Enbridge says it was not covering up the true size of a pipeline leak in the Northwest Territories, which leaked up to 1,500 barrels into the northern environment.

Enbridge had originally reported that only four barrels of oil leaked from its Norman Wells pipeline on May 9. But the company revised its estimate on Monday, saying 700 to 1,500 barrels had spilled.

The pipeline leak, about 50 kilometres south of the community of Wrigley, N.W.T., has area residents wondering why there was such a discrepancy in Enbridge's estimates.

In the company's first on-air interview since the leak, Enbridge executive Leon Zupan told CBC News that crews only discovered the true size of the spill after they started drilling down into the permafrost to obtain core samples.

"It's really only [been] in the last week to 10 days that we've been able to come up with the assessment," Zupan, who is vice-president of liquid pipelines operations, said Tuesday.

"Our initial estimates could only be on what we see," he added. "It's somewhat unique for us because we're dealing with a situation where we have frozen ground. Normally, our pipelines run with warmer product that doesn't allow the frost to trap it in."

Pinhole-sized opening

Enbridge officials say they don't know how the leak began, but they said the oil leaked out of an opening about the size of a pinhole.

Oil coming out of such a small opening has, over time, created a spill about half a hectare in size, according to the company.

Zupan said Enbridge makes regular aerial surveillance checks on the Norman Wells pipeline and uses internal inspection tools to detect leaks.

But in the latest case, those systems failed because the hole was so small, he said.

"We had actually planned to run internal inspection tools this year in the late summer, and we've ramped up those plans so we that will complete the full run of the entire pipeline this year," he said.

Enbridge officials insist the spill has been contained and will not likely exceed 1,500 barrels. The company also maintains the oil leak has had no impact on nearby Willowlake River.

The Norman Wells pipeline resumed operations on May 20. The company says it has spent $1 million to $2 million to date on the spill, but that figure is expected to spike as cleanup efforts continue.

Concerned residents in Wrigley, a community of about 100, are expected to meet on Wednesday to discuss the situation.