Xcel’s Boulder coal ash cleanup is moving forward. Critics say another source of pollution remains.

Colorado regulators are reviewing Xcel Energy’s final plan to clean up groundwater contamination caused by decades of coal ash disposal at the Valmont Power Station. The project, which could begin construction as early as late summer, follows years of contamination that migrated beyond the company’s property and was detected in at least one nearby residential well.

The proposed system would pump contaminated groundwater from beneath the site and transport it for treatment, a first-of-its-kind project in Colorado. But environmental advocates say the cleanup may fall short if a second coal ash landfill near the power plant is also contributing to the pollution.

Almost a century of coal burning at the Valmont Station left behind 1.6 million cubic yards of coal ash stored in landfills just east of Boulder. The waste contains contaminants including arsenic, lead and chromium.

A Boulder Reporting Lab investigation found that a coal ash landfill built at the Valmont site in 1993 has contaminated groundwater since at least 2017. Drawing on Xcel’s own monitoring data, the investigation documented coal ash in contact with groundwater, elevated levels of lithium and selenium in monitoring wells, and a contamination plume that has gradually migrated toward residential properties east of the site. Xcel also found lithium above safe levels in at least one nearby homeowner’s well.

“The timely installation of a well-designed groundwater treatment system is critical to help prevent the continued or further spread of groundwater contamination,” Erin Dodge, a water quality program coordinator with Boulder County Public Health, told Boulder Reporting Lab in a statement.

State and federal regulations require Xcel to address both the source of the contamination — coal ash stored at the site — and the polluted groundwater. The extraction system is one piece of a broader cleanup effort that will later include removing coal ash from the 1993 landfill, a project expected to take about a decade.

Residents and local officials have also raised concerns about potential air quality impacts from that excavation. Earlier this year, Boulder Reporting Lab reported that community members were pursuing independent air monitoring as Xcel prepares to remove and process coal ash at the site.

Under the groundwater treatment plan, contaminated groundwater would be pumped from the site and hauled away for treatment and disposal. Xcel would replenish groundwater removed during the process to offset depletion.

“We are currently anticipating approval from the state on the final design for that remedy,” Xcel spokesperson Sydney Isenberg said in a statement. The company expects installation of the extraction system to begin in late summer and continue through the fall.

Questions about a second landfill

A second coal ash landfill sits northeast of the power plant. Because it was no longer operating when the Environmental Protection Agency adopted nationwide coal ash cleanup requirements in 2015, it was initially excluded from federal regulations.

That changed in 2024, when the EPA expanded its coal ash rules to cover so-called legacy landfills, including the one at Valmont. The updated regulations do not establish firm cleanup deadlines, however, and Xcel has not yet proposed a remediation plan for the site. The Trump administration has since proposed rolling back those requirements.

The uncertainty surrounding that landfill has become one of the central questions hanging over the broader cleanup effort.

According to an EPA fact sheet, legacy landfills are more prone to leaks because many lack liners and groundwater monitoring systems.

Boulder Reporting Lab’s investigation found evidence that the legacy landfill may also be contributing to contamination. But because the landfill was exempt from federal coal ash regulations, Xcel was not required to monitor it for contamination the way it monitored the newer 1993 landfill. The company says the legacy landfill’s role remains unclear. In a 2024 work plan, Xcel stated that “potential contributions from the Legacy Landfill to the groundwater quality is unknown.”

Xcel says contaminated groundwater extends beneath both landfills and that its treatment system is designed to address contamination regardless of the source. But while coal ash would be removed from the 1993 landfill, the legacy landfill would remain in place and could potentially leak pollutants into groundwater for years to come.

“That’s exactly why it’s a problem,” said Abel Russ, a lawyer with the Environmental Integrity Project, a nonprofit focused on environmental accountability and coal ash regulation.

“They’re ignoring what may be an equal contributor to the contamination,” Russ said. “They could get stuck in this perpetual process of pumping and treating groundwater that doesn’t get cleaner.”

If the federal requirements are withdrawn, pressure to address the second landfill would largely fall to Colorado regulators or Xcel itself, Russ said.

“This will impact the Valmont site because the previously unregulated ash areas will again be unregulated,” said Lisa Evans, senior counsel with Earthjustice. She said she expects the rollback to be finalized by the end of the year.

Inside the pump and treat plan

Contamination has spread into two distinct groundwater plumes, according to Xcel’s cleanup design documents. One, northeast of the landfill, contains elevated levels of selenium and lithium. The other, west of the landfill, is primarily contaminated with lithium.

The cleanup system would use a network of extraction wells and collection trenches to intercept contaminated groundwater before it can migrate farther from the site. Collection trenches would use gravity to funnel water to lift stations, while extraction wells would pump it from underground. The captured water would then be stored in tanks and transported to a treatment facility.

Xcel is also preparing a water augmentation plan to replenish groundwater depletion impacts and prevent effects on downstream water rights along Boulder and South Boulder creeks.

The company says nearby residential wells are not expected to be affected by the extraction project.

Other safety measures include high-level alarms designed to prevent overflows, secondary containment around storage tanks capable of holding an entire spill and conveyance pipes buried below the frost line. All pipes used to transport contaminated water would also be lined.

Xcel still needs several federal, state and local permits, including grading, stormwater and floodplain permits from Boulder County.

Construction is expected to take five to seven months. Depending on regulatory approvals, the system could begin operating in early 2027, Isenberg said.

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This story was originally published by Boulder Reporting Lab and distributed through a partnership with The Associated Press.