The Billings Gazette reports that the spill was discovered April 27. More than 50 large dump trucks full of soil have been removed from the site so far, with more to come.
The Environmental Protection Agency is overseeing the cleanup for the reservation.
The well that the spill came from had been shut down and was last inspected in December.
It’s believed that the wellhead froze and cracked over the winter. The crack leaked an estimated 90,000 barrels of brine with 600 barrels of crude oil. The spill was discovered by a rancher flying his plane over the remote area.