The Bureau of Land Management violated federal environmental law in approving more than 58,000 acres of Colorado land for oil and gas leases without a sufficient evaluation, a federal judge ruled on Tuesday.

Environmental advocacy groups, including Rocky Mountain Wild and WildEarth Guardians, had challenged BLM’s June 2018 lease auction in northwest Colorado by asserting the federal government neglected to consider more recent pollution data and disregarded adverse impacts to air quality and wilderness from new extraction activity.

“BLM may not simply ignore evidence of reasonably foreseeable environmental effects that is available to it at the time it makes its decisions,” wrote U.S. District Court Senior Judge Marcia S. Krieger in a September 28 order.

However, she found the violations to be relatively minor and unlikely to alter BLM’s decision-making fundamentally. Krieger declined to cancel the leases outright, calling such an action “excessively disruptive.” Instead, she determined the bureau itself to be in a better position to decide the outcome of the leased land as it followed her directive to reconsider its environmental analysis.

The plaintiffs to the lawsuit celebrated the judge’s order, reiterating that fossil fuel extraction imperils air quality and calling on BLM to revoke the leases in Moffat, Routt, Jackson, and Rio Blanco counties.

“These leases are not only a threat to Dinosaur National Monument, local communities and the climate, but have now been found to be unlawful,” said Tracy Coppola, senior Colorado program manager for the National Parks Conservation Association. “The previous administration had recklessly cut corners in its rush to hand public land over to oil and gas corporations. This court decision has thankfully remanded these leases, but the Biden administration must now finish the job and revoke them permanently.”

A BLM spokesperson said there have been no approved applications for permits to drill on the leased land yet, which will simplify the agency’s reevaluation. Prior to approving permits, BLM completes additional environmental review and may impose conditions to protect specific sites.

In June 2017, BLM conducted an environmental assessment as it considered whether to lease roughly 101,000 acres for oil and gas extraction, and ultimately decided in favor of the lease. The following year, the bureau proposed auctioning off another 58,573 acres in the region. But this time, it did not perform another environmental assessment, concluding the 2017 review was sufficient.

The environmental advocacy groups sued, arguing that oil and gas development contributed to degraded air quality and ozone pollution. They accused BLM of neglecting more accurate air quality data released after the 2017 assessment and of failing to consider an alternative with no additional leases.

BLM “failed to consider the fact that the incremental emissions from these leases will worsen ozone levels that already pose a serious threat to public health and welfare,” the groups wrote to the court.

“BLM must therefore analyze air quality impacts before issuing the leases so that it can determine whether it needs to take measures, such as declining to lease some or all of these lands,” the groups continued.

In making its 2018 decision, BLM did not incorporate the actual, local air quality measurements that became available after the 2017 environmental assessment. The new information showed excessive ozone levels in places even under a “low development” scenario for oil and gas extraction.

Krieger determined BLM’s failure to consider the air quality predictions in its 2018 lease decision violated the National Environmental Policy Act, a signature environmental law that requires the federal government to consider the significant environmental effects of its proposed actions.

Although she found BLM had acted appropriately when it considered a scenario taking no action to lease the 58,000 acres, the judge did find another violation from BLM’s failure to consider the specific effects of extraction activity to areas with wilderness characteristics in the proposed lease region.

“It may be that, upon further evaluation, the wilderness character in the identified lands is not so significant that it outweighs the reasons why the BLM selected those particular parcels for leasing in the first place. But NEPA requires the BLM to at least consider that question,” Krieger concluded.

The Western Energy Alliance, an association of oil and gas companies, wrote to the court asking Krieger not to cancel the 2018 leases, saying it would be a waste of their members’ investment.

The court’s decision comes as the U.S. Senate has confirmed the first permanent BLM director in more than four years this week, and as the Biden administration appeals a June court ruling that prevented the government from pausing new oil and gas leases on public lands and waters. The moratorium was intended to be part of a review of the federal leasing program’s effects on climate change.

The case is Rocky Mountain Wild et al. v. Haaland et al.