Three weeks into an ambitious effort to extinguish a century-old underground coal fire beneath the Marshall Mesa Trailhead, crews are working to mitigate a long-standing wildfire risk in the Boulder area.
On Nov. 4, The Colorado Division of Reclamation Mining and Safety and the City of Boulder Open Space and Mountain Parks began excavating the site, where smoldering coal has burned since the 1920s, when the area was mined by small companies. Officials say the project, funded by the federal Infrastructure Investment and Jobs Act, aims to permanently reduce the fire’s risk by going beyond earlier, less-effective attempts.
The old mining area spans 12 acres beneath the Marshall Mesa Trailhead, a popular access point for biking and hiking trails.
The Marshall Mesa coal fire drew renewed attention after the official investigation into the 2021 Marshall Fire. The investigation, led by the Boulder County Sheriff’s Office and the District Attorney’s Office, concluded that the coal fire likely did not ignite the blaze. Instead, the fire was most likely caused by two separate sources: an unmoored Xcel Energy powerline and a smoldering junk pile on property owned by the Twelve Tribes community. Despite these findings, community concerns about the coal fire’s potential to trigger future wildfires persisted.
If the smoldering coal is exposed to too much oxygen and a fire travels upward through vents, it could ignite nearby vegetation and potentially spread.
The state has made several attempts to address the Marshall Mesa coal fire. In 2005, the state mine safety division used aggregate to cover vents that had caused surface temperatures to reach 375 degrees, igniting vegetation. In 2015, the state tried to cut off the coal’s oxygen supply with a surface seal. Neither effort extinguished the smoldering subsurface coal.
“The challenge is you never know that you’ve been able to plug all the pathways for oxygen,” said Jeff Graves, the director for the Colorado Division of Reclamation, Mining and Safety’s Inactive Mine Reclamation Program.
A new approach
The current project takes a more aggressive approach. Crews are excavating the coal seam — buried about 30 feet below the surface — blending smoldering coal with cooler soil and rock to cool it, and then compacting and replacing the mixture. Unburned coal is also being mixed with soil to minimize the risk of future ignition.
“Recently, we had an influx of funding through the Infrastructure Investment and Jobs Act that’s allowed us to not just take a band-aid approach, but actually do full scale mitigation and extinguishment,” Graves said.
The goal is to reduce coal temperatures below 80 degrees Fahrenheit, a significant drop from the current 175 to 225 degrees. Crews are advancing section by section to minimize exposed coal, a process that is expected to take two to three months, depending on weather conditions.
Wind poses a challenge, as crews are required to pause work involving exposed coal when gusts exceed 15 mph to reduce fire risk. On windy days, crews focus on excavating sediment in safer areas. Snow, on the other hand, aids the process by cooling the coal and keeping the surrounding vegetation wet.
After the coal excavation, trail renovations and revegetation of native plants will take an additional eight months to complete. The trailhead is slated to reopen again in October 2025 with expanded parking and bike services, according to Ilene Flax, senior landscape architect for the City of Boulder.
Long-term goals
“Extinguishment is a relative term,” Graves said. “Coal does oxidize over time, so there’s always that potential for some localized areas to heat up. But the goal is that now you don’t have lots of coal that could potentially combust and create problems at the surface.”
The state has successfully extinguished four other coal fires through excavation, and officials believe this method will work for Marshall Mesa. Of the approximately 40 coal fires burning in Colorado, the one at Marshall Mesa is closer to the surface than most. That’s a blessing and a curse, according to Graves. The proximity makes it more dangerous but also easier to address.
Graves expects federal funding to last another 12 years, allowing crews to address several coal fires each year.
Correction: a previous version of this story said federal funding partially funded the project and would last 10 years. Funding will last 12 years and fully fund the project.
The post In photos: Crews work to extinguish century-old Marshall Mesa coal fire in Boulder appeared first on The Boulder Reporting Lab.