Northern Water has been testing how the uranium minerals leach into water and what concentration to expect when the reservoir fills and its operation begins. To allow time for additional data collection and investigations to advance, we have elected not to fill the reservoir as quickly as initially planned. A small amount of water (less than 2 percent of total capacity) will be moved into Chimney Hollow Reservoir in November 2025. During this time, additional water quality data will be collected and used to evaluate the performance of model simulations, and required dam safety monitoring will begin. Even as the reservoir fills, no water will be released as further assessments are underway and mitigation options continue to be evaluated.
Because the mineralized uranium is coming from materials quarried at the site, excess (unused) rock from construction has been buried under a layer of water-sealing clay. The clay cap will effectively minimize uranium leaching from these materials.
We expect uranium leaching from the dam to decrease over time because there is a finite quantity of soluble uranium at the site. The duration of the leaching process is not yet fully understood and will depend on how the reservoir is operated over time. While the discovery of mineralized uranium has caused Northern Water and the Chimney Hollow participants to modify their plans, it is an issue that can be safely managed. The new reservoir remains an important part of securing water supply needs for Northern Colorado and its future. Please visit the Water Quality page on our website for more information and a list of Frequently Asked Questions.