Minnesota regulators are investigating the release of drilling fluid by crews attempting to route the Enbridge Line 3 oil pipeline under the Willow River near Palisade.

Minnesota Pollution Control Agency spokesperson Cori Rude-Young said approximately 80-100 gallons of drilling fluid (a mixture of bentonite clay, water and xanthan gum — a sucrose-based food additive) was released into the Willow River on July 6.

The fluid surfaced as crews were drilling a tunnel beneath the river and its streambed for the pipeline to follow — a process called "horizontal direction drilling," according to Juli Kellner, a spokesperson for Enbridge.

Drilling has not resumed as Enbridge is consulting with the MPCA before it restarts, as required by project's water quality permit granted by the MPCA, Rude-Young said.

The investigation into the spill is ongoing and the MPCA maintains that additional details are not yet public.

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"The agency is unable to release all information regarding the inadvertent release and follow-up activities, because it is actively investigating the events and such information is classified by state statute as 'nonpublic data,'" Rude-Young said. "Additional information can be released upon conclusion of the active investigation."

Both Kellner and Rude-Young said the spill was "nontoxic."

However, in a news release last week, Honor the Earth, an Indigenous-led environmental group opposed to Line 3, warned even the mud itself provided risks.

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"The mud itself is composed of fine particles, which can smother aquatic life," the group warned.

The release was seen by Line 3 opponents, who call themselves "water protectors," who were at the Willow River on July 6 to block and delay construction.

Kellner said environmental control measures prevented impacts downstream and to aquifers and that cleanup was supervised by "trained environmental inspectors and third-party agency monitors."

"Upon identifying the inadvertent return, the drilling operation was immediately shut down and crews followed the procedure for managing containment and cleanup of material as specified in project permits," Kellner said in a statement Tuesday.

The Willow River crossing is one of more than 200 water crossings the Minnesota section of Line 3 will take.

Work on the 340-mile pipeline across northern Minnesota is more than 60% complete as protests against the pipeline continue. Opponents of the pipeline say it is unneeded, at risk of an oil spill, worsens climate change and violates Indigenous and treaty rights.

Once complete, the pipeline will replace the existing, aging Line 3 and ferry 760,000 barrels of oil (31.92 million gallons) per day from Alberta, Canada, to Enbridge's terminal in Superior, following a new route through much of northern Minnesota. The segments in Canada, North Dakota and Wisconsin are already complete.