One challenge during construction arose through water quality sampling at Chimney Hollow Reservoir that revealed the presence of uranium-bearing minerals in water held at the site. Mineralized uranium naturally occurs in some of Colorado’s geological formations, and uranium exposure in small amounts is common for Colorado residents. Naturally occurring mineral uranium is not a significant source of radioactivity. Uranium in drinking water is monitored by state and federal regulators for its health effects through long-term ingestion. It is occasionally found in groundwater, and some drinking water systems treat for it in their raw water supply.
The uranium at Chimney Hollow is naturally occurring in the granitic rock formations that were quarried to supply material for the dam. As the reservoir fills, these rocks will be in direct contact with water, leading to the leaching of uranium into the water. This issue will initially require close monitoring and management when filling Chimney Hollow Reservoir. The leaching of uranium into water is expected to decrease over time, although it is currently uncertain how long this process will take. No water will be released from Chimney Hollow Reservoir until further assessments are complete.
Dam safety and other tests will continue over the next two to three years as mitigation strategies are being pursued and implemented.