Northern Water opened valves to pour the first 1,500 acre-feet of Western Slope water into its new Chimney Hollow reservoir near Loveland on Tuesday, though the agency is still testing uranium levels at the dam site and has no date yet for when members could start using the supply.
The dam, holding Colorado River water brought through the Continental Divide in the Colorado-Big Thompson system, can store up to 90,000 acre-feet of water just west of a ridge dividing Chimney Hollow from existing Carter Lake. The initial fill will let Northern Water test safety systems and infrastructure at the dam, and none of the new water will be released downstream to the 11 water providers that bought into Chimney Hollow until water quality tests are complete.
After spending years permitting and constructing the dam, Northern Water was surprised in June by a uranium problem. Routine water quality tests ahead of the planned initial reservoir loading found natural uranium leaching out of rocks exposed in a quarry used for dam fill.
The first water fill-up was then delayed for testing, to see how long the leaching might last, and how the uranium would be diluted when the Colorado-Big Thompson supply eventually fills Chimney Hollow. The water provider also wanted to conduct background level testing of groundwater in the area — Colorado’s geology includes pockets of naturally occurring uranium deposits and higher than normal background levels of radioactive elements.
“The addition of a small amount of fresh water in the reservoir, less than 2% of the total reservoir volume, will provide a better opportunity to study real-world conditions,” Northern Water said in a news release.
Many utilities in Western states must filter out uranium along with other potential contaminants as part of their routine water quality screening. Northern Water has said its test results at the quarry site and other locations have been highly variable “even within a few inches of each other” because of the nature of uranium deposits.
The agency “expects uranium levels in the reservoir to decrease over time” as the amount contained in the upstream main dam face washes away. “The movement of mineralized uranium out of the dam will depend on how the reservoir operates over time,” Northern Water said.