On the morning of Nov. 11, Enbridge, an international energy corporation, shut down their 6A oil pipeline after a worker discovered a loose valve at their Cambridge Exchange station. The station located in Oakland, WI — about 20 miles southwest of Madison — leaked 1,650 barrels of crude oil, roughly 69,300 gallons. This makes it one of the largest oil spills in Wisconsin history.

Enbridge initially estimated to the Wisconsin Department of Natural Resources that only two gallons of oil had been spilled. Enbridge revised their estimate to the DNR to 126 gallons Nov. 14. Then, on Dec. 13, Enbridge notified the public that they estimated 69,300 gallons of oil had leaked from the pipeline.

The spill originated in an underground pump transfer pipe within the station. Enbridge spokeswoman Juli Kellner said in an emailed statement to The Badger Herald that a faulty connection between pipes in the transfer station was the cause of the spill. She said the connection had loosened over time and the spill occurred over a period.

According to Enbridge’s final report to federal investigators, 0.10 barrels (4.2 gallons) of oil were believed to have seeped into groundwater. Enbridge claimed it has recovered only 960 (40,000 gallons) of the 1650 barrels spilled and has not stated if there would be an additional risk of oil leaching into groundwater.

In the statement, Keller said no oil had been found in wells in nearby homes or on-site during testing after the spill.

University of Wisconsin professor and expert on groundwater and underground aquifers Michael Cardiff said the amount of oil leaking into groundwater was not a significant amount. With 30,000 gallons yet to be recovered, more oil may leak into groundwater, Cardiff said.

“The four [gallon] number might represent the total identified amount of contamination,” Cardiff said. “There may be some remaining quantity that they haven’t found.”

Cardiff, who works with the DNR on remediation after groundwater contamination, said the four-gallon estimate for groundwater contamination sounded optimistic for the size of the spill.

The biggest danger posed by the spill is contamination of drinking water, Cardiff said. Shallow wells in rural areas are subject to higher contamination risks after spills, Cardiff said.

“All of the groundwater, which is water under the ground in between porous spaces, is connected to all of the surface water,” Cardiff said. “Anytime you have a spill that impacts groundwater, it can really impact any surface water feature nearby.”

Enbridge revised initial estimates of the remediation cost from $890,000 to $1,115,000 between their Dec. 11 report to the Department of Transportation and their final report to DOT on Jan. 10. This number may continue to rise as more of the missing 30,000 gallons has yet to be recovered.

The Line 6 spill raised fears over Enbridge’s fleet of aging pipelines that run across the Great Lakes region according to PBS Wisconsin. Line 6 is one of 10 pipelines operated by Enbridge in Wisconsin and one of three over 55 years old, along with Line 5 and Line 67, according to an Enbridge report.

A valve installed on the pipeline in 1973 was responsible for the spill. Enbridge’s final report to the Department of Transportation said the age of the valve may have played a role in the spill.

Calibrated torquing equipment with precise measurements were not readily available or used at the time of installment, according to the report.

Kellner asserted the pipeline was still safely operational and that Enbridge had investigated all similar connections across the Midwest.

Line 6 has not exceeded its operational lifespan, Kellner said in the statement.

The delay in public notification frustrated environmental groups due to its proximity to the permitting decision regarding Line 5, a crude oil pipeline, according to Sierra Club, an environmental justice group.

The pipeline’s permits for building a section of pipeline that loops around the reservation lands of the Bad River Band of the Lake Superior Chippewa Indians were approved by the DNR on Nov. 14, according to the DNR. The section is located south of the reservation but upstream from the Bad and White Rivers, which flow into Lake Superior.

The reroute comes on the heels of a 2019 lawsuit by Native and environmental groups that asked to stop Enbridge from operating the pipeline on the Bad River Band’s lands. In 2023, U.S. District Judge William Conley ordered Enbridge to pay a $5.1 million fine for trespass and the pipeline to cease operating on reservation lands by mid-2026, according to Wisconsin Public Radio.

Native tribes, environmental groups and businesses filed against a permit for a four-mile section of tunnel for the pipeline in Michigan after the DNR approved the rerouting permits in Wisconsin, according to Michigan Public.

Line 5 was ordered to shut down by Michigan Gov. Gretchen Whitmer in 2020 in a press release. Canada threatened to invoke the U.S.-Canada 1977 Pipeline Treaty in 2021, according to the Library of Congress. The treaty provides that if one of the countries interferes in a pipeline’s operation, an international arbitration court can review the case and order payment for the damages incurred.

Legal proceedings following Gov. Whitmer’s orders have been slowed as courts decide where the lawsuit should be filed. In 2024, the suit was moved back into state court from federal court by the 6th Circuit Court of the United States, according to the complaint.

The Biden administration asked in 2024 for the court to reconsider the shutdown order, according to Wisconsin Public Radio. As of now, Judge Conley’s order still stands if Enbridge continues to operate on the Bad River Band’s lands past the deadline in 2026.