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November 13, 2025

Northern Water will further delay an initial partial filling of its new Chimney Hollow Reservoir into next year to allow time for expanded groundwater tests in the area to make sure unexpected uranium leaching inside the planned pool would not migrate to other supplies.

After spending years permitting and constructing the dam west of Loveland, Northern Water was surprised in June that routine water quality tests ahead of the filling go-ahead found natural uranium leaching out of rocks exposed from a quarry used for dam fill. Initial water fill-up was then delayed for testing, to see how long the leaching might last, and how the uranium would be diluted when water diverted under the Continental Divide by the Colorado-Big Thompson system eventually fills Chimney Hollow.

Now Northern Water says it needs more time to test groundwater outside the reservoir to provide background levels of naturally occurring uranium, and determine whether a filled reservoir would “influence” nearby groundwater with uranium-tainted water. A Northern Water spokesperson used “influence” rather than “leaking” to describe what engineers would be watching.

“Influence or mixing of surface and groundwater can vary greatly, depending on many factors, scenarios and even locations,” spokesperson Amy Parks said. “Without adequate baseline data, we are not able to assess those future conditions, so this short delay allows us to do that work.”

Similar-sized Carter Lake Reservoir is just over a ridge that makes up the east edge of Chimney Hollow.

“At this time, due to the existing bedrock, we do not think that migration of water from Chimney to Carter is likely. However, additional monitoring will help us ensure that can be detected in the future,” Parks said.

Filling of a small portion of the reservoir had been planned for this month, but now is “expected in early 2026,” according to the agency.

The 12 Northern Water members that bought into the project, including the cities of Broomfield and Loveland, are already paying for construction bonds through their rates. The delay in filling the reservoir is not expected to affect their finances, the utility said.

Members were not scheduled to receive Chimney Hollow water for years. “This doesn’t affect water deliveries or anything that project participants have been expecting, so it’s a good time” to widen testing protocols,  Parks said in an interview.

“It’s really just an abundance of caution and making sure we’re putting the health and safety of our public and neighbors into priority, and making sure we’re crossing our t’s and dotting our i’s before we take that step of adding water,” she said.

What mitigation is necessary remains unknown

Northern Water still does not know the scale of mitigation required to keep uranium in Chimney Hollow water at safe levels. The agency earlier this year said it believed uranium leaching would decrease over time as stored water stopped penetrating farther into the naturally occurring seams. Excavators have now capped some unused construction materials that will eventually be underwater with a clay layer that will prevent some leaching.

If uranium levels in the filled pool do not drop far enough, other mitigation measures could include a water treatment plant or system below the reservoir, Parks said. Northern Water does not yet have a cost estimate on how much the testing, delays or treatment will cost, until more testing is complete, she said.

Engineering and testing teams decided “it’s best to delay this for a few months to make sure that we have the groundwater samples from the reservoir, from around the reservoir, before that water goes in there,” Parks said. “We just want to make sure that any water that goes into the reservoir now doesn’t influence groundwater around it.”

Chimney Hollow was built to store 90,000 acre-feet of water for 11 northern Colorado communities and water agencies and the Platte River Power Authority. The project was meant to “firm” or store water rights Northern Water owns in the Windy Gap project near Granby, which collects and pumps Colorado River water into the Adams Tunnel for Front Range buyers. Windy Gap and Chimney Hollow allow the Front Range communities to take advantage of their water rights in wet years when Lake Granby is too full to contain their portion of the river. Northern Water has also suffered setbacks this year on its other major project, the proposed $2.7 billion twin-reservoir Northern Integrated Supply Project. Some members of NISP, a slightly different list than Chimney Hollow members, are warning they will pull out of the two-dam and pipeline construction plan after decades of permitting because costs have risen too high and delays raise uncertainty.

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