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.”

The uranium was detected in samples from a nearby feeder canal as well as from water near a smaller dam created to stop water flow to the work site, known as the Chimney Hollow Cofferdam. At the Hansen Feeder Canal, readings came back at 0.2 parts per billion on Jan. 22 and 0.6 on March 3, Stahla reported, which are well below the 30 ppb federal standard for drinking water.

The readings at the cofferdam, however, were much high than the federal level allowed. The readings were 430 ppb on Oct. 28, 949 on March 12 and 1,890 on April 22, according to information from Stahla.

“The caveat to this data is to recognize this is a small amount of water behind the cofferdam at the Chimney Hollow site that varies greatly depending on precipitation events; in addition, some of the water behind the cofferdam has been used and reused to wash sediments for placement in specific areas of the dam, so uranium has been concentrated in its readings,” he wrote in an email, stressing that with a dry April, no water was added that month.

Northern Water is using those numbers to predict what the level may be in Chimney Hollow once water flows into the reservoir and to inform what mitigations may be necessary, Stahla said. Plus he stressed that when the water flows, the levels will dilute.

Stahla emphasized that uranium occurs naturally in Colorado’s geology and is occasionally detected in other raw water supplies. Many utilities already treat for it, and the presence of uranium in mineral form doesn’t pose an immediate public health threat. Still, federal and state drinking water standards require mitigation if concentrations exceed regulatory thresholds.

A spokesperson from the Colorado Department of Public Health and Environment echoed Stahla’s explanation.

“When water comes into contact with exposed uranium, which is often naturally occurring, it can leach into the water. We do not yet know what those concentrations will be or how they will change over time, as it will be impacted by the concentrations in the geology and the dilution from the water in the reservoir, but it will be monitored closely,” Kaitlyn Beekman said in an email.

“Once the reservoir becomes fully operational, the department is requiring Northern to monitor for many parameters in their water quality certification, including uranium.”

The email added, “At this time, drinking water has not been impacted by the Chimney Hollow project, and there are no plans for utilities to get water from the reservoir for many years. The division will continue to monitor the situation and work with Northern Water to ensure appropriate safeguards are in place for water quality. We understand that Northern Water has contacted all entities that will be using this water for drinking water and alerted them of the situation and subsequent actions.”

Loveland Water and Power is one of the entities with water shares in Chimney Hollow. In response to the announcement, utility officials said there is no risk to Loveland’s drinking water at this time. A spokesperson for the utility noted that Chimney Hollow Reservoir is not yet part of the city’s active water supply, and that even if uranium were detected at concerning levels, “we’re well equipped to manage the risks.”

“Drinking water is our top priority,” said Tracey Hewson, public information officers for the utility. “Our water sources remain safe. We are proactively monitoring with regular testing and oversight. There’s no risk to our community at all.”

Gary Wockner, director of Save the Poudre, a nonprofit environmental group, pointed out that Greeley has a similar problem with the proposed Terry Ranch aquifer storage project. About that, he said, “We’ve been looking into the issue of whether it makes sense to divert clean Poudre River water and inject into an aquifer contaminated with uranium. We are continuing to investigate whether this violates the Save Drinking Water Act.”