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Areas surrounding the Davidson ditch are seen from Marshall and Cherryvale Road, Aug. 15, 2023, south of Boulder. (Olivia Sun, The Colorado Sun via Report for America)

State mining officials will spend the fall and winter digging an enormous $1 million pit to surface and snuff out a long-burning coal fire near the Marshall fire’s launching point, in one of the most urgent battles against smoldering seams that can spark devastating wildfires. 

The Lewis Coal Mine fire, northeast of Marshall and Cherryvale roads a mile south of the city of Boulder, is one of a number of Marshall Mesa coal seam dangers that were at one point a suspect as ignition for the late 2021 Marshall fire that burned more than 1,000 homes.

Investigators later pinned Marshall fire blame on a rekindled trash burn on private property, flames that met up with sparks from a nearby downed Xcel power line in 100 mph winds. (Xcel objects to the conclusions and still points to the nearby coal fires as a possible cause.)

The mesa coal fires, smoldering for decades, are also blamed for periodic grass fires that crop up in southern Boulder County. 

Areas surrounding the Davidson ditch are seen from Marshall and Cherryvale Road, Aug. 15, 2023, south of Boulder. (Olivia Sun, The Colorado Sun via Report for America)

Excavating and extinguishing one section of the Lewis site has extra urgency, state officials said, because they must dig up portions of the Davidson irrigation ditch and then rebuild it before water customers need flow by April 1. Untreated fires could undermine sections of the ditch and cause cave-ins or flooding. Boulder County said it would issue an emergency declaration Tuesday for the Lewis site to help speed the project.

Periodic monitoring of the Lewis site has unveiled “pretty significant changes” since a previous survey in 2018, said Jeff Graves, director of the state’s Inactive Mine Reclamation Program. Those reviews have increased since the Marshall fire. 

Surveys show increased cracking of the surface dirt, and probes inserted into newly-drilled well holes have shown temperatures rising.  The state also takes aerial surveys in winter, and has seen snow patch melting over the Lewis site, much like snow melting in patterns off a warm car hood. 

The state may not know the full extent of the dig until they are deeper into it, but Graves estimates the project at about $1 million. Colorado in 2022 received $10 million in federal infrastructure stimulus funds to attack coal seam fires more aggressively. Graves at the time called the stimulus a “game changer” that would triple the coal firefighting budget. 

Some of that money immediately went toward efforts around Glenwood Springs. Smoldering coal deposits were identified as the cause of the 2002 Coal Seam fire outside Glenwood Springs, which burned 29 homes across 12,000 acres. The ongoing coal fires continued to vent smoke and steam near the city landfill in the South Canyon area last year, and new flare-ups were a constant worry for town residents and officials, said Mayor Jonathan Godes. 

Coal seam fires in Colorado go back so far, officials do not always have records on when they first became a problem. The Lewis site has been smoldering off and on for at least 20 years, Graves said. Coal seam fires can be started by other wildfires or by lightning strikes on exposed flammable spots. Most, though, are caused by spontaneous combustion — when coal is exposed to the air, it oxidizes and heats up to the point of burning, Graves said. 

Part of Marshall Mesa still shows burn scars from previous fires; fighting flareups from coal seam fires burning for decades in the area is a key strategy in combatting future grass and brush fires. These burn scars are visible from near Marshall and Cherryvale Roads, on Aug. 15, 2023, south of Boulder. (Olivia Sun, The Colorado Sun via Report for America)

The initial excavation at Lewis, on private land with permission of the owners, will occur over about an acre, which is the size of a football field. The dig could be 30 feet deep, and 300 feet by 100 feet on the surface, Graves said. 

Excavators uncap the burning materials, then scoop out embers and spread them out, mixing in sand and other inert materials, much like snuffing a campfire. Daily work has to be staged to avoid leaving seams open too long to oxygen that might further ignite the material. When excavators are confident the mined material is cool, and the seam has been dug back to stable areas, the material is repacked into the hole. 

The safe target temperature is less than 100F. Boulder County and state mine reclamation will cooperate to revegetate the Lewis site after the dirt is put back. 

Deadlines and local danger will propel the heaviest work beginning in October, Graves said. 

“It’s time sensitive, certainly from a ditch standpoint, and then also the concern associated with just that increased activity we’ve been observing,” he said. 

Recent testing has found fire activity, though less than at the Lewis site, nearby at the Marshall coal mine. Some of that is under city of Boulder open space, and the state plans fire mitigation work underground there next year, officials said. 

Michael Booth is The Sun’s environment writer, and co-author of The Sun’s weekly climate and health newsletter The Temperature. He and John Ingold host the weekly Sun-Up podcast on The Temperature topics every Thursday. He is co-author with...