Broomfield monitors uranium finds at Chimney Hollow reservoir; no deliveries until assessments complete

City and County of Broomfield staff told council that sampling near Chimney Hollow reservoir — the new Windy Gap storage facility where Broomfield holds roughly 29.4% of the storage allocation — has shown detectable concentrations of naturally occurring uranium in water retained behind a cofferdam.

"As Northern Water has published… minimized uranium was detected in water samples that was behind the cofferdam," Ken Rutt, director of water utilities, told council. He said the apparent source is natural leaching from granite used in nearby quarrying and noted that such mineralization occurs in various Colorado geologies.

Rutt emphasized that Northern Water and the reservoir participants are monitoring the situation and evaluating options. "No water will be delivered from Chimney Hollow Reservoir until the assessments are completed," he said. The city’s presentation cited further sampling planned for August and additional data expected by October.

Public‑health standard and treatment: Rutt referenced the Colorado Department of Public Health and Environment’s primary drinking water regulations and the uranium maximum contaminant level of 30 parts per billion. He told the council that conventional water treatment can remove a large fraction of uranium: "our conventional water treatment plant can remove anywhere from 85 to 95% that comes in the raw water," Rutt said.

Why this matters: Chimney Hollow is designed to store about 90,000 acre‑feet of Windy Gap water; Broomfield’s share is roughly 29.4% (about 26,464 acre‑feet), and project completion (including dam testing and first fill) is scheduled to continue through 2027. Staff said Chimney Hollow is critical to Broomfield’s long‑term water supply planning and that the city and the other 15 participants are working with Northern Water and engineering firms (Black & Veatch and Stantec) to characterize water chemistry and determine treatment options.

Next steps: staff said monitoring will continue, further data are expected in October, and no deliveries from Chimney Hollow will occur until safe‑supply plans are complete. Council members asked follow‑up questions about treatment and health risk; staff described the detection as a water‑quality discovery to be addressed through monitoring and engineering analysis.

Ending: staff said they will report new data as it becomes available and that decisions about treatment, timing and any operational changes will await the assessments now underway.