A broken pipe at Cotter Corp.’s dismantled mill in central Colorado spewed 20,000 gallons of uranium-laced waste — just as Cotter is negotiating with state and federal authorities to end one of the nation’s longest-running Superfund cleanups.

Colorado Department of Public Health and Environment officials said last weekend’s spill stayed on Cotter property.

In addition, uranium and molybdenum contamination, apparently from other sources on the Cotter property, has spiked at a monitoring well in adjacent Cañon City. A Feb. 20 report by Cotter’s consultant said groundwater uranium levels at the well in the Lincoln Park neighborhood “were the highest recorded for this location,” slightly exceeding the health standard of 30 parts per billion. State health data show uranium levels are consistently above health limits at other wells throughout the neighborhood but haven’t recently spiked.

“This isn’t acceptable,” Fremont County Commissioner Tim Payne said of the spill – the fourth since 2010. “(CDPHE officials) told us it is staying on Cotter’s property. But 20,000 gallons? You have to worry about that getting into groundwater.”

Environmental Protection Agency and CDPHE officials are negotiating an agreement with Cotter to guide cleanup, data-gathering, remediation and what to do with 15 million tons of radioactive uranium tailings. Options range from removal — Cotter estimates that cost at more than $895 million — or burial in existing or new impoundment ponds.

Gov. John Hickenlooper intervened last year to hear residents’ concerns and try to speed final cleanup.

Cotter vice president John Hamrick said the agreement will lay out timetables for the company to propose options with cost estimates.

The spill happened when a coupler sleeve split on a 6-inch plastic pipe, part of a 30-year-old system that was pumping back toxic groundwater from a 300-foot barrier at the low end of Cotter’s 2,538-acre property, Hamrick said.

Lab analysis provided by Cotter showed the spilled waste contained uranium about 94 times higher than the health standard, and molybdenum at 3,740 ppb, well above the 100-ppb standard for that metal, said Jennifer Opila, leader of the state’s radioactive materials unit.

She said Cotter’s system for pumping back toxic groundwater is designed so that groundwater does not leave the site, preventing any risk to the public.

In November, Cotter reported a spill of 4,000 to 9,000 gallons. That was five times more than the amount spilled in November 2012. Another spill happened in 2010.

At the neighborhood in Cañon City, the spike in uranium contamination probably reflects slow migration of toxic material from Cold War-era unlined waste ponds finally reaching the front of an underground plume, Hamrick said.

“It is a blip. It does not appear to be an upward trend. If it was, we would be looking at it,” Hamrick said. “We will be working with state and EPA experts to look at the whole groundwater monitoring and remediation system.”

An EPA spokeswoman agreed the spike does not appear to be part of an upward trend, based on monitoring at other wells, but she said the agency does take any elevated uranium levels seriously.

The Cotter mill, now owned by defense contractor General Atomics, opened in 1958, processing uranium for nuclear weapons and fuel. Cotter discharged liquid waste, including radioactive material and heavy metals, into 11 unlined ponds until 1978. The ponds were replaced in 1982 with two lined waste ponds. Well tests in Cañon City found contamination, and in 1984, federal authorities declared a Superfund environmental disaster.

Colorado officials let Cotter keep operating until 2011, and mill workers periodically processed ore until 2006.

A community group, Colorado Citizens Against Toxic Waste, has been pressing for details and expressing concerns about the Cotter site. Energy Minerals Law Center attorney Travis Stills, representing residents, said the data show “the likely expansion of the uranium plume, following the path of a more mobile molybdenum plume” into Cañon City toward the Arkansas River.

The residents deserve independent fact-gathering and a proper cleanup, Stills said.

“There’s an official, decades-old indifference to groundwater protection and cleanup of groundwater contamination at the Cotter site — even though sustainable and clean groundwater for drinking, orchards, gardens and livestock remains important to present and future Lincoln Park residents,” he said. “This community is profoundly committed to reclaiming and protecting its groundwater.

Bruce Finley: 303-954-1700, bfinley@denverpost.com or twitter.com/finleybruce