Northern Water discovered uranium at the Chimney Hollow Reservoir site in the rocks quarried on site, which were used to form the majority of the dam structure. When the reservoir is filled, and the rocks are immersed in water, uranium will leach from the rock.
Chimney Hollow Water Quality
Naturally Occurring Uranium Found at Chimney Hollow
Northern Water and Chimney Hollow participants are committed to keeping its customers, stakeholders and end users, as well as the general public informed as we gather additional information on the discovery of uranium at the Chimney Hollow Reservoir construction site. Collecting data and modeling are crucial steps in the development of mitigation strategies, and we are actively working to learn more by evaluating test results from field investigations and modeling scenarios.
Prior to making mitigation decisions, we want to make sure we have all the information to evaluate operational and treatment options. We are following a rigorous process, starting with geochemical characterization and scoping studies, to inform mitigation alternatives analysis and ultimately select a final approach. Following these steps allows us to make informed decisions, evaluate trade-offs, and determine the best path forward.
Learning. Evaluating. Planning.
Northern Water has been testing how the uranium minerals leach into water and what concentration to expect when the reservoir fills and as 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. After the discovery of naturally occurring uranium at Chimney Hollow Reservoir, many avenues of data collection have been initiated to learn more about this situation and inform mitigation decisions. Northern Water moved approximately 1,500 acre feet of water into Chimney Hollow Reservoir in mid-April 2026. Additional water quality data will now be collected and used to evaluate the performance of model simulations used to predict uranium concentrations in the reservoir. In addition, post-fill dam safety monitoring will begin. Project participants have not expected or planned for delivery of water to occur for several years, and this postponement will not affect community water supplies.
Northern Water and project participants have been studying how the uranium minerals mobilize into water and what concentration to expect when the reservoir fills. All testing to date has been small scale and laboratory based. The addition of a small amount of fresh water in the reservoir, less than 2 percent of the total reservoir volume, will provide a better opportunity to study real-world conditions. In addition, Northern Water has sampled groundwater in the area to characterize baseline water quality conditions. This information, paired with data collected from the initial inflow of water in the reservoir, will inform further management options.
We expect uranium leaching from the dam to decrease over time, since there is a finite quantity of soluble uranium at the site. The duration of the leaching process is not fully understood yet 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 its 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.
Water Quality Monitoring
No water will be released from Chimney Hollow Reservoir until a management plan is identified.
This dashboard displays uranium levels in Chimney Hollow Reservoir and in nearby sampling locations from early April 2026, when water was first introduced into the reservoir. Water sampling for uranium occurs weekly with final results typically available within two months of the sampling date. Final data will be posted to the dashboard when received, and it may not be received in chronological order due to processes at the testing laboratory.
The water currently in Chimney Hollow Reservoir represents less than 2 percent of its total capacity, and no additional water will be added during initial monitoring. The initial monitoring period provides Northern Water time to appropriately sample the new waterbody and determine a strategy to manage the naturally occurring uranium moving forward.
The EPA standard for uranium after it has been treated by the local water provider is 30 µg/L (or parts per billion). The reported values provided below are the sampling results of the raw (untreated) water presently held in the reservoir. For context, and as a comparison to nearby waterbodies, reported values for Flatiron Reservoir (outlet), Carter Lake and Horsetooth Reservoir are also provided. View additional data collected throughout Northern Water’s boundaries.
Background Information:
July 2026 Update:
Uranium is naturally occurring in some of the rocks found at the site. Those rocks were quarried, crushed and used to build the embankments that make up Chimney Hollow Reservoir.
Initial Uranium Monitoring: Uranium monitoring began in December 2021 under the requirements of the short-term construction discharge permit. Through spring 2023, uranium concentrations at the water treatment plant outfall remained below the U.S. Environmental Protection Agency (EPA) standard of 30 µg/L, and often below 10 µg/L.
Extreme Precipitation Events (May-June 2023): In May and June 2023, the site experienced unusually heavy precipitation, including a storm that delivered approximately 2.5 inches of rain. These events overwhelmed stormwater controls and led to emergency discharges from the site into Chimney Hollow Creek and Flatiron Reservoir.
Out of concern for potential impacts to downstream receiving waters, Northern Water proactively initiated additional water sampling at four sites. This monitoring started a baseline of data collection that continued afterward.
During project design, the engineering team completed a geochemical analysis of the onsite quarry that included 33 tests using standard EPA protocols. These tests were performed for 20 different metals; however, uranium was not included in the metals panel considered in the acid mine drainage screening. At the time of design, geochemical results indicated that the rock was non-reactive, which did not trigger the need for further analyses. Throughout the first year of construction, crews tested for uranium in the onsite water associated with construction as required by the construction dewatering permit. Uranium was not measured above EPA standard of 30 µg/L during that time, with results averaging about 4 ug/L, until extreme precipitation events in May-June 2023 began to surface the issue.
