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Great Western Reservoir: A Broomfield Water Supply in the Shadow of Rocky Flats
Great Western Reservoir helped make Broomfield's growth possible. Then contamination from the Rocky Flats nuclear weapons plant immediately upstream changed the reservoir's history—and ultimately changed where Broomfield got its drinking water.
Drive along North Indiana Street near Highway 128 and look southeast, and you'll see a large reservoir with Denver visible in the distance. It's called Great Western Reservoir. Unlike nearby Standley Lake, Great Western isn't surrounded by a regional park or generally open to public recreation. Today it serves primarily as part of Broomfield's non-potable water system.
But Great Western wasn't always on the margins of the community. For decades, it was an important part of Broomfield's water supply. And its transformation tells an important story about what happened when a growing Colorado community found itself immediately downstream from the Rocky Flats nuclear weapons plant.
Before Rocky Flats, there was Great Western Reservoir
The reservoir's history began long before nuclear weapons were manufactured at Rocky Flats.
Adolph J. Zang established the Broomfield Reservoir and Ditch Company to create an irrigation reservoir for agricultural land owned by the Zang family. Construction began around 1903 and was completed around 1911. Water rights and infrastructure brought water into the reservoir to support the surrounding agricultural operation. Then the landscape began changing. In the 1950s, the Turnpike Land Company started developing Broomfield Heights, transforming farmland between Denver and Boulder into a planned suburban community.
The company purchased the former Zang Ranch in 1955. Great Western Reservoir came with it. Suddenly, a reservoir created for agriculture had another purpose: helping provide water for a growing city.
Promotional materials emphasized the reservoir's enormous capacity. Real-estate agents were even taken on tours of Great Western as developers promoted the new community.
Beginning in 1963, the Broomfield Heights Mutual Service Association sold Great Western water to the city. By 1971, Broomfield was obtaining water from both Great Western Reservoir and Denver Water. But immediately west of the reservoir was another rapidly developing part of Colorado's landscape. Rocky Flats.
Rocky Flats was upstream
From 1952 until production ended in 1989, Rocky Flats manufactured components for America's nuclear weapons program. And the geography mattered. Great Western Reservoir sat downstream from the plant. Walnut Creek drained portions of Rocky Flats and flowed toward the reservoir. That meant contaminants carried by surface water could potentially move from the nuclear weapons facility toward a reservoir being used by Broomfield. By the early 1970s, government testing was documenting that connection.
Plutonium was detected in Walnut Creek and reservoir sediment
According to the Broomfield Historical Society's reconstruction, EPA testing in 1970 detected plutonium in Walnut Creek associated with leakage from Rocky Flats. Then came another significant finding. A 1977 Washington Post investigation reported that EPA sampling had found plutonium in layers of sediment at the bottom of Great Western Reservoir in 1973 at concentrations described as 40 times normal levels. That historical wording deserves some caution. "Times normal" isn't the way we'd ideally communicate modern radiological measurements, and it doesn't by itself tell us what dose anyone received or establish a health effect. But the underlying historical point is important: plutonium associated with Rocky Flats had been detected beyond the plant and in the sediment of a reservoir supplying Broomfield.
Then came tritium
Plutonium wasn't the only radionuclide to reach the reservoir. In 1973, contaminated scrap material brought to Rocky Flats resulted in a release of tritium, a radioactive form of hydrogen.
Tritium was subsequently detected in Walnut Creek and Great Western Reservoir. The Broomfield Historical Society cites DOE's own Rocky Flats history for the incident. Federal records indicate that the measured concentrations in Great Western remained below the drinking-water standard cited at the time. That's an important part of the record and should remain in our story. So should the other part: radioactive material released at Rocky Flats had reached a reservoir used by a neighboring community. Those two facts can exist together.
Broomfield pushed back
The contamination issue increasingly strained relations between Broomfield and Rocky Flats.
