The North Dakota Department of Health is testing to see if local drinking water has been contaminated due to an oil field spill near Williston.
On Sunday, more than 2,700 barrels of produced water spilled from a faulty valve at a site operated by Zavanna.
The company had a barrier around the well site, but the North Dakota Department of Health is still determining how the salt water escaped.
Water is being sampled for contamination at a farm to the west of the spill.
Produced water is a mixture of saltwater and oil that can contain drilling chemicals.
Regulators say drinking water in Williston is safe because it is drawn from the Missouri River.
No surface water has been contaminated.
State environmental scientist Bill Suess says they are going to drill holes to find out the depth of the spill.
"It looks like we have a good 50 foot clay layer down 30 feet deep down to ground water, which is about 70 to 80 feet deep. So it looks like, hopefully, it's not going to impact ground water," said Bill Suess, North Dakota Department of Health.
Suess says there have been approximately 11 produced water spills larger than this one.