Colorado health officials were reviewing an explanation from Cotter Corp. on Monday after a spill at Cotter’s defunct uranium mill in central Colorado — one of the nation’s slowest Superfund cleanups.

A pipeline leaked about 1,800 gallons last week on Cotter’s 2,538-acre property uphill from Cañon City and the Arkansas River.

Well tests in July found water in the waste pipeline area contained elevated uranium (577 parts per billion, above a 30 ppb health standard) and molybdenum (1840 ppb, above a 100 ppb standard).

This spill was the latest of at least five since 2010. Federal authorities in 1984 declared an environmental disaster and launched a Superfund cleanup.

From 1958 to 1978, Cotter processed uranium for nuclear weapons and fuel at the mill, discharging liquid waste, including radioactive material and heavy metals, into 11 unlined ponds. The ponds were replaced in 1982 with lined waste impoundments.

Earthen dams and a pump at the low end of Cotter’s property, about 1½ miles from Cañon City’s Lincoln Park neighborhood, trap water and move it to a pond for treatment.

Colorado Department of Public Health and Environment officials last week issued a notice of the spill, saying Cotter reported that it was contained. A report from Cotter, sent Monday, said a coupler on a 6-inch pipeline broke Nov. 24 or early Nov. 25 and was fixed.

Cotter manager Stephen Cohen said it’s unlikely this spill will worsen contamination of groundwater.

“We’re going to have to take a fresh look at what’s going on, in order to prepare a remedial investigation report” for a cleanup, Cohen said.

Cotter is a subsidiary of San Diego-based defense contractor General Atomics.

“They need to eliminate the contamination at its source,” said attorney Travis Stills, who represents the community group Colorado Citizens Against Toxic Waste.

Buried mill tailings and impoundment ponds “continue to be sources of contamination. It’s some of the most toxic mining residue you could have — all of what you’d expect to find at a Gold King disaster, plus an overlay of uranium and radioactive isotopes, flowing into groundwater with a very direct route to people and the Arkansas River, ” Stills said. “What’s it going to take to get real action?”