Preliminary analysis suggests the oil leaked from a quarter-inch (two-thirds of a centimeter) hole corroded in a pipeline, according to Ed Meggert, a spill prevention and response coordinator for the Alaska Department of Environmental Protection in Juneau.
"Both BP and the state are real concerned about that," he said.
The leaky pipe is part of the oil field infrastructure built in the late 1970s. Officials are concerned that other sections of the aging system may be susceptible to leaks in the future.
"That's being examined very closely," Meggert added.
The spill, which covers about 2 acres (0.8 hectares), occurred in one of several caribou-crossing areas where pipes are laid underground and covered with gravel to allow passage by animals.
Brandon said the crossing areas attract water and the pipes underneath are particularly susceptible to corrosion.
While caribou, a migratory species, are currently absent from the North Slope, they'll return to the region this summer.
"Can we get this cleaned up in time for when the caribou get there?" Defenders of Wildlife's Matson asked.
Meggert expects the spill to be nearly 100-percent cleaned up before summer.
The liquid pools of oil have almost all been vacuumed, he said. Snow mixed with oil is being melted and the oil recovered. Crews will also scrape oil residue from the tundra.
He expects the spring melt to wash most of the remaining oil into an adjacent lake where floating booms will prevent further spread and allow for recovery.
"We have a pretty good track record cleaning these things up," he said. "I'm pretty confident we can do it, and if [the tundra] doesn't totally recover this year, in time it will, next year or the year after."
But the cleanup is a slow, cold process. The wind chill at Prudhoe Bay was less than -40ºF (-40ºC) Thursday.
"Right now, they are collecting a few hundred gallons a day basically, because it's so cold," said Brandon of the Alaska Wilderness League. "So that's just longer and longer the oil will be sitting out there."
Prior to this spill, the largest in the North Slope was a 38,850-gallon (147,063-liter) spill in 1989.
By contrast, the oil tanker Exxon Valdez spilled 11 million gallons (41.6 million liters) into Prince William Sound on Alaska's southern coast that same year.
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES