By Logan Smith

BOULDER COUNTY, Colo. (CBS4) — Investigators with the Boulder County Sheriff’s Office are considering a number of possible causes of the Marshall Fire, a grass fire which was propelled by 100-mph winds into southern Boulder County subdivisions and which quickly became the state’s costliest wildfire in terms of homes destroyed.

A new possibility emerged late last week – that the remains of turn-of-the-century coal mines which have been slowly burning underground for decades may have reached the soil surface during the Dec. 30 windstorm and ignited the destructive blaze.

(credit: CBS)

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“We are investigating any and all potential causes of the fire including coal mines in the area, power lines, human activity, etc.,” Carrie Haverfield, a spokesperson with the sheriff’s office, confirmed Friday. “We are working diligently with our investigation partners to ensure the outcome of the investigation is thorough and accurate. We know our community members want to know what happened and we are doing everything we can to make that happen.”

More than 1,000 homes were destroyed in the fire and hundreds more were damaged. Most were located in the city of Louisville and the town of Superior. The western edge of both municipalities lies about two and a half miles directly east of the fire’s presumed ignition point near the intersection of Marshall Road and Highway 93.

(credit: Anjan Sapkota)

Investigators received immediate clues into the source of the fire with the help of video taken during the fire’s early stages. Those images showed flames engulfing a small shed on private property to the north of the intersection.

RELATED: Marshall Fire: Search Warrant Executed At Site Of Shed Fire Seen In Video

(credit: Jack Pommer)

Last week, CBS4 crews found BCSO patrol vehicles guarding the entrance to the Marshall Mesa Trailhead on the intersection’s southeastern corner. A short distance down this trail is where two closed coal mines, referred to simply as Marshall No.1 and Marshall No.2, are known to be buried.

An overhead view of the Marshall Mine site looking southeast over Marshall Road. The southern edge of the Twelve Tribes property where Boulder County investigators have examined the remains of a burned shed is seen at bottom right. (credit: Colorado Division of Reclamation Mining and Safety)

According to a report on active mine fires in the state which was compiled by the Division of Reclamation, Mining and Safety in 2018, the Marshall mines operated from 1863 to 1939.

Fissures, venting, and the odor of coal combustion were noted during an inspection in 2003. On Dec. 20, 2005, a brush fire was ignited by heat from a vent at the site. That fire was quickly contained and extinguished, the report stated. A temperature reading at the vent registered 373 degrees.

The U.S. Department of the Interior responded by dumping 275 tons of aggregate (small rock) on the site the next month, burying the vents another 18 inches. Additional compaction and grading was done 2016. More vents were found, but their exhaust was “warm, moist air” that only measured 90 degrees, according to the report.

The underground fire at that time was deemed to be little risk to the public at that time.

“The fire’s activity is very low and thus presents little potential to start a surface fire,” the report stated. “(But) this fire has behaved erratically in the past, so it is recommended that it be monitored annually to check for increases in fire activity.”

A map of Colorado coal regions and abandoned coal mines, as shown in a 2018 report. (credit: Colorado Geologic Society)

The state report used data collected by – and inspections conducted in 2017 and 2018 by – Tetra Tech, a California-based consulting and engineering firm with an office in Golden. The report identified 38 active underground mine fires in Colorado. Only five were located east of the Continental Divide. This included the Marshall pair, plus another site a short distance away.

A map showing known active coal mine fires in Colorado, according to a 2018 report. (credit: Colorado Division of Reclamation, Mining and Safety)

The Lewis No.1 and Lewis No.2 mine sites are also the vicinity of the Marshall Fire. Both are mostly located on private property near the intersection of Marshall Road and Cherryvale Road, perhaps 2,000 feet from the Marshall mine sites.

An overhead view of the Lewis Mine site showing the locations of active underground fire. (credit: Colorado Division of Reclamation, Mining and Safety)

The Lewis Mine operated from 1914 to 1946. A vent was found on the site in February of 2018, thanks to recent snowfall. But the temperatures here barely measured above those ambient temps elsewhere at the site.

It, too, was declared low risk.

A fire vent at the Lewis Mine site in February 2018. (credit: Colorado Division of Reclamation, Mining and Safety)

Part of the Lewis mine site is on City of Boulder property managed by its Open Space and Mountain Parks department. All of the Marshall site belongs to the City of Boulder.

“We are cooperating with the agencies involved in the investigation,” Phillip Yates, Communications Manager for Boulder Open Space and Mountain Parks, said Monday.

Coal mining began in the area in 1859, sixteen years before Colorado became a state, according to a publication from Boulder Open Space and Mountain Parks and the Colorado Historical Society. The area is in fact named after Joseph Marshall, a coal baron, and 51 mines were officially registered there with the state over the course of production.

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The publication states that Marshall is “cited as one of the oldest coal mining operations in the western United States.”

Most of the marketable coal was excavated by 1939. What was left, according to the publication, “was being consumed by underground mine fires that raged uncontrolled through several old mine workings.”

The City of Boulder began purchasing the land in 1971.

(credit: Jim Mimiaga/Cortez Journal)

An underground mine fire ignited a wildfire in southwestern Colorado in 2019. The Coal Seam Fire was quickly extinguished. The underground fire was extinguished the following year.

(credit: Jim Mimiaga/Cortez Journal)

A separate wildfire by the same name burned 29 homes and 12,000 acres near Glenwood Springs in 2002. It, too, erupted during high winds.

 

CBS4 reached out to the Colorado Division of Reclamation, Mining and Safety on Monday, and also Tetra Tech headquarters in Pasadena, Calif. No responses have been received. This story will be updated with any that are received.

Logan Smith