Water Quality

Frequently Asked Questions

What is the problem?

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.  

Where does the uranium come from?

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.  

When was the uranium first detected?

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.  

Why was uranium only recently discovered?  

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.  

What is uranium?

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.  

How much uranium is found in rock at the site?  

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.   

 How was uranium discovered?  

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.  

What do the preliminary data say?

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.   

Are you planning on putting water into the reservoir soon?

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) will be moved into Chimney Hollow Reservoir in November 2025. 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.  

When will water be delivered out of the reservoir?

No water will be delivered from Chimney Hollow Reservoir until assessments are complete, and a mitigation plan is developed to ensure a safe water supply for downstream water delivery.   

What is Northern Water going to do about it?

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.  

Is there an exposure risk for those who worked on or visited the construction site?

Data collected through air quality monitoring during construction shows that on-site exposure to dust is below workplace safety thresholds.  

Is Chimney Hollow still going to be used for water supply?

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.  

Are there other compounds we are looking at in light of the uranium discovery?  

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.