A pipeline leak caused about 3,500 barrels, or 147,000 gallons, of produced water to spill Friday in McKenzie County.
Bill Suess, spill investigation program manager for the North Dakota Department of Environmental Quality, said cleanup is underway before crews will evaluate the extent of the contamination.
"Based on that, we'll look at what the best plan of attack is," Suess said.
The spill occurred and was reported Friday in a pasture about 8 miles north of Alexander. Caliber Midstream North Dakota LLC owns and operates the pipeline.
No surface water appears impacted, Suess said. Groundwater impacts are under investigation.
How long remediation could take depends on the results of an electromagnetic survey. Suess said an underground concrete vault has contained most of the spill.
Produced water, or brine, is a waste byproduct of the oil production.
State personnel continue to monitor the site.
Alexander is about 20 miles south of Williston.