Cleanup was underway Thursday after nearly 3 million gallons of brine — a toxic byproduct of oil and natural gas production — leaked from a pipeline in western North Dakota in the state’s largest spill of its kind since the current energy boom began.

The brine is an unwanted byproduct of oil and natural gas drilling. It is much saltier than seawater and may contain petroleum and residue from hydraulic fracturing, or fracking, operations.

Two creeks have been affected, and the full environmental impact might not be clear for months.

Pipeline operator Summit Midstream Partners' chief operating officer, Rene Casadaban, said in a statement that the company's "full and undivided attention" is focused on cleaning up the spill and repairing any environmental damage. A contractor was expected to be on site Thursday assessing the damage.

Summit Midstream Partners detected the pipeline spill on Jan. 6 about 15 miles north of the town of Williston and told health officials then. Officials say they were not given a full account of the size until Tuesday.

Inspectors have been monitoring the area, but it will be difficult to measure the effects on the environment and wildlife until the ice melts, said Dave Glatt, chief of the North Dakota Department of Health's environmental health section. Some previous saltwater spills have taken years to clean up. "This is not something we want to happen in North Dakota," he said.

At the moment, the spill doesn't threaten public drinking water or human health, Glatt said. He said a handful of farmers have been asked to keep their livestock away from the two creeks, the smaller of which will be drained.

The new spill is almost three times as large as one that fouled a portion of the Fort Berthold Indian Reservation in July. Another million-gallon saltwater spill, in 2006 near Alexander, is still being cleaned up nearly a decade later.

Summit Midstream said in a statement Wednesday that about 65,000 barrels of a mix of freshwater and brine have been pumped out from Blacktail Creek. Brine also reached the bigger Little Muddy Creek and potentially the Missouri River.

Glatt said the Blacktail Creek will be fully drained as part of the initial cleanup, but the water and soil will have to be continuously tested until after the spring thaw because some of the contaminated water has frozen. The Little Muddy Creek will not be drained because it is bigger than the Blacktail Creek and the saltwater is being diluted.

"We will be monitoring to see how quickly it gets back to natural background water quality conditions, and we are already starting to see that," Glatt said of Little Muddy Creek. "It's getting back pretty quickly."

Summit Midstream spokesman Jonathan Morgan did not immediately confirm when the spill began or what caused the pipeline to rupture. Glatt said that the company has found the damaged portion of pipeline and that it was sent to a laboratory to determine what caused the hole.

North Dakota has suffered scores of saltwater spills since the fracking boom began in earnest in 2006. A network of saltwater pipelines extends to hundreds of disposal wells in the western part of the state, where the brine is pumped underground for permanent storage. Bills to mandate flow meters and cutoff switches on saltwater pipelines was overwhelmingly rejected in the state legislature in 2013.

Wayde Schafer, a North Dakota spokesman for the Sierra Club, called the brine "a real toxic mix" and "an extreme threat to the environment and people's health."

"Technology exists to prevent these spills, and nothing is being done," he said. "Better pipelines, flow meters, cutoff switches, more inspectors — something has got to be done."

Daryl Peterson, a grain farmer from Mohall and a board member of the Northwest Landowners Association who has had spills on his property, said the latest incident underscores the need for tougher regulation and enforcement.

"Until we start holding companies fully accountable with penalties, I don't think we're going to change this whole situation we have in North Dakota," he said.

The Associated Press