High concentration of uranium discovered in Chimney Hollow Reservoir

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High levels of naturally occurring uranium have been detected in water samples at the under-construction Chimney Hollow Reservoir southwest of Loveland, just months before Northern Water planned to start filling the reservoir that will provide water to a dozen cities and water districts.

Water supplier Northern Water plans to flow water into the 90,000-acre-feet reservoir in August, a month later than what was previously reported by the Coloradoan .

That water will serve Loveland, Greeley, Longmont, Erie, Superior, Louisville, Fort Lupton, Lafayette and Broomfield, as well as Platte River Power Authority, Little Thompson Water District and Central Weld County Water District.

"We actually did not know that it (uranium) was there," Northern Water spokesperson Jeff Stahla told the Coloradoan. "Uranium is not uncommon but was unexpected in this group of rocks."

Uranium is found in several areas in Colorado, most notably in the southwest part of the state and in certain areas of the Front Range, according to the Colorado Geological Survey.

Northern Water started construction on the reservoir in August of 2021.

Stahla said the "very low density uranium" is leeching from granitic rocks quarried on the west side of the reservoir and used to build the 350-foot tall dam.

However, uranium was concentrated in levels exceeding 1,000 parts per billion in water behind the project's cofferdam, a temporary holding area built to keep water away from construction, and levels of "hundreds part per billion" in water behind the reservoir dam, Stahla said.

The water behind the cofferdam consists of construction water from small amounts of Windy Gap Project water and runoff. The small amount of water behind the main dam is mostly from precipitation and intermittent creek water.

The U.S. Environmental Protection Agency's standard for safe levels of uranium in drinking water is 30 parts per billion.

Stahla said Northern Water first realized in 2024 that uranium was concentrating in the cofferdam water but that it took several months of testing to determine from where the uranium was leeching. Late last year and early this year, Stahla said it was discovered the uranium was coming from the granitic rock and fine granite from the crushing operation.

"We're still putting together the data from other readings and should have that data in a week or two," Stahla said.

How Northern Water plans to address uranium levels and potential impacts to human health, water supplies and recreation

Long-term exposure to high levels of uranium can lead to chemical toxicity affecting the kidneys, and some studies have linked uranium exposure to an increased risk of certain cancers, though further research is needed to confirm this, according to federal health officials .

Stahla said Northern Water has been working with the Colorado Department of Public Health and Environment and other state and federal agencies to address and monitor the uranium levels.

Kaitlyn Beekman, spokesperson for the Colorado Department of Public Health and Environment's Water Quality Control Division, said the division will work with Northern Water to assure water quality standards are met.

"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," Beekman said in an email. "We understand that Northern Water has contacted all entities that will be using this water for drinking water in the future and alerted them of the situation and subsequent actions."

Stahla said uranium will leach into the reservoir's water from the granitic rock used to build the dam. The amount of uranium leeching into the water is expected to decrease over time, but at what rate is uncertain. He added because Northern Water doesn't plan to release water from Chimney Hollow until 2027, it allows time to address the issue.

"After the first fill and couple of times the water has been through the reservoir, those numbers should decrease based on preliminary testing," Stahla said. "We will address the uranium levels to make sure of the quality of our water but we don't want to put in giant infrastructure (to treat the water) if this will dissipate over time."

Stahla said water from Chimney Hollow will pass through Carter Lake and Horsetooth Reservoir, the later which partially supplies Fort Collins, so Northern Water will work with municipalities those reservoirs serve as well as those municipalities and water districts Chimney Hollow directly serves.

Fort Collins did not show uranium as a contaminant in its 2024 water quality report.

The Coloradoan reached out the morning of June 6 to Nicole Poncelet-Johnson, city of Fort Collins One Water executive director, but the director had not responded by 3:30 p.m. June 6.

Municipalities in areas with unsafe uranium levels in its water supply most commonly treat the water with reverse osmosis or ion exchange that effectively reduce uranium levels by up to 95% to 98%.

Stahla said it's uncertain if water will flow from Chimney Hollow in 2027 as planned, pending how uranium level testing goes.

He added Northern Water has hired a consultant to investigate possible impacts to human health if coming into contact with water containing uranium and will monitor the impacts to fish, which are planned to be stocked in the reservoir.

"We're confident that the data will come back that it won't pose a risk to recreation users," Stahla said. "If we think it will be an issue for fish, let's not stock it until we make sure we have the uranium flushed out to safe levels."

Stahla said Northern Water "learned a lesson" and will test for uranium at Glade Reservoir northwest of Fort Collins. That reservoir is part of Northern Water's Northern Integrated Supply Project , with construction scheduled to start later this year.

The city of Fort Collins is in the process of enlarging Halligan Reservoir, 25 miles north of the city. It is unknown at this time if that site will be impacted by high levels of uranium.

"This is really the first big reservoir built in the last 25 years and when Carter Lake and Horsetooth Reservoir was built in the 1950s, they didn't have fine data gathering like we do now," Stahla said.

This story has been updated to add new information.

This article originally appeared on Fort Collins Coloradoan: High concentration of uranium discovered in Chimney Hollow Reservoir