SAN FRANCISCO (AP/CBS) — The Standing Rock Sioux tribe has promised a legal battle after the Army Corps of Engineers said it’s clearing the way for completion of the Dakota Access oil pipeline.

The Army says it will grant an easement to allow the $3.8 billion pipeline to cross under Lake Oahe near Standing Rock, North Dakota. The crossing would complete work on the pipeline that will move oil from North Dakota through the Dakotas and Iowa to a shipping point in Illinois.

The tribe has long argued a pipeline leak would pollute its drinking water source because their reservation is located just downstream from the crossing. For months, demonstrators dubbed “water protectors” and environmentalists rallied around an encampment to block the 1,180 mile pipeline. Sympathizers staged protests in cities across the U.S., including San Francisco.

Pipeline developer Energy Transfer Partners insists the pipeline is safe.

Fulfilling his campaign promise, President Trump signed an Executive Order on January 24 urging the Army Corps of Engineers to reverse a December 2016 order calling for a broader Environmental Impact Study.

According to the Wall Street Journal, ETP now has “83 days from receipt of the easement to bring it into service after filling and testing it.”