WILLISTON, N.D. — A pipeline leak in Williams County spilled saltwater into a tributary of the Missouri River, the North Dakota Department of Environmental Quality reported Monday, July 15.

The pipeline’s leak detection system notified the company about the spill, which happened on Sunday about 20 miles east of Williston, said Bill Suess, the state's spill investigation program manager.

Pipeline operator Summit Midstream said it shut down the line and estimated that 500 barrels of produced water spilled. That equates to about 21,000 gallons. Produced water is a mixture of saltwater and oil that can contain drilling chemicals.

Most of the fluid was contained on a well pad, according to the company. Suess said the oil seems to have stayed on the pad, with an unknown amount of saltwater spilling into the tributary, which flows into the river's Lake Sakakawea reservoir.

The tributary is a creek that runs through a culvert under a road. It has little flow, and a remediation crew put up dams and is working to pump out the standing water, he said.

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“I’ve got an inspector on site, and we will continue to stay on site until we know there’s no risk to the lake,” Suess said.

Samples do not show any sign that saltwater has reached the lake. Suess said even if the area experiences heavy rainfall resulting in some of the fluid flowing into the lake, it would likely be so diluted that it wouldn’t be detected.

Saltwater poses a threat to invertebrates, and it could impact sediments in the creek as well as vegetation along the shore, according to Suess.

A crew is beginning excavation work around the pipeline to determine what caused the spill, he said.

One of Summit's subsidiaries operates a different line responsible for the state's largest pipeline spill, in 2015. Three million gallons of saltwater leaked, with some of the fluid entering a tributary north of Williston and eventually the Missouri River.