BISMARCK, N.D. (AP) — Cleanup is underway after 21,000 gallons of brine oilfield wastewater leaked from an underground pipeline in western North Dakota and into an unnamed tributary of the Missouri River, the state Health Department said Monday.

State environmental scientist Bill Suess said the pipeline operator, Polar Midstream, on Sunday reported the spill of "produced water," a byproduct of oil production that contains saltwater and oil, and sometimes chemicals from hydraulic fracturing operations.

The spill occurred about a mile from Lake Sakakawea, the largest reservoir on the Missouri River, which then heads into South Dakota before reaching the Nebraska border. Suess said Monday it did not appear the spill reached the lake.

Polar Midstream is a unit of Woodlands, Texas-based Summit Midstream Partners, which was responsible for a 3-million-gallon "produced water" leak from a pipeline in 2014, the largest of its kind in the state.

The 2014 spill leaked for months and contaminated Blacktail Creek. The wastewater also flowed into the Little Muddy and Missouri Rivers, though state officials said there was no harm to drinking water.

Suess said the company then installed spill detection equipment on its pipelines. Automated equipment caught the latest spill right away, he said.

The pipeline was immediately shut down, Suess said. The cause of the spill was not known Monday.

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