Naturally occurring uranium is a weakly radioactive heavy metal. Rocks, soil, surface and ground water, air, plants and animals (including humans) all contain varying amounts of uranium. Some geographical regions of the United States, particularly southwestern states such as Colorado and New Mexico, have concentrated natural deposits of uranium and extensive historic uranium ore mining and milling activities. For these reasons, some areas may have higher than average uranium levels, which may result in increased human exposure. Naturally occurring uranium emits very low levels of radioactivity that travel only short distances and cannot penetrate human skin. In its natural, unprocessed state, chemical toxicity via long-term ingestion or inhalation is the primary health concern, not radioactivity.
Uranium at the site is not concentrated in deposits of uranium ore in quantities significant enough for mining. Uranium at Chimney Hollow Reservoir occurs largely in small rock fractures that are not concentrated and are harder to identify.
Core samples of rock analyzed after discovery of the issue have shown a high degree of variability in uranium levels, even within a few inches of each other. Due to the difference in how uranium is deposited within granitic rock fractures, the concentrations of uranium minerals found at Chimney Hollow Reservoir rock are many times lower than those found in low-grade uranium mines.
Elevated levels of uranium were first detected in samples taken in May 2023 during emergency discharges from the site caused by unusually heavy rainfall. Although uranium monitoring had been ongoing since December 2021 under a construction site discharge permit, it had not previously been a concern due to consistent levels below the EPA standard of 30 ug/L. In response to the emergency discharge, Northern Water proactively initiated additional water quality sampling, which led to the detection of elevated uranium levels—specifically, 226 micrograms per liter (µg/L) in a sample collected from a small reuse pond where water was recycled continuously during construction
However, this initial detection did not fully reveal the scope of the issue. Uranium levels throughout the remainder of 2023 and into early 2024 were only episodically elevated and returned to levels below the EPA standard after the initial spike. Because the source of the uranium was unknown and the elevated levels were not sustained, the issue did not appear to be ongoing at that time.
In spring 2024, a second emergency discharge event from the site occurred, and this time uranium levels rose again—more consistently and at higher concentrations. This resurgence prompted a formal investigation into the source of the uranium. Despite expanded sampling and sediment testing throughout 2024, early efforts did not identify a clear cause. It was not until January-February 2025 that a potential source—granitic rock processed during construction—was suspected. This was confirmed through leaching tests conducted in February-March 2025, which demonstrated that uranium could leach from construction materials into the reservoir water.
Preliminary data from ongoing water quality testing indicate a risk of elevated uranium concentrations in Chimney Hollow Reservoir above the EPA standard of 30 ug/L. The duration of this risk is uncertain, under investigation, and is expected to decrease over time. Field surveys have clearly indicated that quarried granitic rock used to build the main dam and saddle dam for Chimney Hollow Reservoir is the source of uranium. No water will be delivered from Chimney Hollow Reservoir until all assessments are complete, and a mitigation plan is developed to ensure a safe drinking water supply source.
Yes. We have been testing how the uranium minerals leach into water and what concentration to expect when the reservoir fills and as its operation begins. To allow time for additional data collection and investigations to advance, we have decided not to fill the reservoir as quickly as initially planned. A small amount of water (less than 2 percent of total capacity) was moved into Chimney Hollow Reservoir in April 2026. During this time, additional water quality data will be collected, and water quality forecasts will be validated. Even as the reservoir fills, no water will be released as further assessments are underway, and mitigation options continue to be evaluated.
During the 2025 water year, 1,500 acre feet of water was set aside and stored in Lake Granby in anticipation of the first fill. This is the water that was moved into Chimney Hollow in April 2026.
No water will be delivered from Chimney Hollow Reservoir until assessments are complete, and a management plan is developed to ensure a safe water supply for downstream water delivery.
Northern Water and Chimney Hollow Reservoir Project participants are engaged in discussions regarding mitigation strategies.
Northern Water will work collaboratively with all stakeholders to develop a comprehensive mitigation and operational plan to ensure a safe water supply for all Windy Gap and C-BT allottees. Comprehensive monitoring will continue and be used to validate operational strategies and confirm that uranium concentrations are minimized in water supplied to downstream users.
Data collected through air quality monitoring during construction shows that on-site exposure to dust is below workplace safety thresholds.
Yes, although discovery of uranium is a setback to initial plans, it is an issue that can be safely managed and is expected to reduce over time. The new reservoir remains an important part of securing water supply needs for Northern Colorado and its future.
Yes, we have analyzed samples for more than 60 parameters, including uranium-related compounds, unrelated rare-earth metals, and a standard suite of metals and nutrients relevant to water quality and have not identified other parameters of concern.