of the seepage is due to the mechanical failure of well bores in the vicinity of impacted locations,” company president Steve Laut said in a conference call Wednesday. “We have a pretty good idea of the likely well bores.”For weeks now, bitumen has been oozing to the surface at an oil sands project owned by Canadian Natural Resources on the weapons range. The leak has so far released almost a million litres of bitumen.Company officials said the affected area has now been reduced from about 20 hectares to 13.5 hectares. The leak continues to release about 2,400 litres a day.“The bitumen emulsion will continue to seep at an ever-declining rate for a period of time,” said Laut. “There is effectively little to no environmental damage to manage the ever-declining seepage.”The Calgary-based company says each of four locations where bitumen has been oozing to the surface has been secured and cleanup, recovery and reclamation are well underway.Laut said an old well drilled in 1997 by a previous operator is the suspected culprit. The company will test all the well bores on the site in an effort to prevent future failures, he added.“We believe we will be able to identify any potential well bores that may present risk and repair these bores before steaming. If the well bore is not reparable, steaming strategies can be modified to prevent these incidents from occurring in the future.”Laut said there is little chance that the steam injected to soften the bitumen and push it up has damaged the cap rock over the deposit. He said that would require more steam pressure than the company uses.The company also said there is no risk to humans from the seepage, although 16 birds, seven small mammals and 38 amphibians have died as a result.The discoveries were immediately reported to the Alberta Energy Regulator, which is working with the company and Alberta Environment and Sustainable Resource Development to investigate and remediate affected locations.The company’s near-term steaming plan at Primrose has also been modified as a result of the spill, with restrictions on steaming in some areas until the investigation is complete.Laut said the cleanup effort is likely to cost about $40-million. Another $20-million will be spent drilling new monitoring wells.He said it’s not expected that the company’s production figures of between 100,000 and 107,000 barrels a day will change.Laut said the company believes a similar problem in 2009 was also caused by a well failure, that time one of its own. Well designs were modified after that.