The Historical Society recounts a revealing moment from 1976 and 1977. A task-force report discussing contamination from Rocky Flats also emphasized the plant's importance to the local economy. Broomfield officials weren't impressed.
Then-city manager George Di Ciero told the Washington Post:
"They had mistakes, accidents and incidents up there and they were, in effect, telling us we should take it because of their payroll."
That statement captures something statistics alone cannot. For Broomfield, Rocky Flats wasn't merely a distant federal environmental issue. It was becoming a local water issue.
June 1989 changed everything
On June 6, 1989, FBI and EPA agents raided Rocky Flats as part of an investigation into possible violations of environmental law. Broomfield responded immediately. According to the Historical Society's account, the city switched its drinking-water source to Denver Water.
Local citizens also dug a ditch designed to divert Walnut Creek water around Great Western Reservoir rather than allowing water draining from the Rocky Flats direction to enter it.
Think about what that represented physically. For decades, water moving east from the Rocky Flats area had flowed toward Great Western. Now Broomfield was deliberately routing it around the reservoir.
Replacing Great Western as drinking water
The change eventually became permanent. A Department of Energy grant, combined with the sale of Broomfield's Clear Creek water rights, helped finance the 1991 Great Western Reservoir Replacement Project. The project included new water rights, a new water-treatment facility and a pipeline to bring water to it. Broomfield also expanded its secondary supply through an agreement with Denver Water. Great Western's role changed. By 2004, according to the Historical Society, the reservoir had been relegated to Broomfield's water-reuse system, supplying non-potable water for irrigation rather than drinking water. That distinction is important for anyone reading this history today: Broomfield residents do not currently receive their drinking water from Great Western Reservoir.
The plutonium didn't simply disappear
Removing Great Western from the drinking-water system didn't erase its environmental history. The Broomfield Historical Society notes in its 2024 article that plutonium remains in Great Western Reservoir sediments. Federal public-health assessments have also evaluated plutonium measured in Great Western sediments. That doesn't mean merely being near the reservoir exposes someone to a dangerous dose of radiation. Presence, exposure and health effect are three different questions. Nor does historical contamination prove that Rocky Flats caused an individual person's illness. But the contamination itself is part of the documented record. And so is Broomfield's response to it.
A reservoir that tells Broomfield's story
There is something especially revealing about Great Western Reservoir. It wasn't built because of Rocky Flats. It was built for agriculture. Then it helped support the creation of suburban Broomfield. Water once made this landscape valuable for farming. Later, reliable water helped make large-scale residential development possible. Then the nuclear weapons plant immediately upstream introduced an entirely different problem into that history. Plutonium was detected. Tritium reached the reservoir. Broomfield challenged the federal government's response. After the 1989 raid, the city stopped relying on Great Western for drinking water and physically diverted Rocky Flats drainage around it. Eventually, an entirely new drinking-water system replaced it. Today the reservoir remains—but with a different purpose.
Why Great Western matters to Rocky Flats Downwinders
One of the most difficult things about understanding Rocky Flats is recognizing that the plant boundary wasn't necessarily the boundary of its environmental history.
Air moved.
Water moved.
Contaminants moved.
Great Western Reservoir provides a particularly understandable example because we can trace that relationship geographically and historically. There was a nuclear weapons plant.
Immediately downstream was a reservoir. That reservoir supplied water to a growing community. Radioactive materials associated with Rocky Flats were detected in the watershed and reservoir. And eventually Broomfield changed its water infrastructure in response to concerns surrounding Rocky Flats. That doesn't answer every health question.
But it does establish why those questions exist. And it helps explain why communities surrounding Rocky Flats continue asking for better research, monitoring and recognition today. More than a century after it was constructed, Great Western Reservoir remains part of Broomfield's landscape. It is also something else: a visible piece of Colorado's nuclear history.
From the Broomfield Historical Society
This article draws substantially from David R. Feineman's excellent 2024 history of the reservoir for the Broomfield Historical Society. It includes historical photographs, the original reservoir plan, a contamination map and extensive source notes.
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