Just weeks away from the beginning of fill operations at Chimney Hollow Reservoir west of Loveland, Northern Colorado Water Conservancy District has announced that uranium-bearing minerals have been detected in water quality sampling at the site.

According to district officials, the uranium is naturally occurring in the granitic rock formations surrounding the reservoir and is not a significant source of radioactivity.

“The uranium that was detected in mineral form is the type the EPA regulates it because of its chemical effect,” said Jeff Stahla, public information officer for the district. “They have a threshold and it has to do with long-term exposure. So, in other words, the risk is to people who are drinking it year after year.”

Located just west of Carter Lake, Chimney Hollow Reservoir is the centerpiece of the Windy Gap Firming Project, a long-planned effort to improve water resilience for communities across northeastern Colorado. When full, the reservoir will hold 90,000 acre-feet of water that will be shared by 12 municipal partners: Broomfield, Platte River Power Authority, Loveland, Greeley, Longmont, Erie, Little Thompson Water District, Superior, Louisville, Fort Lupton, Lafayette and Central Weld County Water District.

The granitic rock in question was quarried on-site to construct the reservoir’s 350-foot-high asphalt-core dam, just the second of its kind ever built in the United States.

As the reservoir fills, uranium-bearing minerals will leach into the water, a process that will diminish over time.

But exactly how much time that will take is still an open question, Stahla said. Releases from Chimney Hollow were originally scheduled to begin in 2027, but that date is now in doubt.

Though filling will start as planned in August, no water will be released for delivery to municipal partners until further assessments are complete. That includes additional water quality testing and long-term monitoring, plus treatment and mitigation measures.

“In Colorado, uranium appears in groundwater pretty frequently, and so it’s just a matter of treating it and getting it to a level below the EPA threshold and making sure long-term exposure risks are eliminated,” Stahla said. “…Depending on your definition of short-term, this could be several years, but certainly we don’t think that this is going to be an issue of this magnitude for the life of the dam.”